John Norris Bahcall

John Norris Bahcall, professor at the Institute for Advanced Study in Princeton, New Jersey, died on 17 August 2005 in New York City from a rare blood disorder. His scientific output was unflagging and wide ranging. He was a successful “scientific statesman” and a mentor to generations of younger scientists at Princeton.In a commencement address he gave in 2001 at the University of California, Berkeley, John emphasized that science was fun and unpredictable. Nothing better exemplifies its unpredictability than the most sustained and important of his achievements—his research on neutrinos, a 40-year saga in which persistence paid off handsomely, in a quite unexpected way.In the early 1960s, Raymond Davis Jr conceived an extraordinary experiment to detect neutrinos from the Sun. The most energetic of those neutrinos, produced in a reaction involving beryllium-8, can convert chlorine-37 into argon-37. Davis constructed a tank containing 10 000 gallons of cleaning fluid (C2Cl4) and developed a technique that could recover individual argon atoms from it. In back-to-back papers published in 1964, John and Davis presented, respectively, the theoretical predictions and the experiment. Davis found neutrinos, but only around one-third of the predicted number. Had John miscalculated? Or was Davis overconfident in believing he had trawled all the argon from his tank?That discrepancy was the so-called solar-neutrino problem. Davis heroically persisted with his experiment for 30 years; John updated his predictions whenever the physics or astrophysics improved (one of us, May, coauthored one such update in 1967). The factor of 3 discrepancy would not go away.In the 1990s other experiments came on line: in particular Super-Kamiokande in Japan and then, even more crucially, the Sudbury Neutrino Observatory in Ontario. The outcome vindicated both John’s calculations and Davis’s experiment. Neutrinos have mass and oscillate between different particle states: Some electron neutrinos from the Sun turned, while in flight, into tau and mu neutrinos that Davis could not detect. The SNO detector was able to detect those other neutrino species, and thereby clinched the case.The solar-neutrino problem had become the solar-neutrino opportunity—the underground neutrino detectors had achieved one of the most important advances in experimental particle physics since the 1970s. For 40 years, John was the unique solar-neutrino guru—with an overview of all the theory and the experimental details. His reaction to the SNO results was, in his own words, “like that of a man falsely accused of a crime, whose innocence was vindicated 30 years later by a DNA test.”The solar-neutrino results led to half of the 2002 Nobel Prize in Physics being shared by Davis and Masatoshi Koshiba, leader of the Japanese Kamiokande experiment. The other half of the prize went to x-ray astronomer Ricardo Giacconi. That John did not share the prize is one of the more notable hiccups in the Nobel’s history. The following year he was sole recipient of the prestigious Dan David Prize. In parallel with his neutrino work, John immersed himself deeply in a series of astronomical problems. From his doctoral thesis with David Layzer at Harvard University, he had greater expertise in both atomic and nuclear physics than most astrophysicists, and he chose topics in which that expertise gave him an advantage.One sustained interest was quasar absorption lines. In 1965 Edwin Salpeter and John noted that galaxies and gas clouds along the line of sight would leave an imprint in the spectrum of distant objects; quasar spectra should therefore display absorption lines with many different redshifts. John coauthored a classic paper with Jesse Green-stein, Maarten Schmidt, and Wallace Sargent, which reported eight different redshifts in a quasar spectrum. By applying the same analysis to a whole lot of “nonsense” spectra and showing that one would not pick out so many groups by chance, John confirmed that the redshifts were not just accidental matches of lines. He remained active in this subject, leading a key Hubble Space Telescope project on low-redshift absorption lines in the 1990s. As a byproduct of his work on the HST , John was also interested in modeling the distribution of stars in the disc and halo of our galaxy. A further interest was x-ray astronomy. The first x-ray satellite, Uhuru, revealed powerful x-ray emission from compact objects in close binary systems. The optical counterpart of one of those objects, Hercules X1, displayed large-amplitude variations because the side of the “normal” star facing the compact companion was irradiated with intense x rays that were reprocessed into visible light. The discovery of the optical variations stemmed from John’s first collaboration with his wife, Neta, who herself has gone on to a distinguished astronomy career. According to the Institute for Advanced Study, John was born in Shreveport, Louisiana, on 30 December 1934. He began his first year at Louisiana State University—supported by a tennis scholarship—convinced he wanted to study philosophy and perhaps become a rabbi. At LSU he encountered physics, and rapidly decided that it, and eventually astronomy, best suited a lifelong “quest for the truth.” He transferred to the University of California, Berkeley, where he received his AB in physics in 1956. He received an MS in physics from the University of Chicago in 1957, and a PhD in physics from Harvard University in 1961. He was a research fellow at Indiana University before joining the faculty at Caltech, where he was influenced especially by William Fowler’s nuclear astrophysics group. He was appointed to the faculty of the IAS in 1971, and it was there that he spent the rest of his career; he had the title of Richard Black Professor since 1997.John made the IAS a leading training ground for postdoctoral astrophysicists and a magnet for astrophysicists from around the world. One of us (Rees) was a postdoc during John’s first year in Princeton; Rees became, along with some of his own ex-students, a regular and appreciative visitor thereafter. John derived tremendous pleasure from building a culture and community that attracted, encouraged, and stimulated the best young scientists. Many whom he nurtured went on to successful careers and scientific leadership positions; they are part of his enduring legacy. He presided over the weekly “Tuesday lunches”—an institution that became famous far beyond Princeton—at which the latest ideas or theories were debated and visiting speakers were interrogated with Princetonian courtesy and intellectual rigor.A gifted speaker (he was the US national collegiate debating champion), John was a powerful driving force in the astronomy and scientific community. He was prominent and effective in science policy, chairing numerous committees of the National Academy of Sciences, the US National Committee of the International Astronomical Union, and the National Underground Science Laboratory. He chaired the 1990 National Academy of Sciences committee that created the decade road map for US astronomy research, which came to be known as the Bahcall Report. From 1990 to 1992 he served as president of the American Astronomical Society, and he was proud to have followed that up by being elected president of the American Physical Society—though sadly he never took office. He also helped establish the astronomy groups at the Weizmann Institute and Tel Aviv University.His greatest service was his engagement, right from the 1970s, with the HST. His advocacy in Congress and elsewhere was crucial when the entire project was in jeopardy in the 1970s. And in 2004 he chaired a committee to advise on the future of the telescope when the space shuttle’s problems put future refurbishment missions in doubt. John received many honors, including the Heinemann Prize, the Hans Bethe Prize, the Fermi Award, the Comstock Prize, and the National Medal of Science. For his role in giving us the HST , he abundantly deserved NASA’s Exceptional Achievement Award, which was, sadly, awarded posthumously. John Norris Bahcall INSTITUTE FOR ADVANCED STUDY/RANDALL HAGADORNPPT|High resolution© 2006 American Institute of Physics.

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