Alpha experiment. Copyright Cern 201808-267_04

Albert Einstein’s general theory of relativity described the effects of gravity in 1915 and has passed many experimental tests since then. A component of the theory called the weak equivalence principle states that all objects, regardless of their mass or composition, should free-fall the same way in response to gravity. Although the prevailing view is that antimatter should behave in the same way as matter in response to Earth’s gravitational pull, direct observations have been lacking owing to challenges in creating carefully controlled experimental conditions.

"To put it all together, many different pieces have to come together. My own contribution involves calculations and simulations of how the anti-atoms move, especially if they happen to bump into an ordinary atom," says Svante Jonsell, senior lecturer in theoretical atomic physics at Fysikum.

In 2018 the ALPHA collaboration from CERN constructed the ALPHA-g machine, a magnetic trap for antihydrogen atoms, designed to study the effects of gravitation. Antihydrogen atoms suspended within the device are released; and the influence of gravity can then be inferred by tracking their subsequent motion. If more spill out of the bottom than the top it is likely antimatter atoms behave in the same way as regular matter. With this experiment, Jeffrey Hangst and colleagues observed a tendency for magnetically trapped antihydrogen atoms released into ALPHA-g to fall from the bottom of the apparatus.

These findings confirm the prevailing view that antimatter should feel the effects of gravity the same as for ordinary matter, in agreement with the predictions of general relativity. The work paves the way for future tests of the weak equivalence principle, which may improve our understanding of the gravitational nature of antimatter, the authors conclude.

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Observation of the effect of gravity on the motion of antimatter

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