BAHCALL, JOHN NORRIS

(b. Shreveport, Louisiana, 30 December 1934; d. New York, New York, 17 August 2005),

astrophysics, solar physics, x-ray astronomy, gravitational lensing, active galaxies, solar neutrino problem, weak interaction physics, observations from space, science policy.

An American astrophysicist, Bahcall was both a hedgehog and a fox (in Isaiah Berlin’s sense). Best known for more than forty years of engagement with what became “the solar neutrino problem,” he also made important contributions to the understanding of the structure of the Milky Way galaxy, the significance of absorption lines in the optical spectra of quasars, and astronomical constraints on fundamental physics, and to science policy issues, including the championing of the Hubble Space Telescope (HST) and setting of priorities for the 1990s among major ground- and space-based astronomical initiatives.

Background and Family. Bahcall was the son of Mildred (who held degrees in music and social work) and Malcolm (a successful salesman, born a few months after his parents arrived in the United States from Russia) Bahcall. The family name was originally Bachalor. Brother Robert Bahcall followed his father into sales, while John Bahcall, after high school years devoted largely to tennis and debate, entered Louisiana State University thinking of a degree in philosophy and, perhaps, a career in the reform rabbinate. Summer courses at the University of California, Berkeley, and financial support from a cousin, led to his remaining there, where the fulfillment of a science requirement with a standard introductory physics course led to a switch in direction and degrees in physics (AB 1956, UC Berkeley; MS 1957, University of Chicago; PhD 1961, Harvard). The PhD dissertation, directed by David Layzer, concerned the calculation of energy levels of highly ionized atoms for comparison with recent measurements by Bengt Edlén.

During a research fellowship at Indiana University, Bahcall studied theory of the weak interaction (responsible, for instance, for neutron decays) under Emil Konopinski and calculated rates of nuclear decay under conditions different from laboratory ones. Thinking that the effects could never be measured, he was drawn to consideration of nuclear reactions in stars by a conversation with astronomer Marshall Wrubel and by a classic paper on formation of the elements in stars by E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler, and Fred Hoyle (1957, known to astrophysicists as B² FH). Bahcall published a short paper pointing out that certain nuclear reaction rates in stars would be quite different from laboratory ones, which led Fowler to invite him to the California Institute of Technology in 1962 to work on related problems in physics and astrophysics.

Bahcall rose from research fellow to assistant to associate professor of physics at Caltech, but meanwhile had also spent parts of 1968 to 1970 at the Institute for Advanced Study, where he became professor of natural sciences (1971–2000) and the Richard Black Professor of Natural Sciences (1997 until his death), simultaneously holding a visiting position at Princeton University.

In 1965, Bahcall gave lectures on nuclear astrophysics in Israel at the invitation of Yuval Ne’eman (coinventor of a standard model of elementary particle physics). There he met nuclear physics student Neta Assaf (MS 1965, Weiz-mann Institute; PhD 1970, Tel Aviv University). After an extended, largely long-distance courtship, Neta was persuaded to come to California, and they were married in September 1966. Their three children, Safi, Dan, and Orli, all earned PhDs in technical subjects.

Neta and John Bahcall appear as coauthors on more than twenty papers, including studies of quasars, x-ray sources, and globular clusters, as well as some of the early solar neutrino papers, though their interests later diverged: she focusing on large-scale structure in the universe and he on the topics in the following sections. Safi is also a Bahcall coauthor on one paper.

Solar Neutrinos. Raymond Davis Jr., at Brookhaven National Laboratory, had begun development of a radio-chemical detector for reactor neutrinos in the 1950s, based on ideas from Luis Alvarez and the Italian-American-Russian physicist Bruno Pontecorvo. The key reaction is one in which chlorine-37 captures a neutrino, producing argon-37 and an electron. The³⁷ Ar has a half-life near thirty-five days for decaying back to³⁷ C1, during which time the argon atoms can be swept out of the C₂ C1₄ (perchloroethylene) detector liquid and counted by standard radiochemical techniques. Extension to the solar neutrino regime would require a much larger detector than Davis’s original 1,000 gallons and also a location in a deep mine to provide shielding from cosmic-ray noise. William Fowler, who had been interested in the solar case from the beginning, brought Bahcall into the program in 1962, because of the differences in reaction rates in stars versus in the lab that Bahcall had begun to calculate. Detection of neutrinos from the Sun was, and is, important because the light comes from the surface and has scattered its way out over about 100,000 years since it was made by central nuclear reactions, while the neutrinos stream straight out—apart, it turns out, from rotating from electron neutrinos to one or both of the other species, muon and tau neutrinos.

