From PBS Nova:

There are two kinds of matter in the universe: ordinary matter, which makes up all the stuff of everyday life, and antimatter, a sort of mirror image of matter. When the two meet, they annihilate in a flash of energy. It’s our good fortune that, in the early Universe, there was just a tiny bit more matter than antimatter, leaving us with a cosmos almost empty of stuff that could destroy us. Otherwise, we wouldn’t be here to ask what, exactly, antimatter is.

Here’s what we know: Anti-electrons, known as positrons, are nearly identical to electrons, but instead of being negatively charged they are positively charged. The same goes for other antimatter counterparts: antiprotons are negatively charged and made of the antiquarks corresponding to the quarks in normal protons.

But physicists think that the other properties of the particles should be the same. Each antimatter particle should have the same mass, spin, and equal but opposite electric charge, and other important properties. But that “should” hides something interesting: In some cases, we simply don’t know the fundamental properties of an antiparticle, because it’s much harder to experiment on antimatter than on matter. For example, it’s possible antimatter doesn’t feel gravity in the same way matter does.

In other words, antimatter might fall up.

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Up, up and away. Credit: Flickr user Shaun Fisher, adapted under a Creative Commons license.

Now, that’s a very unlikely possibility. As far as we can tell, the differences between matter and antimatter are confined to interactions involving the weak nuclear force, one of the four fundamental interactions in nature. “Everybody including us would be shocked if we were actually to discover any significant differences” between matter and antimatter, says Joel Fajans, physics professor at the University of California at Berkeley who is studying how gravity affects antimatter. It may be a long shot, but if any experiment showed measurably different behavior, “it would really revolutionize our thinking about how the universe behaves.”

The effort isn’t easy, though. First, there’s a lot more matter than antimatter in the universe, so any differences in behavior would be very difficult to observe and measure. Second, experiments must be done quickly, before antimatter runs into ordinary matter and everything goes kablooie.

As a result, we only have rough estimates of some basic properties of antimatter—and some we haven’t measured experimentally at all. Take, for instance, a fundamental quantity called the positron inertial mass, a measure of how difficult it is to accelerate a positron. (The inertial mass is the “m” in E = mc2.) When an electron meets a positron and they annihilate, they give off gamma rays. Researchers can measure the spectrum of gamma rays and figure out how much m was needed to make the E they see. From that, physicists have concluded that the inertial mass of the electron and the positron are very close to equal, if not identical.

We’d like to do better than “very close,” though. To understand antimatter fully, we need measurements as precise and accurate as our measurements of matter, and that’s a hard goal. Similarly, we don’t yet have precision measurements for the electric charge of the positron and the antiproton, though Fajans and his collaborators have shown that their charges are equal and opposite. This experiment, like many modern antimatter tests, involves atoms of antihydrogen, which are made of a single antiproton and positron. To see if antimatter falls up, Fajans and his colleagues at the ALPHA experiment use strong magnetic fields to trap antihydrogen atoms in a sort of virtual bottle.

“If we very slowly turn off the ‘walls,’ the magnetic confining field, [the antihydrogen atoms] eventually get out,” Fajans says. “If we do it slowly enough, even though the effects for gravity are subtle, there’ll be a tendency for them to fall downwards presumably, or upwards if things really are weird.” So far, the results aren’t precise enough to distinguish between falling up and falling down, but that’s merely a sign of how inherently difficult the experiment is.

However, there’s strong indirect evidence that antimatter behaves gravitationally like matter. According to the weak equivalence principle—a key part of the general theory of relativity—the gravitational mass is precisely the same as inertial mass,. (The strong equivalence principle relates to the mathematical structure of gravitational theory.) Researchers have tested the weak equivalence principle to high precision for ordinary matter, using delicate balances capable of detecting tiny variations in gravitational attraction.

While we can’t yet make the same lab equipment out of anti-atoms to test the weak equivalence principle for antimatter, we know that protons and neutrons contain “virtual” pairs of quarks and antiquarks, which don’t have independent existence but contribute to the particles’ overall structure. As Fajans points out, “Different isotopes have different ratios of virtual antimatter particles, and it’s very well known that there are no anomalies there. If virtual antimatter particles gravitate differently, that would have been noticed in all of these experiments.”

There are also theoretical reasons to suspect gravity doesn’t work in reverse for antimatter. Raquel Ribeiro, a physicist at Case Western Reserve University, works on possible modifications to general relativity that could solve the riddle of cosmic acceleration. But Riberio doesn’t include antigravity antimatter, “because it leads to a number of physical violations of energy principles,” she says. While naively all it would take is turning mass from a positive into a negative number, the reality for stars and other astronomical bodies would be “some serious instabilities in the system.”

Theory is a good guide, but we still need experiments to see if our theories are right or if they need modification. In fact, theory is so far unable to solve one of the deepest mysteries in physics. “There simply isn’t enough antimatter in the universe,” says Fajans, “and there isn’t a universally accepted reason as to why matter in the universe predominates by such a large ratio over antimatter. The Big Bang should have created exactly equal amounts of matter and antimatter.”

That’s one reason why researchers will keep studying antimatter, and why some hold out hope for finding even small differences in the behavior of matter and antimatter. Maybe we won’t see antihydrogen falling up, but even a subtle deviation from expectations could open up a new world of possibilities. After all, that’s what the initial discovery of antimatter did.

Go Deeper
Author’s picks for further reading

Ars Technica: Does antimatter fall up?
Matthew Francis’ report on results from the ALPHA experiment.

Ars Technica: CERN traps antimatter for long enough to do serious science on it
Discover how researchers create and trap antihydrogen atoms.

Physical Review: The positive electron
Read Carl Anderson’s 1933 paper, which offered the first estimate of the mass of a positron.