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Would an antimatter apple fall upward from the earth?

Scientists have figured out a technique to measure gravity's effects on antimatter, that mysterious stuff so beloved of Dan Brown fans and physicists alike.

The achievement, made by the ALPHA collaboration at CERN's Large Hadron Collider near Geneva, marks another step toward understanding the differences between ordinary matter and antimatter -- and perhaps, physicists hope, toward revealing more about the origins of the universe.

The scientists reported on their work Tuesday in the journal Nature Communications.

Theorists think that ordinary matter and antimatter, which annihilate when they come into contact with each other, were generated in equal quantities during the Big Bang. But there must be some differences between the two types of matter, they also think, because otherwise matter and antimatter would have canceled each other out completely and there would be no universe.

Scientists at CERN are making atoms of antihydrogen to try to pin down what those differences might be.

But they're also willing to consider the possibility that the ideas were wrong in the first place, said Joel Fajans, a professor of physics at UC Berkeley and a member of the ALPHA team. For example, he said, theory suggests that the universe must be filled with dark matter and dark energy, but no one has seen either one.

"We think it's there, but it's almost an embarassment that we don't have evidence for it yet," Fajans said. "There's the faintest possibility that the theories are fundamentally wrong."

One way to change the theories and make them work would be to allow for antigravity.   

"People have dreamed of doing these experiments: Is it possible that antimatter falls upward instead of downward?" Fajans said. "Could an antimatter apple fall upward when it's close to the earth?"

The answer is probably no, he added. Indirect evidence suggests that gravity's effects are the same on matter and on antimatter. But gravity isn't especially well understood, so there could be "a window of opportunity for other strange things to happen," Fajans said.

The only way to find out for sure is to test gravity's effect on antimatter directly. That's where ALPHA comes in. The experiment uses strong magnets to trap antihydrogen atoms, which are formed by combining an antiproton and a positron.  (For a great primer on how ATLAS works, check out this article by former Times staffer Thomas H. Maugh II. Fajans gave this lecture on the experiment at Berkeley in 2009.)

According to Fajans and Jonathan Wurtele, another Berkeley physicist working on the project, no one had planned to measure gravitational effects with the current version of the experiment. But the scientists figured out a way to use measurements of 434 antihydrogen atoms and computer simulations to tease out some of gravity's very subtle effects. 

They can't yet tell if the particles fall up or down, said Fajans and Wurtele, but they were able to establish a range for antihydrogen's gravitational mass (that is, its mass measured by its gravitational attraction for other bodies).

It's not a level of precision that sheds a whole lot of light on the question of whether antigravity exists. But it's a promising step, Fajans said, because it demonstrates that scientists should be able to test gravity's effects on antimatter and learn more within a few years. Two new experiments at CERN, AEGIS and GBAR, are in the works.

The next version of the ALPHA experiment is already under construction and should also allow scientists to narrow the result -- with less precision, but on a "time scale of years, not decades," Fajans said. 


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