An international team including Nobel Prize-winning physicist Professor Sir Roger Penrose has observed a long-predicted effect of gravity on a falling quantum object for the first time.

The result shows that a fundamental principle at the heart of Einstein’s theory of gravity remains consistent with the behaviour of matter in the quantum world. The study ‘Observation of the quantum phase of free fall and the consistency with the equivalence principle‘, led by Ben-Gurion University of the Negev, The University of Ulm and the University of Oxford, has been published in Science Advances. For more than a century, physicists have relied on two extraordinarily successful descriptions of nature. Quantum mechanics explains the strange behaviour of atoms and other tiny objects. Einstein’s theory of gravity explains how objects fall and how gravity shapes the Universe. Yet physicists still do not fully understand how the two fit together. Now, an international team has performed an experiment that probes the point where they meet. In the study, the researchers observed a distinctive change in the quantum properties of atoms as they fell under gravity. Crucially, the effect they measured is the same one predicted when Einstein’s equivalence principle, a cornerstone of his theory of gravity, is applied to a quantum object. The equivalence principle states that for an observer in free fall, gravity should locally disappear. Someone falling freely in a lift, for example, would experience weightlessness. Whilst this theory has survived extraordinarily precise tests involving ordinary matter, it was unclear how this could be experimentally tested with quantum objects, which can behave as waves and effectively travel along more than one path. At the heart of the experiment is a new apparatus the researchers call the Quantum Galileo Interferometer. It allowed them to do something unusual: effectively split the quantum wave associated with an atom into two paths, hold one in place while allowing the other to fall freely, and then reunite them to see how gravity had changed the falling wave.

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Scientists Observe Einstein’s Gravity in The Quantum World