Physicists have coaxed a black hole's most famous glow out of a strand of optical fiber and, for the first time, watched that light react back on the simulated black hole that produced it.

The result gives researchers a rare, hands-on look at Hawking radiation ‪—‬ the faint thermal emission that Stephen Hawking predicted should leak out of black holes ‪—‬ and offers a first clue about the tiny push that could, in principle, make a real black hole slowly evaporate, the research team said in a new study.

Working with a tabletop experiment in optical fibers, the international team detected both the radiation and its long-sought "back reaction" — the way the radiation feeds energy back and reshapes the object that created it.

According to the new study, published July 1 in the journal Nature, the light behaved exactly as Hawking predicted it should: like the glow of a warm object, with a definite temperature and a spectrum that fades away steadily toward higher frequencies. It did so even in a regime where the usual textbook description of a black hole should break down.

Where three great theories collide

Hawking radiation is famous because it sits at the crossroads of physics' biggest ideas.

"Jacob Bekenstein predicted that black holes have an entropy and a temperature, and Hawking calculated the thermal radiation of the black hole," study co-author Ulf Leonhardt, a physicist at the Weizmann Institute of Science in Israel, told Live Science via email. "In Hawking-Bekenstein radiation, quantum physics, general relativity and thermodynamics come together — subjects that are normally in conflict with each other."

The conflict runs deep: General relativity pictures space and time as smooth and continuous, while quantum mechanics describes a world of discrete, unpredictable jumps ‪—‬ and no one has managed to fully reconcile the two.

That combination is exactly what makes Hawking radiation so hard to study. Astronomers have never seen Hawking radiation from a real black hole and probably never will; the glow is far too faint to pick out across the cosmos. So physicists have turned to laboratory stand-ins that obey the same equations, building black hole analogues out of flowing water, ultracold atoms and, as in this study, light.

Building a black hole from light

The trick behind every black hole analogue is a moving medium. "Imagine a swimmer in the sea with a current faster than he can swim," Leonhardt explained. "He is swept away. This is what happens beyond the [event] horizon, and this is why normally nothing can escape the black hole."

A black hole's event horizon is the boundary where that current — space itself, in real life — starts moving faster than anything can travel. To recreate it, the team needed a material that appears to rush along at the speed of light. Their solution was elegant: use light to make the "material."

"In optics we need a material that appears to move at the speed of light," Leonhardt said. "For this we use light itself — in nonlinear optics, light acts like a material."

In practice, the researchers fired an intense, ultrashort "pump" pulse into a thin photonic-crystal fiber — a strand of glass threaded with a pattern of tiny air channels running along its length, which lets researchers fine-tune how light moves through it. As it traveled, the pulse slightly changed how the glass bent light, creating a moving speed bump that raced along with it. A second, much weaker "probe" pulse then ran into this moving front. Where the probe could no longer keep up, an artificial horizon formed — and the black hole analogue was born.

Catching the glow and its pushback

The payoff came in the ultraviolet. According to theory, Hawking radiation is created in pairs: One partner escapes, while the other, carrying "negative" energy, is the mirror image that would fall into a real black hole. In the fiber, that partner showed up as ultraviolet light.

"We counted photons in the ultraviolet that correspond to the Hawking partners beyond the horizon," Leonhardt explained. "They have a wavelength around 233 nanometers. This was our signal."

Just as important as seeing the glow was understanding how it was made. For years, researchers assumed the fiber built up its Hawking radiation through a cascade — a chain of separate steps in which the light is converted first into one intermediate form, and then another, each feeding the next before the radiation finally emerges. The team found that, instead, a single, direct interaction does the job, with the pump and probe light producing the Hawking pair in one clean step. It is a much simpler picture that the researchers said may carry over to other analogues and perhaps even to real black holes.

Because energy has to come from somewhere, making Hawking radiation should nudge the source that created it. For a real black hole, that nudge is how it loses mass and, over unimaginable timescales, evaporates entirely — the process Hawking described in his landmark 1974 paper. No experiment had ever captured that recoil.

Here, the team saw it. Producing the radiation shifted a small fraction of the pump pulse's own light to a slightly different color, leaving a telltale lopsided pattern in the spectrum. That asymmetry, absent in earlier experiments, is the fingerprint of the back reaction, or recoil — the black hole analogue quietly paying the energetic price for its own glow.

The road to a quantum experiment

The result also speaks to one of the thorniest puzzles in black hole physics: the trans-Planckian problem. Trace Hawking's radiation back to where it was born and the calculation runs into territory no physicist can vouch for — the Planck scale, the vanishingly small size at which space and time are thought to lose their familiar meaning and all known physics gives out. Hawking's prediction, in other words, appears to rest on a foundation that may not exist.

"Any light getting away from the horizon is stretched out enormously," Leonhardt said. "So it must come from waves smaller than the tiniest scale in nature, where the physics is unknown. Would that still give Hawking radiation? That was the question, and we have answered it in our experiment." Remarkably, the glow stayed perfectly thermal even in this extreme regime.

The team's next step is concrete. So far, they have used ordinary laser light, which reproduces the spectrum of Hawking radiation but not its deepest quantum weirdness. Next, the team plans to "go quantum," Leonhardt said. "We will explore how to get into the quantum regime and observe quantum features such as entanglement" — the ghostly link that should tie each escaping Hawking particle to its lost partner.

See how much you know about black holes with our black hole quiz!

A new upgrade to the International Space Station's (ISS) quantum laboratory is enabling NASA to probe the behavior of atoms further than ever before, the space agency has announced.

Combining the ISS's newly upgraded "Cold Atom Laboratory" with the near zero-gravity of low Earth orbit, scientists are attempting to understand the properties of so-called "ultracold" atoms in an environment impossible to replicate on Earth. The aim of the mission is to study how clouds of atoms behave at temperatures close to absolute zero (minus 459.67 degrees Fahrenheit or minus 273.15 degrees Celsius) — the coldest possible temperature in the universe, where atoms lose all their energy of motion.

"At the coldest temperatures, matter behaves drastically different from anything we have experienced," Jason Williams, project scientist for the Cold Atom Lab at NASA’s Jet Propulsion Laboratory in Southern California, which built the facility, said in a statement. "The wavelike nature of matter dominates, and ultracold matter can behave in ways that are not only unexpected, but that also enable extremely precise measurements of time, gravity, and motion. The lab has lots of tools — especially with this latest upgrade — to let us probe the nature of the universe."

Rule-breaking particles

Atoms and their subatomic particles are quantum mechanical objects whose behavior is fundamentally different from that of the large-scale world. For example, the laws of quantum mechanics predict that particles can be in more than one place at the same time (quantum superposition); can be mysteriously linked with each other over great distances (quantum entanglement); and move through spacetime as waves as well as moving like fixed, solid objects.

But observing these behaviors is notoriously difficult. Firstly, atoms are so tiny that if an atom were the size of a golf ball, then a human teeing one off would stand roughly as tall as the distance from Earth to the moon. Secondly, it’s impossible to isolate measurements of these behaviors for atoms in "normal" environments (like on Earth), as the desired quantum behavior is disturbed by energy from heat and gravity.

To overcome these challenges, the ISS's Cold Atom Laboratory — which is the size of a mini-fridge — uses lasers to cool gases of rubidium and potassium to just above absolute zero. At these temperatures, atoms form a state of matter known as a Bose-Einstein condensate, in which many atoms behave like a single wave of quantum matter.

Not only does this setup allow scientists to observe quantum behaviors on a much larger scale than that of single atoms, but the reduced gravity enables the condensate matter waves to expand and evolve undisturbed for much longer periods than would be possible on Earth.

This is the fourth major upgrade to NASA’s Cold Atom Laboratory since it arrived aboard the ISS in 2018. According to NASA, the significant improvements in this most recent upgrade include a redesigned magnetic trap to contain the cloud of atoms, improved atom sources, and better measurement capabilities.

Scientists launched these upgrades to the ISS in April 2026, and they have since been installed, switched on, and started making state-of-the-art measurements. As well as enabling novel tests of fundamental physics, measurements of these effects are critical in demonstrating future, space-based, highly precise quantum technologies related to positioning, navigation, timing, and gravity sensing. These technologies could one day enable astronauts to navigate on the moon without GPS and produce high-precision maps of Earth’s gravity.

"In the previous century, there was a quantum revolution that led to lasers, cellphones, and MRIs for medical imaging," Ethan Elliott, deputy project scientist at NASA’s Jet Propulsion Laboratory in California said in the statement. "We’re performing Quantum 2.0 – direct manipulation of large quantum states – and we hope for similar gains in quantum technology by advancing this science in orbit."

For the first time, physicists have built a working version of quantum mechanics without complex numbers — numbers that have been considered essential to the theory for nearly a century.

Complex numbers combine a regular "real" number with an "imaginary" one — a multiple of the square root of -1, represented by the symbol i — into a single value, like 3 + 4i. The square root of -1 doesn't correspond to any quantity you could count or measure directly (you can't have negative one apple, for instance), which is why mathematicians call it imaginary.

Still, complex numbers have many useful applications. Engineers use them to describe alternating electrical current. Physicists use them to describe waves. And ever since quantum mechanics was first documented in the 1920s, complex numbers have been built directly into its equations. Quantum mechanics describes particles using something called a wave function, and that description relies on complex numbers.

In 2021, a team of physicists predicted that a version of quantum mechanics built with only real numbers would make incorrect predictions in certain experiments involving multiple particles. The following year, other researchers ran those experiments, and the results matched standard quantum mechanics, not the real-number version. Complex numbers seemed unavoidable.

But that 2021 result rested on one specific assumption: a particular mathematical rule for combining particles. That led physicists to ask a question: Are complex numbers actually necessary to describe reality at the quantum level, or are they just a convenience?

Now, in a new study published June 18 in the journal Physical Review Letters, researchers have found a way around the 2021 result.

"Complex numbers are not needed for quantum mechanics," study first author Pedro Barrios Hita, a theoretical physicist and doctoral student at the German Aerospace Center and Heinrich Heine University Düsseldorf, told Live Science.

A different rule

The 2021 result relied on a specific mathematical rule called the tensor product, which combines two separate quantum systems into one. If you have two particles and you want to combine them into a single mathematical description, you can use the tensor product. It's a rule taught in every quantum mechanics textbook.

It works well for ordinary complex-number quantum mechanics, but past attempts to build a real-number version around that same rule ran into trouble. They couldn't reproduce the correlations seen in experiments involving three or more entangled particles.

In their new study, Barrios Hita and his colleagues found that the tensor product isn't the only option. They built quantum mechanics around a different rule based on an idea: An action taken on one part of a system shouldn't have any effect on a separate part of it.

In ordinary quantum mechanics, multiplying a particle's state by i is undetectable on its own. But when two particles combine, that i can shuffle over and effectively attach itself to the other particle instead. Physicists call this phase kickback, and it's built automatically into the tensor product.

Barrios Hita's team had to recreate that shuffling using only real numbers. They attached a small "flag" to each particle to keep track of what the imaginary part used to store. Then, they treated certain flag combinations as physically identical, even though they looked different on paper. That grouping step allowed their real-number version to match every prediction of standard quantum mechanics, including the multiparticle cases that had tripped up earlier attempts.

At its core, the trick is simple. A complex number, like 3 + 4i, is really just a pair of ordinary real numbers (3 and 4) — the i is only a label marking which one is the imaginary part. "A complex number is nothing but two real numbers," Barrios Hita said. His team essentially built a bookkeeping system that tracks those two real numbers separately, instead of combining them into one complex number. It took a long time to figure out how to make that work consistently across multiple combined particles. But once they did, Barrios Hita said, the underlying structure turned out to be elegant.

The result puts quantum mechanics in the same boat as other physics theories that are often written using complex numbers purely for convenience, Barrios Hita said.

"There are many other theories, like, for example, electromagnetism," Barrios Hita added, "which has complex numbers at its core. So, these theories are formulated using complex numbers, but [they] are not fundamental. They're just helpful tools to help express equations."

The work doesn't change any experimental predictions or point to new quantum technology. It's also currently limited to systems with a finite number of quantum states. Extending it to infinite-dimensional systems, which show up in many real physics problems, is a natural next step, and other researchers are already looking into it. Barrios Hita is moving on to different research, on how quantum properties like entanglement can be used as a resource.

Still, the study settles a decades-long debate. Complex numbers make quantum mechanics easier to write down, but they aren't required to make it work.

For decades, physicists have pursued a goal that sounds nearly impossible: to build a clock that keeps time using an atom's nucleus rather than the electrons orbiting it.

Now, researchers have demonstrated the first functioning nuclear clock ‪—‬ an advancement that could eventually lead to more robust timekeeping devices and new ways to search for dark matter and physics beyond the Standard Model.

"Having worked in this field for more than 15 years, it is just beautiful, how a very 'wild' idea such as manipulating an atomic nucleus with a laser has turned into reality," Thorsten Schumm, a professor of quantum metrology at the Vienna University of Technology and a member of the research team, told Live Science via email.

How is a nuclear clock different from an atomic clock?

Today's most accurate clocks are optical atomic clocks, which measure the frequency of electrons jumping between different energy levels inside atoms. These clocks are so precise that they would lose less than a second over a 100 million years.

A nuclear clock works similarly, but it uses a transition within the nucleus itself, where the nucleus jumps between energy levels. Because the nucleus sits deep inside the atom, it's far less affected by external disturbances from things like electric or magnetic fields. According to Schumm, the nuclear transition can be 1,000 to 10,000 times less sensitive to environmental noise than atomic transitions are.

"This means that it would be easier to stabilize a nuclear clock over long periods of time," Jacob Higgins, a postdoctoral researcher at Northwestern University who previously worked on thorium clock experiments at JILA in Colorado but was not affiliated with the study, told Live Science in an email. "The transition used for the nuclear clock experiment has a higher quality factor than optical atomic clock transitions, which means that in principle, it can be measured more precisely given the same amount of measurement time."

Together, those advantages could allow nuclear clocks to outperform even today's best atomic clocks according to Higgins.

Why thorium-229 is special

The nuclear clock relies on a rare isotope called thorium-229, whose nucleus contains an unusually low-energy excited state that can be manipulated with ultraviolet laser light.

For decades, scientists had suspected thorium had a low transition, but identifying and controlling it proved extremely challenging. Researchers spent years testing different thorium-containing materials, laser systems and detection methods before finally pinning down the transition.

"It was a long road," Higgins said.

One key advancement was the development of continuous-wave lasers that operate at the precise wavelength needed to excite the thorium nucleus. Before those lasers existed, researchers had to excite the nucleus and then wait several minutes for it to decay and emit a detectable signal. That process was too slow to build a practical clock.

"With the continuous lasers, we can measure the nucleus in absorption and get an immediate response, whether the laser is still at the right frequency (and if not, correct it back)," Schumm said. "Once we had that, it was 'just' implementing some electronics and atomization to have the clock stabilize itself to the nucleus."

Thanks to this set-up, the researchers kept the nuclear clock running continuously for 24 hours.

Unlike many optical atomic clocks, which require ultracold atoms to be suspended in a vacuum chamber, the thorium nuclei are embedded inside a crystal at room temperature.

Because the thorium transition remains stable inside a solid material, researchers may eventually be able to build compact clocks that are useful for navigation systems, telecommunication networks and data synchronization.

A tool for studying the universe

Some physicists are more excited about what nuclear clocks could reveal about fundamental physics, rather than the clocks' timekeeping abilities.

Atomic clocks primarily probe electromagnetic interactions involving electrons. Nuclear clocks, by contrast, are sensitive to the strong nuclear force, weak nuclear force and electromagnetism — three of the four fundamental forces of the universe, along with gravity. This can make them useful detectors of new physics, in a way.

"The nuclear clock is foremost a different clock, ticking on different fundamental physics mechanisms," Schumm said. "Essentially all modern theories beyond the standard model predict additional particles or 5th forces … which can be probed with the nuclear clock in some parameter regime."

Thorium-229 is particularly intriguing because the energy difference between its two nuclear states results from a delicate balance between electromagnetic and nuclear forces. Because those large contributions nearly cancel each other out, even tiny changes in the underlying forces could shift the clock's frequency.

"So small shifts in these forces — like if the nucleus were to couple to certain forms of dark matter or if there were an oscillation of a fundamental constant — will be amplified in our measurement," Higgins said.

Researchers, including Higgins, have already used early versions of the clock to place constraints on some dark matter models, and they expect its sensitivity to improve as the technology itself gets better.

Although the first functioning nuclear clock is a major achievement, these timekeepers remain in their infancy. Scientists still need to gain a better understanding of how the thorium transition responds to factors like temperature and magnetic fields while developing more powerful and stable laser systems.

"I think it will be many years before the thorium clock can compete with today's best optical atomic clocks," Higgins said, "but we will learn a lot of new science on the pathway to getting there."

A new theoretical study adds fresh support to the idea that a mathematical pattern of ripples in space-time geometry could give rise to naked singularities and microscopic black holes. The new finding advances research into a subject that has vexed physicists for decades.

In 1997, Stephen Hawking famously conceded defeat on a 1991 bet with fellow theoretical physicists Kip Thorne and John Preskill about the possible existence of naked singularities: objects like black holes but without an event horizon (a point beyond which light, and all other matter, cannot escape), making them observable. Hawking eventually admitted that such objects could exist. Thorne and Preskill’s prize? T-shirts to cover their "nakedness."

The evidence that swayed Hawking came from physicist Matthew Choptuik. In 1993, Choptuik studied a specific set of solutions to Albert Einstein's general relativity equations. When solved numerically, on what was then considered a supercomputer, he showed how naked singularities could hypothetically occur under very specific conditions.

Choptuik found that by modeling the gravitational collapse of a simple form of matter, such as a field, and fine-tuning the initial conditions, an unstable state can be constructed. This theoretical state later became known as a space-time crystal — a self-organized repetitive mathematical pattern of ripples in space-time geometry — containing a singularity with infinite curvature (a naked singularity). Because such a singularity wouldn’t form inside a black hole, it could theoretically be observable.

But much like the phase transition from liquid water to ice, this state is delicate, with the field teetering on the edge between dissipating to become empty space or forming a microscopic black hole.

However, significant doubt remained about such a state's existence, even theoretically.

"Whenever you formulate a system in numerical code, you always have a problem because you can only represent a finite number of digits on a computer," study co-author Christian Ecker, an astrophysicist at Goethe University in Germany, told Live Science. "The historic computer simulations could only go so far before inaccuracies became unavoidable."

Though more recent numerical methods offer much higher accuracy, they are not exact and can never provide deep understanding of the phenomenon that traditional analytical methods (such as manipulating equations using algebra and calculus) offer.

In the new study published May 12 in the journal Physical Review Letters, the researchers mathematically described the formation of space-time crystals, naked singularities and microscopic black holes precisely.

A pen and paper solution

They succeeded using just pen and paper, and some mathematical sleight of hand. "Whenever physicists find a small parameter, they are happy because they can first solve the equations when this parameter is zero, then add small corrections to it with standard perturbation theory," co-author Daniel Grumiller, an astrophysicist at the Institute for Theoretical Physics, Vienna University of Technology, told Live Science. "General relativity by itself doesn’t have a small parameter, but if you inject a small parameter [one over the number of dimensions and let this number be huge]… then you can use these perturbative tools and get a handle on otherwise very tough equations."

When taking the number of dimensions to be infinite, the team's exact solution could fit on just a few lines. This solution is unrealistic given we are most certainly not living in an infinite dimensional universe. However, as they brought the number of dimensions down to more realistic numbers, the solution required additional terms that made the expressions ever more complicated.

"The lowest dimension that we can consistently connect with so far is 52, but the numerical data extends only up to dimension 14 — so there's a gap," Grumiller said, referring to the fact that neither pen-and-paper nor numerical techniques are accurate enough to cross paths yet.

"In the future, we plan to extend the numerics to higher dimensions, so that we can actually connect the two," Grumiller added.

Doing so would provide a compelling case that space-time crystals, naked singularities and microscopic black holes are mathematically possible in a universe like ours — however, this would still not prove they actually exist in reality. In the end, Hawking may have awarded those T-shirts too soon.

See how much you know about black holes with our black hole quiz!

Researchers at ETH Zurich have demonstrated a means of generating "perfect randomness" by using entangled superconducting qubits.

Creating true randomness is extremely difficult. Even the most sophisticated conventional random number generator can carry tiny biases. While in most everyday uses those biases are harmless, in cryptography — where the security of encrypted systems depends on unpredictability — even the most subtle pattern can become an exploitable weakness.

The team at ETH Zurich, led by physics professors Renato Renner and Andreas Wallraff, say they have shown how to overcome this flaw and create perfectly random numbers using quantum physics, a milestone they describe as the first certified realization of perfect randomness.

Random acts of qubits

Traditional random-number generators often rely on physical processes such as photon behavior, but those systems can still be slightly skewed and exhibit a bias that causes certain numbers to appear more frequently than others. The ETH team's approach uses quantum entanglement to push randomness beyond that limit.

The experiment revolves around two superconducting chips cooled to temperatures near absolute zero. Each chip acts as a qubit, the quantum equivalent of a binary bit. The chips are connected by a 98-foot (30-meter) tube that is also supercooled, allowing microwave photons to shuttle between them and create entanglement — a "spooky" quantum state where two particles can become linked such that measuring one instantly affects the other.

By keeping the qubits nearly 100 feet apart, the researchers ensured that, during measurement, even light-speed signals could not travel between the qubits quickly enough to influence the outcome. In the language of quantum physics, that helps preserve the integrity of the entanglement and prevents unwanted communication from spoiling the randomness.

The team then started with an imperfect random-number generator to choose the measurement basis for the qubits. After the quantum measurement, they used a special algorithm to amplify the randomness in the results. The key idea is that the quantum system can cleanse the input of bias and produce an output sequence of zeros and ones that is certifiably random, meaning its randomness is not merely assumed or inferred from standard statistical tests.

Practical randomness

The method also significantly reduces computational cost, Renner told Live Science by email.

"Our method does not really require a computation," Renner said, "as all the randomness is generated by measuring quantum bits. In this sense, the computational cost of our approach is negligible compared to that of pseudo-random number generators."

The researchers argue that the output remains perfect for all practical and analytical purposes, no matter how future methods might try to assess it.

The practical implications are significant. The ETH team compares the advance to an atomic clock for timekeeping: a physically reliable reference that other systems can rely on. Future potential applications include message encryption, digital identities, lottery systems and blockchain operations.

Renner stated that their work would be most useful in network architectures. "Our experiment would be most useful in networks where every node has access to a 'server' that implements it to produce randomness."

