Physicists have developed an experiment involving super cold matter and an empty elevator shaft that will test one concept crucial to Einstein's Theory of General Relativity. The experiment may shed light onto the tension between quantum physics and Einstein's relativity. "Einstein's 'equivalence principle', which underpins general relativity, says that if you stand in a falling elevator, your acceleration should effectively cancel out the pull of gravity, leaving you unable to determine whether you are in free fall or whether there is simply no gravity present at all," says the New Scientist. "Determining whether quantum systems are also subject to this equivalence principle might help pin down why quantum theory has so far resisted any merger with general relativity – a mystery that haunts physics."