https://www.theregister.co.uk/2018/06/05/nist_atomic_clock_einstein_theorem/ > 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.