Scientists Watched Clocks for 14 Years to Prove Einstein's Theory of Relativity

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Einstein saw the Earth as a freefalling elevator, the NIST team explained in a press statement. The famous physicist theorized that objects found in such a slow moving elevator would accelerate at the same rate almost like they were in a uniform gravitational field. Einstein also predicted that these objects properties relative to each other would stay the same during the fall. 

And it was those thoughts the researchers put to the test. The NIST study makes it possible to have continual improvements in atomic clocks by using the best atomic clocks around. NIST compared the data of ticks from two different styles of atomic clocks found throughout the world in order to show that they remained in sync for 14 years -- despite the gravitational pull from the 'elevator' changed with Earth's rotation and orbit. 

The team explained further in a statement: 

"In their experiment, the NIST team regarded Earth as an elevator falling through the Sun's gravitational field. They compared recorded data on the "ticks" of two types of atomic clocks located around the world to show they remained in sync over 14 years, even as the gravitational pull on the elevator varied during the Earth's slightly off-kilter orbit around the sun. Researchers compared data from 1999 to 2014 for a total of 12 clocks -- four hydrogen masers (microwave lasers) in the NIST time scale with eight of the most accurate cesium fountain atomic clocks operated by metrology laboratories in the United States, the United Kingdom, France, Germany and Italy."

The team's work resulted in a record low number, a number so small that it aligns with Einstein's prediction of zero for the figure. 

"The researchers constrained the violation of LPI to a value of 0.00000022 plus or minus 0.00000025 -- the most miniscule number yet, consistent with general relativity's predicted result of zero, and corresponding to no violation," they said. "This means the ratio of hydrogen to cesium frequencies remained the same as the clocks moved together in the falling elevator."

Those figures were also more accurate than their previous best measurement. The improvement largely comes from more accurate cesium fountain atomic clocks and better time transfer processes.

"But the main reason we did this work was to highlight how atomic clocks are used to test fundamental physics; in particular, the foundations of general relativity," Patla said. "This is the claim made most often when clockmakers strive for better stability and accuracy. We tie together tests of general relativity with atomic clocks, note the limitations of the current generation of clocks, and present a future outlook for how next-generation clocks will become very relevant."