When a beam of light passes through a cloud of atoms, photons (particles of light) sometimes appear to spend a negative amount of time there, with light seeming to exit the cloud before it even enters. Now, physicists have confirmed this quantum quirk by asking the atoms themselves.

"This doesn't mean that we're on the verge of building a time machine or anything like that," study co-author Howard Wiseman, a theoretical quantum physicist at Griffith University in Australia, told Live Science. "It can all be understood with standard physics, but it's yet one more weird property of quantum physics that people hadn't suspected."

Photons that pass through an atomic cloud can be temporarily absorbed. They vanish as particles of light and reappear as atomic excitations — a kind of stored energy — before being reemitted. Some photons, called transmitted photons, make it through in roughly the same direction they entered Others scatter off in random directions.

Experiments dating back to 1993 had already hinted that transmitted photons tend to arrive at a detector before the center of their own pulse even enters the cloud. That implies a negative transit time.

But there was a problem with this setup: Photons at the front of a pulse may be more likely to make it through than photons at the back. If you look only at the ones that are transmitted, of course, they look early. But this left a door open for a simpler explanation.

"People were convincing themselves that this is not actually as crazy as it sounds," Wiseman told Live Science.

Confirming the crazy

In a new paper published April 13 in the journal Physical Review Letters, physicists tried a different approach. Rather than watching when a photon arrived at a detector, they monitored whether the atoms were in an excited state while the photon was passing through.

When a photon is absorbed by an atom, it is stored as energy, causing the atom to enter what physicists call an excited state. The atom remains in this excited state until it reemits the photon. Therefore, measuring the duration of the atom's excited state reveals how long the photon was absorbed by the atom.

The team measured this using a second beam of light, which picked up a tiny phase shift depending on the atoms' excitation levels. The light beam acted as a live readout of what the atoms were experiencing from moment to moment.

This atomic readout confirmed the quantum craziness of the earlier experiments.

"You get the same answer if you ask the atoms, 'How long was the photon staying with you?'” Wiseman said. "They will also tell you an answer, which is a negative time."

A million-test milestone

Getting that answer wasn't easy, because measuring quantum systems disturbs them. In this case, it potentially prevents the photon from being absorbed at all. So the team used "weak measurements," which are gentle but extremely noisy. Any single run of the experiment was swamped by noise — random fluctuations that made it impossible to tell signal from static in any individual measurement. Only after averaging roughly 1 million runs did a clear signal emerge. Across roughly seven sets of experimental parameters, the total data collection ran to approximately 70 hours.

"Even in this really simple thing — a photon interacting with atoms — people were already doing calculations on that almost 100 years ago," Wiseman said. "Just the fact that it can still show surprises after all this time is interesting."

The team's next target is the photons that don't make it through the cloud. Theory predicts that those scattered photons carry extra positive excitation time. That is enough to balance the negative time of the transmitted ones, keeping the overall average for the beam of light at zero or above. That prediction has never been tested.

A new theoretical study suggests that black holes may never fully evaporate, which contradicts an infamous Stephen Hawking theory that seems to violate fundamental laws of quantum mechanics. Instead, black holes could leave behind tiny, stable remnants that store all the information they once consumed, the study suggests.

But there's a twist — literally. For the theory to work, the universe must have three extra hidden dimensions that humans cannot perceive, making space-time seven-dimensional. As these hidden dimensions fold and twist, they create a repulsive force that prevents black holes from evaporating entirely.

The work, while hard to test directly, links black holes to the geometry of extra dimensions, offering a fresh approach to one of the deepest puzzles in physics.

A paradox that challenges the foundations of physics

Black holes are often thought of as cosmic traps from which nothing escapes. Yet, since the 1970s, physicists have known that these cosmic behemoths are not entirely black. Famed theoretical physicist Stephen Hawking proposed that black holes emit radiation and slowly evaporate over time, which leads to a troubling contradiction known as the information loss paradox.

"Imagine you throw a book into a fire," study co-author Richard Pinčák, a senior researcher at the Slovak Academy of Sciences' Institute of Experimental Physics, told Live Science via email. "The book is destroyed, but in principle you could reconstruct every word from the smoke, ash, and heat — the information is scrambled, not lost."

But when a black hole evaporates completely, the information about everything that fell into it appears to vanish, violating a core principle of quantum mechanics.

For decades, physicists have struggled to resolve this paradox. Now, the new study, published March 19 in the journal General Relativity and Gravitation, suggests the answer may lie in the hidden structure of space-time itself.

Extra dimensions and the hidden structure of space-time

The new research explores a universe with more dimensions than the familiar four. In this framework, the cosmos contains seven dimensions, three of which are compact and invisible at everyday scales.

"We experience three dimensions of space and one of time — four dimensions in total," Pinčák said. "Our model proposes that the universe actually has seven dimensions: the four we know, plus three tiny extra dimensions curled up so tightly that we cannot directly perceive them."

These extra dimensions are arranged in a highly symmetrical structure known as a G₂ geometry. This mathematical framework, often explored in advanced theories such as a version of string theory known as M-theory, determines how the hidden dimensions are "folded."

"Think of it like origami," Pinčák said. "The way you fold the paper determines what the final shape can do."

In the new model, this geometric structure produces a physical effect called torsion, which can be thought of as a twisting of space-time. This torsion field turns out to play a crucial role in black hole physics.

Torsion and the birth of stable black hole remnants

The study shows that torsion generates a repulsive force that becomes important at extremely small scales, near the end of a black hole's life. As the black hole shrinks through Hawking radiation, this force eventually counteracts further collapse.

"This repulsive force acts as a brake, halting the evaporation before the black hole vanishes completely," Pinčák said.

Instead of disappearing, the black hole stabilizes into a tiny remnant. According to the model, this leftover object has a mass of about 9 × 10⁻⁴¹ kilograms ‪—‬ some 10 billion times smaller than an electron.

Crucially, this remnant can store the information that fell into the black hole, avoiding any violation of quantum mechanics. The information is encoded in subtle oscillations known as quasinormal modes, which act as carriers of the lost data.

The model also reveals an unexpected connection to particle physics: The existence of three hidden dimensions, together with the presence of torsion, produces the pattern of particle interactions responsible for the Higgs mechanism, the phenomenon that gives mass to elementary particles like electrons and quarks.

"The same torsion field… generates a potential energy landscape that is identical in form to the one responsible for giving mass to the W and Z bosons — the carriers of the weak nuclear force," Pinčák said.

This link ties the behavior of black holes to the electroweak scale, a well-known energy scale in particle physics.

Where the new theory reaches its limits

Despite its appeal, the model faces important challenges. The standard description of black hole evaporation relies on a semiclassical approximation, which is expected to break down at extremely small scales near the Planck mass — approximately 10^-5 grams. This is the mass scale at which quantum gravitational effects become strong and impossible to ignore.

"As the black hole shrinks toward the Planck scale, all existing models — ours included — must eventually confront the transition into the deep quantum-gravity regime," Pinčák noted.

In this regime, a full theory of quantum gravity is required, but such a theory remains incomplete. The new work does not claim to solve this problem entirely. Instead, it provides a concrete mechanism for how new physics could emerge at the final stage of evaporation.

"What distinguishes our approach is that we do not claim semiclassical evaporation operates all the way down to the remnant mass," Pinčák said. "At that point, a new physical effect … takes over and stabilises the configuration."

Testing the theory directly will be extremely difficult; the relevant energy scales are far beyond the reach of current particle accelerators. However, the model makes clear predictions that could, in principle, be tested.

For example, it predicts that hypothetical Kaluza-Klein particles associated with extra dimensions should have masses of around 10¹⁶ gigaelectronvolts ‪—‬ about 14 orders of magnitude heavier than the top quark, the most massive known elementary particle. Detecting lighter versions of these particles with current or future accelerators would rule out the model.

Another possibility involves observing the final stages of black hole evaporation, particularly for primordial black holes. Future gamma-ray telescopes or gravitational wave detectors could provide indirect evidence for stable remnants.

"The important point is that the predictions are concrete — the model can be wrong, which is what makes it scientific," Pinčák said.

Looking ahead, the researchers aim to connect their framework more directly to fundamental theories such as M-theory and to better understand how information is stored in the remnants. If confirmed, the idea that black holes leave behind tiny, information-rich remnants could reshape our understanding of gravity, quantum mechanics and the fundamental structure of the universe.

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For the first time, researchers have detected empty voids moving faster than the speed of light — and they blazed past that cosmic speed limit without breaking the laws of relativity.

A recent study shows the voids' acceleration. Researchers used recent advances in ultrafast electron microscopy to measure voids in phonon-polariton waves zooming around inside a thin flake of boron nitride. Phonon-polaritons are quasiparticles formed from photons (quantized light) coupled with tiny vibrations, and they act like light and sound waves combined.

Waves are often visualized as a single squiggle, but in many applications, imagining them as a lake could give a better idea of what's going on. Lakes are full of waves and ripples that interfere with each other. If the waves interact when they're at their maximum height, they combine to create an even higher wave. But if they make contact when they're at their lowest points, they create deeper troughs than they would on their own.

Sometimes, waves cancel each other out, creating points where the waves' magnitude drops to zero. In a lake, this would make a temporary whirlpool (a vortex) that moves around that empty point, also called a singularity. These singularities are found throughout nature and mathematics and, since the 1970s, have been theorized to move faster than light speed in some instances, according to a recent statement from the Technion-Israel Institute of Technology.

Blazing past the limit

Einstein's theory of special relativity states that the speed of light in a vacuum ‪—‬ 299,792,458 meters per second, or about 186,000 miles per second ‪—‬ is the fastest speed information, matter and energy can travel through space. So how do singularities move faster than light speed? Because singularities are empty points of nothingness, they contain no information, no matter and no energy. They are tiny voids, so they don't have to obey the cosmic speed limit.

These voids don't just exceed the speed of light ‪—‬ they blaze past it. When two singularities encounter each other, they can sometimes exponentially speed up toward each other until their velocities approach infinity just before they cancel each other out. However, the faster they go, the harder it is to observe them. The recent study, published March 25 in the journal Nature, shows researchers doing just that.

"Our discovery reveals universal laws of nature shared by all types of waves, from sound waves and fluid flows to complex systems such as superconductors," Ido Kaminer, an electrical and computer engineering professor at the Technion-Israel Institute of Technology and a member of the research team, said in the statement.

The study's results apply to more than just tiny whirlpools; the null points act enough like particles that scientists can study them to better understand particle interactions. To do this, researchers need to know where the comparison breaks down. The new study shows the voids' need for speed is a point where the singularities stop acting like particles, since particles obey the cosmic speed limit that voids ignore.

In addition, the team's new techniques for observing very small, very fast things could light up some previously unexplored pockets across multiple scientific disciplines.

"We believe these innovative microscopy techniques will enable the study of hidden processes in physics, chemistry, and biology, revealing for the first time how nature behaves in its fastest and most elusive moments," Kaminer added.

For the first time, scientists have observed quantum entanglement in the way atoms physically move — bringing a phenomenon once described by Albert Einstein as "spooky action at a distance" into even sharper reality.

In the new study, published in the journal Nature Communications, researchers demonstrated that pairs of ultracold helium atoms can be quantum mechanically linked through their momentum — a measure of how fast and in which direction a particle moves, factoring in its mass.

Quantum entanglement is one of the strangest features of quantum mechanics. When two particles are entangled, a measurement of one instantly affects the other. Scientists had demonstrated this before in photons (packets of light) and in the internal spin states of atoms but never in the motion of particles with mass. This is important because atoms have mass, and mass responds to gravity; photons don't. Momentum-entangled atoms could one day power quantum sensors precise enough to detect space-time ripples called gravitational waves or to map Earth's interior.

Catching entanglement in the act

First, the team chose helium as their atom, because it can be held in a long-lived excited state with a lifetime of around two hours — which is “essentially infinite” in experiments that only last 20 to 30 seconds, Sean Hodgman, an experimental physicist at the Australian National University and senior author of the study, told Live Science. That internal energy means each atom hits a detector with enough force to register individually. It allows the team to reconstruct the full three-dimensional momentum of the cloud with single-atom resolution.

To create momentum-entangled atom pairs, the team started with a cloud of helium cooled to near absolute zero. Normally, atoms zip around independently. But if you cool them enough, they slow to a near standstill. Their quantum identities blur together into a single collective object called a Bose-Einstein condensate.

Then, they used tuned laser pulses to split that condensate into three groups: one kicked upward, one kicked downward, and one left stationary. As the moving clouds passed through the stationary one, pairs of atoms collided and scattered in opposite directions, forming spherical shells of correlated pairs. Physicists call it "scattering halos." At low enough density, only a single pair scatters per experimental shot. "You either have a pair at one position, or a pair at another," Hodgman said. "Your entangled state is a superposition of both."

To prove the entanglement was real, the team used a device called a Rarity-Tapster interferometer. This method, first demonstrated with photons in 1990, now extended to matter waves for the first time.

"The atoms scatter apart; then you reflect them back onto themselves and interfere with them together," Hodgman explained. "Interference only occurs if the atom is truly in a superposition of both states." The correlations the team measured cannot be explained by any classical theory.

To get their final result, the team collected data continuously for nearly a month and spent a month to a year just setting up the experiment.

"This has kind of been a long-term goal for our lab for probably 20 years or so," Hodgman said. "To be able to finally demonstrate it is really exciting."

A surreal win for quantum mechanics

The result, while exciting, mainly served to validate “textbook” physics theories, Hodgman added. Quantum mechanics predicts this exact kind of behavior, but that doesn't make it any less disorienting.

"Our brains aren't really equipped to process it," Hodgman added. "Atoms appear as smeared out at small scales, not concrete blobs or little balls. And that just seems really, really weird."

The team is already working on a stronger version of the test. But the experiment Hodgman describes as the most consequential next step involves colliding two isotopes of helium ‪—‬ helium-3 and helium-4, which are fundamentally different kinds of particles — to create pairs entangled in both momentum and mass simultaneously.

"From a quantum gravity point of view, how do you even write down the gravitational description of that kind of state?" Hodgman said. "You can't really describe it in a general relativity framework at all. These sorts of states would provide a real challenge for quantum gravity theories to explain."

Athreya, Y. S., Kannan, S., Yan, X. T., Lewis-Swan, R. J., Kheruntsyan, K. V., Truscott, A. G., & Hodgman, S. S. (2026). Bell correlations between momentum-entangled pairs of 4He* atoms. Nature Communications, 17(1). https://doi.org/10.1038/s41467-026-69070-3

How much do you know about Albert Einstein and quantum physics? Try your luck with our Einstein quiz!

The Big Bang is often described as the moment everything began — a point of infinite density where the laws of physics broke down. But what if that picture is incomplete?

A new study proposes a different account of the universe's birth: Instead of an abrupt beginning from a singularity, as predicted by Einstein's theory of general relativity, the early cosmos may have passed through a more controlled high-energy phase governed by a modified theory of gravity known as QQG.

"QQG stands for quadratic quantum gravity," study co-author Niayesh Afshordi, a professor of physics at the University of Waterloo and the Perimeter Institute for Theoretical Physics, told Live Science via email. "In simple terms, it is an extension of Einstein's theory of gravity that includes additional terms which become important at extremely high energies, such as those that would have existed near the beginning of the universe."

The study was published March 18 in the journal Physical Review Letters.

Why Einstein's theory may not be enough

Einstein's theory of general relativity has been extraordinarily successful in describing gravity on large scales. It explains the motion of planets, the behavior of black holes, and the expansion of the universe. However, it struggles to explain the ultra-small world of quantum mechanics and is widely believed to contain some fundamental inconsistencies.

"The main problem is that Einstein's general relativity predicts its own failure under extreme conditions, most famously at the Big Bang singularity," Afshordi said.

At that point, densities and space-time curvature become infinite — a clear indication that the theory is incomplete. Physicists have long sought a deeper framework that can describe gravity under such conditions.

"What makes [quadratic quantum gravity] interesting is that it may provide a mathematically consistent way to describe gravity at very short distances and very high energies, where ordinary general relativity is expected to break down," Afshordi said. "In that sense, it offers a possible conservative route toward a quantum theory of gravity, while still remaining close to Einstein's theory at ordinary scales."

A universe without a singularity

In the new study, the researchers explored how QQG would reshape the earliest moments of the cosmos if it is indeed a correct completion of Einstein’s theory. Their results suggest that the universe may not have started from a singular point at all.

"Our main result is that, within quadratic gravity, the very early universe can avoid the usual Big Bang singularity and instead pass through a better-controlled high-energy phase," Afshordi said.

Rather than emerging from an infinitely dense state, the universe would have begun in a smoother, more stable configuration with finite density and finite temperature, with its precise properties depending on the particles and fields present at extremely high energies and temperatures. This avoids one of the most troubling predictions of standard cosmology.

The theory also offers a fresh perspective on cosmic inflation, the brief period of extremely rapid expansion thought to have occurred just after the Big Bang.

"In our analysis, this framework can also generate an inflation-like period without having to introduce an extra hypothetical field by hand," Afshordi said.

In standard models, inflation is typically driven by a mysterious field known as the inflaton. That field has never been directly observed. In contrast, QQG produces inflation naturally as a consequence of gravity itself.

"In other words, some of the key ingredients we normally add separately to cosmology may arise directly from the gravitational theory itself," Afshordi added.

From exotic physics to the familiar universe

One striking feature of QQG is that it behaves very differently depending on the energy scale. At extremely high energies, it follows new quantum rules. But as the universe expands and cools, it transitions back to the familiar physics described by Einstein.

The theory suggests that gravity becomes simpler at very high energies — a property known as asymptotic freedom — before evolving into the form we observe today. Eventually, the universe enters the hot, radiation-filled phase described by standard cosmology.

This framework provides a continuous bridge between an exotic early universe and the well-tested physics of later times. The key question, however, is whether this idea can be tested.

"Yes, at least in principle," Afshordi said. "The most promising tests come from cosmology, especially from the imprint of the early universe on primordial gravitational waves and the cosmic microwave background."

These ancient signals carry information about the universe's earliest moments. According to the new theory, these signals should contain subtle differences compared with predictions from standard inflation models.

"One particularly interesting aspect of our scenario is that it can lead to distinctive predictions for the gravitational-wave signal produced in the early universe," Afshordi noted. "As observational sensitivity improves over the coming years and decades, future measurements of primordial gravitational waves could begin to distinguish this kind of model from more conventional inflationary scenarios."

Although the idea is still being explored, it offers a compelling possibility: that the Big Bang may not have been a singular beginning but rather part of a deeper, quantum description of gravity. If confirmed, this framework could reshape how scientists understand the origin of the universe — replacing a breakdown of physics with a new, more complete picture of cosmic beginnings.

Think you know about Einstein's theories? Test your knowledge with our Albert Einstein quiz!

Scientists just watched a bizarre phase of matter turn into an even stranger one. For the first time, they saw a superfluid turn into a supersolid — a transition they weren't sure was even possible.

In a Jan. 28 study in the journal Nature, researchers observed a group of excitons — quasiparticles that combine an electron and an electron hole — transforming from a superfluid into a supersolid and back again. It is the first time excitons have been seen condensing into a supersolid, undergoing a reversible phase transition the way water can transform from a liquid to ice and back.

Secret phases of matter

There are many more phases of matter than the typical three we encounter every day (gases, liquids and solids), although most of these other matter states exist only under extreme conditions. Superfluids are one type that occurs only when some particles, like helium isotopes and excitons, are cooled to just above absolute zero — the complete absence of heat. They're not quite liquids — they flow without resistance from friction — and when stirred, they form tiny eternal tornadoes called quantum vortices.

Supersolids, on the other hand, are a state of matter theorized to exist when superfluids are cooled even more. They keep superfluidity's zero viscosity, but instead of particles moving around in a liquid-like blob, they form an orderly structure, like a crystal lattice, while maintaining their ability to flow and form quantum vortices.

Supersolids have been made in labs before, including in 2021, when researchers created 2D supersolid dysprosium and in 2024 when they saw quantum vortices in a supersolid. However, they achieved this only by using extra equipment and energy to force particles into an orderly lattice. The new study, by contrast, demonstrates a natural phase transition.

"For the first time, we've seen a superfluid undergo a phase transition to become what appears to be a supersolid," Cory Dean, a physicist at Columbia University and co-author of the study, said in a statement.

Exploring new boundaries

To do it, researchers put two pieces of graphene — which is like a very thin sheet of paper made entirely of carbon atoms — very close together. Then, they added a strong magnetic field and cooled the system to form an exciton "soup."

When cooled to between 2.7 and 7.2 degrees Fahrenheit (1.5 to 4 degrees Celsius) above absolute zero, the excitons formed a superfluid. When cooled more than that, the excitons changed into an electrically insulative mysterious new phase that the team suspects is the theorized supersolid state.

"Superfluidity is generally regarded as the low-temperature ground state," Jia Li, a physicist at the University of Texas at Austin and co-author of the study, said in the statement. "Observing an insulating phase that melts into a superfluid is unprecedented. This strongly suggests that the low-temperature phase is a highly unusual exciton solid."

The team is looking at other materials to test, as well as finding new ways to measure and study the exciton supersolid state.

"For now, we're exploring the boundaries around this insulating state, while building new tools to measure it directly," Dean said. Further study will help scientists understand how supersolids and superfluids behave, deepen our understanding of particle physics and work toward applications of higher-temperature supersolids.

Physicists have put thousands of atoms into a "Schrödinger's cat" state — smashing the record for the most macroscopic object to be observed in a quantum state.

In a new study, researchers observed nanoparticles of 7,000 sodium atoms acting as a cohesive wave, pushing the strange world of quantum mechanics to new limits. Building on this research, future experiments could finally put biological molecules into a quantum state, opening up new ways to investigate their physical properties.

In the experiment, the team produced a beam of sodium nanoparticles and aimed it at a narrow slit. Their results, published Jan. 21 in the journal Nature, show that the sodium nanoparticles spread out to produce an interference pattern, exhibiting the strange quantum behavior known as wave-particle duality. These sodium nanoparticles have now collectively set the record for the most macroscopic objects to be observed in a quantum superposition.

"Usually when people think of quantum mechanics, they associate it with small, tiny things, maybe photons, maybe electrons," lead study author Sebastian Pedalino, a physicist at the University of Vienna, told Live Science. "But quantum mechanics itself doesn't state any limits. And that's what we are testing."

Both here and there

In the quantum realm, particles can be both here and there. This strange phenomenon is known as quantum superposition.

The quantum physicist Erwin Schrödinger likened this to placing a cat in a sealed box with a vial of poison that is set to be released when a radioactive source decays, meaning the cat could be killed at any moment after the box has been sealed. This puts the cat into a superposition of being both dead and alive. It is only if the box is opened and the cat is observed that the superposition collapses and the cat is defined as either dead or alive.

