GREAT BEAMS OF ANTIMATTER

By J. MADELEINE NASH

Astrophysicist William Purcell knew that if he looked at the center of the Milky Way, he would see what is known as antimatter: bizarre subatomic particles that resemble ordinary protons and electrons but carry an opposite charge. But when NASA controllers trained the orbiting Compton Gamma Ray Observatory on this core region and beamed the data back, Purcell saw something on his computer screen at Northwestern University that nobody could have predicted: a veritable colossus of antimatter, a vast fountain spewing out from the center of our galaxy and reaching trillions of miles into space.

What could have produced such a huge outpouring? That's what mystified astrophysicists meeting in Williamsburg, Va., last week. As most college freshmen know, antimatter is unstable stuff. Whenever antimatter and matter collide, they annihilate each other, disappearing in a blast of intense radiation. Thus while the Big Bang probably created almost as much antimatter as matter, virtually all of it, scientists believe, was consumed in a frenzy of annihilation long ago. In today's universe, antimatter must be created anew. And it is--in the form of subatomic particles, at least--in giant particle accelerators on earth and, in space, by one of several physical processes.

When massive stars explode as supernovas, for example, they create a periodic table's worth of radioactive elements, some of which decay into antielectrons, known as positrons. A black hole, scientists believe, can also produce electron-positron pairs by superheating the material that spirals into its gravitational sinkhole. It was the radiation produced by annihilating positrons and electrons, not the antimatter itself, that was actually observed by Purcell at Northwestern and his collaborators at the Naval Research Laboratory in Washington.

The real mystery, scientists say, is not that the positrons were created. It's that they were lobbed so many thousands of light-years above the galactic plane, like water droplets scattered by a giant geyser. Scientists offered several competing explanations last week. Rice University astrophysicist Edison Liang thinks black holes may be the key. While most of the stuff that falls into a black hole stays there, he observes, some of it gets blasted out in the form of a hot wind. Liang's hypothesis draws strength from the fact that there appear to be a good half a dozen black holes near the center of the Milky Way.

A competing theory, which Purcell favors, suggests that exploding supernovas may be the force that creates the positrons and catapults them to such great heights. There are certainly plenty of massive stars close to the Milky Way's core that are capable of generating explosions with sufficient force. The rate at which such explosions would have to occur, however, is mind-boggling: around one a century, Purcell estimates. Since supernovas have never been observed to go off at that rate in our galaxy, this theory suggests that the antimatter fountain originated in a more violent epoch in the distant past.

It's a puzzle, in other words, that could take years to solve. And that's what Purcell and others find most exciting. The Milky Way--so familiar and in many ways so humdrum--still hasn't lost its ability to surprise.

--By J. Madeleine Nash