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Missing: Half a universe. Last seen: 13.7 billion years ago

By Bob Berman|Monday, October 24, 2005
RELATED TAGS: DARK MATTER, COSMOLOGY

In 1931 the shy, brilliant British physicist Paul Dirac predicted the existence of a weird class of particles called antimatter. When Carl Anderson of Caltech quickly proved him right, Dirac should have become a household name, but his aversion to publicity—he almost turned down the Nobel Prize—discouraged media attention. Today he is known only among serious science fans, even though antimatter lies at the heart of some of the deepest  mysteries in modern physics.

Despite its esoteric connotations, antimatter is stone-simple to understand. In most respects—mass, for instance—particles of antimatter are identical to those of matter. The primary difference is that their electric charges are reversed. An antimatter nucleus is negative instead of positive, and it is orbited by positrons, electrons that are positive instead of negative.

In fact, the two are so much alike that nobody can explain why matter so thoroughly dominates its near twin. Nearly every model of the Big Bang says that the explosive beginning of our universe 13.7 billion years ago should have created antimatter and matter in equal amounts, and yet we see no sign of antistuff except, fleetingly, in particle accelerators.  So where did all the antimatter go?

There is no way to hide large amounts of antimatter on Earth. When antimatter touches matter, both vanish in a flash of violent energy. Dirac had suggested that masses of antimatter might instead be tucked away in distant corners of the universe. At the time it seemed plausible: A galaxy made of antimatter would be indistinguishable from a normal one. Not even spectroscopic analysis could tell them apart.

These days, astronomers are quite sure that antimatter is rare in the depths of space too. Contact between electrons and positrons produces gamma rays characteristically having an energy of 511,000 electron volts. If antimatter galaxies existed, they would interact with ordinary particles floating through intergalactic space to produce halos of gamma-ray energy around galaxies. Researchers have looked and found no such halos. We live in a matter-oriented universe.

Currently the best explanation for the imbalance is that, contrary to long-held theory, the laws of physics are slightly rigged in favor of matter. That notion got a boost last year when a team at the Stanford Linear Accelerator Center observed a small but distinct difference in the behavior of certain matter and antimatter particles. This result may hint at a pro-matter bias built into the laws of physics.

A good thing too. A universe containing lots of antimatter would be a dangerous place. When matter and antimatter meet, the result is a 100 percent efficient E = mc2 conversion of mass into energy, 143 times more efficient than an exploding H-bomb. If a one-ounce marble rolled into an equivalent antimarble, the reaction would release 50 billion trillion ergs of energy, enough to light every bulb in the United States for a day. An antimatter meteor shower would be like hellfire raining down on our planet.

Even traces of antimatter produce dramatic results. In 1997 a detector aboard the orbiting Compton Gamma Ray Observatory analyzed a geyser of positrons near the center of our galaxy. These antiparticles annihilate any electrons they meet, giving rise to a maelstrom of gamma rays that extend outward about 3,500 light-years. If you had gamma-ray vision, the nucleus of the Milky Way—low in the southwest at nightfall this month—would be a brilliant, glowing cloud.

The source of this activity does not correspond to any known object. “Some theorists believe that dark matter might be generating the antimatter,” says James Kurfess of the U.S. Naval Research Laboratory, a principal investigator on Compton. “But since the lifetime of positrons in the Milky Way is millions of years, they might also be coming from an old supernova.” Translation: Nobody has a clue.

Compton was decommissioned in 2000, but a new European gamma-ray observatory called Integral is taking up the quest. The origin of the Milky Way’s antimatter fountain is one of the millennium’s juicier enigmas. Too bad Dirac is not around to shyly enjoy it.

On Earth, antimatter can be created in particle accelerators, but only in minuscule quantities. Current annual global production is one-hundredth of a microgram; its value translates to about a quadrillion dollars per ounce. The main stored supply of antimatter consists of antihydrogen atoms, first synthesized in 1995.


The sky this month
October 1: Saturn returns, rising in the east-northeast around 2 a.m. at the beginning of the month and two hours earlier by month’s end. Its rings are tilted increasingly edgewise.

October 16: Dazzling Venus passes near the first-magnitude star Antares fairly low in the western twilight at dusk. The whitish planet appears 130 times brighter than ruddy Antares.

October 29: Mars passes 43.1 million miles from Earth, its closest approach until 2018. It rises in the east by 7 p.m. and outshines every star.

October 31: Mercury is an evening star, hovering within the bright western twilight, but it is extremely low.

All Month: The famed Andromeda galaxy is high in the sky at 10 p.m., a faint, fuzzy oval. Binoculars make it much easier to glimpse. The galaxy is best seen during the month’s moonless first week.

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