Incredibly, this is how particles behave at the quantum scale; they are in multiple places at once and act as both a particle and a wave until they are observed.

This bizarre world raises a question: Where is the boundary between the quantum world and the one we observe every day? At what point does a particle start acting like a wave?

The reason we don't see quantum superposition all around us is because of a process called decoherence. If something in a quantum superposition interacts with its environment, it will decohere and no longer be both here and there; instead, it will be forced into one place. Larger objects are constantly interacting with their environment, so they can't maintain a quantum superposition. So the real challenge when trying to observe larger particles acting as a wave is to isolate them so they can stay in a coherent quantum superposition.

Searching for interference

For the new study, Pedalino attempted to observe the large nanoparticles of sodium in a quantum superposition. To do this, he and his team converted a few grams of sodium into a beam of nanoparticles, which he then aimed at a narrow slit.

If the sodium nanoparticle was in a quantum superposition, this would mean that it spread out like a wave after passing through the slit. This would then produce an interference pattern. However, if it decohered and started acting like a normal particle, the sodium would pass straight through the slit and the team would see a flat line.

"For two years, I was looking at flat lines," Pedalino said. "We were trying to see the interference pattern, but we had flat lines. And in the end, the flat line is not really helpful, as it is inconclusive."

Finally, the single line they had been seeing on the detector widened and became the unmistakable interference pattern that meant the sodium nanoparticles were behaving as both particles and waves.

"That moment was unbelievable," Pedalino said. "It was already late in the night, and I called my professor. And he came back to the lab, and we took measurements until 3 a.m., when we ran out of the sodium."

The team determined the "macroscopicity" — a quantity that describes how much a quantum object pushes into the classical world — of the sodium nanoparticles to be 15.5, beating the previous record for macroscopicity by an order of magnitude.

This discovery opens the door for future experiments where scientists could feasibly observe biological materials, such as a virus or proteins, in a quantum superposition. The experiment represents a major step forward and brings this strange quantum phenomenon tantalizingly close to the real world.

Some atoms are stable, while others seem to fall apart. Lead-208 will probably last forever, while the synthetic isotope technetium-99 exists for just hours. The difference lies in the structure of the atom's nucleus, with certain "magic numbers" of nuclear particles making some isotopes especially resistant to radioactive decay.

So what are these magic numbers, and why are they so special?

The stability of atomic nuclei varies wildly with the number of nuclear particles they contain. Some, like lead-208 and calcium-40, have been around since Earth first formed. Known as primordial isotopes, they will likely survive until the end of time. Others, like oganesson-294 and tennessine-294, are lost to radioactive decay in an instant, with fleeting half-lives of just 0.89 and 0.80 milliseconds, respectively.

This stability seems partly connected to the mass of the atom, with heavier elements proving less stable. But in the 1940s and '50s, scientists observed that many of the lighter elements also had radioactive isotopes; both carbon-14 and potassium-40 undergo radioactive decay slowly and are responsible for much of the planet's background radiation.

Intriguingly, these scientists noticed that very particular numbers of protons and neutrons appeared to result in unusually stable nuclei, and these values became known as magic numbers.

"The magic numbers are 2, 8, 20, 28, 50, 82 and 126," said David Jenkins, a nuclear physicist at the University of York in the U.K. "If you take the lightest one — two protons and two neutrons — that's the nucleus of the helium atom, and we know that's a very stable combination of protons and neutrons."

Related: Why isn't an atom's nucleus round?

Shell game

Helium nuclei, also known as alpha particles, are spontaneously emitted from heavier, unstable atoms as they undergo nuclear decay.

"If you think about it, that's very weird," Jenkins said. "If an atom is going to decay, why doesn't it lose protons or neutrons one at a time? The reason is that the alpha particle is very very stable, and that's related to this idea of magic numbers."

Other magic nuclei include oxygen-16 (eight protons and eight neutrons), calcium-40 (20 protons and 20 neutrons) and lead-208 (82 protons and 126 neutrons), the heaviest stable element known.

To understand these bizarre observations, physicists proposed the "nuclear shell model," which draws parallels with the electronic shells used to explain the chemical behavior of atoms.

"The idea was that protons and neutrons sit in shells, a bit like the electrons in an atom, and nuclear excitations would involve protons and neutrons jumping up and down between those shells," Jenkins explained.

Like their electron analogues, these nuclear shells have fixed energy values known as quantized states, and the system is most stable when these shells are completely filled. The exact reasoning behind this is a complex combination of quantum mechanical factors, but it's thought that the strong force — the fundamental interaction that holds the protons and neutrons together in the nucleus — is higher than expected per particle in completed shells.

Magic numbers are therefore simply the numbers of particles required to fill each of these nuclear shells, with separate levels for protons and neutrons. Individual isotopes can correspondingly be singly magic, with a magic number of either protons or neutrons (for example, the primordial isotope iron-56), or doubly magic, with magic numbers of both protons and neutrons (like oxygen-16 and lead-208).

These doubly magic systems are few and far between, but they possess some intriguing quantum properties, Jenkins said.

"The doubly magic systems have a spherical distribution of matter and charge" — a completely round nucleus, he said. "Most nuclei are deformed and rotate. They have a very different structure."

No one knows how far this model will stretch. Tin-100 — the heaviest doubly magic nucleus, with 50 protons and 50 neutrons — has a half-life of just 1.2 seconds, while unbihexium, the next magic element after lead, has never been synthesized. Therefore, whether this magic stability boost will be enough to allow scientists to add an eighth row to the periodic table remains an open question.

Periodic table of elements quiz: How many elements can you name in 10 minutes?

Tiny black holes created in the aftermath of violent cosmic collisions could offer unprecedented insight into the quantum structure of space and time, a new theoretical study proposes.

What's more, signals from these "black hole morsels" could potentially be detected by current instruments, scientists reported in the study, which was published in the journal Nuclear Physics B.

"Our work shows that if these objects are formed, their radiation might already be detectable using existing gamma-ray observatories," Francesco Sannino, a theoretical physicist at the University of Southern Denmark and co-author of the study, told Live Science via email.

Hawking radiation and the smallest black holes

One of the deepest mysteries in modern physics is how gravity behaves at the quantum level. The new study offers a bold proposal to explore this regime by looking for the glow produced by tiny black holes created in the aftermath of giant black hole collisions.

The idea that black holes are not entirely black, and therefore could emit faint radiation, was first proposed by Stephen Hawking in the 1970s. His calculations revealed that quantum effects near a black hole's event horizon would cause it to emit radiation and lose mass — a process now known as Hawking radiation. The black hole temperature is predicted to be inversely proportional to its mass. So for massive astrophysical black holes, the effect is minuscule, with temperatures so low that the radiation is effectively undetectable. But for very small black holes, the situation is different.

"Black hole morsels are hypothetical micro-black holes that could be formed during the violent merger of two astrophysical black holes," Giacomo Cacciapaglia, a senior researcher at the French National Centre for Scientific Research (CNRS) and co-author of the study, said in an email. "Unlike the larger parent black hole, these morsels are much smaller — comparable in mass to asteroids — and thus much hotter due to the inverse relationship between black hole mass and Hawking temperature."

Related: Scientists detect most massive black hole merger ever — and it birthed a monster 225 times as massive as the sun

Because of this elevated temperature, these morsels would evaporate relatively quickly, releasing bursts of high-energy particles such as gamma-rays and neutrinos. The team's analysis suggests that this radiation could form a distinct signal that may already be within reach of present-day detectors.

A new handle on quantum gravity

Although no such morsels have been observed yet, the researchers argue that the formation of these tiny black holes is theoretically plausible. "The idea is inspired by analogous processes in neutron star mergers," Stefan Hohenegger, senior researcher at the Institut de Physique des Deux Infinis de Lyon and co-author of the study, explained in an email. "It's supported by estimates from beyond-General Relativity frameworks, including string theory and extra-dimensional models."

In such extreme environments, small-scale instabilities might pinch off tiny black holes during the merger process. These objects, in turn, could evaporate through Hawking radiation over timescales ranging from milliseconds to years, depending on their mass.

Crucially, if such radiation is detected, it could open a window into new physics. "Hawking radiation encodes information about the underlying quantum structure of spacetime," Sannino said. "Its spectral properties could reveal deviations from the Standard Model at extreme energy scale, potentially leading to discoveries of unknown particles or such phenomena as extra dimensions predicted by various theories."

Such energy scales lie far beyond the reach of even the most powerful particle colliders, like the Large Hadron Collider at CERN. The possibility that black hole morsels might provide a natural "accelerator" for probing these physics is what makes them so compelling.

According to the team, the signature of a black hole morsel would be a delayed burst of high-energy gamma-rays radiating in all directions — unlike typical gamma-ray bursts, which are usually beamed.

Instruments capable of detecting such high-energy signals include atmospheric Cherenkov telescopes, like the High Energy Stereoscopic System (HESS), in Namibia; the High-Altitude Water Cherenkov Observatory (HAWC), in Mexico; and the Large High Altitude Air Shower Observatory (LHAASO) in China, as well as satellite-based detectors, like the Fermi Gamma-ray Space Telescope. "Some of these instruments already have the sensitivity required," Hohenegger noted.

The researchers didn't stop at theorizing. They used existing data from HESS and HAWC to place upper bounds on how much mass could be emitted in the form of morsels during known black hole mergers. These limits represent the first observational constraints on such phenomena.

"We showed that if black hole morsels form during mergers, they would produce a burst of high-energy gamma rays, with the timing of the burst linked to their masses," Cacciapaglia said. "Our analysis demonstrates that this novel multimessenger signature can offer experimental access to quantum gravitational phenomena.”

What comes next

While the study provides a compelling case for morsels, many uncertainties remain. The exact conditions for their formation are still poorly understood, and no full simulations have been performed at the scales necessary to model them. But the researchers are optimistic.

"Future work will involve refining the theoretical models for morsel formation and extending the analysis to include more realistic mass and spin distributions," Sannino said. The team also hopes to collaborate with observational astronomers to perform dedicated searches in both archived and upcoming datasets.

"We hope this line of research will open a new window into understanding the quantum nature of gravity and the structure of spacetime," Hohenegger said.

If black hole morsels exist, they may not only illuminate the sky with exotic radiation but could also shed light on some of the deepest unsolved questions in physics.

On a quiet pandemic afternoon in 2021, Zhiyuan Wang, then a graduate student at Rice University, was alleviating his boredom by working on a weird mathematical problem. After he found an exotic solution, he started to wonder if the math could be interpreted physically. Eventually, he realized that it seemed to describe a new type of particle: one that's neither a matter particle nor a force-carrying particle. It appeared to be something else altogether.

Wang was eager to develop the accidental discovery into a full theory of this third kind of particle. He brought the idea to Kaden Hazzard, his academic adviser.

"I said, I'm not sure I believe this can be true," Hazzard recalled, "but if you really think it is, you should put all your time on this and drop everything else you're working on."

This January, Wang, now a postdoctoral researcher at the Max Planck Institute of Quantum Optics in Germany, and Hazzard published their refined result in the journal Nature. They say that a third class of particles, called paraparticles, can indeed exist, and that these particles could produce strange new materials.

When the paper appeared, Markus Müller, a physicist at the Institute for Quantum Optics and Quantum Information in Vienna, was already contending with the notion of paraparticles for a different reason. According to quantum mechanics, an object or observer can be in multiple locations at once. Müller was thinking about how you can, on paper, switch between the perspectives of observers in these coexisting "branches" of reality. He realized that this came with new constraints on the possibility of paraparticles, and his team described their results in a preprint in February that's now under review for publication in a journal.

The close timing of the two papers was a coincidence. But taken together, the work is reopening the case of a physics mystery that was believed to be solved decades ago. A basic question is being reevaluated: What kinds of particles does our world allow?

Related: In a first, physicists spot elusive 'free-range' atoms — confirming a century-old theory about quantum mechanics

Hidden worlds

All known elementary particles fall into one of two categories, and the two behave almost as opposites. There are the particles that make up matter, called fermions, and the particles that impart the fundamental forces, called bosons.

The defining characteristic of fermions is that if you switch the positions of two fermions, their quantum state gains a minus sign. The presence of that measly minus sign has enormous ramifications. It means that no two fermions can be in the same place at the same time. When packed together, fermions cannot be compressed past a certain point. This feature prevents matter from collapsing in on itself — it's why the electrons in every atom exist in "shells." Without this minus sign, we couldn't exist.

Bosons have no such restriction. Groups of bosons will happily all do exactly the same thing. Any number of particles of light, for instance, can be in the same place. This is what makes it possible to build lasers, which emit many identical light particles. This ability comes down to the fact that when two bosons swap places, their quantum state stays the same.

It's not obvious that fermions and bosons should be the only two options.

That's in part due to a fundamental feature of quantum theory: To calculate the probability of measuring a particle in any particular state, you have to take the mathematical description of that state and multiply it by itself. This procedure can erase distinctions. A minus sign, for example, will disappear. If given the number 4, a Jeopardy! contestant would have no way to know if the question was "What is 2 squared?" or "What is negative 2 squared?" — both possibilities are mathematically valid.

It's because of this feature that fermions, despite gaining a minus sign when swapped around, all look the same when measured — the minus sign disappears when quantum states are squared. This indistinguishability is a crucial property of elementary particles; no experiment can tell two of a kind apart.

But a minus sign may not be the only thing that disappears. In theory, quantum particles can also have hidden internal states, mathematical structures not seen in direct measurements, which also go away when squared. A third, more general category of particle, known as a paraparticle, could arise from this internal state changing in a myriad of ways while the particles swap places.

While quantum theory seems to allow it, physicists have had difficulty finding a mathematical description of a paraparticle that works. In the 1950s, the physicist Herbert Green made a few attempts, but further inspection revealed that these paraparticle models were really just mathematical combinations of typical bosons and fermions.

In the 1970s, the mystery of why no one could find a proper model of paraparticles seemed to be solved. A collection of theorems called DHR theory, after the mathematical physicists Sergio Doplicher, Rudolf Haag and John Roberts, proved that if certain assumptions are true, only bosons and fermions are physically possible. One assumption is "locality," the rule that objects can only be affected by things in their vicinity. ("If I poke my table, I better not affect the moon instantaneously," as Hazzard put it.) The DHR proof also assumed that space is (at least) three-dimensional.

The results discouraged new ventures into paraparticles for decades, with one exception. In the early 1980s, the physicist Frank Wilczek came up with a theory of particles called anyons that can't be described as either bosons or fermions. To get around the DHR theorems, anyons come with a big catch: They can only exist in two dimensions.

Physicists now widely study anyons for their potential in quantum computing. Even confined to two dimensions, they could manifest on a flat surface of a material, or in a 2D array of qubits in a quantum computer.

But paraparticles in three dimensions that could form a solid still seemed impossible. That is, until now.

Shifting sights

While developing their model, Wang and Hazzard noticed that the assumptions behind DHR theory went beyond typical concerns of locality. "I think people overinterpreted what limitations or constraints were actually imposed by these theorems," Hazzard said. Paraparticles, they realized, may be theoretically possible after all.

In their model, in addition to the usual properties of a particle like charge and spin, groups of paraparticles share extra hidden properties. As with the minus sign that gets squared away during a measurement, you can't directly measure these hidden properties, but they change how the particles behave.

When you swap two paraparticles, these hidden properties change in tandem. As an analogy, imagine that these properties are colors. Start with two paraparticles, one that's internally red and another that's internally blue. When they swap places, rather than keeping these colors, they both change in corresponding ways, as prescribed by the mathematics of the particular model. Perhaps the swap leaves them green and yellow. This quickly turns into a complex game, where paraparticles affect each other in unseen ways as they move around.

Meanwhile, Müller was also busy rethinking the DHR theorems. "It's not always super transparent what they mean, because it's in a very complicated mathematical framework," he said.

His team took a new approach to the paraparticle question. The researchers considered the fact that quantum systems can exist in multiple possible states at once — what's called a superposition. They imagined switching between the perspectives of observers who exist in these superposed states, each of whom describes their branch of reality slightly differently. If two particles are truly indistinguishable, they figured, then it won't matter if the particles are swapped in one branch of the superposition and not in the other.

"Maybe if the particles are close by, I swap them, but if they are far away I do nothing," Müller said. "And if they're in a superposition of both, then I do the swapping in one branch, and nothing in the other branch." Whether observers across branches label the two particles in the same way should make no difference.

This stricter definition of indistinguishability in the context of superpositions imposes new restrictions on the kinds of particles that can exist. When these assumptions hold, the researchers found that paraparticles are impossible. For a particle to be truly indistinguishable by measurement, as physicists expect elementary particles to be, it must be either a boson or fermion.

Although Wang and Hazzard published their paper first, it's as though they saw Müller's constraints coming. Their paraparticles are possible because their model rejects Müller's starting assumption: The particles are not indistinguishable in the full sense required in the context of quantum superpositions. This comes with a consequence. While swapping two paraparticles has no effect on one person's measurements, two observers, by sharing their data with each other, can determine whether the paraparticles have been swapped. That's because swapping paraparticles can change how two people's measurements relate to each other. In this sense, they could tell the two paraparticles apart.

This means there's a potential for new states of matter. Where bosons can pack an endless number of particles into the same state, and fermions can't share a state at all, paraparticles end up somewhere in the middle. They are able to pack just a few particles into the same state, before getting crowded and forcing others into new states. Exactly how many can be crammed together depends on the details of the paraparticle — the theoretical framework allows for endless options.

"I find their paper really fascinating, and there's absolutely no contradiction with what we do," Müller said.

The road to reality

If paraparticles exist, they'll most likely be emergent particles, called quasiparticles, that show up as energetic vibrations in certain quantum materials.

"We might get new models of exotic phases, which were difficult to understand before, that you can now solve easily using paraparticles," said Meng Cheng, a physicist at Yale University who was not involved in the research.

Bryce Gadway, an experimental physicist at Pennsylvania State University who sometimes collaborates with Hazzard, is optimistic that paraparticles will be realized in the lab in the next few years. These experiments would use Rydberg atoms, which are energized atoms with electrons that roam very far from their nuclei. This separation of the positive and negative charge makes Rydberg atoms especially sensitive to electric fields. You can build quantum computers out of interacting Rydberg atoms. They are also the perfect candidates for creating paraparticles.

"For a certain kind of Rydberg quantum simulator, this is kind of just what they would do naturally," Gadway said about creating paraparticles. "You just prepare them and watch them evolve."

But for now, the third kingdom of particles remains wholly theoretical.

"Paraparticles might become important," said Wilczek, the Nobel Prize–winning physicist and inventor of anyons. "But at present they're basically a theoretical curiosity."

Original story reprinted with permission from Quanta Magazine, an editorially independent publication supported by the SimonsFoundation.

Physicists have developed a novel approach to solving one of the most persistent problems in theoretical physics: uniting gravity with the quantum world.

In a recent paper published in the journal Reports on Progress in Physics, the scientists outline a reformulation of gravity that could lead to a fully quantum-compatible description — without invoking the extra dimensions or exotic features required by more speculative models, like string theory.

At the heart of the proposal is a rethinking of how gravity behaves at a fundamental level. While the electromagnetic, weak and strong forces are all described using quantum field theory — a mathematical framework that incorporates uncertainty and wave-particle duality — gravity remains the outlier. General relativity, Einstein's theory of gravity, is a purely classical theory that describes gravity as the warping of space-time geometry by mass and energy. But attempts to blend quantum theory with general relativity often run into fatal mathematical inconsistencies, such as infinite probabilities.

The new approach reinterprets the gravitational field in a way that mirrors the structure of known quantum field theories. "The key finding is that our theory provides a new approach to quantum gravity in a way that resembles the formulation of the other fundamental interactions of the Standard Model," study co-author Mikko Partanen, a physicist at Aalto University in Finland, told Live Science in an email.

Instead of curving space-time, gravity in their model is mediated by four interrelated fields, with each one similar to the field that governs electromagnetism. These fields respond to mass in much the same way that electric and magnetic fields respond to charge and current. They also interact with each other and with the fields of the Standard Model in a way that reproduces general relativity at the classical level while also allowing quantum effects to be consistently incorporated.

Related: 'Einstein's equations need to be refined': Tweaks to general relativity could finally explain what lies at the heart of a black hole

Because the new model mirrors the structure of well-established quantum theories, it sidesteps the mathematical problems that have historically hindered efforts to quantize general relativity. According to the authors, their framework produces a well-defined quantum theory that avoids common problems — such as unphysical infinities in observable quantities and negative probabilities for physical processes — that typically arise when general relativity is quantized using conventional, straightforward methods.

A key advantage of the approach is its simplicity. Unlike many models of quantum gravity that require undetected particles and additional forces, this theory sticks to familiar terrain.

"The main advantages or differences in comparison with many other quantum gravity theories are that our theory does not need extra dimensions that do not yet have direct experimental support," Jukka Tulkki, a professor at Aalto University and co-author of the paper, told Live Science in an email. "Furthermore, the theory does not need any free parameters beyond the known physical constants."

This means the theory can be tested without waiting for the discovery of new particles or revising existing physical laws. "Any future quantum gravity experiments can be directly used to test any (forthcoming) predictions of the theory," Tulkki added.

Looking ahead

Despite the promising features, the model is still in its early stages. Although preliminary calculations indicate that the theory behaves well under the usual consistency checks, a complete proof of its consistency remains to be worked out.

Moreover, the framework has yet to be applied to some of the deepest questions in gravitational physics, such as the true nature of black hole singularities or the physics of the Big Bang. "The theory is not yet capable of addressing those major challenges, but it has potential to do so in the future," Partanen said.

Experimental verification may prove even more elusive. Gravity is the weakest of the known forces, and its quantum aspects are incredibly subtle. Direct tests of quantum gravity effects are beyond the reach of current instruments.

"Testing quantum gravity effects is challenging due to the weakness of gravitational interaction," Tulkki said. Still, because the theory includes no adjustable parameters, any future experiment that probes quantum gravitational behavior could potentially confirm — or rule out — the new proposal.

"Given the current pace of theoretical and observational advancements, it could take a few decades to make the first experimental breakthroughs that give us direct evidence of quantum gravity effects," Partanen said. "Indirect evidence through advanced observations could be obtained earlier."

For now, Partanen and Tulkki's work opens up a fresh direction for theorists searching for a quantum theory of gravity — one that stays grounded in the successful frameworks of particle physics while potentially unlocking some of the most profound mysteries of the universe.

For the first time, scientists have observed solo atoms floating freely and interacting in space. The discovery helps to confirm some of the most basic principles of quantum mechanics that were first predicted more than a century ago but were never directly verified.

Individual atoms are notoriously difficult to observe due to their quantum nature. Researchers cannot, for example, know both an atom's position and its velocity at the same time, due to quantum weirdness. But using certain laser techniques, they have captured images of clouds of atoms.

