If Antimatter Is The Opposite Of Matter, Does It Fall Up?

To find out, physicists go Newton on it, and drop some.
Now you know what they look like. John Fowler via Flickr/CC licensed

Antimatter, the mirror opposite of normal matter, is strange stuff. Particles and atoms of antimatter have the opposite charge of their regular counterparts, and they behave accordingly. When the two collide, you don’t want to be nearby–they annihilate each other in a flash of light.

Physicists wonder whether they also respond differently to gravity–does antimatter “fall” up? Does it weigh the same as regular matter? Does it exhibit anti-gravity? To find out, physicists at CERN and Lawrence Berkeley National Laboratory went Newton on it and dropped some.

It would make sense if gravity is the same for antimatter and regular matter. But there is no direct evidence proving this. Scientists who work with CERN’s ALPHA experiment, an antimatter experiment, tried to measure it in a somewhat roundabout way. First the researchers made antimatter, by joining antiprotons with positrons–the opposites of protons and electrons, respectively–and producing atoms of antihydrogen. The anti-atoms were suspended in a strong magnetic field so they wouldn’t touch the walls of their container, annihilating themselves and their counterparts.

Then the researchers turned off the magnetic field, allowing the antimatter to free-fall. If they knew where the anti-atom was when the field shut down, and they knew its velocity while hanging out in magnetic limbo, they could calculate how long it took to fall, and thereby measure the effect of gravity. Antihydrogen did not behave strangely, they found.

The results were very unclear, though. The magnetic fields don’t turn off instantly; it takes about 30 thousandths of a second to dissipate. When you are talking about individual atomic particles, that can add up to a long time. And individual flashes of annihilation happened throughout the antimatter trap, confusing the results. It could take several years to achieve the statistical accuracy physicists would need to prove how antimatter behaves with gravity.

The good news: The technical achievement is the real breakthrough here. It’s the first time anyone has directly measured the effect of gravity on free-falling antimatter. “Is there such a thing as antigravity? Based on free-fall tests so far, we can’t say yes or no,” Joel Fajans of Berkeley Lab said in a statement. “This is the first word, however, not the last.”

A paper describing the experiment appears this week in Nature Communications.