The first experimental measurements of the gravitational mass of antihydrogen are reported in Nature Communications this week. The results mark an important step towards testing our understanding of the behaviour of antimatter.

Many arguments exist to suggest that antimatter and normal matter should behave in gravitationally the same way as each other, but no direct measurements exist that can categorically rule out the possibility that they do not. Scientists in the ALPHA collaboration, at CERN, are working on just such an experiment. They trap antihydrogen atoms and then release them, measuring their free-fall under gravity. This enables the researchers to determine the ratio of antihydrogen’s gravitational to inertial mass, which should be exactly 1 if antimatter and matter respond identically to gravity. From their first measurements, the team is able to put an upper limit on this ratio of 75, in the absence of systematic errors. They also obtain a similar limit for the behaviour under antigravity. Although the data are currently not able to pin down a more accurate limit, future developments to the apparatus and methodology should allow the researchers to improve their measurements and test more accurately for possible deviations from our expectations.