"It's like seeing a cloud in the sky, but not the individual water molecules that make up the cloud," Martin Zwierlein, a physicist at MIT and co-author of the new research, said in a statement.

The new method goes one step further, allowing scientists to capture images of "free-range" atoms in free space. First, Zwierlein and his colleagues corralled a cloud of sodium atoms in a loose trap at ultracold temperatures. Then, they shot a lattice of laser light through the cloud to temporarily freeze the atoms in place. A second, fluorescent laser then illuminated the individual atoms' positions.

Related: There may be a 'dark mirror' universe within ours where atoms failed to form, new study suggests

The observed atoms belong to a group called bosons. These particles share the same quantum mechanical state and, as a result, behave like a wave, bunching together. This concept was first proposed by French physicist Louis de Broglie in 1924 and has subsequently become known as a "de Broglie wave."

Sure enough, the bosons Zwierlein and his team observed displayed de Broglie wave behavior. The researchers also captured images of lithium fermions — a type of particle that repels similar particles rather than bunching together.

The results were published May 5 in the journal Physical Review Letters. Two other groups reported using a similar technique to observe pairs of bosons and fermions in the same issue of the journal.

"We are able to see single atoms in these interesting clouds of atoms and what they are doing in relation to each other, which is beautiful," Zwierlein said.

In the future, the team plans to use the new technique — called "atom-resolved microscopy" — to investigate other quantum mechanical phenomena. For example, they may use it to try observing the "quantum Hall effect," in which electrons sync up under the influence of a strong magnetic field.

Quantum physics is the branch of science that deals with the tiniest particles in the universe, such as atoms, electrons, photons (light particles), and other subatomic particles like quarks.

In the everyday world, at the scale that we can see, things tend to follow the laws of classical physics. However, when you zoom all the way in to the smallest particles, classical physics stops working quite as well, and the rules of quantum mechanics come into play.

Some of the key concepts of quantum physics are that particles like electrons can behave as waves, and vice versa (known as wave-particle duality); two particles can be linked in such a way that if you measure one, you instantly know something about the other (quantum entanglement); and a quantum particle can be in multiple states at once until it's observed (quantum superposition).

What is quantum superposition?

In everyday life, something can only be in one state at a time: a light switch is either on or off, a cat is either dead or alive. In the quantum world, things don't work quite the same way. Quantum superposition describes how a quantum particle, like an electron, a photon, or even an atom, can exist in multiple different states at the same time — until it's measured. Before it's observed, it's not halfway between states, but is instead a "superposition" of the two at once.

In quantum physics, the state of a particle is described by a wave equation, which tells us the probabilities of where a particle might be or what its properties might be. This probability wave can exist in a blend of multiple states.

What is Schrödinger’s Cat?

Schrödinger’s Cat is a famous thought experiment that illustrates how superposition works. Imagine a cat in a box with a mechanism that has a 50/50 chance of killing it, depending on whether or not a quantum particle decays radioactively, spontaneously changing into a different type of atom and releasing radioactive particles like electrons.

Until someone opens the box and observes it, the cat is considered to be in a superposition of both alive and dead. When you measure or observe the system — or in the case of Schrödinger’s Cat look inside the box — the superposition settles into one definite state, and the cat's fate is discovered.

Related: Physicists create hottest Schrödinger's cat ever in quantum technology breakthrough

Quantum superposition has been experimentally observed by scientists on multiple occasions. One famous example is the double-slit experiment, where photons are fired at a barrier with two slits, behind which is a a screen that records where the particles land. If you send particles through one slit, you get a single band on the screen, but if you open both, you get a wave-like interference pattern with multiple bands on the screen, which also proves that particles and waves can act like each other. Sending one particle at a time, you would expect each one to go through one slit or the other. However, the interference pattern still builds up, as if each single particle is interfering with itself. This means that each single particle is somehow going through both slits at once, and therefore is in a superposition of both possibilities

If you try to measure which slit the particle goes through, the superposition collapses: the particle does appear to have passed through a single slit, and the interference pattern disappears, leaving only two bands on the screen.

Additionally, ions and larger molecules have been experimentally trapped in a superposed state, and chlorophyll in the leaves of plants has been discovered to use quantum superposition to more efficiently harvest light from the sun.

Why is superposition so important in quantum computing?

Quantum superposition is also used as a tool in quantum computing and is the main reason quantum computers can be so powerful.

A classical binary bit can only be in one state at a time: 0 or 1. These bits are encoded on transistors, usually made from silicon, germanium or other semiconductors. With three bits present, they can have a potential of 8 different states: 000, 001, 010, 011, 100, 101, 110, and 111. To process all possibilities, a classical computer has to check them one at a time.

In quantum computers, particles such as electrons or photons act as a qubit (quantum bit), which can be in a superposition of both 0 and 1. Three qubits can be in a superposition of all 8 possible states at once, meaning that quantum computers can process a much larger number of calculations simultaneously. With three qubits present, a quantum computer could process all eight states listed above at once.

This much greater processing power than traditional computers could mean that quantum computers could one day be used to perform complex simulations in pharmaceuticals, climate modeling, and manufacturing. In theory, a quantum computer powerful enough can perform calculations in seconds that would have taken the most powerful supercomputers millions of years to complete.

When is World Quantum Day?

World Quantum Day, an international celebration held to promote public understanding of quantum science, is held annually on April 14.

The date, 4/14, was chosen because 4.14 represents the first three digits of Planck’s constant (4.135667696 x 10^-15 electron volts per hertz, rounded to 4.14 x 10^-15) — an important number in quantum physics.

Physicists claim they may have found a long-awaited explanation for dark energy, the mysterious force that's driving the accelerated expansion of the universe, a new preprint study hints.

Their calculations suggest that, at the smallest scales, space-time behaves in a profoundly quantum way, differing drastically from the smooth, continuous structure we experience in everyday life. According to their findings, the coordinates of space-time do not "commute" — meaning the order in which they appear in equations affects the outcome. This is similar to how a particle's position and velocity behave in quantum mechanics.

One of the most striking consequences of this quantum space-time, as predicted by string theory, is that it naturally leads to cosmic acceleration. Moreover, the researchers found that the rate at which this acceleration decreases over time aligns remarkably well with the latest observations from the Dark Energy Spectroscopic Instrument (DESI).

"Viewed through the lens of our work, you could think of the DESI result as the first observational evidence supporting string theory and perhaps the first observable consequences of string theory and quantum gravity," study co-author Michael Kavic, a professor at SUNY Old Westbury, told Live Science via email.

The mystery of the universe's expansion

In 1998, two independent teams — the Supernova Cosmology Project and the High-Z Supernova Search Team — discovered that the universe's expansion was not slowing down, as previously thought, but was instead accelerating. They reached this conclusion by studying distant supernovas, which appeared dimmer than expected. This acceleration implied the presence of a mysterious entity permeating space, later dubbed dark energy.

Related: 'The universe has thrown us a curveball': Largest-ever map of space reveals we might have gotten dark energy totally wrong

However, the origin of dark energy has remained elusive. A popular hypothesis suggests it arises from quantum fluctuations in the vacuum, similar to those seen in the electromagnetic field. Yet, when physicists attempted to compute the expansion rate based on this idea, they arrived at a value that was 120 orders of magnitude too large — a staggering discrepancy.

Recent DESI observations further complicated the picture. According to the Standard Model of elementary particles, if dark energy were simply a vacuum energy, its density should remain constant over time. However, DESI data indicate that the acceleration rate is not fixed but that it decreases over time — something the Standard Model does not predict.

Solving the mystery with string theory

To address these inconsistencies, the researchers turned to string theory, one of the leading candidates for a quantum theory of gravity. Unlike the Standard Model, which treats elementary particles as point-like, string theory proposes that they are actually tiny, vibrating, one-dimensional objects called strings. These strings, depending on their modes of vibration, give rise to different particles — including the graviton, the hypothetical quantum carrier of gravity.

In a new paper that was posted in the preprint database arXiv but has not been peer-reviewed, physicists Sunhaeng Hur, Djordje Minic, Tatsu Takeuchi (Virginia Tech), Vishnu Jejjala (University of the Witwatersrand), and Michael Kavic applied string theory to analyze space-time at the quantum level.

By replacing the Standard Model's description of particles with the framework from string theory, the researchers found that space-time itself is inherently quantum and noncommutative, meaning the order in which coordinates appear in equations matters.

This radical departure from classical physics allowed them to derive the properties of dark energy not just from experimental data, but directly from a fundamental physical theory. Their model not only yielded a dark energy density that closely matches observational data but also correctly predicted that this energy should decrease over time, aligning with DESI's findings.

One of the most striking aspects of their result is that the value of dark energy depends on two vastly different length scales: the Planck length, the fundamental scale of quantum gravity, which is about 10⁻³³ centimeters; and the size of the universe, which is billions of light-years across. Such a connection between the smallest and largest scales in the cosmos is highly unusual in physics and suggests that dark energy is deeply tied to the quantum nature of space-time itself.

"This hints at a deeper connection between quantum gravity and the dynamical properties of nature that had been supposed to be constant," Kavic said. "It may turn out that a fundamental misapprehension we carry with us is that the basic defining properties of our universe are static when in fact they are not."

Experimental tests and future prospects

Although the team's explanation of the universe's accelerated expansion is a significant theoretical breakthrough, independent experimental tests are needed to confirm their model. The researchers have proposed concrete ways to test their ideas.

One line of evidence "involves detecting complicated quantum interference patterns, which is impossible in standard quantum physics but should occur in quantum gravity," Minic added.

Interference occurs when waves, such as light or matter waves, overlap and either amplify or cancel each other out, creating characteristic patterns. In conventional quantum mechanics, interference follows well-understood rules, typically involving two or more possible quantum paths. However, higher-order interference—predicted by some quantum gravity models—suggests more complex interactions that go beyond these standard patterns. Detecting such effects in the lab would be a groundbreaking test of quantum gravity.

"These are tabletop experiments that could be performed in the near future — within three to four years."

"There are many implications of our approach to quantum gravity," said Djordje Minic, a physicist at Virginia Tech and co-author of the paper, in an email. One line of evidence "involves detecting complicated quantum interference patterns, which is impossible in standard quantum physics but should occur in quantum gravity," Minic added.

Interference occurs when waves, such as light or matter waves, overlap and either amplify or cancel each other out, creating characteristic patterns. In conventional quantum mechanics, interference follows well-understood rules. However, some quantum gravity models suggest more complex interactions that go beyond these standard patterns. Detecting such effects in the lab would be a groundbreaking test of quantum gravity.

"These are tabletop experiments that could be performed in the near future — within three to four years."

In the meantime, the researchers are not waiting for experimental confirmations. They are continuing to refine their understanding of quantum space-time, as well as exploring additional avenues for testing their theory.

If confirmed, their findings would mark a major breakthrough not only in explaining dark energy but also in providing the first tangible evidence for string theory — a long-sought goal in fundamental physics.

For the first time, researchers have transformed light into a "'supersolid" — a strange state of matter that is both solid and liquid at the same time.

Although scientists have made supersolids out of atoms before, this is the first instance of coupling light and matter to create a supersolid and it opens new doors for studying condensed-matter physics, researchers explained in a paper published March 5 in journal Nature.

But what exactly is a supersolid, and why is this new development so exciting? Here's everything you need to know.

Related: 32 physics experiments that changed the world

Scientists have used AI to discover an easier method to form quantum entanglement between subatomic particles, paving the way for simpler quantum technologies.

When particles such as photons become entangled, they can share quantum properties — including information — regardless of the distance between them. This phenomenon is important in quantum physics and is one of the features that makes quantum computers so powerful.

But the bonds of quantum entanglement have typically proven challenging for scientists to form. This is because it requires the preparation of two separate entangled pairs, then measuring the strength of entanglement — called a Bell-state measurement — on a photon from each of the pairs.

These measurements cause the quantum system to collapse and leave the two unmeasured photons entangled, despite them never having directly interacted with one another. This process of “entanglement swapping” could be used for quantum teleportation.

In a new study, published Dec. 2, 2024 in the journal Physical Review Letters, scientists used PyTheus, an AI tool that has been specifically created for designing quantum-optic experiments. The authors of the paper initially set out to reproduce established protocols for entanglement swapping in quantum communications. However, the AI tool kept producing a much simpler method to achieve quantum entanglement of photons.

Related: Quantum data beamed alongside 'classical data' in the same fiber-optic connection for the 1st time

"The authors were able to train a neural network on a set of complex data that describes how you set up this kind of experiment in many different conditions, and the network actually learned the physics behind it," Sofia Vallecorsa, a research physicist for the quantum technology initiative at CERN, who was not involved in the new research, told Live Science.

Tapping into AI to simplify quantum entanglement

The AI tool proposed that entanglement could emerge because the path of photons were indistinguishable: when there are several possible sources the photons could have come from, and if their origins become indistinguishable from one another, then entanglement can be produced between them when none existed before.

Although the scientists were initially skeptical of the results, the tool kept returning the same solution so they tested the theory. By adjusting the photon sources and ensuring they were indistinguishable, the physicists created conditions where detecting photons at certain paths guaranteed that two others emerged entangled.

This breakthrough in quantum physics has simplified the process by which quantum entanglement can be formed. In future, it could have implications for the quantum networks used for secure messaging, making these technologies much more feasible.

"The more we can rely on simple technology, the more we can increase the range of applications," Vallecorsa said. "The possibility to build more complex networks, that could branch out in different geometries, could have a big impact with respect to the single end-to-end case."

Whether it is practical to scale the technology into a commercially viable process remains to be seen, however, as environmental noise and device imperfections could cause instability in the quantum system.

The new study has also provided a convincing argument for the use of AI as a research tool by physicists. "We are looking more into introducing AI, but there is still a little bit of scepticism, mostly due to what the role of the physicist is going to be once we start going that way," Vallecorsa said. "It is an opportunity for getting a very interesting result and shows in a very compelling way how this can be a tool that physicists use."

Theoretical physicists have proposed a potential solution to one of the most puzzling problems in modern physics: the black hole singularity paradox. By modifying Einstein's theory of general relativity, the center of a black hole with infinite curvature could be replaced by a highly curved but regular region of space-time, the researchers suggest in a new study.

"Singularities are regions of the universe where space, time and matter are crushed and stretched into nonexistence," study co-author Robie Hennigar, a postdoctoral researcher at Durham University in the U.K., told Live Science via email. "This is a very serious problem, as if singularities were to really exist in our universe, it would be catastrophic for science.

"We could no longer use the equations of physics to predict the future from the past and present," he continued. "For these reasons, most practising scientists expect that singularities are not physical, but are telling us that general relativity must be replaced by a more complete theory to describe the universe near singularities."

Related: Scientists may have just discovered 300 of the rarest black holes in the universe

Correcting Einstein

Since its introduction in 1915, general relativity has been remarkably successful in explaining astrophysical and cosmological phenomena, including the formation of black holes, the structure of neutron stars, and the large-scale structure and evolution of the universe.

However, the theory has fundamental limitations. It is incompatible with quantum mechanics, which governs the behavior of particles at the smallest scales, and it predicts singularities — points of infinite density — at the centers of black holes and at the Big Bang.

To address this issue, the researchers used a concept known as quantum gravity, which is commonly applied in attempts to unify Einstein's general relativity with quantum mechanics, which predicts continuous particle creation and annihilation in empty space, along with perpetual fluctuations in all fields, including gravity. Their study, published in February in the journal Physics Letters B, suggests that at extremely high energies or incredibly small distances, general relativity should be modified by an infinite series of additional terms in its equations.

"In quantum gravity, one considers all corrections to the equations relating the energy and momentum of a system with the spacetime curvature that are consistent with known physical principles," Hennigar said. "Different approaches to quantum gravity will place different importance on different terms in the equations, but they all suggest that Einstein's equations need to be refined."

By incorporating these modifications into their calculations, the researchers examined how black holes would behave under this revised framework. Their results showed that when an infinite number of new terms are included, the singularity vanishes. Instead of an infinitely dense point, the black hole's core becomes a highly curved but regular region of space-time.

Testing the theory

Although the new model resolves the singularity problem mathematically, scientific theories must ultimately be tested through observation. The researchers acknowledged that directly confirming their idea presents a significant challenge.

"The absence of singularities itself is hard to test experimentally, because it would occur inside a black hole, or at the very beginning of the universe," Pablo Cano, a postdoctoral researcher at the University of Barcelona and another co-author of the study, told Live Science in an email. "However, we can look for signatures of the theories that lead to singularity resolution.

"The modifications of general relativity that we consider become larger in stronger gravitational fields, but are very small otherwise," Cano added. "This means that, for instance, gravitational waves coming from collisions of black holes — where gravitational fields are much stronger than in the solar system — provide a way of searching for these effects."

Another promising avenue is the study of the early universe. If the effects of this modified gravity theory influenced cosmic inflation — the rapid expansion that followed the Big Bang — evidence of these changes might be imprinted in primordial gravitational waves. Future experiments targeting these signals could help test the validity of the theory.

Next steps

In addition, further theoretical work is needed to determine whether singularity-free black holes can form naturally through gravitational collapse and whether the team's approach can address other types of singularities, such as those associated with the Big Bang.

"We have recently shown that the collapse of a certain type of matter gives rise, within this framework, to the formation of these regular black holes," said Pablo Bueno, a research fellow at the University of Barcelona and a co-author of the study. "We would like to test this under more general assumptions. This may give rise to intriguing features in other areas, such as explicit models of bouncing cosmologies in which the usual Big Bang scenario is replaced by a never ending series of expansive and contracting phases."

Black hole quiz: How supermassive is your knowledge of the universe?

In 1974, Stephen Hawking put forward an intriguing idea: Using the principles of quantum physics, he predicted that even though nothing is supposed to escape a black hole's event horizon, these cosmic beasts can actually emit particles. And by ejecting these particles, black holes will shrink over a very long time, eventually evaporate and possibly explode.

Hawking sparked a debate that has continued for more than 50 years. The idea that black holes evaporate puts two fundamental tenets of physics — general relativity and quantum mechanics — into conflict.

But, assuming that Hawking and others are correct, how would a black hole evaporate, leaving nothing behind? Before we explore this mind-bending idea, it's important to know that theoretical physicists are still debating how it's possible. But in the past few decades, a number of potential explanations have emerged for how black holes seem to defy the rules of our universe by disappearing.

"We spent the last 50 years fighting about it," Daniel Harlow, a physicist at MIT, told Live Science. "I would say now we understand it a lot better than Hawking did."

Related: Could a black hole devour the universe?

Albert Einstein predicted the existence of black holes in 1915 with his theory of general relativity, which explains how gravity is a property of space-time's curvature. Based on this theory, black holes are objects with a lot of mass compressed into a singular area, where gravity is so strong that even light can't escape its pull.

"According to general relativity, everything can only go inside [a black hole] and nothing can ever come out," said Heino Falcke, an astrophysicist at Radboud University in the Netherlands who was involved in capturing the first image of a black hole in 2019. "Everything thrown in is completely crushed into a point."

But around 60 years later, Hawking's calculations showed that perhaps not everything is crushed by black holes. In quantum mechanics, pairs of particles — particles and antiparticles — blink in and out of existence. These particles usually cancel each other out.

But Hawking argued that fluctuations of fields at the event horizon, a black hole's "point of no return" beyond which nothing can escape, means that these particles don't always cancel out: One of those particles can get sucked into the black hole, while the other gets ejected into space, leaving behind a cloud called Hawking radiation. As more and more particles are ejected, black holes begin to lose energy and mass, and eventually disappear, according to this theory.

This process would be very slow. A black hole with a mass of the sun could take 10^67 years to fully evaporate — longer than the current age of the universe. And scientists have yet to find evidence for this; black holes do not seem to release thermal radiation, meaning that Hawking radiation may not be detectable. Some scientists are trying to get a peek at this elusive radiation in labs and on tiny black holes, which are posited to evaporate more quickly than the ones in our galaxy would.

Black hole paradox

However, Hawking's idea has a few caveats that lead to confounding questions. Evaporation introduces a conundrum called the black hole information paradox. If a black hole evaporates and disappears, the particles it leaves behind are missing information on the matter's original state. This violates a core concept in physics — that a system in one point in time should determine, or reflect, its state in another — also known as predictability.

Scientists are still debating how to resolve this paradox. "The amazing thing about Hawking's paradox is that any resolution of it requires you to give up some sacred principle of physics," Harlow said. Hawking's solution was to give up predictability, as he lamented in a 1976 paper.

Some physicists are examining the laws around thermodynamics to solve this discrepancy, and how entropy affects quantum information. Another group of physicists is examining locality, the principle that objects are directly influenced only by their immediate surroundings. They believe the information paradox can be resolved through something called quantum nonlocality — the idea that particles inside a black hole share their quantum state with correlated particles outside it.

Despite the progress in understanding black hole evaporation, mysteries continue to pile up. In a 2023 study in the journal Physical Review Letters, Falcke and colleagues argued that the information paradox may not be limited to black holes. By rederiving Hawking's calculations, the team proposed that all objects may have the same problem. All things may be evaporating, thus deepening the puzzle.

"There's something in the world we cannot explain," Falcke said. "But, you know, by creating more mysteries, we may actually be a step closer to a solution eventually."

Researchers in Birmingham have created the first image of a photon, a lemon-shaped particle of light emitted from the surface of a nanoparticle. The theory that made this image possible, reported Nov. 14 in the journal Physical Review Letters, enables scientists to calculate and understand various properties of these quantum particles — which could open up a range of new possibilities across fields such as quantum computing, photovoltaic devices and artificial photosynthesis.

Light's quantum behavior is well established, with over 100 years of experiments showing it can exist in both wave and particle form. But our fundamental understanding of this quantum nature is much further behind, and we only have a limited grasp of how photons are created and emitted, or of how they change through space and time.

"We want to be able to understand these processes to leverage that quantum side," first author Ben Yuen, a research fellow at the University of Birmingham in the U.K., told Live Science. "How do light and matter really interact at this level?"

However, the very nature of light means the answer to this question has almost limitless possibilities. "We can think of a photon being a fundamental excitation of an electromagnetic field," explained Yuen. These fields are a continuum of different frequencies, each of which could potentially become excited. "You can split up a continuum into smaller parts and between any two points, there's still an infinite number of possible points you could pick," Yuen added.

The result is that the properties of a photon are heavily dependent on the properties of its environment, leading to some incredibly complex math. "At first glance, we would have to write down and solve an infinite number of equations to reach an answer," Yuen said.

