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Does Antimatter Fall Up?

Who knows? But at least researchers have discovered a way to figure it out.
April 30, 2013, 3:00pm

Antigravity: Mark Sebastian/Creative Commons

It almost sounds like a joke question: does antimatter fall up? Or a cartoonish sci-fi question. In the real-world, however, it remains one of the more pressing queries about the very real if exceedingly rare antimatter.

Antimatter is one of the most bizarre substances in the universe (at least if you live in a world of matter, like we do), and is simply defined by being the opposite of "normal" matter--so opposite that when the two meet, they annihilate completely, giving themselves up totally in a burst of energy. The conversion is so efficient that it makes even nuclear fission look like burning wet leaves. (At 100 percent efficiency, an antimatter collision bests fission 1,000 times over.)

Most of the antimatter hype tends to revolve around the above, containing the stuff (very hard to do) and using it for purposes like spaceship fuel and killing other humans. Angels and Demons, blech. Antimatter is really interesting because of what it is, not what it does or can do for us. Antimatter exists because we exist, because normal matter exists: whenever a particle of normal matter is created (by two photons colliding, for example), a second particle of equal energy is created.

The difference is that the second particle has the opposite charge as the first. So, if we get an electron, we also get a positron (positive electron), which doesn't hang around very long because almost immediately it's going to find a particle of normal matter to annihilate with.

So, a perfectly opposite form of matter. Shouldn't it then fall up? Shouldn't there be an opposite force to gravity, antigravity? Maybe, maybe not. We're still rather vague on the nature of gravity. Humans have been thinking about gravity for a whole lot longer than they've been thinking about Higgs bosons, yet we've yet to observe a gravitational wave or graviton, the supposed gravity particle. And, though we've created and stored antimatter particles, their world is even more of a fundamental mystery, particularly what happened to almost all of that world when the universe-of-matter was created.

It seems we're missing a universe-of-antimatter. Though, that universe could just be "up there" somewhere, protected by some as yet unknown force-field from our universe. The anti-universe would look pretty much the same; the antiparticles function the same as particles, building into atoms the same way. So, there could be well be an anti-you out there, totally unaware that there's anything different until the anti-you were to shake hands with the you-you, and both of you would annihilate in an explosion big enough to take out entire planets.

Fortunately, researchers are now able to make progress towards an answer to the antimatter/gravity question--and the missing antimatter universe question--and they're able to do it using old results from CERN's ALPHA experiment, which trapped particles of antihydrogen in a magnetic field so that they could be compared with normal hydrogen. When you turn that trap off, the antiparticles are released and left to collide with whatever they encounter outside the trap. Based on where the particles collide, things like their velocity, initial position, and energy can be inferred.

“Late-escaping particles have very low energy, so gravity’s influence is more apparent on them," says Berkley's Jonathan Wurtele, one of the authors of a new paper in Nature Communications describing the technique. "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.”

So, we don't actually have an answer after this round of analysis, but we do have a pretty good method of finding an answer once ALPHA's successor, ALPHA-2, begins delivering good data. Exciting times for our mirror universe. And with apologies to Dan Brown fans (not really), the idea of antigravity is way more interesting than blowing things up.

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