For the first time, physicists have measured the tiny quantum shift an object acquires by falling through Earth’s gravity — an effect of Einstein’s relativity that was predicted almost a century ago but never observed.
Researchers placed ultracold rubidium atoms into a superposition, a quantum state in which a single particle takes two paths at once. In this experiment, one path put the atom in free fall while the other kept it motionless. Recombining the atoms revealed an almost imperceptible difference between the two paths.