Related: High school students who came up with 'impossible' proof of Pythagorean theorem discover 9 more solutions to the problem

To tackle this seemingly impossible task, Yuen and co-author Angela Demetriadou, professor of theoretical nanophotonics at the University of Birmingham, employed a clever math trick to dramatically simplify the equations.

Introducing imaginary numbers — multiples of the impossible square root of -1 — is a powerful tool when handling complex equations. Manipulating these imaginary components allows many of the difficult terms in the equation to cancel each other out. Provided all imaginary numbers are converted back to real numbers before reaching the solution, this leaves a much more manageable calculation.

"We transformed that continuum of real frequencies into a discrete set of complex frequencies," explained Yuen. "By doing that, we simplify the equations from a continuum into a discrete set which we can handle. We can put those into a computer and solve them."

The team used these new calculations to model the properties of a photon emitted from the surface of a nanoparticle, describing the interactions with the emitter and how the photon propagated away from the source. From these results, the team generated the first image of a photon, a lemon-shaped particle never seen before in physics.

Yuen stressed, however, that this is only the shape of a photon generated under these conditions. "The shape changes completely with the environment," he said. "This is really the point of nanophotonics, that by shaping the environment, we can really shape the photon itself."

The team's calculations provide a fundamental insight into the properties of this quantum particle — knowledge that Yuen believes will open up new lines of research for physicists, chemists and biologists alike.

"We could think about optoelectronic devices, photochemistry, light harvesting and photovoltaics, understanding photosynthesis, biosensors, and quantum communication," Yuen said. "And there will be a whole host of unknown applications. By doing this kind of really fundamental theory, you unlock new possibilities in other areas."

From the most distant stars in the sky to the screen in front of your face, light is everywhere. But the exact nature of light, and how it travels, has long puzzled scientists. One question in particular has vexed thinkers from Issac Newton to Albert Einstein: Is light a particle or a wave?

"Whether light is a particle or a wave is a very old question," Riccardo Sapienza, a physicist at Imperial College London, told Live Science. As a species, we seem driven to understand the fundamental nature of the world around us, and this particular puzzle kept 19th-century scientists busy.

Today, there's no doubt about the answer: Light is both a particle and a wave. But how did scientists reach this mind-bending conclusion?

The starting point was to scientifically distinguish between waves and particles. "You would describe an object as a particle if you can identify it as a point in space," Sapienza said. "A wave is an object that you don't define as a point in space and you need to give a frequency of oscillation and distance between maximum and minimum."

The first conclusive evidence of the wave nature of light came in 1801, when Thomas Young performed his now-famous double-slit experiment. He placed a screen with two holes in front of a light source and observed the behavior of the light after it had passed through the slits. The light hitting the wall showed a complicated pattern of bright and dark bands, known as interference fringes.

As the light waves passed through each hole, they generated partial waves that radiated spherically, intercepting each other and adding or subtracting to the final intensity.

"If the light was a particle, you would have ended up with two bunches on the other side of the screen," Sapienza said. "But we have interference, and we see light everywhere after the screen, not just at the position of the holes. That's proof that light is indeed a wave."

Eighty-six years later, Heinrich Hertz became the first to demonstrate the particle nature of light. He noticed that when ultraviolet light shone on a metal surface, it generated a charge — a phenomenon called the photoelectric effect. However, the significance of his observation wasn't fully understood until many years later.

Related: What is the speed of light?

Atoms contain electrons in fixed energy levels. Shining light on them is therefore expected to give the electrons energy and enable them to escape from the atom, with brighter light liberating electrons faster. But in experiments following Hertz's work, several unusual observations seemed to completely contradict this classical understanding of physics.

It was Einstein who finally solved this puzzle, for which he was awarded a Nobel prize in 1921. Rather than absorbing light continuously from a wave, atoms actually receive energy in packets of light called photons, explaining odd observations such as the existence of a cutoff frequency.

But what determines whether light behaves as a wave or as a particle? According to Sapienza, this isn't the right question to be asking. "Light is not sometimes a particle and sometimes a wave," he said. "It is always both a wave and a particle. It's just that we highlight one of the properties depending on which experiment we do."

In day-to-day life, we mostly experience light as a wave, and it's this form that physicists find most useful to manipulate.

"There's a full field called metamaterials — by shaping a material with the same features as light, we can enhance the interaction of light with the material and control the waves,” Sapienza said. "For example, we can make solar absorbers that can absorb light more efficiently for energy generation or metamaterial MRI probes which are much more effective."

However, light's double nature, known as wave particle duality, is absolutely fundamental to the existence of the world as we know it. This strange twinned behavior also extends to other quantum particles, like electrons.

"You could not have an atom be stable if you didn't have quantum mechanics with the electrons in specific states," Sapienza said. "If you remove the fact that it is a particle, you remove the fact that it has a specific energy and life could not exist."

Quantum physicists are familiar with wonky, seemingly nonsensical phenomena: atoms and molecules sometimes act as particles, sometimes as waves; particles can be connected to one another by a "spooky action at a distance," even over great distances; and quantum objects can detach themselves from their properties like the Cheshire Cat from Alice's Adventures in Wonderland detaches itself from its grin. Now researchers led by Daniela Angulo of the University of Toronto have revealed another oddball quantum outcome: photons, wave-particles of light, can spend a negative amount of time zipping through a cloud of chilled atoms. In other words, photons can seem to exit a material before entering it.

"It took a positive amount of time, but our experiment observing that photons can make atoms seem to spend a *negative* amount of time in the excited state is up!" wrote Aephraim Steinberg, a physicist at the University of Toronto, in a post on X (formerly Twitter) about the new study, which was uploaded to the preprint server arXiv.org on September 5 and has not yet been peer-reviewed.

The idea for this work emerged in 2017. At the time, Steinberg and a lab colleague, then doctoral student Josiah Sinclair, were interested in the interaction of light and matter, specifically a phenomenon called atomic excitation: when photons pass through a medium and get absorbed, electrons swirling around atoms in that medium jump to higher energy levels. When these excited electrons lapse to their original state, they release that absorbed energy as reemitted photons, introducing a time delay in the light's observed transit time through the medium.

Sinclair's team wanted to measure that time delay (which is sometimes technically called a "group delay") and learn whether it depends on the fate of that photon: Was it scattered and absorbed inside the atomic cloud, or was it transmitted with no interaction whatsoever? "At the time, we weren't sure what the answer was, and we felt like such a basic question about something so fundamental should be easy to answer," Sinclair says. "But the more people we talked to, the more we realized that while everyone had their own intuition or guess, there was no expert consensus on what the right answer would be." Because the nature of these delays can be so strange and counterintuitive, some researchers had written the phenomenon off as effectively meaningless for describing any physical property associated with light.

After three years of planning, his team developed an apparatus to test this question in the lab. Their experiments involved shooting photons through a cloud of ultracold rubidium atoms and measuring the resulting degree of atomic excitation. Two surprises emerged from the experiment: Sometimes photons would pass through unscathed, yet the rubidium atoms would still become excited — and for just as long as if they had absorbed those photons. Stranger still, when photons were absorbed, they would seem to be reemitted almost instantly, well before the rubidium atoms returned to their ground state — as if the photons, on average, were leaving the atoms quicker than expected.

The team then collaborated with Howard Wiseman, a theoretical and quantum physicist at Griffith University in Australia, to devise an explanation. The theoretical framework that emerged showed that the time these transmitted photons spent as an atomic excitation matched perfectly with the expected group delay acquired by the light — even for cases where it seemed as though the photons were reemitted before the atomic excitation had ebbed.

Related: Time might be a mirage created by quantum physics, study suggests

To understand the nonsensical finding, you can think of photons as the fuzzy quantum objects they are, in which any given photon's absorption and reemission through an atomic excitation is not guaranteed to occur over a certain fixed amount of time; rather, it takes place across a smeared-out, probabilistic range of temporal values. As demonstrated by the team's experiments, these values can encompass instances when an individual photon's transit time is instantaneous — or, bizarrely, when it concludes before the atomic excitation has ceased, which gives a negative value.

"I can promise you that we were completely surprised by this prediction," Sinclair says, referring to the matchup between the group delay and the time that the transmitted photons spent as atomic excitations. "And as soon as we were confident we hadn't made a mistake, Steinberg and the rest of the team — I had moved on to do a postdoc at [the Massachusetts Institute of Technology] by this point — began planning to do a follow-up experiment to test this crazy prediction of negative dwell time and see if the theory would hold up."

That follow-up experiment, the one led by Angulo that Steinberg touted on X, can be understood by considering the two ways a photon can be transmitted. In one, the photon wears blinders of sorts and ignores the atom entirely, leaving without even a nod. In the other, it interacts with the atom, boosting it to a higher energy level, before getting reemitted.

"When you see a transmitted photon, you can't know which of these occurred," Steinberg says, adding that because photons are quantum particles in the quantum realm, the two outcomes can be in superposition—both things can happen at the same time. "The measuring device ends up in a superposition of measuring zero and measuring some small positive value." But correspondingly, Steinberg notes, that also means that sometimes "the measuring device ends up in a state that looks not like 'zero' plus 'something positive' but like 'zero' minus 'something positive,' resulting in what looks like the wrong sign, a negative value, for this excitation time."

The measurement results in Angulo and her colleagues' experiment suggest that the photons moved through the medium faster when they excited the atoms than when the atoms remained in their ground state. (The photons aren't communicating any information, so the outcome does not contradict the "nothing can travel faster than light" speed limit set by Einstein's special theory of relativity.)

"A negative time delay may seem paradoxical, but what it means is that if you built a 'quantum' clock to measure how much time atoms are spending in the excited state, the clock hand would, under certain circumstances, move backward rather than forward," Sinclair says. In other words, the time in which the photons were absorbed by atoms is negative.

Even though the phenomenon is astonishing, it has no impact on our understanding of time itself — but it does illustrate once again that the quantum world still has surprises in store.

"[Angulo] and the rest of the team have accomplished something really impressive and produced a beautiful set of measurements. Their results raise interesting questions about the history of photons traveling through absorptive media and necessitate a reinterpretation of the physical meaning of the group delay in optics," Sinclair says.

A version of this article originally appeared in Spektrum der Wissenschaft and was reproduced with permission.

This article was first published at Scientific American. © ScientificAmerican.com. All rights reserved. Follow on TikTok and Instagram, X and Facebook.

A new study suggests that black holes may not be the featureless, structureless entities that Einstein's general theory of relativity predicts them to be. Instead, the cosmic monsters might be bizarre quantum objects known as "frozen stars."

While these would share some similarities with black holes, the hypothetical celestial bodies differ in crucial ways that could potentially resolve the infamous Hawking radiation paradox (named for the late physicist Stephen Hawking, who proposed the phenomenon). This paradox arises because the theoretical radiation emitted by a black hole's event horizon seemingly carries no information about the matter that formed the black hole, which contradicts a fundamental principle of quantum mechanics stating that information cannot be destroyed.

Moreover, unlike the conventional black holes, frozen stars are not expected to harbor a singularity — a point of infinite density at their centers — which resolves another contradiction between the classical picture of black holes and the general rule in physics that infinities cannot exist in nature. When infinities do appear in a theory, it usually signals the theory's limitations.

"Frozen stars are a type of black hole mimickers: ultracompact, astrophysical objects that are free of singularities, lack a horizon, but yet can mimic all of the observable properties of black holes," Ramy Brustein, a professor of physics at Ben-Gurion University in Israel, told Live Science in an email. "If they actually exist, they would indicate the need to modify in a significant and fundamental way Einstein's theory of general relativity."

Brustein is a co-author of a study describing the frozen star theory, published in July in the journal Physical Review D.

Resolving the paradox

The classical model of a black hole, first described by Karl Schwarzschild in 1916, portrays black holes as having two key features: a singularity where all the mass is concentrated and an event horizon, a boundary from which nothing, not even light, can escape.

However, this model encounters a serious problem when quantum mechanics is introduced. In the 1970s, Stephen Hawking famously discovered that quantum effects near the event horizon should lead to the creation of particles out of the vacuum of space, a process known as Hawking radiation. This radiation would cause the black hole to gradually lose mass and eventually evaporate completely.

Related: 'Twisty' new theory of gravity says information can escape black holes after all

The paradox arises because this radiation appears to carry no information about the matter that originally formed the black hole. If the black hole evaporates completely, this information seems to be lost forever, violating the principles of quantum mechanics, which dictate that information must be conserved. This contradiction is known as the information loss paradox, and it has been one of the most significant challenges in theoretical physics.

In their new study, Brustein and fellow co-authors A.J.M. Medved of Rhodes University and Tamar Simhon of Ben-Gurion University performed a detailed theoretical analysis of the frozen stars model, and found that it resolves the paradoxes of the traditional model because it lacks both a horizon and a singularity.

The authors found that if black holes are actually very compact objects composed of ultra rigid matter whose properties are inspired by string theory, the leading candidate for the theory of quantum gravity, they don't collapse into infinitely dense points, and have a size slightly larger than the conventional event horizon, preventing the latter from forming.

"We have shown how frozen stars behave as (nearly) perfect absorbers although lacking a horizon and act as a source of gravitational waves," said Brustein, noting that these objects can absorb almost everything that falls onto them, much like black holes. "Moreover, they source the same external geometry as that of a conventional model of black holes and reproduce their conventional thermodynamic properties."

Testing the frozen star hypothesis

While the frozen star model presents a potential solution to the paradoxes associated with traditional black holes, scientists still need to test it experimentally.

But unlike conventional black holes, frozen stars are expected to have an internal structure, albeit one with bizarre properties dictated by quantum gravity. This paves the way to observationally discriminate between the two. The evidence could be present in gravitational waves — ripples in the fabric of space-time — generated during black hole mergers.

"This is when the distinctions would be most pronounced," explained Brustein.

The team still needs to work out exactly what the internal structure of a frozen star would look like, and how it would differ from other extreme cosmic objects like neutron stars, but it's achievable, Brustein said. From there, they could analyze data from existing and future gravitational wave observatories, because the gravitational waves emitted during the mergers are extremely powerful and can carry information about these ultracompact objects' structure.

"A discovery of any of the predictions of the frozen star model will have a revolutionary impact," Brustein said.

Editor's note: This article was updated on Sep. 25 to list Ramy Brustein as a study co-author, rather than the lead author. All researchers contributed to the work equally.

Scientists have found a key process required for superconductivity occurring at higher temperatures than previously thought. It could be a small but significant step in the search for one of the "holy grails" of physics, a superconductor that operates at room temperature.

The discovery, made inside the unlikely material of an electrical insulator, reveals electrons pairing up at temperatures of up to minus 190 degrees Fahrenheit (minus 123 degrees Celsius) — one of the secret ingredients to the near-lossless flow of electricity in extremely cold superconducting materials.

So far, the physicists are baffled by why this is happening. But understanding it could help them find room-temperature superconductors. The researchers published their findings Aug. 15 in the journal Science.

"The electron pairs are telling us that they are ready to be superconducting, but something is stopping them," co-author Ke-Jun Xu, a graduate student in applied physics at Stanford University, said in a statement. "If we can find a new method to synchronize the pairs, we could apply that to possibly building higher temperature superconductors."

Superconductivity emerges from the ripples left in the wakes of electrons as they move through a material. At low enough temperatures, these ripples draw atomic nuclei to each other, in turn causing a slight offset in charge that attracts a second electron to the first.

Normally, two negative charges should repel each other. But instead, something strange happens: the electrons become bound together into a "Cooper pair."

Related: Room-temperature superconductors: The facts behind the 'holy grail' of physics

Cooper pairs follow different quantum mechanical rules than those of lone electrons. Instead of stacking outward in energy shells, they act like particles of light, an infinite number of which can occupy the same point in space at the same time. If enough of these Cooper pairs are created throughout a material, they become a superfluid, flowing without any loss of energy due to electrical resistance.

The first superconductors, discovered by Dutch physicist Heike Kamerlingh Onnes in 1911, transitioned into this zero electrical resistivity state at unimaginably cold temperatures — near absolute zero (minus 459.67 F, or minus 273.15 C). Yet, in 1986, physicists found a copper-based material, called a cuprate, which becomes a superconductor at a much warmer (but still very cold) minus 211 F (minus 135 C).

Physicists hoped this discovery would lead them to room-temperature superconductors. Yet insights into what causes cuprates to display their unusual behavior slowed and, last year, viral claims of viable room-temperature superconductors ended in allegations of data falsification and disappointment.

To investigate further, the scientists behind the new research turned to a cuprate known as neodymium cerium copper oxide.This material's maximum superconducting temperature is relatively low at minus 414.67 F (minus 248 C), so scientists haven't bothered to study it much. But when the study researchers shone ultraviolet light onto its surface they observed something strange.

Usually, when packets of light, or photons, strike a cuprate which carries unpaired electrons, the photons give the electrons enough energy to be ejected from the material, causing it to lose a lot of energy. But electrons in Cooper pairs can resist their photonic eviction, causing the material to lose only a little bit of energy.

Despite its zero resistance state occurring only at very low temperatures, the researchers found that the energy gap persisted in the new material up to 150 K, and that the pairing was, bizarrely, the strongest in the most samples best at resisting the flow of electrical current.

This means that, even though the cuprate is unlikely to reach room temperature superconductivity, it could contain some hints in finding a material that can.

"Our findings open a potentially rich new path forward. We plan to study this pairing gap in the future to help engineer superconductors using new methods," senior author Zhi-Xun Shen, a professor of physics at Stanford, said in the statement. "On the one hand, we plan to use similar experimental approaches to gain further insight into this incoherent pairing state. On the other hand, we want to find ways to manipulate these materials to perhaps coerce these incoherent pairs into synchronization."

Microscopic wormholes may be driving the accelerated expansion of the universe, scientists say. These tiny wormholes are constantly being born from the vacuum of space due to subtle quantum effects.

If confirmed through experiments and observations, the wormholes could become a valuable source of information on quantum gravity — a theoretical unification of the fundamental forces of the universe, often considered to be the Holy Grail of theoretical physics.

Numerous astronomical observations show that our universe is expanding at an ever-increasing rate. However, Einstein's general theory of relativity states that if the universe contains only the species of particles and radiation we know, such behavior of the fabric of space is impossible.

To reconcile the observations of universe expansion with this theory, scientists have proposed that space is filled with an enigmatic entity that can't be detected in ground or space-based experiments.

This mysterious substance, called dark energy, interacts very weakly with other types of matter and fields, so, there is currently no reliable information about its structure or origin.

In a recent study published April 5 in the journal Physical Review D, researchers proposed a bold new candidate for dark energy: subatomic-size wormholes — or tiny tunnels connecting disparate points in space.

Related: Wormholes might bend light like black holes do — and that could be the key to finding them

According to the authors, these wormholes are constantly being born and destroyed in the vacuum of space due to quantum effects. This is similar to how particles are produced near the event horizons of black holes, leading to Hawking radiation; or how electron-positron pairs are generated by a strong electric field — a phenomenon known as the Schwinger effect.

However, the creation of these wormholes is somewhat different from those other phenomena because their mathematical description requires quantum effects in gravity to be accounted for — a task that's much more complicated and poorly understood.

These difficulties in calculating quantum gravitational phenomena prevented the authors from accurately deriving the wormhole birth rate. However, using an approach known as Euclidean quantum gravity, they showed that if about 10 billion wormholes are spontaneously created per cubic centimeter per second, the energy they generate would be sufficient to explain the currently observed rate of the universe's expansion.

"Although our result was derived on the grounds of Euclidean quantum gravity… it is likely that our modification may hold for other quantum gravity theories as well," study co-author Stylianos Tsilioukas, a doctoral student at the University of Thessaly and National Observatory of Athens, told Live Science via email.

Moreover, the team's analysis showed that their model of dark energy is even better observationally than the most widely accepted theory, known as the Standard Cosmological Model, which posits that dark energy has a time-independent energy density.

"According to our proposal dark energy can change as time flows," Tsilioukas said. "This is a major advantage because recent observations suggest that the rate of expansion of the universe is different in recent times than it was in the early universe."

However, no matter how successful the researchers' model is at explaining the general properties of dark energy, the validity of any physical theory must be tested with experimental data. And for now, the theory remains untestable.

In the future, the ever-increasing accuracy of space experiments and observations should enable astronomers to deduce the universe expansion rate in more detail, as well as to measure other observable manifestations of dark energy. This could enable researchers to test whether this newly proposed model of dark energy is correct.

In the meantime, the authors plan to further improve their theoretical analysis. "We are working right now on a model which calculates the rate of wormhole formation. " Tsilioukas said. "The research seems promising and we hope to publish the results very soon."

The year 2025 marks the 100th anniversary of the birth of quantum mechanics. In the century since the field's inception, scientists and engineers have used quantum mechanics to create technologies such as lasers, MRI scanners and computer chips.

Today, researchers are looking toward building quantum computers and ways to securely transfer information using an entirely new sister field called quantum information science.

But despite creating all these breakthrough technologies, physicists and philosophers who study quantum mechanics still haven't come up with the answers to some big questions raised by the field’s founders. Given recent developments in quantum information science, researchers like me are using quantum information theory to explore new ways of thinking about these unanswered foundational questions. And one direction we're looking into relates Albert Einstein's relativity principle to the qubit.

RELATED: Tweak to Schrödinger's cat equation could unite Einstein's relativity and quantum mechanics, study hints

Quantum computers

Quantum information science focuses on building quantum computers based on the quantum "bit" of information, or qubit. The qubit is historically grounded in the discoveries of physicists Max Planck and Einstein. They instigated the development of quantum mechanics in 1900 and 1905, respectively, when they discovered that light exists in discrete, or "quantum," bundles of energy.

A computer based on a quantum bit rather than a classical bit could have a significant computing advantage. And that's because a classical bit produces a binary response — either a 1 or a 0 — to only one query.

In contrast, the qubit produces a binary response to infinitely many queries using the property of quantum superposition. This property allows researchers to connect multiple qubits in what's called a quantum entangled state. Here, the entangled qubits act collectively in a way that arrays of classical bits cannot.

That means a quantum computer can do some calculations much faster than an ordinary computer. For example, one device reportedly used 76 entangled qubits to solve a sampling problem 100 trillion times faster than a classical computer.

But the exact force or principle of nature responsible for this quantum entangled state that underlies quantum computing is a big unanswered question. A solution that my colleagues and I in quantum information theory have proposed has to do with Einstein's relativity principle.

Quantum information theory

The relativity principle says that the laws of physics are the same for all observers, regardless of where they are in space, how they're oriented or how they're moving relative to each other. My team showed how to use the relativity principle in conjunction with the principles of quantum information theory to account for quantum entangled particles.

Quantum information theorists like me think about quantum mechanics as a theory of information principles rather than a theory of forces. That's very different than the typical approach to quantum physics, in which force and energy are important concepts for doing the calculations. In contrast, quantum information theorists don't need to know what sort of physical force might be causing the mysterious behavior of entangled quantum particles.

