Physics Nobelists. (Left to right) Ray Davis, Masatoshi Koshiba, and Riccardo Giacconi.

Physics Nobelists Expanded View of the Cosmos

Three physicists have won the ultimate scientific accolades for giving humanity new eyes. Half of this year's Nobel Prize in physics goes to Ray Davis of the University of Pennsylvania in Philadelphia and Masatoshi Koshiba of the University of Tokyo for using neutrinos to illuminate the cosmos. The other half goes to Riccardo Giacconi of Associated Universities Inc. in Washington, D.C., for a 4-decade-long effort to view the universe with x-ray spectacles.

Davis and Koshiba dedicated their careers to hunting neutrinos, nearly massless elementary particles. Even though Earth is flooded with neutrinos emanating from the sun's nuclear furnace, neutrinos seldom interact with matter, making them very hard to detect. By the late 1950s, Davis realized that he could use a rare reaction--a neutrino striking a chlorine atom, turning one of its neutrons into a proton and creating argon--to detect neutrinos generated by the decay of boron-8 in the sun. Surprisingly, Davis's experiments, carried out at the Homestake Mine in South Dakota, showed a deficit: The detector was picking up only about a third of the neutrinos expected from the nuclear reactions in the sun. This "solar neutrino problem" became an enduring problem in physics--and one of the first signs that neutrinos have mass.

Koshiba and his colleagues made the next step. In 1982, they built a detector, Kamiokande, that used a big tank of water instead of chlorine to detect the capture of a neutrino. Buried deep in a mine near Kamioka, Japan, the Kamiokande detector and its successors not only confirmed the solar neutrino paradox but also spotted neutrinos coming from the explosion of a star 170,000 light-years away.

Giacconi, on the other hand, worked with light, but a particularly troublesome type. Unlike visible light, x-rays zoom right through mirrors and are absorbed by the atmosphere. This made x-ray astronomy all but impossible until scientists could propel compact detectors high above Earth. In 1960, Giacconi and Bruno Rossi of the Massachusetts Institute of Technology in Cambridge figured out a clever way to focus x-rays onto a detector by skipping them along a surface. In 1962, Giacconi's detector aboard an Aerobee rocket made the first observation of x-rays from the sun. Since then, Giacconi has taken part in most of the major advances in x-ray astronomy, which have given astronomers crucial information about black holes, star formation, galactic nuclei, and other energetic events and objects. "X-rays give you the key to phenomena of cosmic evolution," says Giacconi. "I was lucky enough to get involved right at the beginning."

Related sites
Nobel Foundation site
Neutrino basics
Background on x-ray astronomy from NASA