Atomic clocks measure Einstein's general relativity at millimeter scale


Date:
February 16, 2022
Source:
National Institute of Standards and Technology (NIST)
Summary:
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.



FULL STORY ==========================================================================
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, consistent with predictions. (While much too small
for humans to perceive directly, the differences add up to major effects
on 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.

"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.

========================================================================== Story Source: Materials provided by National_Institute_of_Standards_and_Technology_(NIST).

Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
*
Video_describing_Albert_Einstein's_theory_of_general_relativity_and_an
image_of_a_tiny_cloud_of_strontium_atoms ========================================================================== Journal Reference:
1. Bothwell, T., Kennedy, C.J., Aeppli, A. et al. Resolving the
gravitational redshift across a millimetre-scale atomic
sample. Nature, 2022 DOI: 10.1038/s41586-021-04349-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220216112213.htm

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