That gives us an advantage for explaining quantum entanglement because, as physicist John Bell proved in 1964, any explanation for quantum entanglement in terms of forces requires what Einstein called "spooky actions at a distance."

That's because the measurement outcomes of the two entangled quantum particles are correlated — even if those measurements are done at the same time and the particles are physically separated by a vast distance. So, if a force is causing quantum entanglement, it would have to act faster than the speed of light. And a faster-than-light force violates Einstein's theory of special relativity.

Many researchers are trying to find an explanation for quantum entanglement that doesn't require spooky actions at a distance, like my team's proposed solution.

Classical and quantum entanglement

In entanglement, you can know something about two particles collectively — call them particle 1 and particle 2 — so that when you measure particle 1, you immediately know something about particle 2.

Imagine you're mailing two friends, whom physicists typically call Alice and Bob, each one glove from the same pair of gloves. When Alice opens her box and sees a left-hand glove, she'll know immediately that when Bob opens the other box he will see the right-hand glove. Each box and glove combination produces one of two outcomes, either a right-hand glove or a left-hand glove. There's only one possible measurement — opening the box — so Alice and Bob have entangled classical bits of information.

But in quantum entanglement the situation involves entangled qubits, which behave very differently than classical bits.

Qubit behavior

Consider a property of electrons called spin. When you measure an electron's spin using magnets that are oriented vertically, you always get a spin that's up or down, nothing in between. That's a binary measurement outcome, so this is a bit of information.

If you turn the magnets on their sides to measure an electron's spin horizontally, you always get a spin that's left or right, nothing in between. The vertical and horizontal orientations of the magnets constitute two different measurements of this same bit. So, electron spin is a qubit — it produces a binary response to multiple measurements.

Quantum superposition

Now suppose you first measure an electron's spin vertically and find it is up, then you measure its spin horizontally. When you stand straight up, you don't move to your right or your left at all. So, if I measure how much you move side to side as you stand straight up, I'll get zero.

That's exactly what you might expect for the vertical spin up electrons. Since they have vertically oriented spin up, analogous to standing straight up, they should not have any spin left or right horizontally, analogous to moving side to side.

Surprisingly, physicists have found that half of them are horizontally right and half are horizontally left. Now it doesn't seem to make sense that a vertical spin up electron has left spin (-1) and right spin (+1) outcomes when measured horizontally, just as we expect no side-to-side movement when standing straight up.

But when you add up all the left (-1) and right (+1) spin outcomes you do get zero, as we expected in the horizontal direction when our spin state is vertical spin up. So, on average, it's like having no side-to-side or horizontal movement when we stand straight up.

This 50-50 ratio over the binary (+1 and -1) outcomes is what physicists are talking about when they say that a vertical spin up electron is in a quantum superposition of horizontal spins left and right.

Entanglement from the relativity principle

According to quantum information theory, all of quantum mechanics, to include its quantum entangled states, is based on the qubit with its quantum superposition.

What my colleagues and I proposed is that this quantum superposition results from the relativity principle, which (again) states the laws of physics are the same for all observers with different orientations in space.

If the electron with a vertical spin in the up direction were to pass straight through the horizontal magnets as you might expect, it would have no spin horizontally. This would violate the relativity principle, which says the particle should have a spin regardless of whether it's being measured in the horizontal or vertical direction.

Because an electron with a vertical spin in the up direction does have a spin when measured horizontally, quantum information theorists can say that the relativity principle is (ultimately) responsible for quantum entanglement.

And since there is no force used in this principle explanation, there are none of the "spooky actions at a distance" that Einstein derided.

With quantum entanglement's technological implications for quantum computing firmly established, it's nice to know that one big question about its origin may be answered with a highly regarded physics principle.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Time may not be a fundamental element of the universe but rather an illusion emerging from quantum entanglement, a new study suggests. 

Time is a thorny problem for physicists; its inconsistent behavior between our best theories of the universe contributes to a deadlock preventing researchers from finding a "theory of everything," or a framework to explain all of the physics in the universe. 

But in the new study, researchers suggest they may have found a clue to solving that problem: by making time a consequence of quantum entanglement, the weird connection between two far-apart particles. The team published their findings May 10 in the journal Physical Review A. 

"There exists a way to introduce time which is consistent with both classical laws and quantum laws, and is a manifestation of entanglement," first author Alessandro Coppo, a physicist at the National Research Council of Italy, told Live Science. "The correlation between the clock and the system creates the emergence of time, a fundamental ingredient in our lives." 

 It's about time 

In quantum mechanics, our best theory of the microscopic world, time is a fixed phenomenon — an inexorable, unidirectional flow from the past to the present. It remains external from the bizarre and ever-changing quantum systems it measures and can be seen only by observing changes to outside entities, such as the hands of a clock. 

Related: Atoms squished closer together than ever before, revealing seemingly impossible quantum effects

Yet, according to Einstein's theory of general relativity — which describes larger objects, such as our bodies, stars and galaxies — time is interwoven with space and can be warped and dilated at high speeds or in the presence of gravity. This leaves our two best theories of reality at a fundamental impasse. Without its resolution, a coherent theory of everything remains out of reach.

"It seems there is a serious inconsistency in quantum theory," Coppo said. "This is what we call the problem of time."

To resolve this problem, the researchers turned to a theory called the Page and Wootters mechanism. First proposed in 1983, the theory suggests that time emerges for one object through its quantum entanglement with another acting as a clock. For an unentangled system, on the other hand, time does not exist, and the system perceives the universe as frozen and unchanging. 

By applying the Page and Wootters mechanism to two entangled but noninteracting theoretical quantum states — one a vibrating harmonic oscillator and the other a set of tiny magnets acting as a clock — the physicists found that their system could be perfectly described by the Schrödinger equation, which predicts the behavior of quantum objects. Yet, in place of time, their version of the famous equation ran according to the states of the tiny magnets acting as a clock. 

This insight is not new, but the team's next step was. They repeated their calculations twice, assuming first that the magnet clock and then the harmonic oscillator were macroscopic (larger) objects. Their equations simplified into those for classical physics, suggesting that time's flow is a consequence of entanglement even for objects on large scales. 

"We strongly believe that the correct and logical direction is to start from quantum physics and understand how to reach classical physics, not the other way around," Coppo said. 

Other physicists have expressed caution. Despite finding the Page and Wootters mechanism a fascinating idea for the quantum origins of time, they said it has yet to produce anything testable.

"Yes, it is mathematically consistent to think of the universal time as the entanglement between quantum fields and quantum states of 3D space," Vlatko Vedral, a professor of quantum information science at the University of Oxford who was not involved in the work, told Live Science. "However, no one knows if anything new or fruitful will come out of this picture — such as modifications to quantum physics and general relativity, and corresponding experimental tests."

Despite these doubts, building ground-up theories of time from quantum mechanics may nonetheless be a promising place to start — so long as they can be shaped to fit experiments. 

"Maybe there is something about entanglement where it plays a role," Adam Frank, a theoretical physicist at the University of Rochester in New York who was not involved in the study, told Live Science. "Maybe the only way to understand time is not from some God's-eye perspective, but from the inside, from a perspective of asking what is it about life that manifests such an appearance of the world." 

Scientists have squished two layers of ultracold magnetic atoms to within 50 nanometers of each other — 10 times closer than in previous experiments — revealing bizarre quantum effects not seen before.

The extreme proximity of these atoms will allow researchers to study quantum interactions at this length scale for the first time and could lead to important advances in the development of superconductors and quantum computers, the scientists reported in a new study published May 2 in the journal Science.

Unusual quantum behaviors begin to emerge at ultracold temperatures as the atoms are forced to occupy their lowest possible energy state. "In the nanokelvin regime, there's a type of matter called Bose Einstein condensate [in which] all the particles behave like waves," Li Du, a physicist at MIT and lead author of the study, told Live Science. "They are basically quantum mechanical objects."

Interactions between these isolated systems are particularly important for understanding quantum phenomena such as superconductivity and superradiance. But the strength of these interactions typically depends on the separation distance, which can create practical problems for researchers studying these effects; their experiments are limited by how close they can get the atoms.

"Most atoms used in cold experiments, such as the alkali metals, have to have contact in order to interact," Du said. "We're interested in dysprosium atoms which are special [in that they] can interact with each other at long range through dipole-dipole interactions [weak attractive forces between partial charges on adjacent atoms]. But although there's this long-range interaction, there are still some types of quantum phenomena that cannot be realized because this dipole interaction is so weak."

Related: Inside the 20-year quest to unravel the bizarre realm of 'quantum superchemistry'

Bringing cold atoms into close proximity while maintaining control of their quantum states is a significant challenge, and until now, experimental limitations have prevented researchers from fully testing theoretical predictions about the effects of these quantum interactions.

"In ordinary experiments, we trap atoms with light, and that's limited by the diffraction limit — in the order of 500 nanometers," Du said. (For comparison, a human hair measures between 80,000 - 100,000 nanometers wide, according to the National Nanotechnology Initiative.)

Using a laser beam focused through a lens, researchers can create a "Gaussian focal point," which is like an energy well within the laser beam that traps particular atoms in position. This is known as an optical tweezer, but the size of the tweezer (the width of the energy well) is limited by the wavelength of the laser light. This minimum width is called the diffraction limit.

Du's team came up with a clever trick to beat this diffraction limit, using another quantum property of dysprosium atoms: their spin. Atomic spin can point either up or down — but crucially, they have slightly different energies. This means the team could use two different laser beams at slightly different frequencies and polarization angles to trap the spin-up and spin-down of dysprosium atoms separately.

"If atom A doesn't see light B and atom B doesn't see light A, they basically have independent control," he explained. "As the atoms always sit precisely at the center of the Gaussian beam, you can move [the two different trapped particles] arbitrarily close." By carefully controlling the two optical tweezers, Du's team brought the spin-up and spin-down dysprosium atoms to within 50 nanometers of each other, increasing the interaction strength by 1,000 times from 500-nanometer levels.

With this bilayer established, the team began a series of experiments to study quantum interactions at close range. They heated up one of the dysprosium layers, completely separated from the other by a vacuum gap. Incredibly, they observed heat transfer to the second layer across the empty space.

"Typically, you need contact or radiation for heat to transfer, which we don't have here," Du said. "But we still see heat transfer, and this must be due to long range dipole-dipole interactions."

Seemingly impossible heat transfer was just one of the bizarre effects the team studied. Now, they're eager to further explore the potential of quantum interactions at this scale. The group is already beginning to study how these bilayers interact with light. But Du is particularly interested in another quantum effect, called Bardeen-Cooper-Schrieffer (BCS) pairing — a quantum bound state experienced by some subatomic particles called fermions at low temperatures.

"BCS pairing between layers is very important to superconductivity," he said. "Several years ago, a theoretical paper predicted that if we have this kind of bilayer system, coupled by long range dipole-dipole interactions, you could form a BCS pair. Previously we were not able to see this experimentally, but now it could be possible with our system."

For the first time ever, physicists have captured a clear image of individual atoms behaving like a wave.

The image shows sharp red dots of fluorescing atoms transforming into fuzzy blobs of wave packets and is a stunning demonstration of the idea that atoms exist as both particles and waves — one of the cornerstones of quantum mechanics. 

The scientists who invented the imaging technique published their findings on the preprint server arXiv, so their research has not yet been peer reviewed. 

"The wave nature of matter remains one of the most striking aspects of quantum mechanics," the researchers wrote in the paper. They add that their new technique could be used to image more complex systems, giving insights into some fundamental questions in physics.  

First proposed by the French physicist Louis de Broglie in 1924 and expanded upon by Erwin Schrödinger two years later, wave particle duality states that all quantum-sized objects, and therefore all matter, exists as both particles and waves at the same time. 

Related: Tweak to Schrödinger's cat equation could unite Einstein's relativity and quantum mechanics, study hints

Schrödinger's famous equation is typically interpreted by physicists as stating that atoms exist as packets of wave-like probability in space, which are then collapsed into discrete particles upon observation. While bafflingly counterintuitive, this bizarre property of the quantum world has been witnessed in numerous experiments. 

To image this fuzzy duality, the physicists first cooled lithium atoms to near-absolute zero temperatures by bombarding them with photons, or light particles, from a laser to rob them of their momentum. Once the atoms were cooled, more lasers trapped them within an optical lattice as discrete packets.

With the atoms cooled and confined, the researchers periodically switched the optical lattice off and on — expanding the atoms from a confined near-particle state to one resembling a wave, and then back. 

A microscope camera recorded light emitted by atoms in the particle state at two different times, with atoms behaving like waves in between. By putting together many images, the authors built up the shape of this wave and observed how it expands with time, in perfect agreement with Schrödinger's equation

"This imaging method consists in turning back on the lattice to project each wave packet into a single well to turn them into a particle again — it is not a wave anymore," study co-author Tarik Yefsah, a physicist at the French National Centre for Scientific Research and the École normale supérieure in Paris, told Live Science. "You can see our imaging method as a way to sample the wavefunction density, not unlike the pixels of a CCD camera." A CCD camera is a common type of digital camera that uses a charge-coupled device to capture its images.

The scientists say this image is just a simple demonstration. Their next step will be using it to study systems of strongly interacting atoms that are less well understood.

"Studying such systems could improve our understanding of strange states of matter, such as those found in the core of extremely dense neutron stars, or the quark-gluon plasma that is believed to have existed shortly after the Big Bang," Yefsah said.  

Theoretical physicists have proposed a new solution to the Schrödinger's cat paradox, which may allow the theories of quantum mechanics and Einstein's relativity to live in better harmony.

The bizarre laws of quantum physics postulate that physical objects can exist in a combination of multiple states, like being in two places at once or possessing various velocities simultaneously. According to this theory, a system remains in such a "superposition" until it interacts with a measuring device, only acquiring definite values as a result of the measurement. Such an abrupt change in the state of the system is called a collapse.

Physicist Erwin Schrödinger summarized this theory in 1935 with his famous feline paradox — using the metaphor of a cat in a sealed box being simultaneously dead and alive until the box is opened, thus collapsing the cat's state and revealing its fate.

However, applying these rules to real-world scenarios faces challenges — and that's where the true paradox arises. While quantum laws hold true for the realm of elementary particles, larger objects behave in accordance with classical physics as predicted by Einstein's theory of general relativity, and are never observed in a superposition of states. Describing the entire universe using quantum principles poses even greater hurdles, as the cosmos appears entirely classical and lacks any external observer to serve as a measuring device for its state.

"The question is can the Universe, which does not have a surrounding environment, be in such a superposition?" lead author Matteo Carlesso, a theoretical physicist at the University of Trieste in Italy, told Live Science in an email. "Observations say no: everything goes along the classical predictions of General Relativity. Then, what is breaking such a superposition?"

Related: Quantum 'yin-yang' shows two photons being entangled in real-time

To tackle this question, Carlesso and his colleagues proposed modifications to the  Schrödinger equation, which governs how all states, including those in superposition, evolve over time.

"Specific modifications of the Schrödinger equation can solve the problem," Carlesso said. In particular, the team added terms to the equation that captured how the system interacts with itself, as well as adding some other specific terms. This in turn leads to superposition breaking down.

"Such effects are stronger the larger the system," Carlesso added.

Crucially, these modifications have little impact on microscopic quantum systems, such as atoms and molecules, but allow larger systems — like the universe itself — to collapse at frequent intervals, giving them definite values that fit with our observations of the cosmos. The team described their modified Schrödinger equation in February in the Journal of High Energy Physics.

Taking the cat out of purgatory

In their tweaked version of quantum physics, the researchers eliminated the distinction between objects subject to measurement and measuring devices. Instead, they proposed that each system's state undergoes spontaneous collapse at regular intervals, leading to the acquisition of definite values for some of their attributes.

For large systems, spontaneous collapse occurs frequently, rendering them classical in appearance. Subatomic objects interacting with these systems become part of them, leading to rapid collapse of their state and the acquisition of definite coordinates, akin to measurement.

"With no action from external entities, any system localizes (or collapses) spontaneously in a particular state. In place of having a cat being dead AND alive, one finds it dead OR alive," Carlesso said.

The new model may explain why our universe's space-time geometry doesn't exist in a superposition of states and obeys the classical equations of Einstein's relativity.

"Our model describes a quantum Universe, which eventually collapsed thus becoming effectively classical," Carlesso said. "We show that spontaneous collapse models can explain the emergence of a classical Universe from a quantum superposition of Universes, where each of these Universes has a different space-time geometry."

While this theory may explain why the universe seems to be governed by classical laws of physics, it doesn't make new predictions about large-scale physical processes. 

However, it does make predictions about how atoms and molecules will behave, albeit with minimal deviations from conventional quantum mechanics. 

As a result, testing their modified quantum model won't be so simple. Future work will be aimed at coming up with such tests.

"Together with experimental collaborators, we are trying to test the effects of the collapse modifications or derive bounds on their parameters. This is completely equivalent to testing the limits of quantum theory."

Scientists recently created a never-before-seen four-atom molecule — the coldest of its kind ever made.

Researchers created the oddball molecule — a strange configuration of sodium-potassium with an ultralong chemical bond —  at 134 nanokelvin, or just 134 billionths of a degree above absolute zero. They described the ultracold material Jan. 31 in the journal Nature.

Ultracold systems are crucial to understand quantum behavior because quantum mechanics, the rules governing subatomic particles, dominate at low temperatures. These setups also let scientists precisely control the energy of particles to create quantum simulations, which model other quantum systems with physics we don't fully understand. For instance, studying the quantum behavior in a system of ultracold molecules could one day help scientists identify the material properties needed in high-temperature superconductors.

Related: Inside the 20-year quest to unravel the bizarre realm of 'quantum superchemistry'

The problem is that there's an inherent tradeoff: an ultracold system that is too simple may not capture the full array of behavior in interesting quantum systems. But add more complexity, and designing an effective experiment gets trickier.

"Usually people use atoms or ions and what makes them somewhat controllable is the fact that you have a relatively limited number of quantum states," Roman Bause, a quantum optics researcher at the University of Groningen in the Netherlands, told Live Science.

"But if I draw all the quantum states of a molecule, it will fill quite a thick book. It's a factor of a million or so more states."

All these additional quantum states open up more interesting quantum questions, but also make the molecules difficult to cool.

To solve that problem, in the new study, Xinyu Luo, a physicist at the Max Planck Institute of Quantum Optics in Germany, and international collaborators used a multi-step cooling process, beginning with laser cooling to create the record-breaking molecules.

Related: How do lasers work?

This cooling method uses laser beams fired from all directions at a moving atom. The atom absorbs light and enters an excited quantum state, then immediately releases energy to return to its ground state. But, because of how the atom is moving relative to the laser beams (known as the Doppler effect), the atom releases a little more energy than it absorbs, cooling itself.

"The problem with using this technique for molecules is that there's not just one ground state. You would potentially need thousands of laser beams and it's just too much technical effort," Bause said.

However, ultracold atoms are an excellent starting point to build ultracold molecules. Using a mixture of ultracold sodium (Na) and potassium (K) atoms, Shi's team weakly associated these single particles into diatomic NaK molecules.

This is where the technical difficulties really started. "The problem with associating cold atoms is you heat them while doing this so then you need another cooling technique, evaporative cooling," Bause said.

For reasons no one quite understands, under these cooling conditions the molecules stick together and the experimenter can no longer precisely control them. This particular challenge has stumped researchers across the field for years.

But, by shining in precisely controlled microwaves, Shi's team overcame the clumping issue in the diatomic NaK molecules as they were cooled down to 134 nanokelvin.

The microwaves also had a unique advantage when getting the two NaK molecules to weakly associate and form one four-atom-molecule of (NaK)2.  "If you shape the microwaves exactly right, what you have is a potential that's not just repulsive at short ranges but it's also attractive at longer ranges," Bause said.

As such, this first-of-its-kind four-atom molecule has a central bond 1000 times longer than the bond between the sodium and potassium atoms and was created at a temperature more than 3000 times colder than any previous four-atom molecule.

The new finds are exciting because they  "will ultimately bring us to interesting places where we currently have no theoretical handle — high temperature superconductors and materials for better lithium batteries for example," Bause said.

Editor's Note: This story was updated at 9:30 a.m. EDT on Monday, April 8 to correct the name of the researcher leading the team. Xinyu Luo oversaw the experimental efforts; Tao Shi worked primarily on theory.

Scientists have created a giant quantum tornado inside a helium superfluid, and they want to use it to probe the enigmatic nature of black holes.

The whirlpool — made from liquid helium cooled to near absolute zero — moves without friction, making it mimic the way rotating black holes warp the space-time that surrounds them.

By studying the vortex, physicists could glean important insight into the behavior of the cosmic monsters. The researchers published their findings March 20 in the journal Nature.

"Using superfluid helium has allowed us to study tiny surface waves in greater detail and accuracy than with our previous experiments in water," lead author Patrik Svancara, a physicist at the University of Nottingham in the U.K., said in a statement. "As the viscosity of superfluid helium is extremely small, we were able to meticulously investigate their interaction with the superfluid tornado and compare the findings with our own theoretical projections."

Related: Supermassive black hole at the heart of the Milky Way is approaching the cosmic speed limit, dragging space-time along with it

The workings of black holes remain a persistent mystery for physicists. The known laws of physics break in the presence of these extreme objects' infinite gravitational pulls. For those looking to combine Einstein's theory of general relativity with quantum mechanics, this means black holes' warping of space-time offers an alluring pull.

In the absence of a cataclysmic space-time rupture on Earth, the team behind the new study looked to a model system that could simulate some of the extreme eddies that exist around black holes. After supercooling liquid helium to a few fractions above absolute zero, they placed it inside a tank with a propeller at the bottom to stir up a vortex inside the fluid.

Then, by watching how the superfluid (which flows roughly 500 times more easily than water) moved, the researchers observed how thousands of tiny vortices inside it combined into a giant whirlpool.

"Superfluid helium contains tiny objects called quantum vortices, which tend to spread apart from each other," Svancara said in the statement. "In our set-up, we've managed to confine tens of thousands of these quanta in a compact object resembling a small tornado, achieving a vortex flow with record-breaking strength in the realm of quantum fluids."

By studying the quantum whirlpool, the scientists found convincing similarities to how black holes behave in space. Most notably, they observed a similar black hole phenomenon called ringdown, which is when a newly merged black hole wobbles on its axis.

Now that the simpler parallels have been observed, the researchers will train their experiment on more mysterious aspects of black hole behavior.

This "could eventually lead us to predict how quantum fields behave in curved spacetimes around astrophysical black holes," co-author Silke Weinfurtner, a professor of physics at the University of Nottingham, said in the statement.

