First Matter-Antimatter Molecule Created

Positrons and electrons still want to destroy each other, but physicists force them into groups

20 September 2007—Antimatter permeates the realm of science fiction, from Isaac Asimov’s robot brains to the warp drives of Star Trek ’s Enterprise . Not so in our real universe, where what we see, eat, touch, or smell is made of normal, run-of-the-mill matter. When bits of antimatter do show up, they tend to interact with matter and disappear into a burst of energy.

But two physicists from the University of California at Riverside have pulled off a seemingly impossible feat: creating molecules of equal parts matter and antimatter. These long-sought dipositronium molecules don’t look like normal molecules—they each have two electrons and two of their antimatter counterparts, positrons, that swirl around each other in a quantum mechanical dance.

”It’s basically an experimental tour de force, an enormous technical achievement,” says Mike Charlton, the head of the physics department at Swansea University, in Wales, an antimatter expert who was not involved in the research.

Most matter-antimatter interactions lead to immediate annihilation, but on a rare occasion one electron and one positron will combine to form a positronium atom.

Positronium atoms can happily bounce around for a while, but they’re still an unstable mix ready for self-destruction. The key to forming a molecule, says David Cassidy, lead author of the report of the discovery in the 13 September issue of Nature , is getting a lot of positronium atoms in the same place at the same time.

Famed physicist John Wheeler theorized dipositronium’s existence as early as 1946, but the technology needed more than 60 years to catch up. In that time, researchers have continually improved methods of collecting and storing the positrons that some radioactive elements naturally emit. Cassidy and his coauthor, physics professor Allen Mills, pushed the latest antimatter traps to the extreme, filling them to capacity with positrons and releasing them in a burst at a target of porous silica film.

At impact, most of the positrons annihilate as they collide with electrons or form unstable atoms. The positrons that survive are those paired with electrons of the same quantum mechanical state, called spin, a relatively stable configuration. Having lost much of their energy, they slowly diffuse throughout the sample. If two positronium atoms with opposite spin meet at the surface of one of the silica’s vacuous pores, they can combine to form dipositronium.

But there’s no honeymoon following that marriage; the molecules typically annihilate in a flash of gamma rays within a quarter of a nanosecond. Cassidy and Mills identified their existence only through their signature demise, which occurs after the initial burst of destruction but before the remaining positronium atoms meet their end.

The dipositronium molecules may not last long, but actually that’s exactly what Cassidy wants, because their destruction leaves behind spin-polarized positronium atoms that can be cooled to form a Bose-Einstein condensate—what Cassidy describes as a superatom, where all the atoms share the same quantum mechanical state.

Although Cassidy emphasizes that such work is far in the future, annihilating a positronium Bose-Einstein condensate would produce coherent gamma rays, the first step toward a laser beam at least 10 times as powerful as anything available today.

The task of actually making a gamma-ray laser is both ”far-off” and difficult, according to Clifford Surko, a physics professor at the University of California at San Diego.

Even a gamma-ray laser wouldn’t match the antimatter miracles of science fiction, but it’s a step in that direction.

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