Atomic clocks have just proved Einstein’s “experimental elevator” theory

A fundamental part of Albert Einstein’s theory of general relativity has just been put to the test using the world’s best atomic clocks.

Physicists at the National Institute of Standards and Technology (NIST) used the clocks to perform the most accurate assessment of Einstein’s so-called equivalence principle, or “Einstein’s experimental elevator” idea.

Einstein’s experimental elevator

As far back as 1907, almost a decade before the eminent physicist published his full theory of general relativity, Einstein devised a thought experiment designed to study the forces of gravity.

Einstein realised that if you were to stand in a lift and the cable were to snap, causing the lift to fall freely towards Earth, you’d experience a feeling of weightlessness. Your feet would leave the floor and you’d temporarily appear to “float.”

Einstein proposed that the acceleration you’d experience while falling would cancel out the feeling of gravity’s pull, and this would happen regardless of time and place. In summary, all objects located in the elevator would accelerate at the same rate, as if they were in a uniform gravitational field – or as if there were no gravity at all – and this would be independent of the elevator’s position.

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To put this thought experiment to the test, the team at NIST used a very large “elevator” – the Earth, falling through the sun’s gravitational field – and extremely precise atomic clocks locationed in different regions of the planet.

The team compared data from four hydrogen maser clocks in the NIST time scale, with eight cesium fountain atomic clocks in the United States, the United Kingdom, France, Germany and Italy. According to the theory of equivalence, objects on Earth should accelerate at the same rate as the planet orbits around our nearest star, known as local position invariance or LPI. By pulling readings from 1999 to 2014, the scientists found that these clocks remained in-sync, even though the gravitational pull on the “elevator” varied during the Earth’s orbit.

“General relativity is one of the fundamental theories of physics,” Bijunath R. Patla, one of the researchers on the project, tells Alphr. “It explains the phenomenon of gravitation very well. But recent discoveries which suggest that the universe is accelerating, and invoke dark matter to explain the rotation of galaxies, have made people refocus on alternatives to general relativity.

“General relativity is also incompatible with quantum mechanics. It is widely believed that there should be a more unified theory whose low energy limit will be asymptotic to general relativity and high energy limit to quantum mechanics. Since general relativity is based on a set of postulates, testing these postulates is the best way to test the limits of the theory itself.”

Comparing the frequencies of electromagnetic radiation from the atomic clocks at different locations, the researchers measured LPI readings a value of 0.00000022, plus or minus 0.00000025. General relativity predicts a result of zero, signaling no violation, and this result, published in Nature Physics, is the closest reading yet to that figure.

“By comparing the best clocks in the world, over a duration of 14 years, we looked for any violation in one of the postulates of general relativity,” says Patla. “Based on our study, we found that general relativity passes this test. We tested one postulate better than ever.”

Patla adds that future attempts to test LPI are unlikely to be done with comparisons between hydrogen and caesium clocks. Instead, experimental next-generation clocks based on ytterbium and strontium atoms are predicted to yield even better results, getting closer and closer to zero. What could be revelatory is any indication from these new clocks that inviolation breaks down when you start assessing it on a minute scale. Given that general relativity is taken to be an imperfect natural model, there’s every possibility that this could happen.