Washington (AFP) – US scientists have measured Albert Einstein’s theory of general relativity — which holds that gravity slows time down — at the smallest scale ever, demonstrating that clocks tick at different rates when separated by fractions of a millimeter. Jun Ye, of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, told AFP it was “by far” the most precise clock ever built — and could pave the way for new discoveries in quantum mechanics, the rulebook for the subatomic world. Ye and colleagues published their findings in the prestigious …