Spot the difference: matter and antimatter

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In a small experiment hall at Europe's Center for High-Energy Physics, Gerald Gabrielse's set-up is a study in contrasts: a piece of hardware that looks as though it could sit on a large tabletop surrounded by walls of enormous concrete blocks.

Such are the radiation precautions scientists must take when they try to build atoms out of antimatter. Such atoms could help physicists answer one of the most vexing questions about the evolution of the early universe: Why is the universe here?

"Given that we believe the big-bang scenario, it's a surprise to us that we exist at all, because all of the matter and antimatter should have been created in equal amount," explains Dr. Gabrielse, chairman of the physics department at Harvard University. This occurred at a time when the universe was much smaller and more dense than it is now - and should have led to matter and antimatter annihilating each other in an enormous burst of energy. To live in the universe we see today, a slight excess of matter survived.

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"I don't know exactly what the solution to that is. No one does," Gabrielse says. "But it means that looking for any possible difference between matter and antimatter is more interesting because of the cosmic stakes involved."

Physicists are looking for those differences with high-speed beams of particles at accelerators worldwide.

"These are really long-shot experiments," says David Christian, a physicist at the Fermi National Accelerator Laboratory in Batavia, Ill. But if the teams find something, the effect on physicists' understanding of the subatomic world and evolution of the forces of nature "would be dramatic."

He notes that studying antihydrogen atoms built from the anti-matter counterparts of electrons and protons could yield clues. Scientists have been able to produce antihydrogen in particle accelerators for at least three years. But while researchers could imagine accelerator experiments that could probe key properties of the atoms as they whizzed around at nearly the speed of light, he says, these approaches looked to be too difficult to execute.

Gabrielse is taking another tack by trying to build extremely lethargic antihydrogen atoms. Then he hopes to probe them with lasers and compare the results with those from hydrogen. He says this could lead his team into uncharted territory where key differences show up - or where similarities between matter and antimatter are established to accuracies up to a thousand times greater than those achieved so far.

The ingredients for Gabrielse's antihydrogen stew include antimatter counterparts to electrons, called positrons, plus some antiprotons and a cloud of electrons.

So far, the team has successfully captured positrons and anti-protons and gotten them to pass through each other. But, Gabrielse adds, it's tougher to get them to stay put in the same place long enough to form antihydrogen.

If the team can pull together enough antihydrogen and hang on to it, the researchers can begin their search for subtle differences between hydrogen and antihydrogen.

Gabrielse notes that he and his colleagues have been working on the antihydrogen experiment for 13 years. If it works, researchers will have to run the apparatus for another 10 years before they have enough data to be confident of their results.

"Based on our theoretical understanding, we won't see a difference," acknowledges Gabrielse. But he adds that those theories don't explain how the apparent imbalance between matter and antimatter emerged from the early universe "in a satisfying way. There's something rattling around loose in there, so we're looking in the general area to see what we can find."

(c) Copyright 2001. The Christian Science Publishing Society

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