A high-precision atomic clock could not only provide the most precise test of time dilation ever but could point the way to exciting new physics including a quantum description of gravity.
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One of the most startling outcomes of Einstein’s general theory of relativity, when it was introduced in 1915 and still to physics students today, is the idea that mass does not just curve space, it affects time too.
The concept of time dilation means that time runs differently in the presence of a strong gravitational field generated by an object of great mass. This means that a person at the top of a mountain experiences time differently from a person at sea level closer to where Earth’s gravity is strongest.
That the revelation of this effect is so shocking to us reflects the fact that for bodies with the mass of Earth it is negligibly small. We don’t see disputes in dates or times with people who live at high and low altitudes, for example.
Though this effect is tiny, without considering it technology like global positioning systems and other devices that rely on satellites would quickly become unusable due to this small time difference.
Consequently, measuring the time dilation effect predicted by Einstein requires incredibly sensitive devices - a normal clock simply won’t do. That means that researchers have to turn to sensitive timepieces called atomic clocks.
This is exactly the approach taken by physicists at JILA and researchers from the University of Wisconsin–Madison and the University of Colorado Boulder, who have used one of the most high-performance atomic clocks ever created to measure the effect of time dilation.
Their research is published in the journal Nature and represents one of the most precise measurements of general relativity ever made.
Atomic Clocks and Time Dilation
General relativity says that atomic clocks at different elevations within a gravitational field should “tick” at different rates, with the frequency of radiation more strongly red-shifted when emitted in stronger gravity.
Researchers have repeatedly demonstrated that atomic clocks run slower in stronger gravity - for example when they are placed closer to the Earth. As an example of this, in 2010 the National Institute of Standards and Technology (NIST)² used two independent atomic clocks, one positioned 33 centimeters above the other to obtain a “personal scale” picture of relativity.
This revealed that a person who lived two steps above another on a staircase would experience a time difference of around 90 billionths of a second over a 79-year lifetime.
The instrument used by the team is known as an optical lattice atomic clock. The device, consisting of a “multiplexed” optical clock with six separate clocks existing in the same space, is so accurate that it loses just one second every 300 billion years.
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The device consists of around 100,000 ultracold strontium atoms loaded into an optical lattice, which could be roughly imagined as a stack of pancakes created by laser beams. The “pancakes” in this clock are large, flat, and thin, and are generated by less intense light than usually employed.
The advantage this confers is a reduction in the distortion within the lattice caused by the scattering of photons and atoms. This has the net result of spreading the wavefunction of the atoms which gives the probability of finding the atoms in a certain location at a given time.
This means that the energy states of the atoms are well controlled and were in unison for around 37 seconds as they alternated between two energy levels. This represents a record for what is known as quantum coherence — the ability to describe the elements of a quantum system with a single wave function allowing it to simultaneously take different states.
Using a new imaging technique, the researchers created a map of the distribution of the wavefunctions of the atoms and compare two areas of the cloud of the strontium atoms, a method that differed from the usual approach of using two clocks.
The team measured a redshift of 0.0000000000000000001 which lines up with the predictions of general relativity, again much too small a time dilation effect for humans to detect but significant enough to play havoc with precision GPS technology.
Following 90 hours of experimentation, the team obtained a measurement with a precision 50 times better than any atomic clock comparison test in the past.
Exploring New Physics
The team’s work isn’t just an important further confirmation of a century-old theory - the device they used could be a way to explore future physics.
One of the most existing things that such a precision atomic clock could deliver is a way to unify quantum physics and Einstein’s theory of gravity, general relativity. While both these disciplines are the most accurate and precise descriptions of their domains, the very small and the very larger respectively, they don’t unite as there is no “quantum theory of gravity.”
The fact that gravity cannot be described at a quantum level but the other three fundamental forces of the Universe (electromagnetism, the strong nuclear force, and the weak nuclear force) can, is a major stumbling block to a “unified theory of everything.”
Improving the precision optical lattice atomic clock could break this deadlock by allowing physicists to measure the matter wavefunction of the atoms across the curvature that mass caused in the unified entity of spacetime suggested by relativity. This would allow for the study of complex physics that arises when particles are distributed throughout the curved spacetime predicted by general relativity.
It would also reveal how gravity disrupts quantum coherence, something that could show why quantum effects govern the microscopic world and classic physics defines our everyday macroscopic world.
JILA Fellow Jun Ye believes that these precise atomic clocks could also help investigate dark matter, which makes up the majority of matter in the Universe.
This mysterious substance, whose gravitational influence holds galaxies together, is difficult to investigate as it does not interact with other matter or light like everyday matter made for protons and neutrons do.
Dark matter does, however, interact with gravity, if it didn't galaxies would fly apart, which means that these atomic clocks could be the perfect probe of its effect.
References and Further Reading
Bothwell. T., Kennedy. C. J., Aeppli. A. A., et al, , “Resolving the gravitational redshift in a millimeter-scale atomic sample,” Nature, [https://www.nature.com/articles/s41586-021-04349-7
“NIST Pair of Aluminum Atomic Clocks Reveal Einstein’s Relativity at a Personal Scale,” NIST, , [https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale]