If Einstein made one point very apparent, it was that time does not pass in the same way everywhere. Gravity, in effect, impacts the speed at which clocks register the passage of time, therefore the ‘ticking’ of a clock will be faster the further it is from a huge body, such as the earth, according to General Relativity. Numerous tests using pairs of atomic clocks, one in space and one on Earth, have been successful in measuring this tiny change in the temporal flow.

In theory, similar discrepancies should exist at smaller distances, such as a few cms or millimeters. But there were no accurate clocks to measure them. Until now.

A team of scientists at the University of Colorado Boulder has created an atomic clock that is so sensitive that it has been able to detect a one-millimeter-long temporal acceleration in an atom sample.

The pre-publication server ‘arXiv‘ hosts the results of this remarkable project.

Scientists have used atoms as clocks for decades because of one of its properties: atoms exist at multiple energy levels, and a specific frequency of light causes them to ‘jump’ from one level to the next. That frequency, the rate of oscillation of light waves, serves as the second hand on a clock, marking the ticking. Because time moves quicker at atoms farther from the earth, a higher frequency of light is required to make the energy leap. Scientists had previously measured this frequency shift, known as ‘gravitational redshift,’ over a record distance of 33 cm.

This mark, however, has been pulverized by physicist Jun Ye and his colleagues thanks to a watch made of approximately 100,000 ultracold strontium atoms. Mesh-like arrangement meant that the height of each atom varied, as though the atoms were ladder-like on their perches.

The results were obtained by mapping how the frequency varied at those heights. After accounting for nongravitational influences, the clock’s frequency changed by around a hundredth of a trillionth of a second at both ends of a millimeter, exactly as anticipated by general relativity.

Furthermore, after collecting data for approximately 90 hours and comparing the ticking of the top and lower sections of the clock, the scientists discovered that their technique could quantify relative ticking rates with a precision of 0.76 millionths of a billionth of a second. Setting a new record for the most precise frequency measurement ever performed. Jun Ye and his colleagues, in effect, detected a disparity between the two parts of the clock, resulting in a one-second delay after roughly 4 billion years.

On the same day that Jun Ye presented his results, another team of researchers, this time led by Shimon Kolkowitz of the University of Wisconsin-Madison, performed the same in another article in ‘arXiv‘. The researchers used accuracy of 8.9 millionths of a billionth of a second to measure the relative speeds of the ticks of two of the six clocks in their experiment, which were roughly 6 mm apart. They were able to identify a discrepancy between two clocks that advance at such a slightly different rate that they would have been misaligned within a second after 300,000 million years because to their sensitivity. Without Jun Ye’s research, the results would have set a new record.

In any case, both experiments demonstrate that the accuracy of atomic clocks has already reached a point where they could be used to solve some of the Universe’s biggest mysteries, such as the existence of dark matter, which according to some study results could also alter the rhythm of clock ticking, or the supposed immutability of nature’s fundamental constants.

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