Researchers at the world's biggest particle accelerator have put forward proposals to build a new, even larger atom smasher.

The $17 billion Future Circular Collider (FCC) would be 57 miles (91 kilometers) long,  dwarfing its predecessor, the 16.5-mile-long (27 kilometers) Large Hadron Collider (LHC), located at the European Organization for Nuclear Research (CERN) near Geneva.

Physicists want to use the FCC's increased size and power to probe fringes of the Standard Model of particle physics, the current best theory that describes how the smallest components of the universe behave. By smashing particles at even higher energies (100 tera electron volts, compared with the LHC's 14), the researchers hope to find unknown particles and forces; discover why matter outweighs antimatter; and probe the nature of dark matter and dark energy, two invisible entities believed to make up 95 percent of the universe.

Related: Our universe is merging with 'baby universes,' causing it to expand, new theoretical study suggests

"The FCC will not only be a wonderful instrument to improve our understanding of the fundamental laws of physics and nature," Fabiola Gianotti, CERN's director-general, said at a news conference Monday (Feb. 5). "It will also be a driver of innovation, because we will need new advanced technologies, from cryogenics to superconducting magnets, vacuum technologies, detectors, instrumentation — technologies with a potentially huge impact on our society and huge socioeconomic benefits."

Atom smashers like the LHC collide protons together at near light speed while looking for rare decay products that could be clues to new particles or forces. This helps physicists scrutinize their best understanding of the universe's most fundamental building blocks and how they interact, described by the Standard Model of physics.

Though the Standard Model has enabled scientists to make remarkable predictions — such as the existence of the Higgs boson, discovered by the LHC in 2012 — physicists are far from satisfied with it and are constantly looking for new physics that might break it.

This is because the model, despite being our most comprehensive one yet, includes enormous gaps, making it totally incapable of explaining where the force of gravity comes from, what dark matter is made of, or why there is so much more matter than antimatter in the universe.

To unlock these new frontiers, physicists at CERN will use the sevenfold increase in beam energy of the FCC to accelerate particles to even higher speeds.

But the detector, despite having taken a promising step forward, is far from built. The proposals put forward by CERN are part of an interim report on a feasibility study set to be finished next year. Once it's complete and if the detector plans go ahead, CERN — which is run by 18 European Union member states, as well as Switzerland, Norway, Serbia, Israel and the U.K. — will likely look for additional funding from nonmember states for the project.

Despite the high hopes for what the new collider could find, some scientists remain skeptical that the expensive machine will encounter new physics.

"The FCC would be more expensive than both the LHC and LIGO [Laser Interferometer Gravitational-Wave Observatory] combined and it has less discovery potential," Sabine Hossenfelder, a theoretical physicist at the Munich Center for Mathematical Philosophy, wrote in a 2019 post on the platform X, formerly Twitter. "It would, at the present state of knowledge and technology, not give a good return on investment. There are presently better avenues to pursue than high energy physics."

Member states will meet in 2028 to decide whether to greenlight the project. Then, the first phase of the machine — which would collide electrons with their animatter counterparts, positrons — would come online in 2045. Finally, in the 2070s, the FCC would begin slamming protons into one another.

How do you cut a quantum string the size of the universe? New research shows how the chaos of the Big Bang could have done it, and how those cuts could have led to a cosmos filled with rippling gravitational waves.

Cosmic strings are the hypothetical leftovers from the earliest moments of the universe. Within the first second of the Big Bang, the cosmos underwent several intense rounds of phase transitions as the forces of nature split off from each other. Many cosmologists believe that these transitions were far from perfect and that each one left behind flaws in space-time itself.

These defects could have taken on a variety of shapes and properties, depending on the details of the (largely unknown and complicated) physics happening in those early days. Cosmic strings are likely one of the most common of these defects, and if they exist, they would manifest as truly exotic objects. They would have thicknesses no wider than a proton, would stretch from one end of the universe to the other, and would be so dense that a mile of string would outweigh the entire Earth.

They would also be nearly indestructible. Before the new research, which has not been submitted for peer review yet, scientists believed cosmic strings were stable. Once cosmic strings formed, they simply existed, from the earliest moments of the Big Bang all the way to the present day. The only way to diminish a cosmic string would be if two of them intersected, or if one looped over on itself. Once that happened, the ensuing vibrations would force the cosmic string to decay into a shower of particles and high-energy radiation.

But no such signals from cosmic strings have ever been found, and to date, astronomers have uncovered no evidence of their existence. This presents a challenge for cosmology, because many models of the early universe predict the existence of cosmic strings. So, if quantum strings were made in abundance during the Big Bang, where did they all go?

Perhaps the cosmic strings are not as stable as we once thought, according to the new study, led by researchers from the Kavli Institute for the Physics and Mathematics of the Universe in Japan. Instead of being stable, the strings might be "metastable." Metastability is a phenomenon found throughout the physical world and occurs when a system can find itself in a stable position and remain there for an indefinite amount of time, but any shift from that position will cause the system to find a new arrangement.

As an example, imagine tumbling down the side of a hill — a very dynamic, unstable situation. If you come to a temporary stop in a divot on the side of the hill, you are now metastable. If nothing disturbs you, you can stay in that divot forever. But if someone nudges you, you will fall out of the divot and continue falling down the hill.

The researchers found that other exotic entities, known as magnetic monopoles, can destabilize a cosmic string. Magnetic monopoles — pure-north or pure-south magnetic particles — can also be created during the phase transitions of the early universe. Near cosmic strings, monopoles can form with anti-monopoles and then promptly annihilate each other. The energy released can snip a cosmic string in half, triggering a destabilization process that forces the cosmic string to eventually dissolve, according to the study authors.

As the cosmic string dissolves, it vibrates, and that vibration can trigger the formation of gravitational waves. Thus, there may not be many cosmic strings left in the universe, explaining why we haven't found any so far — but their gravitational signals may remain.

Indeed, the researchers pointed to the recent discovery of a background of gravitational waves as the possible signal of these metastable cosmic strings. Astronomers will need to study the background waves in more detail to determine if cosmic strings or some other source, like colliding black holes, is responsible for the faint, universe-spanning vibrations.

The "forbidden" light of a distant spiral galaxy shines brightly in a new image from the Hubble Space Telescope.

Located about 275 million light-years from Earth, the galaxy, called MCG-01-24-014, has two prominent, well-defined spiral arms and an energetic glowing core known as an active galactic nucleus (AGN). The galaxy is seen face-on with its arms creating a nearly perfect circular shape.

MCG-01-24-014 is classified as a Type-2 Seyfert galaxy, which is one of the two largest groups of active galaxies scientists know of, along with quasars. Seyfert galaxies exhibit a characteristic bright core, but are less detectable when compared to quasars — whose incredibly luminous AGNs can outshine the entire host galaxies within which they reside, according to a statement from the European Space Agency (ESA).

Seyfert galaxies can also be further categorized based on the intensity of light being emitted from their active cores. Depending on the wavelengths of light, or spectra, Seyfert galaxies are classified as either Type-1 or Type-2. The latter emit spectral lines associated with so-called "forbidden" emissions, given they should not exist according to certain rules of quantum physics.

"To understand why emitted light from a galaxy could be considered forbidden, it helps to understand why spectra exist in the first place," ESA officials said in the statement. "Spectra look the way they do because certain atoms and molecules will absorb and emit light very reliably at very specific wavelengths."

Related: James Webb telescope finds an 'extreme' glow coming from 90% of the universe's earliest galaxies

Electrons — the tiny particles that orbit the nuclei of atoms — lose or gain specific amounts of energy, which correspond to certain light wavelengths being absorbed or emitted. However, certain spectral emission lines are considered to be "forbidden" because they are observed in space but do not occur under normal conditions on Earth.

"Quantum physics is complex, and some of the rules used to predict it use assumptions that suit laboratory conditions here on Earth," ESA officials said in the statement. "Under those rules, this emission is 'forbidden' — so improbable that it’s disregarded. But in space, in the midst of an incredibly energetic galactic core, those assumptions don’t hold anymore, and the 'forbidden' light gets a chance to shine out towards us."

Indeed, the bright light from MCG-01-24-014 shines radiantly in the new Hubble photo, which was taken using the telescope's Advanced Camera for Surveys (ACS). The spiral galaxy appears in the center of the image, with two large bright stars in the foreground, one blue and one red, positioned directly above the galaxy itself. Several more distant galaxies are scattered across the otherwise pitch black backdrop of space. ESA released the new Hubble photo online on Dec. 18.

Originally posted on Space.com.

Scientists at the world's largest gravitational wave observatory have just squeezed light beyond a key quantum limit.

The new technique, called frequency-dependent squeezing, will increase the number of tiny ripples in space-time detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO), boosting the number of neutron star and black hole collisions the detector can find. 

Related: Quantum 'yin-yang' shows two photons being entangled in real-time

"Now that we have surpassed this quantum limit, we can do a lot more astronomy," co-lead author Lee McCuller, an assistant professor of physics at Caltech, said in a statement.Gravitational waves ripple out when objects with mass move through space. Bigger objects — such as neutron stars or black holes — produce more prominent gravitational waves. Scientists first detected these space-time ripples in 2015 and have steadily gotten better at spotting the waves as they lap at our cosmic shores.

The LIGO detector spots these cosmic ripples from the way they distort space-time as they pass through it. Made up of two intersecting L-shaped detectors — each with two 2.48-mile-long (4 kilometers) arms and two identical laser beams inside — the experiment is designed such that if a gravitational wave passes through Earth, the laser light in one arm of the detector will get compressed while the other expands, creating a tiny change in relative path lengths of the beams arriving at the detector.

But because these distortions are so tiny — often the size of a few thousandths of a proton or neutron — LIGO's detectors must be incredibly sensitive. So sensitive, in fact, that their most precise measurements are muddied by noise from quantum effects, or the spontaneous interactions of subatomic particles.

High frequency noise comes from tiny particles randomly popping in and out of existence. Low frequency noise comes from the rumble of reflecting light particles that cause the mirrors to wobble. Both sources  limit the number and types of gravitational waves LIGO can detect.

To break through these quantum limitations, the physicists turned to another principle of physics: Heisenberg's uncertainty principle, which states that we can only simultaneously know specific pairs of a particle's physical properties to a set level of certainty.

This means that there is a trade-off in how well scientists can measure both the amplitude (or power) and frequency of the light inside LIGO, but it also means that either property can be amplified at the cost of the other. By using crystals that split individual photons, or packets of light, into two entangled photons, the physicists tuned the light so that the uncertainty behind its amplitude or its frequency could be "'squeezed"' as required.

Frequency-dependent squeezing works a bit like pinching a balloon, the researchers say. Just as pinching a balloon at one end helps the other end get bigger, pinching one property of light to know it with greater certainty moves the overall uncertainty to the other. This means that at low frequencies, a squeezed amplitude reduces noise from the mirror rumbling, and at high frequencies a squished phase makes the signal stronger than noise from quantum perturbations.

"It is true that we are doing this really cool quantum thing, but the real reason for this is that it's the simplest way to improve LIGO's sensitivity," co-lead author Dhruva Ganapathy, a graduate student at MIT, said in the statement. "Otherwise, we would have to turn up the laser, which has its own problems, or we would have to greatly increase the sizes of the mirrors, which would be expensive."

The findings were published Sept. 6 in the journal Physical Review X.

Scientists have used a first-of-its-kind technique to visualize two entangled light particles in real time — making them appear as a stunning quantum "yin-yang" symbol.

The new method, called biphoton digital holography, uses an ultra high-precision camera and could be used to massively speed up future quantum measurements.

The researchers published their findings Aug. 14 in the journal Nature Photonics.

Related: Bizarre particle that can remember its own past created inside quantum computer

Quantum entanglement — the weird connection between two far-apart particles that Albert Einstein objected to as "spooky action at a distance" — enables two light particles, or photons, to become inextricably bound to each other, so that a change to one causes a change in the other, no matter how far apart they are. 

To make accurate predictions about a quantum object, physicists need to find its wavefunction: a description of its state existing in a superposition of all the possible physical values a photon can take. Entanglement makes finding the wavefunction of two connected particles a challenge, as any measurement of one also causes an instantaneous change in the other. 

Physicists usually approach this hurdle through a method known as quantum tomography. By taking a complex quantum state and applying a projection to it, they measure some property belonging to that state, such as its polarization or momentum, in isolation from others. 

By repeating these measurements on multiple copies of the quantum state, physicists can build up a sense of the original from lower-dimensional slices — like reconstructing the shape of a 3D object from the 2D shadows it casts on surrounding walls. 

This process gives all the right information, but it also requires a lot of measurements and spits out plentiful "disallowed" states that don't follow the laws of physics to boot. This leaves scientists with the onerous task of painstakingly weeding out nonsensical, unphysical states, an effort that can take hours or even days depending on a system's complexity.

To get around this, the researchers used holography to encode information from higher dimensions into manageable, lower-dimensional chunks.

Optical holograms use two light beams to create a 3D image: one beam hits the object and bounces off of it, while the other shines on a recording medium. The hologram forms from the pattern of light interference, or the pattern in which the peaks and troughs of the two light waves add up or cancel each other out. The physicists used a similar method to capture an image of the entangled photon state through the interference pattern they made with another known state. Then, by capturing the resulting image with a nanosecond precise camera, the researchers teased apart the interference pattern they received — revealing a stunning yin-yang image of the two entangled photons.

"This method is exponentially faster than previous techniques, requiring only minutes or seconds instead of days," study co-author Alessio D'Errico, a postdoctoral fellow at the University of Ottawa in Canada, said in a statement. 

British theoretical physicist Paul Dirac was one of the most significant figures in the early days of quantum physics, who along with Erwin Schrödinger won the Nobel Prize for physics in 1933. But it was in 1927 that this quiet, but brilliant mind set to work looking for "pretty mathematics," and in doing so formulated what would become one of his greatest achievements — the Dirac equation.

In this extract from the Antimatter chapter of his book "'The One Thing You Need to Know'," author Marcus Chown explains how Dirac's unusual methods and mannerisms helped guide us towards understanding the fundamental physics that forms the world around us.

Related: What is antimatter?

Nature has chosen to double the number of its basic building blocks. For every subatomic particle, remarkably there exists an "'antiparticle"' with opposite properties such as electric charge. Before 1927, nobody had the slightest suspicion that such a world of "'antimatter"' existed. But that year, the British physicist Paul Dirac wrote down an equation that described an electron travelling at close to the speed of light and noticed that it contained something odd.

Dirac was one of the pioneers of quantum theory, the revolutionary description of the submicroscopic realm of atoms and their constituents. The theory reconciled two seemingly contradictory characteristics of the world revealed in experiments in the first quarter of the twentieth century: the ability of atoms and their like to behave both as localized particles and as spread-out waves. In 1926, the Austrian physicist Erwin Schrödinger encapsulated this in the Schrödinger equation, which describes quantum waves of probability spreading through space.

The problem with the Schrödinger equation is that it does not incorporate the other revolution of twentieth-century physics. In his special theory of relativity of 1905, Einstein showed that strange things happen to space and time as a body with mass approaches the speed of light. Although the Schrödinger equation works fine when describing an electron in a small atom, where the electric force of only a handful of protons in the nucleus causes it to orbit at much less than the speed of light, in heavier atoms, where there are lots of protons in the nucleus and an electron is whirled around at close to the cosmic speed limit, the equation breaks down. What was needed was an equation that was compatible with the special theory of relativity — relativistic — and that was what Dirac set out to find.

Dirac was a strange man who today would probably be diagnosed as being on the autism spectrum. Tall, gangly and reminiscent of a stick insect, his habit was to work hard all week and on Sundays take long walks in the countryside around Cambridge, where he would climb tall trees dressed in his suit and tie. Literal to the point of obtuseness, he was the Mr. Spock of physics. When a student put up their hand during one of his lectures and said, "'Professor Dirac, I don't understand the equation on the blackboard," he replied: "'That's a comment not a question'," and continued with his lecture.

Dirac's approach to physics was no less strange than his character. Whereas other physicists sought everyday analogues of the phenomena they wanted to describe, which they then tried to encapsulate in a mathematical equation, Dirac had the courage to simply sit with a pen and paper and guess at the form of an equation. "It's a peculiarity of myself that I like to play about with equations, just looking for beautiful mathematical relations which maybe don't have any physical meaning at all," said Dirac. 'Sometimes they do.'

It was while looking for "pretty mathematics" in his spartan rooms at St. John's College in late November 1927 that Dirac literally plucked from thin air what would become known as the Dirac equation. Today, it is one of two equations inscribed on flagstones on the floor of London's Westminster Abbey. The other is Stephen Hawking's equation for the temperature of a black hole. "Of all the equations of physics, perhaps the most magical is the Dirac equation," says American physicist Frank Wilczek (in the book "It Must Be Beautiful: Great Equations Of Modern Science" by Graham Farmelo (Granta, 2003)). "It is the most freely invented, the least conditioned by experiment, the one with the strangest and most startling consequences."

Dirac had found it impossible to describe a relativistic electron's properties, such as its energy, with a mere number, so instead had to use a two-by-two table of numbers known as a matrix. This "two-ness" explained a puzzling feature of the electron. Experiments had revealed that the particle behaved as if it was spinning in one of two ways: clockwise or anticlockwise. However, if an electron was really spinning, its behavior could be understood only if it was spinning faster than light, which, according to Einstein, was impossible. Physicists were forced to conclude that the "spin" of an electron was something entirely new. It was an intrinsic quantum property with no analog in the everyday world. And here it was, Dirac saw, just popping unbidden out of the formula he had written down. "My equation gave just the properties one needed for an electron," said Dirac. "That was really an unexpected bonus for me, completely unexpected." According to the American physicist John Hasbrouck Van Vleck, Dirac's explanation of an electron's spin was comparable to "a magician's extraction of rabbits from a silk hat."

Spin was strange. But another aspect that emerged from Dirac's equation was even stranger. When Dirac wrote down his equation, he noticed that its machinery was oddly duplicated. It appeared to describe not only a negatively charged electron, but also a particle with the same mass as an electron that had a positive charge. At the time, only three subatomic particles were known: the proton in the nucleus of the atom; the electron, which orbited the nucleus; and the photon, the particle of light. There appeared to be no need for another one. Even the great physicists of the day such as Werner Heisenberg and Wolfgang Pauli thought that the Dirac equation must be wrong. However, Dirac was right and they were wrong, as an experiment 8,000 kilometers away from Cambridge would later show.

In 1932, Carl Anderson, an American physicist at the California Institute of Technology in Pasadena, was trying to understand cosmic rays, extremely high-energy particles from space. He expected them to smash into atoms in the atmosphere, kicking out their electrons. If he could just measure the energy of such ejected electrons, he reasoned, he would have a handle on the energy of the cosmic rays. To this end he used an extremely strong magnetic field to bend the electrons, deducing that if they had high energy and so were moving fast, they would spend little time in the vicinity of his magnetic field and be bent less sharply than if they had low energy and spent more time there.

Anderson made his electrons visible by means of a "cloud chamber." Inside the device, tiny trails of water droplets formed along the tracks of electrons, and he could photograph these trails. On 2 August 1932, Anderson developed a photographic plate and was astonished to see a particle of the mass of an electron that was bent by the magnetic field in the opposite way to an electron. He knew nothing of Dirac's prediction. Nevertheless, he had stumbled on Dirac's positively charged electron, a particle which he immediately christened the "positron."

Excerpted from The One Thing You Need to Know. Copyright © 2023 by Marcus Chown.

Published by Michael O'Mara.

Scientists could soon test Einstein's theory of general relativity by measuring the distortion of time. 

According to new research published June 22 in the journal Nature Astronomy, the newly proposed method turns the edge of space and time into a vast cosmic lab to investigate if general relativity can account for dark matter — a mysterious, invisible form of matter that can only be inferred by its gravitational influence on the universe's visible matter and energy — as well as the accelerating expansion of the universe due to dark energy. The method is ready to be tested on future surveys of the deep universe, according to the study authors.

Related: The expansion of the universe could be a mirage, new theoretical study suggests

General relativity states that gravity is the result of mass warping the fabric of space and time, which Einstein lumped into a four-dimensional entity called space-time. According to relativity, time passes more slowly close to a massive object than it does in a mass-less vacuum. This change in the passing of time is called time distortion.

Since its introduction in 1915, general relativity has been tested extensively and has become our best description of gravity on tremendous scales. But scientists aren't yet sure if it can explain invisible dark matter and dark energy, which together account for around 95% of the energy and matter in the universe.

"Time distortion predicted by general relativity has already been measured very precisely at small distances," Camille Bonvin, lead study author and an associate professor at the University of Geneva, told Live Science via email. "It has been measured for planes flying around the Earth, for stars in our galaxy, and also for clusters of galaxies. We propose a method to measure the distortion of time at very large distances."

The method suggests testing time distortion by measuring redshift, the change in the frequency of light an object emits as it moves away from us. Bonvin said the difference here is that this technique measures redshift caused as light attempts to climb out of a gravitational well, a "dent" in space-time created by a massive object. 

"This climb changes the frequency of the light because time passes at different rates inside and outside of the gravitational well," she said. "As a consequence, the color of the light is changed; it is shifted to red. … By measuring gravitational redshift, we obtain a measurement of the distortion of time."

Time to test general relativity

Time distortion suggests that time is not absolute in our universe but rather passes at varying rates depending on gravitational fields.This idea is not exclusive to general relativity.

"Time distortion exists in all modern theories of gravity," Bonvin said. "However, the amplitude of the time distortion  —  how much the presence of a massive object slows down time —  varies from theory to theory."

In general relativity, the distortions of time and space are predicted to be the same; in other theories of gravity, this is not always the case. That means that by measuring the distortion of time and comparing it to the distortion of space, physicists can test the validity of general relativity.

The team's new method could also test another leading theory of the cosmos: Euler's formula, which astronomers use to calculate the movement of galaxies. Specifically, the team's proposed measurement of time distortion could prove whether dark matter obeys Euler's equation, as prior studies of time distortion have presumed.

"We have never observed a particle of dark matter directly. We have only felt its presence gravitationally," Bonvin said. "As a consequence, we don't know if dark matter obeys the Euler equation. It may very well be that dark matter is affected by additional forces or interactions in our universe besides gravity. If this is the case, then dark matter will not obey the Euler equation."

The team's method could be employed by future missions, including the European Space Agency's Euclid telescope, which is set to launch in July, and the Dark Energy Spectroscopic Instrument, which is three years into its five-year survey of the universe.

"It will be possible to measure the distortion of time with the data delivered by these surveys," Bonvin said. "This is very interesting because, for the first time, we will be able to compare the distortion of time with that of space, to test if general relativity is valid, and we will also be able to compare the distortion of time with the velocity of galaxies, to see if Euler's equation is valid. With one new measurement, we will be able to test two fundamental laws."

