Physicists have put a decades-old idea about why the strange rules of quantum mechanics disappear in our everyday world to one of its toughest experimental tests yet. Deep beneath Italy’s Gran Sasso mountain, researchers searched for an extremely faint radiation signal predicted by a theory suggesting that tiny fluctuations in spacetime, caused by gravity, gradually destroy quantum superpositions. After 62 days of measurements with a highly shielded germanium detector, they found no such signal.