Sam Ting is on a mission: find the other half of the universe.
- By Andrew Lawler
- Air & Space magazine, May 2001
(Page 2 of 5)
By the mid-1950s, physicists using large particle colliders had succeeded in manufacturing an anti-
proton by smashing together two ordinary protons at fantastic speeds. Since then, giant accelerators have sprung up—or, more accurately, sprung down—in Europe, the United States, Russia, and China. In 1995, researchers at a vast underground complex called CERN (Centre Européen de Recherche Nucléaire), located on the border of Switzerland and France, opened a new door into the anti-world. By colliding anti-protons and xenon atoms, they produced anti-atoms of the most basic element, hydrogen—one anti-proton and one positron. The anti-atoms lasted only 0.00000004 second before being annihilated by ordinary matter, but they left signals that confirmed their existence.
Scientists have also seen anti-hydrogen in nature. Mimicking what happens in accelerators, cosmic ray particles crashing into particles in the atmosphere produce a secondary shower of anti-protons and positrons. But high-altitude balloons have yet to detect anti-atoms of heavier elements—which could only be forged inside distant anti-matter stars by nuclear fusion, just as ordinary carbon, iron, and other elements are created in the furnaces of ordinary stars. In other words, we see no evidence of anti-stars and anti-galaxies wheeling in the sky. And that presents astrophysicists, who like symmetry, with an embarrassing question: If the birth of the universe created matter and anti-matter in equal parts, as Dirac’s equations demand, where’s the other half?
The best explanation offered to date is that our universe is all that remains of the mutual annihilation of matter and anti-matter that took place shortly after the Big Bang. The two materials duked it out until only a small amount of matter—what we today call the universe—was left standing. That means matter was granted some slight advantage. Scientists call this puzzlement the charge parity violation—“CP violation” for short.
High-energy physicists are busy investigating the theory in accelerators, while astrophysicists look for signs of anti-matter stars and galaxies. Until one or the other succeeds, says Steve Ahlen, a Boston University physicist involved in the early stages of the AMS project, the jury is still out: “No one really can demonstrate how the universe could have no anti-matter,” he says.
Enter an experimentalist like Ting, who has little patience with theorizing. “If you listen to the theorists, you would do nothing,” he says. So when the anti-matter question caught his attention in 1994, Ting ignored the warnings of colleagues and starting working on ideas that could turn up primordial anti-matter.
Though his is the most ambitious, it is not the first. As far back as the 1970s, fellow Nobelist Luis Alvarez was on the trail. More recently, two high-altitude instruments—the Balloon-borne Experiment with a Superconducting Solenoidal magnet (BESS), run by NASA and Japanese researchers, and the High-Energy Anti-matter Telescope (HEAT), sponsored by a consortium of universities, have counted about 1,000 anti-protons to date, the results of cosmic ray collisions in the atmosphere. But still no sign of heavier anti-atoms forged inside anti-stars. The CP theorists doubt they are there to be discovered, and even some experimentalists have grave reservations about finding them.
One reason is distance. No significant amount of anti-matter is believed to exist in our own supercluster of galaxies, or within about 30 million light-years of Earth. If it did, we would see enormous flashes of gamma rays from the mutual destruction of matter and anti-matter—and we don’t. Any anti-atoms created in anti-galaxies must have originated near the edge of the visible universe. In theory, those anti-particles could have crossed that vast distance to reach Earth, but most would have been trapped by the magnetic fields surrounding stars and galaxies along the way. “Even if anti-stars and anti-galaxies exist, they are so far away it would be quite hard for particles to come close enough for observation,” laments Dietrich Müller, a University of Chicago physicist and spokesperson for the HEAT project.