Instrumentation and predictions developed in parallel. A critical pair of papers in 1964 from Davis and Bah-call made clear that the expected neutrino flux was, just barely, within experimental grasp and that the largest uncertainties appeared to be in rates for certain nuclear reactions, both for production in the Sun and for capture by chlorine nuclei. Construction of the 100,000-gallon detector tank, in the Homestake Mine in Lead, South Dakota, began in 1966 and data collection in 1968. The 1968 prediction was 7.5 ± 3 SNU, where SNU, or solar neutrino unit, means 10^-36 captures per chlorine-37 atom per second. But the data were setting, at first, only upper limits around 3 SNU and, eventually, a measured value of

2.6 SNU—about one-third of expectations.

At this point, Bahcall the hedgehog came into his own. Sometimes alone, sometimes with collaborators, he continued to refine the physics in the calculations of expected capture rate. The result never varied outside the 7.5 ± 3 SNU range of 1968, though the error bars shrank with better numbers for nuclear reaction rates, solar composition, and other contributing physics. About twenty possible explanations for the discrepancy were published between 1969 and 1977. A few said Davis was somehow not getting all the argon-37 atoms out of his tank (only about 1.5 per day were expected). Most proposed models of the Sun that were different from Bahcall’s standard one. And Bruno Pontecorvo put forward the idea of oscillations between the one sort of neutrino (electron) the Davis detector could see and another species (the mu neutrino) as early as 1969.

But it took more than thirty years and a succession of additional experiments to show this was the right answer. Davis’s experiment was working just fine, the nuclear numbers were OK, and Bahcall’s calculation was right. But weak-interaction physics was different from the standard model (in ways that were still being incorporated into early twenty-first-century theories of elementary particle physics). Over the years, additional detectors were built to look at the Sun, especially ones using gallium-71, which can capture the low-energy solar neutrinos from the main hydrogen-fusion process. These too were deficient.

A Japanese group, headed by Masatoshi Koshiba, built a succession of ever-better experiments at Kamioka, originally intended to look for proton decay. These confirmed the deficiency of high-energy solar neutrinos and, later, showed that neutrinos produced by cosmic rays hitting the Earth’s upper atmosphere oscillate between species (mostly mu neutrinos to the third kind, the tau neutrino). In the 1990s, the Sudbury Neutrino Observatory (SNO) in Canada, using heavy (deuterated) water as the detector, began also seeing fewer electron neutrinos than were expected, and, finally, early in the twenty-first century confirmed that the missing ones were arriving from the Sun as mu neutrinos.

Bahcall continued to improve the calculations throughout these decades, and his last papers dealt with reconciling the solar neutrino flux (at last well understood!) with the best numbers for solar composition and measurements of the deep interior structure that come from the frequencies of subtle quivering of the gaseous body of the Sun (helioseismology).

Other Scientific Contributions. Quasars were discovered in 1963, soon after Bahcall arrived at Caltech, by astronomy faculty member Maarten Schmidt. Debate quickly arose between a majority of astronomers (including Schmidt) who concluded that these bright, compact sources of radio and visible radiation were at the large distances implied by their redshifts and Hubble’s law, and the sources therefore very bright, and a minority who thought that the sources were nearby and the redshifts due to some new physical principle. In 1965, Bahcall and Edwin E. Salpeter (a nuclear physicist at Cornell who had suggested a model for quasars involving black holes the previous year) pointed out that distant quasars would surely have gas clouds between Earth and them, which would absorb some of the light in a recognizable pattern. Such absorption lines turned up about two years later in many observations (some with Bahcall as a collaborator), and Bahcall was in the forefront of developing the mathematical tools to demonstrate that many (not all) of the absorption clouds were in deep intergalactic space, and the sources themselves therefore as distant as their redshifts indicate.

