True to Albert Einstein’s calculations all the way back in 1915, gravity does indeed have an effect on the passage of time. To be precise, the stronger gravity distorts spacetime, the slower the passage of time becomes. Scientists have been able to measure the effects of what’s known as time dilation within the scales of thousands of kilometers, but a recent study from a collaboration between the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder (UCB) just managed to whittle down our measurements—all the way down to its effects across a single millimeter.
Humans may not feel the effects of time dilation in everyday life as it’s far too small to be perceived, but it’s definitely there; for that reason, your feet age slower than your head, and someone living on the uppermost penthouse of an apartment ages faster than someone living closer to the lobby down below. This is because the gravity of the Earth itself affects time around it, and satellites in orbit need to take this into account when they help let you know how far you are from your current GPS destination.
Now, recent research published in the journal Nature managed to give us our most precise measurement of time dilation to date. The remarkable feat was the product of the JILA collaboration between NIST and UCB.
“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,” said NIST/JILA Fellow Jun Ye. “For timekeeping, it also shows that there is no roadblock to making clocks 50 times more precise than today—which is fantastic news.”
Of course, the details surrounding this landmark study stretch far beyond just pulling out your nearest stopwatch and watching the seconds pass. The team needed the help of atomic clocks, which themselves make use of atoms that interact with specific electromagnetic frequencies as they transition between different energy levels.
Ye and the team measured the frequency shifts between the top and bottom of their own atomic clock, which contained 100,000 ultracold strontium (Sr) atoms in an “atomic cloud” as they switched between two energy states. To do so, the team had to keep the ticking of these atoms in the “cloud” synced for a record total of 37 seconds.
These atoms were also loaded into what was called an optical lattice, which the team described in the NIST press release as “a stack of pancakes created by laser beams.” From there, they used precise imaging techniques to measure the differences between the atomic ticking of the upper and bottom portions of the “pancake stack.”
It was there that they measured a difference between the two regions, which was in turn described as a redshift across the atomic cloud. (This is analogous to how an ambulance’s siren changes pitch—and hence sound frequency—depending on whether it’s moving towards or away from you.) The measured redshift was in the realm of 0.0000000000000000001—unimaginably small to us humans, but nevertheless measurable.
Said Ye about their find: “This a completely new ballgame, a new regime where quantum mechanics in curved space-time can be explored. “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.”
According to Ye and the research team, their find could make atomic clocks about 50 times more precise than they are now, and can also help us study the effects of gravity on smaller scales—with the latter perhaps giving us clues into the as-of-yet unexplainable relationship between gravity and quantum physics.
References
- Advanced Atomic Clock Makes a Better Dark Matter Detector. (2020, November 12). [Text]. NIST. https://www.nist.gov/news-events/news/2020/11/advanced-atomic-clock-makes-better-dark-matter-detector
- Bothwell, T., Kennedy, C. J., Aeppli, A., Kedar, D., Robinson, J. M., Oelker, E., Staron, A., & Ye, J. (2022). Resolving the gravitational redshift across a millimetre-scale atomic sample. Nature, 602(7897), 420–424. https://doi.org/10.1038/s41586-021-04349-7
- Irving, M. (2022, February 18). Physicists measure gravitational time warp to within one millimeter. New Atlas. https://newatlas.com/physics/gravity-time-dilation-one-millimeter/
- JILA Atomic Clocks Measure Einstein’s General Relativity at Millimeter Scale. (2022, February 16). [Text]. NIST. https://www.nist.gov/news-events/news/2022/02/jila-atomic-clocks-measure-einsteins-general-relativity-millimeter-scale
- Kennedy, C. J., Oelker, E., Robinson, J. M., Bothwell, T., Kedar, D., Milner, W. R., Marti, G. E., Derevianko, A., & Ye, J. (2020). Precision metrology meets cosmology: Improved constraints on ultralight dark matter from atom-cavity frequency comparisons. Physical Review Letters, 125(20), 201302. https://doi.org/10.1103/PhysRevLett.125.201302
