Physicists Build Interferometer to Test Free Fall of a Quantum Wave
A team led by Ron Folman of Ben-Gurion University has built an atom interferometer that sends a single atom down two paths at once — one in free fall, one held still — to test a century-old prediction about quantum waves.
The measurement concerns what free fall should do to a quantum wave. If the solution physicists arrived at nearly a century ago is wrong, quantum mechanics and Einstein's theory of gravity flatly contradict each other. Testing that question had been impossible because nobody had managed to build an interferometer capable of performing the necessary measurement, Ars Technica reported.
The new instrument routes one atom along two branches simultaneously. In one branch the atom falls freely; in the other it is held motionless. Because both paths end at the same location at the same instant, the team can compare them and measure what the fall does to a wave-like property of the atom.
Folman described the reasoning behind the experiment in terms of wave behavior. "Every particle, doesn't matter if it's a car or a spaceship or an atom, is a wave," he said. "Everything that is a wave, like sea waves or sound waves, goes up and down. And if you're up or down, this is measured by something called a phase. A phase just tells you if you are at the top of the wave or at the bottom."
The experiment sits at the junction of two descriptions of nature that physicists have never fully reconciled. Since Galileo, they have known how to describe a falling object — where it is, how fast it goes, or how quickly it accelerates. Quantum mechanics, however, insists that every object is also a wave, and the phase of that wave is what the interferometer is designed to read out. The account published by Ars Technica on 11 September 2026 did not report the outcome of the measurement.