Scientists first observe gravity’s effect on antimatter

Though antimatter is a popular topic in science fiction, it is also real in our world. Antimatter is simply the “partner” to ordinary matter, and when a matter particle meets its antimatter partner, they annihilate each other. In theory, the only difference between matter and antimatter is an opposite electric charge and different quantum numbers, however antimatter is much harder to discover.

The reason for this imbalance is one of the great unsolved problems in physics. Scientists have been studying antimatter's physical properties for some time to learn more about it. On Sept. 27, a group of scientists released a paper regarding gravity’s effects on antimatter in Nature.

One of the main challenges these scientists faced was how to measure gravity’s effects on antimatter, since antimatter only exists in small quantities and gravity is considered comparatively weak to electromagnetic forces. This group of scientists used the “ALPHA-g machine,” which can trap antihydrogen, the antimatter counterpart to hydrogen that is made up of a positron and antiproton.

The ALPHA-g machine is a vertical apparatus that can release antihydrogen through its top or bottom. Depending on the difference between how many particles are released through the bottom versus the top, the scientists could see the effect gravity had on the antihydrogen.

Before physically testing, the scientists ran numerical simulations of hydrogen under ALPHA-g machine conditions, and found that they would see about 80 percent of the hydrogen to come out of the bottom. As such, if antimatter behaved like matter under the effects of gravity, the team should see about the same ratio for antihydrogen. During the actual experiment, the scientists saw that under gravity alone, about 71.9 percent of the antihydrogen escaping through the top or bottom escaped through the bottom, showing bias for the “normal” gravitational direction. When adding additional forces in the “normal” direction of gravity, the scientists saw results that agreed with antimatter behaving like matter under gravity.

The group of scientists concluded their paper with their best fit for the effects of gravity on antimatter toward Earth, which is (0.75 ± 0.13 (statistical + systematic) ± 0.16 (simulation))g, giving a probability of 0.00029 that gravity has no effect on antimatter and a less than 10^-15 percent chance that gravity repulses antimatter.

They also note that this experiment and result are just the start of more detailed inquiries into the gravitational nature of antimatter, as their uncertainty is still quite sizable. They look toward future experiments that will utilize “offline magnetometry using electrons, nuclear magnetic resonance (NMR) probes, and possibly trapped, laser-cooled ions” to improve their results.