The 2002 Nobel Prize for Physics

The 2002 Nobel Prize for Physics recognizes work that led to the establishment of two new branches of astrophysics, those involving x rays and neutrinos. The award will be presented to Raymond Davis (University of Pennsylvania and Brookhaven Natl. Lab), Masatoshi Koshiba (University of Tokyo), and Riccardo Giacconi (Associated Universities Inc.).

In the 1960s Davis was the first to detect neutrinos coming from the sun. The number of nu's recorded fell short of predictions made by John Bahcall (Institute for Advanced Study) and thus was born the "solar neutrino problem." Later detector experiments, such as SAGE and Gallex, also failed to observe the expected number of neutrinos from the sun. The best explanation for the shortfall was that electron neutrinos made in the solar core, as products of nuclear fusion reactions, might be transforming while in flight toward Earth into other types of neutrino such as muon neutrinos, which could not be recorded in terrestrial detectors.

This hypothesis was put to the test in the Kamiokande detector, which had earlier sought to find evidence for proton decay. Koshiba and his collaborators enlarged the detector (Super-Kamiokande) and finally affirmed (by observing asymmetries in cosmic-ray-engendered nu's coming through the Earth to the detector or directly into the detector from Earth's atmosphere) that nu's were indeed transforming, or "oscillating."

Still more proof for the oscillation principle arrived this past spring when the Sudbury Neutrino Observatory (SNO), capable of directly detecting all three types of neutrino, reported that all solar nu's (albeit not the same mix as was produced in the sun) were accounted for.

Neutrinos are important in astrophysics since they might have played a considerable role in shaping or herding early galaxies; they are the form of energy coming directly from the solar core (photons scatter around inside the sun for up to a million years before escaping); and they account of the largest share of energy released during supernovas; indeed, after the 1987A supernova, a dozen or so nu's from the event were observed in terrestrial detectors.

As for x-ray astrophysics, Giacconi was the first to employ an x-ray telescope in space (1962) and observe specific x-ray sources outside our solar system. There followed decades of new orbiting x-ray telescopes (e.g., ASCA, RXTE, ROSAT, Einstein, Yokhoh, Chandra) and notable x-ray discoveries, such as the detection of an x-ray background, resolving that background mostly into point sources, and the detection of x rays from a variety of sources, such as comets, black holes, quasars, and neutron stars.

For more information, see special Physics Today website on the prize; also historic APS journal articles relating to the Prize will be posted at the APS Media website. Some other websites include Royal Swedish Academy page: SNO website: US-Kamiokande: Beamline, Winter 99 (PDF format): Chandra X-Ray Telescope; Physics News Graphics page with some Super-K graphics.

Some past Update items related to the Prize include: solar neutrino problem: ; x rays from a supernova: ; x-ray background: background pt. sources: Chandra ; quark stars: neutrino (nu) oscillation; nu mass limits; recent SNO results.)