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The 18 Biggest Unsolved Mysteries in Physics

What happens inside a black hole?

This artist's concept shows a black hole's surroundings, including its accretion disk, jet and magnetic field.

(Image credit: ESO/L. Calçada)

What happens to an object's information if it gets sucked into a black hole? According to the current theories, if you were to drop a cube of iron into a black hole, there would be no way to retrieve any of that information. That's because a black hole's gravity is so strong that its escape velocity is faster than light — and light is the fastest thing there is. However, a branch of science called quantum mechanics says that quantum information can't be destroyed. "If you annihilate this information somehow, something goes haywire," said Robert McNees, an associate professor of physics at Loyola University Chicago. [How to Teleoport Info Out of a Black Hole]

Quantum information is a bit different from the information we store as 1s and 0s on a computer, or the stuff in our brains. That's because quantum theories don't provide exact information about, for instance, where an object will be, like calculating the trajectory of a baseball in mechanics. Instead, such theories reveal the most likely location or the most likely result of some action. As a consequence, all of the probabilities of various events should add up to 1, or 100 percent. (For instance, when you roll a six-sided die, the chances of a given face coming up is one-sixth, so the probabilities of all the faces add up to 1, and you can't be more than 100 percent certain something will happen.) Quantum theory is, therefore, called unitary. If you know how a system ends, you can calculate how it began.

To describe a black hole, all you need is mass, angular momentum (if it's spinning) and charge. Nothing comes out of a black hole except a slow trickle of thermal radiation called Hawking radiation. As far as anyone knows, there's no way to do that reverse calculation to figure out what the black hole actually gobbled up. The information is destroyed. However, quantum theory says that information can't be completely out of reach. Therein lies the "information paradox."

McNees said there has been a lot of work on the subject, notably by Stephen Hawking and Stephen Perry, who suggested in 2015 that, rather than being stored within the deep clutches of a black hole, the information remains on its boundary, called the event horizon. Many others have attempted to solve the paradox. Thus far, physicists can't agree on the explanation, and they're likely to disagree for some time. 

Do naked singularities exist?

An artist's drawing shows a large stellar-mass black hole pulling gas away from a companion star.

(Image credit: NASA E/PO, Sonoma State University, Aurore Simonnet)

A singularity occurs when some property of a "thing" is infinite, and so the laws of physics as we know them break down. At the center of black holes lies a point that is infinitely teensy and dense (packed with a finite amount of matter) — a point called a singularity. In mathematics, singularities come up all the time — dividing by zero is one instance, and a vertical line on a coordinate plane has an "infinite" slope. In fact, the slope of a vertical line is just undefined. But what would a singularity look like? And how would it interact with the rest of the universe? What does it mean to say that something has no real surface and is infinitely small?

A "naked" singularity is one that can interact with the rest of the universe. Black holes have event horizons — spherical regions from which nothing, not even light, can escape. At first glance, you might think the problem of naked singularities is partly solved for black holes at least, since nothing can get out of the event horizon and the singularity can't affect the rest of the universe. (It is "clothed," so to speak, while a naked singularity is a black hole without an event horizon.)

But whether singularities can form without an event horizon is still an open question. And if they can exist, then Albert Einstein's theory of general relativity will need a revision, because it breaks down when systems are too close to a singularity. Naked singularities might also function as wormholes, which would also be time machines — though there's no evidence for this in nature.

Violating charge-parity symmetry

Quantum Entanglement

(Image credit: agsandrew |

If you swap a particle with its antimatter sibling, the laws of physics should remain the same. So, for example, the positively charged proton should look the same as a negatively charged antiproton. That's the principle of charge symmetry. If you swap left and right, again, the laws of physics should look the same. That's parity symmetry. Together, the two are called CP symmetry. Most of the time, this physics rule is not violated. However, certain exotic particles violate this symmetry. McNees said that's why it's strange. "There shouldn't be any violations of CP in quantum mechanics," he said. "We don't know why that is."

When sound waves make light

Sound waves in the dark.

(Image credit: DmitrySteshenko/Shutterstock)

Though particle-physics questions account for many unsolved problems, some mysteries can be observed on a bench-top lab setup. Sonoluminescence is one of those. If you take some water and hit it with sound waves, bubbles will form. Those bubbles are low-pressure regions surrounded by high pressure; the outer pressure pushes in on the lower-pressure air, and the bubbles quickly collapse. When those bubbles collapse, they emit light, in flashes that last trillionths of a second.

The problem is, it's far from clear what the source of the light is. Theories range from tiny nuclear fusion reactions to some type of electrical discharge, or even compression heating of the gases inside the bubbles. Physicists have measured high temperatures inside these bubbles, on the order of tens of thousands of degrees Fahrenheit, and taken numerous pictures of the light they produce. But there's no good explanation of how sound waves create these lights in a bubble.

