The effect of time dilation due to gravity has been measured to the smallest scale yet. Predicted by Einstein’s theory of general relativity, this has been measured using stars, supermassive black holes, and atomic clocks on a scale of 30 centimeters (12 inches).

The new work, published in the journal Nature, pushed the record to just one millimeter. The time dilation was measured thanks to a phenomenon called gravitational redshift. This is the relativity effect in question.

If a photon – a particle of light – is escaping a gravitational well (in this case, our planet), the photon’s wavelength is stretched by the gravity of a massive object and moving towards the end scale of the electromagnetic spectrum.   

As we reported when the research was placed on the ArXiv a few months ago, this measurement not only demonstrates how revolutionary the new optical atomic clocks are, but is getting closer to the point where we can measure gravitational effects in the quantum world. A clock 50 times more precise would do it.

“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,” senior author professor Jun Ye, from the Joint Institute for Laboratory Astrophysics and National Institute of Standards and Technology, said in a statement.

“For timekeeping, it also shows that there is no roadblock to making clocks 50 times more precise than today – which is fantastic news.”

The clock used in the experiment has broken a record for quantum coherence – the way that the energy stats ticked between energy levels. They were in unison for a whopping 37 seconds. The team measured the redshift by studying two different regions of an atom cloud, made of 100,000 atoms of strontium at temperatures close to absolute zero.

By studying how these atom cloud regions behaved, they were able to detect a difference in the timekeeping. The measured gravitational redshift across the atom cloud was tiny, in the realm of 0.0000000000000000001 (one part in 10 billion billions), but perfectly consistent with predictions from theory.

“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.”

The applications of these extremely precise clocks are not just in pushing the limit of known particle physics. They could be used as instruments to study dark matter, the mysterious invisible substance that outweighs regular matter five-to-one. They could also help map the interior of Earth by measuring gravity to incredible precisions.

“There will be very interesting discoveries that are waiting for us if we get to the times that are sensitive to the very small space-time curvature,” Professor Ye told IFLScience when it was announced he had won the 2022 Breakthrough Prize in Fundamental Physics.