https://phys.org/news/2018-06-nist-atomic-clock-comparison-key.html

As described in a Nature Physics paper postedonline June 4, NIST researchers used the solarsystem as a laboratory for testing Einstein'sthought experiment involving Earth as a freefallingelevator. Einstein theorized that all objectslocated in such an elevator would accelerateat the same rate, as if they were in a uniformgravitational field—or no gravity at all. Moreover,he predicted, these objects' properties relativeto each other would remain constant during theelevator's free-fall.

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