Distortions in space-time could put Einstein's theory of relativity to the ultimate test

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By Robert Lea
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Observing time distortions could show whether Einstein's theory of general relativity accounts for the mysteries of dark matter and dark energy.

A telescope image of warped yellow starlight forming a smily face against a black background
Gravitational lensing -- a phenomenon predicted by Albert Einstein's theory of relativity -- warps starlight into a cosmic smily face. (Image credit: NASA/ESA/JPL-Caltech)

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."

When distant starlight bends around the gravity of a closer foreground object, it may make an 'Einstein ring' like this. The name is an homage to predictions about space-time made in Einstein's theory of relativity.  (Image credit: ESA/Hubble & NASA/S. Jha/Acknowledgement: L. Shatz)

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.

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"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."

Robert Lea
Robert Lea

Robert Lea is a science journalist in the U.K. who specializes in science, space, physics, astronomy, astrophysics, cosmology, quantum mechanics and technology. Rob's articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University

3 Comments Comment from the forums
  • Debed
    Where are the protests?
    Wasn’t «Science» settled by consensus?
    How dare they question settled «Science».
    Reply
  • ClintMcKenzieNicholas
    admin said:
    Observing time distortions could show whether Einstein's theory of general relativity accounts for the mysteries of dark matter and dark energy.

    Distortions in space-time could put Einstein's theory of relativity to the ultimate test : Read more
    Distortion.... Similar to a photon passing an electron I bet?
    Reply
  • Hartmann352
    JILA physicists have measured Albert Einstein’s theory of general relativity, or more specifically, the effect called time dilation, at the smallest scale ever, showing that two tiny atomic clocks, separated by just a millimeter or the width of a sharp pencil tip, tick at different rates.

    The experiments, described in the Feb. 17 issue of Nature, suggest how to make atomic clocks 50 times more precise than today’s best designs and offer a route to perhaps revealing how relativity and gravity interact with quantum mechanics, a major quandary in physics.

    JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

    “The most important and exciting result is that we can potentially connect quantum physics with gravity, for example, probing complex physics when particles are distributed at different locations in the curved space-time,” NIST/JILA Fellow Jun Ye said. “For timekeeping, it also shows that there is no roadblock to making clocks 50 times more precise than today — which is fantastic news.”

    Einstein’s 1915 theory of general relativity explains large-scale effects such as the gravitational effect on time and has important practical applications such as correcting GPS satellite measurements. Although the theory is more than a century old, physicists remain fascinated by it. NIST scientists have used atomic clocks as sensors to measure relativity more and more precisely, which may help finally explain how its effects interact with quantum mechanics, the rulebook for the subatomic world.

    According to general relativity, atomic clocks at different elevations in a gravitational field tick at different rates. The frequency of the atoms’ radiation is reduced — shifted toward the red end of the electromagnetic spectrum — when observed in stronger gravity, closer to Earth. That is, a clock ticks more slowly at lower elevations. This effect has been demonstrated repeatedly; for example, NIST physicists measured it in 2010 by comparing two independent atomic clocks, one positioned 33 centimeters (about 1 foot) above the other.

    The JILA researchers have now measured frequency shifts between the top and bottom of a single sample of about 100,000 ultracold strontium atoms loaded into an optical lattice, a lab setup similar to the group’s earlier atomic clocks. In this new case the lattice, which can be visualized as a stack of pancakes created by laser beams, has unusually large, flat, thin cakes, and they are formed by less intense light than normally used. This design reduces the distortions in the lattice ordinarily caused by the scattering of light and atoms, homogenizes the sample, and extends the atoms’ matter waves, whose shapes indicate the probability of finding the atoms in certain locations. The atoms’ energy states are so well controlled that they all ticked between two energy levels in exact unison for 37 seconds, a record for what is called quantum coherence.

    Crucial to the new results were the Ye group’s imaging innovation, which provided a microscopic map of frequency distributions across the sample, and their method of comparing two regions of an atom cloud rather than the traditional approach of using two separate clocks.

    The measured redshift across the atom cloud was tiny, in the realm of 0.0000000000000000001 (10-19 in scientific notation), consistent with predictions. (While much too small for humans to perceive directly, the differences add up to major effects when extrapolated on the size of the universe as well as technology such as GPS.) The research team resolved this difference quickly for this type of experiment, in about 30 minutes of averaging data. After 90 hours of data, their measurement precision was 50 times better than in any previous clock comparison.

    https://www.nist.gov/sites/default/files/styles/480_x_480_limit/public/images/2022/02/11/Einstein-relativity-04.png?itok=JTdG-e9RSee more about relativity and age in a larger infographic. Credit: N. Hanacek/NIST
    “This a completely new ballgame, a new regime where quantum mechanics in curved space-time can be explored,” Ye said. “If we could measure the redshift 10 times even better than this, we will be able to see the atoms’ whole matter waves across the curvature of space-time. Being able to measure the time difference on such a minute scale could enable us to discover, for example, that gravity disrupts quantum coherence, which could be at the bottom of why our macroscale world is classical.”

    Better clocks have many possible applications beyond timekeeping and navigation. Ye suggests atomic clocks can serve as both microscopes to see minuscule links between quantum mechanics and gravity and as telescopes to observe the deepest corners of the universe. He is using clocks to look for mysterious dark matter, believed to constitute most matter in the universe. Atomic clocks are also poised to improve models and understanding of the shape of the Earth through the application of a measurement science called relativistic geodesy.

    Funding was provided by the Defense Advanced Research Projects Agency, National Science Foundation, Department of Energy Quantum System Accelerator, NIST and Air Force Office for Scientific Research.

    Paper: T. Bothwell, C.J. Kennedy, A. Aeppli, D. Kedar, J.M. Robinson, E. Oelker, A. Staron and J. Ye. Resolving the gravitational redshift in a millimetre-scale atomic sample. Nature. Published online Feb. 16, 2022. DOI: 10.1038/s41586-021-04349-7
    See: https://www.nist.gov/news-events/news/2022/02/jila-atomic-clocks-measure-einsteins-general-relativity-millimeter-scale
    Scientists taking their readings at two separate points, measured the redshift across the cloud of about 100,000 ultracold strontium atoms. The redshift shows the change in the frequency of the atoms' radiation along the electromagnetic spectrum – or in other words, how quickly the atomic clock is ticking. While the difference in redshift across this tiny distance was just 0.0000000000000000001 or so, it follows the predictions made by general relativity. Those differences can make a difference when you get out to the scale of the entire Universe, or even when you're dealing with systems that need to be ultra-accurate, such as GPS navigation.
    Hartmann352
    Reply