Physicists have, for the first time, experimentally verified that Einstein’s equivalence principle—the idea that gravity and acceleration are indistinguishable—holds true even at the quantum level. The research, published in *Science Advances*, confirms a nearly century-old prediction regarding the phase shift of quantum objects in a gravitational field.
The experiment, led by a team including Vlatko Vedral, a physicist at the University of Oxford, involved placing ultracold rubidium atoms into a quantum superposition. This allowed each atom to exist in two states simultaneously: one in free fall and the other stationary. By recombining these atoms, researchers detected a minuscule difference in their phases, confirming the predicted quantum shift.
Einstein’s equivalence principle is famously illustrated by the “elevator thought experiment,” where an individual inside a closed elevator cannot determine if they are at rest on Earth or accelerating through space. Einstein considered this realization his “happiest thought” and used it as the foundation for his theory of general relativity. While the principle has been rigorously tested with macroscopic objects, its validity in the quantum realm remained unproven until now.
Charles Galton Darwin and Earle Kennard first predicted the quantum phase shift in 1927, theorizing that a wave in free fall would accumulate phase relative to a stationary wave, with the effect growing proportionally to the cube of the falling time. The current experiment successfully measured this effect, providing a crucial link between general relativity and quantum mechanics. According to Vedral, “The principle says that acceleration and gravity cannot be distinguished locally.”
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