Which Weighs More? Matter or Anti-Matter?

Scientists try to determine the effect of gravity on anti-matter.

(PHYSORG)- Does antimatter behave differently in gravity than matter? Physicists at the University of California, Riverside have set out to determine the answer. Should they find it, it could explain why the universe seems to have no antimatter and why it is expanding at an ever increasing rate.

In the lab, the researchers took the first step towards measuring the free fall of "positronium" – a bound state between a positron and an electron. The positron is the antimatter version of the electron. It has identical mass to the electron, but a positive charge. If a positron and electron encounter each other, they annihilate to produce two gamma rays.

Physicists David Cassidy and Allen Mills first separated the positron from the electron in positronium so that this unstable system would resist annihilation long enough for the physicists to measure the effect of gravity on it.

"Using lasers we excited positronium to what is called a Rydberg state, which renders the atom very weakly bound, with the electron and positron being far away from each other," said Cassidy, an assistant project scientist in the Department of Physics and Astronomy, who works in Mills's lab. "This stops them from destroying each other for a while, which means you can do experiments with them."

Rydberg atoms are highly excited atoms. They are interesting to physicists because many of the atoms' properties become exaggerated.

In the case of positronium, Cassidy and Mills, a professor of physics and astronomy, were interested in achieving a long lifetime for the atom in their experiment. At the Rydberg level, positronium's lifetime increases by a factor of 10 to 100. Keep on reading...