Physicists devise a method of measuring the gravitational mass of antihydrogen

Physicists at the ALPHA collaboration at CERN have placed upper and lower bounds on the gravitational mass of antihydrogen, in the first direct observation of the gravitational interactions of antimatter. The ALPHA experiment was designed to trap the antihydrogen, and measure the spectral lines of the atom in order to compare it to its counterpart, regular hydrogen, in order to test CPT symmetry.

It was realised that the apparatus for trapping the antihydrogen, a magnetic trap surrounded by an annihilation detector, could be used to measure the free fall of the particle as the magnetic fields were switched off and the particle subsequently annihilated with the wall of the apparatus. The point of annihilation could then be pinpointed, and the particles’ trajectories mapped and compared to predicted trajectories of varying gravitational mass.

Until now, no experiments had directly measured the interaction of antimatter particles with gravity, however the inertial mass of antimatter particles are known extremely accurately and so the measurement of their gravitational mass provides insight into whether antiparticles interact with gravity in the same manner as normal matter. This experiment managed to rule out extremely large deviations in the behaviour of antihydrogen, placing its gravitational mass between -65 and 110 times that of its inertial mass.

According to the weak equivalence principle, the gravitational mass of antihydrogen should be the same as that of hydrogen; however the results are not yet precise enough to rule out the possibility of antimatter being repelled by gravitational fields. Over the next year, the ALPHA experiment is being upgraded, which should more precisely measure this quantity when it reopens in 2014.

http://www.nature.com/ncomms/journal/v4/n4/full/ncomms2787.html