You don’t weigh the same in Boston as you do in Berlin. Granted, the difference—a consequence of Earth’s rotation and unevenly distributed mass—is minuscule, but it’s easily detected with state-of-the-art gravimeters. In fact, atom interferometers can detect Earth’s gravitational pull to better than one part in 1011, which makes them sensitive enough to register the change in gravity due to a change in elevation of just a few millimeters.1
The interferometers exploit the wave nature of matter: Pulses of light are used to deliver momentum kicks that split a free-falling atom into two wavepackets, steer them along separate paths through space, and recombine them. The constructive or destructive nature of the wavepackets’ interference reveals information about the respective distances they traveled and the acceleration they experienced.
The resulting ultraprecise measurements are more than curiosities. They can be used to locate buried mineral deposits and to monitor aquifers,...
