Researchers Say Time Is Key to Understanding Overlap Between Quantum Mechanics and General Relativity

By Shawn Radcliffe

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Physicists from Israel have developed a new way to investigate the intersection between Einstein’s general theory of relativity and quantum mechanics. They hope that their proof-of-concept test—which uses interferometry and a “self-interfering clock”—will allow them to probe the mysteries of time and the role of gravity in quantum systems.

Although quantum mechanics and general relativity both have strong support in modern physics, the two fields don’t always agree. Time is a good example of this. In quantum theory, the passage of time is the same throughout the universe. General relativity, though, says that time can be affected by gravitational fields—something that has been shown experimentally by placing clocks at different elevations.

In a new experiment, published August 6 in the journal Science, researchers sent a “quantum clock” along two paths of an interferometer. Physicists can use these devices to make electrons or photons take two paths simultaneously. This only occurs when the particle is unobserved along the way, which leads to a characteristic interference pattern and the “superposition” of the particles in both states at once (also known as Schrödinger’s cat paradox).

In this case, the “clock” used by researchers consisted of ultra-cold rubidium atoms at nano-kelvin temperatures. Quantum theory states that if a clock passes down two paths simultaneously, time should flow at different rates along each path as a result of variations in gravity. In this way, researchers should be able to distinguish between which path was taken by the clock, because time would act as a witness to the specific path taken (also known as a “which path” witness).

In this experiment, the researchers did not use a conventional interferometer. Instead they separated two copies of the clock (wavepackets) in space, which essentially created the two paths of an interferometer. Also, the clocks were not sensitive enough to feel the effect of gravity, so the researchers used different magnetic fields along each path to change the ticking rate of the two clock wavepackets.

In their tests, the researchers were able to make the interference pattern disappear, something that occurs when a quantum particle is observed along its path. This strengthens their belief that time can act as a “which path” witness.

This work is still in its early stages (it will need to be made more sensitive to the effects of gravity), but the researchers hope this approach will enable them to test the overlap between quantum mechanics and general relativity. Eventually, they said, this could help them “learn more about time itself.”

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