How deep does the asymmetry between matter and antimatter go? Each type of particle (electrons, protons, etc.) have antimatter partners: positrons, antiprotons, and so forth. These antiparticles have an opposite electric charge (unless they're neutral), but otherwise behave much like their matter counterparts. But one interesting question remains unanswered: does antimatter possess antigravity, experiencing a repulsive force when matter experiences attraction? And, even if antimatter experiences plain old gravity, does it behave in exactly the same way as matter does?

Researchers from the ALPHA experiment at CERN realized their antihydrogen trap could help answer that question. By releasing antihydrogen atoms—consisting of an antiproton and a positron—they could measure whether the atoms fell up or down. Using data they had already obtained, the ALPHA team determined they didn't yet have enough data to rule out antigravity or strange behavior in antimatter.

That may sound like a weak result, but the experiment was not originally designed to perform this test. The implication is that ALPHA could be deliberately used to answer this question in the future, with mindful experiments designed to test the gravity of antimatter.

Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment.

However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. Angels and Demons aside, physicists have yet to produce and maintain macroscopically measurable amounts of antihydrogen, much less store it long-term.

The most successful anti-atom factory, the ALPHA experiment, produces and traps hundreds of antihydrogen atoms, using magnetic fields to keep them from bumping into matter atoms. The authors of the current study realized that those antiatoms eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter.

The researchers quantified this effect using the ratio of the antihydrogen's gravitational mass M g to its inertial mass M i —its resistance to change of motion under a force. According to the weak equivalence principle, inertial and gravitational mass are identical; this has been tested to high precision for matter, though there is still room for very tiny deviations. (As the name suggests, a strong or Einstein equivalence principle also exists, but that's not important for this discussion.)

If the weak equivalence principle holds, then the ratio F = M g /M i is exactly equal to 1. If antimatter experiences antigravity but otherwise obeys the equivalence principle, then we expect F = -1. Any other strange behavior will result in some other value for F. We've already found that each type of antimatter particle has the same inertial mass as its matter partner, to the best current experimental standards, but gravitational masses have been harder to gauge.

Based on archival data from ALPHA, the researchers determined that F is between -65 and 110, based on a very conservative estimate of potential errors. (A more optimistic estimate shows F to be less than 75.) That's hardly a precise measurement, and it's unlikely that F is hugely different from +1 or -1. However, these data were gleaned from previous uses of ALPHA, not those dedicated to testing the weak equivalence principle in antihydrogen. If researchers set out to measure F in the first place, they could design experiments better suited to getting a precision measurement.

Therein lies the value of these results: they showed in principle how gravitational tests on antihydrogen can be performed. (Other experiments have used electron-positron pairs called positronium, which pose their own challenges.) With upgrades to ALPHA in the works and other antihydrogen-based experiments in process, the question of whether antimatter behaves differently under gravity could be answered.

Nature Communications, 2013. DOI: 10.1038/ncomms2787 (About DOIs).