A rubidium atom can behave as a wave that researchers can split between two paths. This allows the same atom to effectively follow both paths at once, without being broken into pieces. Physicists have now used that property to hold one part of the wave still while letting the other fall under gravity, then bring them back together to check a prediction rooted in Einstein’s theory of gravity.
When the two parts of the wave reunited, the researchers measured a difference in their quantum phase, which describes where a wave is in its cycle. That difference matched the prediction obtained by applying Einstein’s equivalence principle to a quantum object. The principle says that, for an observer falling freely, gravity’s effects should locally disappear.
The researchers describe the result as the first direct measurement of the predicted quantum phase of a freely falling object. Earlier experiments have used quantum particles to measure gravity, but this experiment specifically compared a freely falling atomic wave with one held stationary.
The study was led by Ben-Gurion University of the Negev, the University of Ulm, and the University of Oxford, with Nobel Prize-winning physicist Professor Sir Roger Penrose among its coauthors. It was published September 2 in Science Advances.
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