# The atom that fell and the atom that didn't > Physicists finally split a single atom between free fall and perfect stillness, and watched what gravity did to its wave. - Title: The atom that fell and the atom that didn't · Ars Technica - Summary: Physicists finally split a single atom between free fall and perfect stillness, and watched what gravity did to its wave. Einstein's equivalence principle… - Keywords: quantum physics, gravity, relativity, interferometry, experiment, technology, science, atom, fell, didn't, Ars Technica - Source: Ars Technica — https://arstechnica.com/science/2026/09/what-happens-when-quantum-mechanics-and-relativity-meet - Published: 2026-09-11T11:20:02+00:00 - Read time: 2 min - Topics: quantum physics, gravity, relativity, interferometry, experiment, technology, science ## A hundred year old homework Physicists worked out what free fall should do to a quantum wave almost a century ago. Nobody could build an instrument capable of checking the answer. ## Someone finally built it Ron Folman's team at Ben-Gurion University, with collaborators in Germany, the UK, the US, and Roger Penrose, made an interferometer that gives one atom two fates at once. ## Fall and stay, together One path drops the atom in free fall. The other holds it perfectly still. Both end at the same spot at the same instant so they can be compared. ## Your car is a wave Quantum mechanics says everything is also a wave, with a phase marking whether it sits at a crest or a trough. > Every particle, doesn't matter if it's a car or a spaceship or an atom, is a wave. ## Why it took so long An atom only shows its wave nature when it is nearly motionless, which means cooling it to near absolute zero. That only became possible in the late 1990s. ## Phase is always relative You cannot measure a phase by itself. You have to split one particle down two routes and bring them back together, the same logic as the double-slit experiment. > It can only be a relative measure done by splitting a single particle into two trajectories. ## You can't just catch it Grabbing a falling atom with a trap would add so much noise the real effect would vanish. Folman's word for the result: fake news. > This very violent action would create so much noise that we wouldn't have known what is fake news. ## Shoot it like artillery Instead they fire one wave packet upward and let gravity bring it home, while its twin never moves. The cannon has to push one packet and ignore the other. > You have to shoot one with a cannon, into a ballistic motion like artillery. ## Speed is a witness A packet returning from flight moves fast, its twin does not. That speed gap reveals which path was taken, and any such clue kills the interference outright. ## Twenty thousand rubidium atoms The Quantum Galileo Interferometer holds a Bose-Einstein condensate with wires on a chip hanging upside down about 113 micrometers above it. ## The magnetic sleight of hand A microwave pulse splits each atom into a magnetism-feeling state and a blind one. A magnetic kick launches the sensitive half, then a flip swaps the roles mid-flight. ## A parachute at the end The falling packet coasts with no forces on it while its twin hovers against gravity. A second magnetic pulse kills the returning speed right where it started. ## The phase showed up Trajectories separated by 7.5 micrometers, flights of two milliseconds. Longer falls built more phase, landing 2.5 percent from theory. Equivalence survives superposition. > This has never been shown before, that the equivalence principle can work in a quantum system in a superposition. ## Next stop, nanodiamonds Folman's lab wants objects ten orders of magnitude heavier in superposition, which would mean two curvatures of spacetime at once. Penrose says that has to collapse. > Roger Penrose says that the Universe cannot sustain a superposition of curved spacetime. ## Key takeaway Einstein's equivalence principle holds up even for an atom in two places at once, but the real test of quantum gravity is still a nanodiamond away.