Physicists have created the world's heaviest Schrödinger's cat, bringing the bizarre behavior of the quantum world to larger scales than ever before.

The trick, performed by vibrating 100 million billion atoms inside a sand-grain-sized sapphire crystal, created the world's heaviest quantum superposition as the crystal simultaneously oscillated in two different directions. Despite weighing just 16 micrograms (16 millionths of a gram), the crystal is trillions of times heftier than the molecules put into previous large-scale quantum states, and is visible to the naked eye. 

Taking its name from Erwin Schrödinger's famous thought experiment, the new Schrödinger's cat crystal could be used to design more robust quantum computers and reveal clues behind why quantum effects are not seen in the real world, according to the researchers. Their findings were published April 20 in the journal Science. 

Related: Scientists find a loophole in Heisenberg's uncertainty principle

"Of course, in the lab we can't realize … an experiment with an actual cat weighing several kilograms," lead author Yiwen Chu, a physics professor at the Laboratory for Solid State Physics at ETH Zurich, said in a statement. But "by putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrödinger cat weighing 16 micrograms."

In Schrödinger's thought experiment, the weird rules of the quantum world are envisioned by imagining a cat placed inside an opaque box with a poison vial whose release mechanism is controlled by radioactive decay — a completely random quantum process. Until the box is opened and the cat is observed, Schrödinger said, the rules of quantum mechanics mean that the unfortunate feline should exist in a superposition of states, simultaneously dead and alive.

As most quantum effects typically decohere and disappear at macroscopic scales, Schrödinger's analogy was meant to demonstrate the fundamental differences between our world and the world of the very small. Yet no hard limit exists between the two realms, enabling physicists to begin cajoling complex, near-macroscopic-scale objects into showing freaky quantum behavior.

To achieve this, the physicists connected the vibrating part of the sapphire crystal to a superconducting circuit, shaking it in such a way that it began to vibrate in a superposition of two directions at once. 

Then, to confirm that they had indeed created a quantum cat, the researchers measured the spatial separation of the crystal's two vibrating states. Though the vibrations were subatomic in scale (vibrating over a distance of a billionth of a billionth of a meter), they were clearly distinguishable from random thermal and quantum vibrations — the cat was real.

In the future, Chu would like to increase the mass of the Schrödinger's cat crystal even more, creating macroscopic quantum objects that could be used to more robustly store information in quantum computers, look for gravitational waves and dark matter, and figure out how quantum effects disappear at the scale of real cats.

The Big Bang may have been accompanied by a shadow, "Dark" Big Bang that flooded our cosmos with mysterious dark matter, cosmologists have proposed in a new study. And we may be able to see the evidence for that event by studying ripples in the fabric of space-time.

After the Big Bang, most cosmologists think, the universe underwent a period of rapid, remarkable expansion in its earliest moments, known as inflation. Nobody knows what triggered inflation, but it’s necessary to explain a variety of observations, like the extreme geometrical flatness of the universe at large scales.

Inflation was presumably driven by some exotic quantum field, which is a fundamental entity that soaks all of spacetime. At the end of inflation, that field decayed into a shower of particles and radiation, triggering the "Hot Big Bang" that physicists commonly associate with the beginning of the universe. Those particles would go on to coalesce into the first atoms when the cosmos was around 12 minutes old and — hundreds of millions of years later  — begin clumping into stars and galaxies.

But there's another ingredient to the cosmological mix: dark matter. Once again, cosmologists aren't sure what dark matter is, but they see the evidence for its existence through its gravitational influence on normal matter. 

In the simplest models, the end of inflation and the ensuing Hot Big Bang also flooded the universe with dark matter, which evolved along an independent track. But this assumption is made merely for the sake of simplicity, two cosmologists proposed in a paper appearing in February on the preprint database arXiv. Scientists see no evidence for the existence of dark matter until far later in the evolution of the universe, after the elusive substance had enough time to exert gravitational influence, so there's no need for it to have filled the universe in the Hot Big Bang alongside normal matter. Plus, because dark matter does not interact with normal matter, it might have had its own "Dark" Big Bang, the researchers claim.

The Dark Big Bang

In their paper the researchers explored what a Dark Big Bang would look like. First,  they hypothesized the existence of a new quantum field — a so-called "dark field," that is necessary to allow dark matter to form completely independently.

In this new scenario, the Dark Big Bang only gets underway after inflation fades away and the universe expands and cools enough to force the dark field into its own phase transition, where it transforms itself into dark matter particles.

The researchers found that the Dark Big Bang had to obey certain constraints; if too early, there would be too much dark matter today, and if too late, there would be too little. But if the Dark Big Bang happened when the universe was less than a month old, it could agree with all known observations.

Introducing a Dark Big Bang has several advantages. First, it's consistent with what scientists know about dark matter: if it doesn't interact with normal matter, then there's no reason for them to share a common origin. Second, it allows the researchers to create models of dark matter without having to worry about how they'll affect the behavior of normal matter at very early times, which gives scientists much more flexibility in creating models.

But most importantly, the researchers found that a Dark Big Bang produces a particular signature in gravitational waves, which are ripples in space-time that still slosh around the universe in the present day. That means the theory could one day be testable.

The researchers admit that current gravitational wave experiments do not have the sensitivity to find signatures of the Dark Big Bang. But another probe of gravitational waves using distances to far-flung pulsars, known as Pulsar Timing Arrays like the NANOGrav experiment, might just be able to do the trick.

Observations suggest that the intergalactic gas in our universe is a little hotter than it should be. Recently, a team of astrophysicists have used sophisticated computer simulations to propose a radical solution: an exotic form of dark matter known as "dark photons" could be heating the place up.

These strange particles would be the carriers of a new, fifth force of nature that normal matter does not experience, but occasionally these dark photons can flip their identities to become regular photons, providing a source of heat.

Feeling neutral

We could find such dark photons by observing the intergalactic gas using what's known as the Lyman-alpha forest. When we observe light from a distant, bright object, like a quasar (glowing objects powered by black holes at the centers of distant galaxies), there is a series of gaps in an otherwise smooth spectrum of light from that faraway object. 

Here's why: that light has to filter through billions of light-years of gas to reach us. Occasionally that light will pass through a relatively dense clump of neutral hydrogen — a type of hydrogen that consists of one proton and one neutron, and which permeates gas clouds throughout the universe. 

Most of that light will pass through unaffected, but a very specific wavelength of light will get absorbed. This wavelength corresponds to the energy difference needed to bump an electron from its first to its second energy level inside the hydrogen atoms.

When astronomers look at the light coming from that object, it will look otherwise unremarkable except for a gap at the wavelength of that specific energy transition, known as the Lyman-alpha line.

The light from the distant object will pass through multiple clouds and clumps of neutral hydrogen. The expansion of the universe causes the gaps to redshift to different wavelengths, with a new gap appearing at a different wavelength depending on the distance to the particular gas clouds.. The end result of this is the "forest": a series of lines and gaps in the spectrum. 

Getting hot in here

These Lyman-alpha gaps can also be used to measure the temperature of each gas cloud. If the neutral hydrogen were perfectly still, the gap would appear as an incredibly thin line. But if the individual molecules are moving, then the gap will widen because of the kinetic energy of those molecules. The hotter the gas, the more kinetic energy the molecules have, and the wider the gap.

In a paper appearing in November in the journal Physical Review Letters, a team of astrophysicists have pointed out that by using this method, it seems that the clouds of gas that scatter between galaxies are a little too hot. Computer simulations of the evolution of those gas clouds predict them to be just a little bit colder than we observe, and so perhaps something is heating up those clouds that isn't currently accounted for in our astrophysical simulations.

One possible explanation for this discrepancy is the presence of "dark photons" in our universe, the study authors claim. This is a very hypothetical form of dark matter, the mysterious, invisible substance that accounts for roughly 80% of all the mass in the universe, yet doesn't seem to interact with light.

Since astronomers do not currently understand the identity of dark matter, the field is wide open with possibilities as to what it could be. In this model, instead of the dark matter being made of invisible particles (like a phantom version of electrons, for example), it would instead be made of a new kind of force carrier — that is, a type of particle that mediates interactions between other particles.

A warm and fuzzy darkness

The familiar photon is the force carrier of electromagnetism - it's what creates electricity, magnetism, and light. Dark photons would be a force carrier for a new force of nature that does not operate at the usual scales in the usual scenarios (for example, in our laboratories or within the solar system, where we otherwise would have already observed it).

According to the study authors, the dark photons would still have a tiny bit of mass, and hence they could still account for the dark matter. Plus, because they're force carriers, they may also interact amongst themselves and with other potential dark matter particles. In the models investigated by the team of astrophysicists, the dark photons are capable of one more trick: they can occasionally turn into a regular photon.

In physics terms, the dark photons can "mix" with regular photons, very rarely swapping identities. When they do, the newly created photon goes on to do what regular photons always do: heat things up. The researchers performed the first ever simulations of the evolution of the universe, including the effects of these sneaky shapeshifting dark photons. They found that a particular combination of dark photon mass and the probability of changing into a regular photon could explain the heating discrepancy.

This result is a very far from a slam-dunk case for the existence of dark photons. A range of possibilities could also explain the Lyman-alpha results, like inaccurate observations or a poor understanding of (normal) astrophysical heating between galaxies. But it is an intriguing clue, and the results can be used as a springboard to continue exploring the viability of this exotic idea. 

The standard interpretation of quantum mechanics places a lot of emphasis on the act of measurement. Before measurement, quantum systems exist in many states at once. After measurement, the system "collapses" into a specific value, so it's natural to ask what's really going on when measurements don't take place. There isn't a clear answer, and different ideas can go in some really wild directions.

One of the first lessons that physicists learned when they started examining subatomic systems in the early 20th century was that we do not live in a deterministic universe. In other words, we cannot precisely predict the outcome of every experiment. 

For example, if you shoot a beam of electrons through a magnetic field, half of the electrons will curve in one direction while the other half will curve in the opposite direction. While we can build mathematical descriptions of where the electrons go as a group, we cannot say which direction each electron will take until we actually perform the experiment.

In quantum mechanics, this is known as superposition. For any experiment that can result in many random outcomes, before we make a measurement, the system is said to be in a superposition of all possible states simultaneously. When we make a measurement, the system "collapses" into a single state that we observe.

The tools of quantum mechanics are there to make some sense out of this chaos. Instead of giving precise predictions for how a system will evolve, quantum mechanics tells us how superposition (which represents all the various outcomes) will evolve. When we make a measurement, quantum mechanics tells us the probabilities of getting one outcome over another.

And that's it. Standard quantum mechanics is silent as to how this superposition actually works and how measurement does the job of collapsing the superposition into a single result.

Schrödinger's cat

If we take this line of thinking to its logical conclusion, then measurement is the most important act in the universe. It transforms fuzzy probabilities into concrete results and changes an exotic quantum system into verifiable results that we can interpret with our senses.

But what does that mean for quantum systems when we're not measuring them? What does the universe really look like? Does everything exist but we are simply unaware of it, or does it not really have a defined state until measurement takes place?

Ironically, Erwin Schrödinger, one of the founders of quantum theory (it's his equation that tells us how the superposition will evolve in time), railed against this line of thinking. He developed his famous cat-in-a-box thought experiment, now known as Schrödinger's cat, to show how ridiculous quantum mechanics was.

Here's a highly simplified version. Put a (live) cat in a box. Also put in the box some sort of radioactive element that is tied to the release of a poisonous gas. It doesn't matter how you do it; the point is to introduce some ingredient of quantum uncertainty into the situation. If you wait awhile, you won't know for sure if the element has decayed, so you won't know if the poison has been released and thus if the cat is alive or dead.

In a strict reading of quantum mechanics, the cat is neither alive nor dead at this stage; it exists in a quantum superposition of both alive and dead. Only when we open the box will we know for sure, and it's also the act of opening the box that allows that superposition to collapse and the cat to (suddenly) exist in one state or the other.

Schrödinger used this argument to express his astonishment that this could be a coherent theory of the universe. Are we really to believe that until we open the box that the cat doesn't really "exist" — at least in the normal sense that things are always definitely alive or dead, not both at the same time? To Schrödinger, this was too far, and he quit working on quantum mechanics shortly thereafter.

Decoherence

One response to this bizarre state of affairs is to point out that the macroscopic world does not obey quantum mechanics. After all, quantum theory was developed to explain the subatomic world. Before we had experiments that revealed how atoms worked, we had no need for superposition, probabilities, measurement or anything else quantum-related. We just had normal physics.

So it doesn't make sense to apply quantum rules where they don't belong. Niels Bohr, another founder of quantum mechanics, proposed the idea of 'decoherence" to explain why subatomic systems obey quantum mechanics but macroscopic systems do not.

In this view, what we understand as quantum mechanics is true and complete for subatomic systems. In other words, things like superposition really do happen for tiny particles. But something like a cat in a box is most definitely not a subatomic system; the cat is made of trillions of individual particles, all constantly wiggling, colliding and jostling.

Every time two of those particles bump into each other and interact, we can use quantum mechanics to understand what goes on. But once a thousand, or a billion, or trillions upon trillions of particles enter the mix, quantum mechanics loses its meaning — or "decoheres" — and regular macroscopic physics takes its place.

In this view, a single electron — but not a cat — in a box can exist in an exotic superposition.

However, this story does have limitations. Most important, we have no known mechanism for translating quantum mechanics into macroscopic physics, and we can't point to a specific scale or situation where the switch takes place. So, even though it sounds good on paper, this model of decoherence doesn't have a lot of firm backing.

So does reality exist when we're not looking? The ultimate answer is that it appears to be a matter of interpretation.

It's the stuff of science fiction — parallel worlds that fan out in time and space. 

But do such parallel worlds exist?

It turns out that at least some physics theories do allow for the existence of parallel universes — at least on the quantum level. 

In several interpretations of quantum mechanics, like the Many-Worlds interpretation and the Pilot Wave theory, the universe can be described by a single giant equation, known as a quantum wavefunction. Any time a quantum (or subatomic) process occurs anywhere in the universe, this wavefunction splits in two, meaning parallel universes are constantly created. 

But these interpretations have never been shown to be correct, and they have some major weaknesses that prevent them from being widely accepted.

Related: If we live in a multiverse, where are these many worlds hiding?

The problem of measurement

Quantum mechanics is the physics framework that describes the behavior of tiny particles. One quirk of this theory is that no one is sure what results they get until they look. For example, the canonical interpretation of the physics theory says that electrons exist in multiple states at once. Then once someone makes a measurement, the electron "picks" one of those states.

This idea can be pretty frustrating, because the whole point of physics is to make predictions for how the objects in our universe will behave. If I throw a ball to you, you can use your knowledge of physics (for example, Newton's laws) to predict where the ball will go. But if I throw an electron at you, you have no way of knowing exactly where it will land.

However, quantum mechanics does give us one tool to make predictions: the Schrödinger equation. The Schrödinger equation assigns something called the wavefunction to every particle, and describes how that wavefunction evolves with time. In the standard picture of quantum mechanics, that wavefunction is a cloud of probability that describes where there's a chance to see the particle once people look for it. Where the wavefunction has high values, there's a strong possibility, and where it has low values, there's a small possibility.

However, this standard picture runs into a problem when scientists actually make a measurement. When they're not looking, the wavefunction evolves on its own according to the Schrödinger equation. No big deal. But when scientists make a measurement, this wavefunction "collapses", essentially disappearing, with the particle appearing at one of the possible locations.

Introducing many worlds

How can the quantum world have two completely different sets of rules for how the wavefunction behaves? In the standard picture, the wavefunction obeys Schrödinger's equation when people are not looking, and then immediately collapses when people are. That seems…weird.

In response to this, some other interpretations of quantum mechanics, most notably the Many-Worlds Interpretation and Pilot Wave theory, promote the wavefunction from a mere mathematical tool into a real, existing object. In these interpretations, there's no such thing as measurement. There's no special process or magic trick that makes the wavefunction disappear. Instead every particle in the universe gets assigned its own private wavefunction, and those wavefunctions just keep on evolving according to the Schrödinger equation without end.

When particles interact, their wavefunctions briefly overlap. In quantum mechanics, once this happens those particles are forever linked: a single wavefunction describes both particles simultaneously, a process known as "quantum entanglement." When scientists make a measurement, they are just triggering a series of entanglements beginning with the particle hitting a detector, and ending with molecules shifting around in their brains to make them consciously aware of what just happened.

But the entanglements don't stop there: every particle in the universe becomes entangled with every other particle, leading to a single universal wavefunction that describes the entirety of the cosmos in one fell swoop.

Split personalities

But even with a universal wavefunction, randomness is still a fact of life in quantum mechanics. To account for this, these interpretations say that the wavefunction splits every time a quantum interaction takes place, with each duplicate universe containing one of the possible results. So if we send an electron through a screen and it has a 50/50 chance of going up or down, for example, there's one universe where the electron goes up and one where it goes down.

This process creates a quantum multiverse. Because essentially every interaction is at some level a quantum interaction, there are universes containing every possible alternative choice you could have made in your entire life. In fact, you are being constantly split at this very moment, fragmenting and splitting into multiple copies of you with every choice, every movement, and every action.

This is where the multiverse starts to get a little heavy, because it's not just conscious decisions that lead to splits, but every quantum interaction. Just by reading this article on a device, you are triggering the splitting of countless universes that are exactly identical except for the tiny, insignificant quantum details happening inside the electronics.

That's…a lot. But there's a bigger issue. Humans experience consciousness as seamless, and it takes time for the brain to integrate all sensory inputs into a conscious experience of the world. But if we're constantly splitting and fragmenting, how can we maintain a consistent history of our own identity?

Beyond that, none of these physics theories explain how this splitting of the universes actually takes place. How quickly does it happen, and why can't people detect it? And how do people recover the probabilities of quantum mechanics with all these splitting universes — in other words, how do the universes "know" how much splitting to produce with every quantum interaction?

These questions are areas of active research, so it's not clear if the quantum multiverse truly exists or not.

This is part of an ongoing series describing potential interpretations of quantum mechanics.

One of the most perplexing aspects of quantum mechanics is that tiny subatomic particles don't seem to "choose" a state until an outside observer measures it. The act of measurement converts all the vague possibilities of what could happen into a definite, concrete outcome. While the mathematics of quantum mechanics provides rules for how that process works, that math doesn't really explain what that means in practical terms. 

One idea is that consciousness — an awareness of our own selves and the impact we have on our surroundings   — plays a key role in measurement and that it's our experience of the universe that converts it from merely imagined to truly real. 

But if this is the case, then is it possible that human consciousness could explain some of the weirdness of quantum mechanics?

Quantum measurement

Quantum mechanics are the rules that govern the zoo of subatomic particles that make up the universe. Quantum mechanics tells us that we live in a fundamental nondeterministic world. In other words, at least when it comes to the world of tiny particles,  it's impossible, no matter how clever scientists are in their experimental design or how perfectly they know that experiment's initial conditions, to predict with certainty the outcome of any experiment. Know the force acting on a proton? There's no set location where it's certain to be a few seconds from now — only a set of probabilities of where it could be.

Related: Spooky action is real: Bizarre quantum entanglement confirmed in new experiments

Thankfully, this indeterminism surfaces only in the subatomic world; in the macroscopic world, everything operates according to deterministic laws of physics (and no, we're not exactly sure why that split happens, but that's a problem for a different day).

When physicists perform an experiment on quantum systems (for example, trying to measure the energy levels of an electron in an atom), they're never quite sure what answer they'll get. Instead, the equations of quantum mechanics predict the probabilities of these energy levels. Once scientists actually conduct the experiment, however, they get one of those results, and all of a sudden the universe becomes deterministic again; once scientists know the energy level of the electron, for example, they know exactly what it's going to do, because its "wavefunction" collapses and the particle chooses a certain energy level.

This flip from indeterminism to determinism is outright odd, and there is no other theory in physics that operates the same way. What makes the act of measurement so special? Myriad quantum interactions happen in the universe all the time. So do those interactions experience the same kind of flipping even when no one is looking?

The role of consciousness

The standard interpretation of quantum mechanics, known as the Copenhagen interpretation, says to ignore all this and just focus on getting results. In that view, the subatomic world is fundamentally inscrutable and people shouldn't try to develop coherent pictures of what's going on. Instead, scientists should count themselves lucky that at least they can make predictions using the equations of quantum mechanics.

But to many people, that's not satisfying. It seems that there's something incredibly special about the process of measurement that appears only in quantum theory. This specialness becomes even more striking when you compare measurement to, say, literally any other interaction.

For instance, in a faraway gas cloud, deep in the vastness of interstellar space, nobody is around; nobody is watching. If, within that gas cloud, two atoms bump into each other, this is a quantum interaction, so the rules of quantum mechanics should apply. But there is no "measurement" and no result — it's just one of trillions of random interactions happening every day, unobserved by humans. And so the rules of quantum mechanics tell us that the interaction remains indeterministic. 

But if those same two atoms bump together inside a laboratory, scientists can measure and record what happened. Because a measurement occurred, the same rules of quantum mechanics tell us that the indeterminism flipped to become deterministic — that's what allowed me to write down a concrete result.

What's so different between these two cases? Both involve subatomic particles interacting with other subatomic particles. And every step of the measurement process involves subatomic particles at some level, so there shouldn't be an escape from the usual quantum rules that say the outcome should be indeterminate.

Some theorists, such as pioneering quantum physicist Eugene Wigner, point out that the only difference between these two scenarios is that one involves a conscious, thinking observer and the other does not. Thus, what's called a "collapse" in quantum mechanics (the transition from indeterministic probabilities to a concrete result) relies on consciousness.

Dreams of the universe

Because consciousness is so important to humans, we tend to think there is something special about it. After all, animals are the only known conscious entities to inhabit the universe. And one way to interpret the rules of quantum mechanics is to follow the above logic to its extreme end: What we call a measurement is really the intervention of a conscious agent in a chain of otherwise mundane subatomic interactions.

This line of thinking requires consciousness to be  different from all the other physics in the universe. Otherwise, scientists could (and do) argue that consciousness is itself just the sum of various subatomic interactions. If that's the case,  there's no end point in the chain of measurement. And if so, then what scientists do in the laboratory really isn't any different from what happens in random gas clouds.

While not strictly a physical theory, the concept of consciousness as different and separate from the material universe does have a long tradition in philosophy and theology.

However, until someone can figure out a way to test this concept of consciousness as separate from the rest of the physical laws in a scientific experiment, it will have to stay in the realm of philosophy and speculation.

This is part of an ongoing series describing potential interpretations of quantum mechanics.