Weighing The Universe

By MICHAEL D. LEMONICK

Physicists love nothing more than announcing a discovery that rewrites the textbooks--unless, perhaps, it's a discovery that rewrites two textbooks at once. That's evidently what happened last week at a scientific conference in Japan. An international team of 120 physicists reported that the neutrino, a subatomic particle long thought to be utterly without mass, actually weighs in at a tiny fraction of the mass of the electron (until now, the lightest particle known). For elementary-particle physicists, that means their most basic theories will have to be rewritten; for astronomers, it means that the missing "dark matter" believed to pervade the cosmos and far outweigh the visible stars may no longer be missing.

Evidence that neutrinos have mass has been reported before, but the measurements were so marginal that they left more room for doubt than confidence. Not this time. The Super-Kamiokande neutrino observatory, a stainless-steel chamber filled with 12.5 million gal. of water, lined with sensitive light detectors and located deep underground in an old zinc mine near the city of Takayama, is among the most sensitive instruments of its kind in the world. The physicists who use it are widely recognized as extremely careful experimenters. And, says University of Hawaii physicist John Learned, there wasn't much doubt about what they saw. "It was," he says, "something like a big fish slapping you on the face."

Even so, the detection was indirect: what the physicists actually saw was flashes of light caused by fallout from rare but occasional collisions between neutrinos and water molecules. There were fewer flashes than expected from so-called muon neutrinos, suggesting that some of them had changed into another type, called tau neutrinos. Arcane theory dictates that neutrinos can't change form unless they have mass--though scientists can't say precisely what that mass is.

For particle physicists, the answer hardly matters; the mere fact of neutrino mass will force a rethinking of the "standard model"--the theoretical framework of all subatomic physics. For cosmology, there could be even more tangible consequences. The universe is teeming with neutrinos--so many that trillions of them pass through every human being on Earth every second (fortunately, without doing any damage). Even a minuscule mass could give them enough combined gravity to slow the expansion of the universe that followed the Big Bang. They might conceivably even reverse it, leading to a cosmic catastrophe aptly named the Big Crunch. No need to panic, though; it won't happen for tens of billions of years.