Fountain of Annihilation
WILLIAM PURCELL'S GOAL WAS modest, if any research on the mysteries of deep space can be called modest. Back in 1974 astronomers had discovered that the center of our Milky Way galaxy teems with antimatter, the stuff of science fiction. A few years later instruments on satellites and high-altitude balloons found that the antimatter seemed to appear and disappear: the high-energy gamma rays that signal the presence of antimatter were growing fainter and brighter, as if God were fooling around with the galaxy's dimmer switch. Purcell, an astrophysicist at Northwestern University, intended to study this dimming with NASA's Compton Gamma Ray Observatory, which the space shuttle dropped off in a 270-mile-high orbit in April 1991. But GRO failed to detect any dimming and brightening; it remains a mystery. So Purcell and his colleagues switched gears: they decided to instead pinpoint where the antimatter is. Last winter they got the surprise of their careers. In November and December, the scientists reported last week, GRO discovered that a huge cloud of antimatter jets up from some place toward the center of our Milky Way galaxy. Reaching some 20,000 trillion miles high, the antimatter gusher "was completely unexpected," says Purcell. "And it suggests that there may be other unexpected clouds of [antimatter] in the galaxy."
For all its parallel-universe connotations, antimatter is a frequent visitor to physics labs. In 1928 theorist Paul Dirac proved that every fundamental particle (electrons, protons and the like) has a Through the Looking Glass partner. These "antiparticles" have exactly the same properties, including mass, as the regular particles, but with opposite charges. The antimatter partner of the negatively charged electron is the positively charged positron. In 1932 the positron was discovered in a device that captured cosmic rays from space; vapor in the "cloud chamber" records the characteristic track of any particle that enters. Today physicists regularly create antimatter in powerful particle accelerators, a.k.a. atom smashers.
Can nature do as much? In 1974 researchers led by astronomer Robert Haymes of Rice University discovered positrons toward the galactic center. (The Milky Way is shaped like a fried egg; the positrons seemed to come from the bulging yolk.) Actually, the scientists did not detect positrons themselves but the antiparticles' death throes. When an electron meets up with a positron, the two annihilate instantly in a burst of pure energy that usually takes the form of two invisible gamma rays, each with an energy of 511,000 electron volts, or 250,000 times the energy of visible light. It was these gamma rays that GRO, too, detected. The characteristic energy left no doubt that the gamma rays were produced by positrons annihilating electrons.
A little antimatter loafing around the galaxy was no surprise. A veritable Old Faithful of antimatter was. What could account for it? One candidate is the black hole that slurps up matter at the center of the galaxy. When matter falls in, it acquires so much energy that it eventually becomes hot enough to create electron-positron pairs. Contrary to the notion that nothing can escape the gravitational embrace of the black hole, these particles sometimes do. Other suspects are massive new stars forming in the galactic center. These newborns, says astrophysicist James Kurfess of the U.S. Naval Research Laboratory, quickly "pop off as supernovae." The stellar shrapnel contains radioactive cobalt and aluminum. The radioactive decay of these atoms produces positrons. Finally, merging neutron stars would produce "a fireball with tremendous energy," says Kurfess. High energy means gamma rays. A gamma ray can "break up" into a positron and an electron. Trouble is, no one can figure why black-hole emissions, supernovae shrapnel or neutron star jets should shoot out in only one direction--up.
An even bigger puzzle is where, exactly, the antimatter geyser is. Despite newspaper accounts that the antimatter is gushing from the center of the Milky Way, a comfortable 147,000 trillion miles distant, in fact "it could be anywhere between there and here," says Rice's Haymes. Scientists assume it's at the galactic center because that's where the exploding stars and black holes are. In that case, the antimatter fountain is intrinsically bright but far away. But an equally plausible possibility is that the antimatter is intrinsically dim but close. "There could be many more, closer clouds of antimatter," says Purcell, "not 25,000 light-years away but just a few thousand." That's still not close enough to worry about antimatter annihilating the solar system, but it suggests that the encounters with antimatter so common in science fiction just might become science fact.
ANTIMATTER MATTERS Aiming the orbiting Compton Gamma Ray Observatory at the center of our Milky Way galaxy, astronomers discovered a geyser of antimatter 20,000 trillion miles high. It's just the latest antimatter milestone.
1928: Theoretical physicist Paul Dirac predicts the existence of antimatter
1932: Carl Anderson detects an antiparticle in a lab apparatus called "cloud chamber."
1974: Robert Haymes discovers antimatter toward the center of the Milky Way.
1997: Huge fountain of antiparticles discovered toward galactic center