What lies beyond the Standard Model?

abstract image represents string theory

(Image credit: Robert Spriggs | Shutterstock)

The Standard Model is one of the most successful physical theories ever devised. It's been standing up to experiments to test it for four decades, and new experiments keep showing that it is correct. The Standard Model describes the behavior of the particles that make up everything around us, as well as explaining why, for example, particles have mass. In fact, the discovery of the Higgs boson — a particle that gives matter its mass — in 2012 was a historic milestone because it confirmed the long-standing prediction of its existence. 

But the Standard Model doesn't explain everything. The Standard Model has made many successful predictions — for example, the Higgs boson, the W and Z boson (which mediate the weak interactions that govern radioactivity), and quarks among them — so it is difficult to see where physics might go beyond it. That said, most physicists agree that the Standard Model is not complete. There are several contenders for new, more complete models — string theory is one such model — but so far, none of these have been conclusively verified by experiments. 

Fundamental constants


(Image credit: Andreas Guskos |

Dimensionless constants are numbers that don't have units attached to them. The speed of light, for example, is a fundamental constant measured in units of meters per second (or 186,282 miles per second). Unlike the speed of light, dimensionless constants have no units and they can be measured, but they can't be derived from theories, whereas constants like the speed of light can be.

In his book "Just Six Numbers: The Deep Forces That Shape the Universe" (Basic Books, 2001), astronomer Martin Rees focuses on certain "dimensionless constants" he considers fundamental to physics. In fact, there are many more than six; about 25 exist in the Standard Model. [The 9 Most Massive Numbers in Existence]

For example, the fine structure constant, usually written as alpha, governs the strength of magnetic interactions. It is about 0.007297. What makes this number odd is that if it were any different, stable matter wouldn't exist. Another is the ratio of the masses of many fundamental particles, such as electrons and quarks, to the Planck mass (which is 1.22 ´1019 GeV/c2). Physicists would love to figure out why those particular numbers have the values they do, because if they were very different, the universe's physical laws wouldn't allow for humans to be here. And yet there's still no compelling theoretical explanation for why they have those values. 

What the heck is gravity, anyway?


(Image credit: koya979 | Shutterstock)

What is gravity, anyway? Other forces are mediated by particles. Electromagnetism, for example, is the exchange of photons. The weak nuclear force is carried by W and Z bosons, and gluons carry the strong nuclear force that holds atomic nuclei together. McNees said all of the other forces can be quantized, meaning they could be expressed as individual particles and have noncontinuous values.

Gravity doesn't seem to be like that. Most physical theories say it should be carried by a hypothetical massless particle called a graviton. The problem is, nobody has found gravitons yet, and it's not clear that any particle detector that could be built could see them, because if gravitons interact with matter, they do it very, very rarely — so seldom that they'd be invisible against the background noise. It isn't even clear that gravitons are massless, though if they have a mass at all, it's very, very small — smaller than that of neutrinos, which are among the lightest particles known. String theory posits that gravitons (and other particles) are closed loops of energy, but the mathematical work hasn't yielded much insight so far.

Because gravitons haven't been observed yet, gravity has resisted attempts to understand it in the way we understand other forces – as an exchange of particles. Some physicists, notably Theodor Kaluza and Oskar Klein, posited that gravity may be operating as a particle in extra dimensions beyond the three of space (length, width, and height) and one of time (duration)we are familiar with, but whether that is true is still unknown. 

Do we live in a false vacuum?

Multiverse Membrane Illustration

(Image credit: Shutterstock/Sandy MacKenzie)

The universe seems relatively stable. After all, it's been around for about 13.8 billion years. But what if the whole thing were a massive accident?

It all starts with the Higgs and the universe's vacuum. Vacuum, or empty space, should be the lowest possible energy state, because there's nothing in it. Meanwhile, the Higgs boson — via the so-called Higgs field — gives everything its mass. Writing in the journal Physics, Alexander Kusenko, a professor of physics and astronomy at the University of California, Los Angeles, said the energy state of the vacuum can be calculated from the potential energy of the Higgs field and the masses of the Higgs and top quark (a fundamental particle).

So far, those calculations appear to show that the universe's vacuum might not be in the lowest possible energy state. That would mean it's a false vacuum. If that's true, our universe might not be stable, because a false vacuum can be knocked into a lower energy state by a sufficiently violent and high-energy event. If that were to happen, there would be a phenomenon called bubble nucleation. A sphere of lower-energy vacuum would start growing at the speed of light. Nothing, not even matter itself, would survive. Effectively, we'd be replacing the universe with another one, which might have very different physical laws. [5 Reasons We May Live in a Multiverse]

That sounds scary, but given that the universe is still here, clearly there hasn't been such an event yet, and astronomers have seen gamma-ray bursts, supernovas, and quasars, all of which are pretty energetic. So it's probably unlikely enough that we wouldn't need to worry. That said, the idea of a false vacuum means that our universe might have popped into existence in just that way, when a previous universe's false vacuum was knocked into a lower energy state. Perhaps we were the result of an accident with a particle accelerator.