Is antimatter the key to anti-gravity?

Scott Sutherland

Back in 2000, there was this great IBM commercial with actor Avery Brooks asking "Where are the flying cars?" Well, it's taken more than a decade since then, but scientists are starting to hone in on answers about anti-gravity, and antimatter may be the key.

Matter and antimatter are mirror-image twins. Any particle of matter (proton, neutron, electron) has a counterpart (antiproton, antineutron, positron) that is exactly the same size and mass, but otherwise it is opposite in every other way that they interact with the universe... well, nearly every way, as far as we know.

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Antimatter particles have the opposite electric charge of matter particles. For example, an electron is negative (-1), but a positron is positive (+1). They also have opposite 'spin', which is a way of describing how they react to magnetic fields — an electron would be pushed in one direction by a magnetic field, while a positron would be pushed in the opposite direction. Scientists aren't quite sure about how antimatter reacts to gravity yet, though.

There's two 'types' of mass, the mass that's measured as a resistance to being accelerated (inertial mass) and the mass measured in response to gravity (gravitational mass). For matter these are the same, and their ratio is equal to 1. However, if antimatter 'follows suit' with the rest of how it interacts with the universe, it could have a ratio of -1.

According to Joel Fajans of Berkeley Lab in California, "in the unlikely event that antimatter falls upwards, we’d have to fundamentally revise our view of physics and rethink how the universe works."

Up until now, there really hasn't been a way to actually test how antimatter reacts to gravity, but scientists with Berkeley Lab and CERN's Alpha experiment are looking into it.

CERN's Alpha experiment locked 'antihydrogen' atoms (an antiproton orbited by a positron) inside a magnetic field, and then watched where these 'anti-atoms' went when they turned the field off. They were specifically looking for anti-atoms that took the longest time to leave the field (about 20- to 30-thousandths of a second after it was turned off), since they would have the lowest energy, and thus would be affected by gravity more than higher energy anti-atoms.

"Late-escaping particles have very low energy, so gravity's influence is more apparent on them," said Berkeley Lab's Jonathan Wurtele, in a press release. "But there were very few late escaping anti-atoms; only 23 of the 434 escaped after the field had been turned off for 20-thousandths of a second."

With only that many, the question about whether antimatter falls upwards or downwards is still up in the air it seems, but they were at least able to rule out the most extreme cases.

Their results showed that the ratio of gravitational mass to inertial mass for these antihydrogen atoms was somewhere between 110 and -65 (remember that they're looking for it to be somewhere around 1 or -1).

Basically, what that mean is, if antihydrogen falls downward, its gravitational mass is less than 110 times stronger than its inertial mass. If it falls upward, its gravitational mass is less than 65 times stronger than its inertial mass. That's a pretty wide range, but models of the experiment apparently set the limit of the ratio at between 200 and -200, so while their results aren't mind-blowing, they have narrowed it down.

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So, we're not exactly at 'flying car' results, but this is really just the first step. The Alpha experiment is due for an upgrade soon, to Alpha-2, which will cool the anti-atoms with a laser before they're released from the magnetic field. That will give them more low-energy atoms to get results from, and that will put us closer to fulfilling that promise to Mr. Brooks.

"Is there such a thing as antigravity? Based on free-fall tests so far, we can’t say yes or no," says Fajans. "This is the first word, however, not the last."

(Images courtesy: CERN/ALPHA collaboration)

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