Four hydrogen + eight caesium clocks = one almost-proven Einstein theory

Time team comes closest it ever has to magical zero result

A team at the National Institute of Science and Technology (NIST) has used a range of atomic clocks from around the globe to test the equivalence principle* of Einstein's theory of general relativity.

The research, published in Nature Physics, follows Einstein's thought experiment whereby the occupant of a windowless lift (or elevator, for our US chums) is unable to tell the difference between gravitational pull and acceleration.

Furthermore everyone within the lift would feel the same gravity or acceleration, and the properties of each person relative to the other would remain constant with zero deviation.

The NIST team thought big, and used the Earth as the lift falling through the gravitational field of the Sun. They then took 14 years worth of data from 12 atomic clocks scattered around the world, four hydrogen microwave lasers and eight caesium fountain clocks.

The team then compared data on the "ticks" of the clocks to show that they had remained in sync over the time period.

The principle of Local Position Invariance (LPI) holds that in the falling lift (or Earth, in this case), measurements of non-gravitational effects are independent of time and place. The researchers were able to show that the difference in the ratio between hydrogen and caesium clock frequencies as the "lift" fell was within a gnat's whiskers of the predicted result of zero – 0.00000022 plus or minus 0.00000025.

While this is five times better than a previous attempt by NIST in 2007, due in part to better clocks and improved data on the Earth's position and velocity in space, the team hopes future testing using clocks based on optical frequencies will yield results even closer to zero variance.

There was no word on why time seems to slow interminably when a lift is full of sweaty people during a summer lunchtime rush. ®

* Dealing with the equivalence of gravitational and inertial mass

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