For the first time, physicists have measured the small quantum shift that an object acquires as it falls through Earth’s gravity, an effect of Einstein’s relativity that was predicted nearly a century ago but never observed. The researchers placed ultracold rubidium atoms in 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 still. The recombination of the atoms revealed an almost imperceptible difference between the two paths. The measurement, published September 2 in the journal Science Advances, shows that Einstein’s equivalence principle, the central idea of general relativity, still holds when introduced to the quantum world, creating a small but tantalizing link between relativity and quantum mechanics. Latest Live Science Videos “The principle says that acceleration and gravity cannot be distinguished locally,” Vlatko Vedral, a physicist at the University of Oxford and co-author of the new study, told LiveScience via email. Einstein’s Happiest Thought The classic illustration of the equivalence principle is a thought experiment known as Einstein’s elevator. A person trapped in a windowless elevator cannot tell whether the ground is pressing against his feet because the elevator is parked motionless on Earth or because it is being accelerated through empty space. Einstein called that realization the “happiest thought” of his life and built general relativity around it. You may like For heavy, everyday objects, the principle of equivalence has been tested with extraordinary precision. Quantum objects are a different matter. They behave like waves, and each wave carries a quantity called phase, essentially where its crests and troughs are located. The phase cannot be seen directly, but when two versions of the same particle are recombined, any mismatch between their phases shows up as an interference pattern. The two waves reinforce each other in some places and cancel each other in others, thus changing the probabilities of where the particle appears. The theory says that a wave in free fall should accumulate a phase relative to an identical wave held still and that this phase should grow with the cube of the fall time. Therefore, doubling the decay time multiplies the effect by eight. Charles Galton Darwin, grandson of the famous naturalist, and Earle Kennard documented this prediction in 1927, but no one had measured it until now. Get the world’s most fascinating discoveries delivered straight to your inbox. An atom that falls and remains still at the same time This device, called 2D MOT, feeds the scientific chamber with cold atoms. (Image credit: Or Dobkowski) Capturing the effect required an instrument with one arm actually at rest relative to the Earth and the others in true free fall, something no previous experiment could provide. Matter wave experiments dating back to neutron interferometry of the 1970s had measured gravity in other ways, but never with that pair of trajectories. The team behind the new study, led by physicist Ron Folman of Ben-Gurion University of the Negev in Israel, cooled approximately 20,000 rubidium atoms into an exotic state of matter called a Bose-Einstein condensate, freed them from their magnetic trap, and ran the interferometer about 113 micrometers beneath an “atomic chip” patterned with gold wires just 2 micrometers thick. Radio and microwave pulses place each atom in a superposition of two magnetic states. A magnetic kick launched one of them upward, and a fraction of a millisecond later, a second pulse made that half insensitive to magnetic fields, so that it rose and fell only under gravity. The other half remained magnetically sensitive and felt a force tuned to exactly cancel gravity, leaving it suspended in place. What to read next At the top of the arc, the two halves of a single atom were separated by about 7.5 micrometers (about seven times the width of the atomic wave itself) before the sequence was reversed to bring them back together. The team named the device Galileo quantum interferometer, after the scientist who first argued that all objects fall at the same rate. “Two different accelerations can be superimposed, zero [acceleration] and the Earth’s gravitational acceleration, and the resulting quantum interference was measured,” Vedral said. A phase predicted almost 100 years ago. The researchers tracked the interference signal while extending the free fall time to about 2.4 milliseconds. In 633 runs over 5.3 hours, they recorded 13 complete oscillations, each of which was a complete cycle of accumulated phase between the two halves of the atom. The growth followed the cube of the time of fall, matching the theoretical model “The phase between the two elements of the superposition, which grows as the cube of the duration of the experiment and which was predicted long ago, in 1927, has finally been observed for the first time,” Vedral said. The result is important because the same phase can be derived in two completely different ways: one treats gravity as a force acting on a quantum wave, the other moves towards the descending frame, where gravity disappears, and both invoke the principle of equivalence. The same answer, and that agreement is what allows quantum mechanics and general relativity to coexist. “They tell us that, at this level of precision, there is no conflict between quantum physics and gravity,” Vedral said. “In other words, the equivalence principle complies perfectly with quantum mechanics.” of the atomic cloud, distorting the two halves in different ways. The contrast of the interference fringes started at 80% for the short cycles and faded to 20% for the longer cycles, setting a practical limit on how long the experiment can last. What comes next The team now wants to test the equivalence principle under more general conditions, such as the “same experiment in a rotating frame,” Vedral said. overlap and can gravitationally affect each other” would further test the results, he added. That last idea points to one of the field’s central open questions. Heavier objects, such as nanodiamonds, would allow physicists to ask whether gravity itself follows quantum rules and would open a route to testing the conjecture, championed by study co-author and Nobel laureate Roger Penrose, that gravity is what destroys quantum superpositions in the first place. Guridi, IF, Penrose, R., Vedral, V., Schleich, W.P. and Folman, R. (2026) Observation of free-fall quantum phase and coherence with the equivalence principle Science Advances, 12(36).