Bahcall continued to work on aspects of quasar astronomy right through to the Hubble Space Telescope era, heading the key project team working on absorption lines in QSOs (the radio-quiet equivalent of quasars). Less happily, Bahcall attempted HST imaging of the galaxies of which QSOs are the bright centers. He set worrisomely small upper limits to how big and how bright some of these galaxies could be, but other groups soon found the expected host galaxies.

In 1972, the Bahcalls were in Israel, with access to a one-meter telescope at Wise Observatory. X-ray astronomy was an exciting, relatively new field, and the Uhuru satellite had just produced a better position for the bright source called Hercules X-1. Stellar astronomer William Liller, looking at catalogued objects in the right part of the sky, had suggested that the known variable star HZ Herculis and Her X-1 might be the same object. The Bahcalls were able to confirm this by showing that the light varies with the same period and phase as the x-rays, because the source consists of a normal star orbiting and periodically eclipsing a neutron star. In 1975, Bahcall and Jeremiah P. Ostriker (then at Princeton University) were among the first to suggest that some other x-ray sources, those in old globular clusters of stars, might not be binary systems but single black holes of 100–10,000 solar masses accreting gas from the clusters. This issue was again under consideration thirty years later.

In the lead-up to the launch of what became the Hubble Space Telescope, Bahcall was concerned both about aspects of the operations process, particularly the availability of sufficient guide stars to keep the telescope correctly aimed, and about using resulting data to learn about the various populations of stars in the Milky Way. With postdoc Raymond Soneira, he showed that guide stars would sometimes be in short supply, and the planned

operation procedure was modified accordingly. Their model of galactic structure was subsequently widely used in dark-matter searches and other studies. Information on the numbers and stability of binary stars with very wide separations was one of many byproducts of the Bahcall group’s observations and calculations.

Both solar neutrino and quasar data can be used to address problems in fundamental physics. Among those considered by Bahcall were possible changes with time in the strength of the electromagnetic force (below detectability in 1967 and still so in a 2004 paper), limits on the extent to which electric charge might not be conserved (a 1996 collaboration with Maurice Goldhaber, who had been director of Brookhaven National Laboratory when Davis and Bahcall first started working together on solar-neutrino detection), and tests of the combined conservation of charge, parity, and time reversal (again, only limits—very tight ones).

The cooling of neutron stars, the gravitational lensing of quasar images by galaxies between us and them, the structure of globular clusters, limits on dark matter in the disk of the Milky Way, neutrinos from supernovae, and sources of the highest-energy cosmic rays and their implications for neutrino astrophysics of the future were among the other topics on which Bahcall the fox said something important.

Public Policy Issues. Bahcall was the first astrophysicist on the staff of the Institute for Advanced Study, and it was expected that he would build up a group there. This happened, and it flourished, with a few additional permanent staff members and some interaction with Princeton University and its students, but largely on the basis of outstanding postdoctoral researchers, who arrived, were often mentored by Bahcall, and then went on to permanent positions elsewhere. He was also instrumental in building the astrophysics groups at the Weizmann Institute and Tel Aviv University in Israel.

The idea of a large optical telescope in space, having originally been championed by Lyman Spitzer, was in the air at Princeton in the 1970s. In addition, in 1973, when Robert O’Dell (of Marshall Space Flight Center, Huntsville, Alabama) and Nancy Roman (then at NASA headquarters) began canvassing the astronomical community for ideas and support for a Large Space Telescope, Bahcall responded with both enthusiasm for what might be done and an offer to help. By the end of 1973, he was part of the working group. In the second phase of development, he became head of the photometry team and then a proponent of the idea of taking many short-duration exposures that could be used for serendipitous searches and perhaps education. This eventually happened.

Over the next few years, the Space Telescope was several times in trouble and at risk of being removed completely from the NASA program. In response, Bahcall, together with Spitzer and a few others, orchestrated one of the first and most successful campaigns of congressional visits, letters to members of Congress from the community, and lobbying by potential contractors for the telescope.

By 1977, when a NASA advisory panel helped select the first instrument package and associated science working group, it was inevitable that Bahcall would be part of it. He championed charge-coupled devices as light detectors and the retention of spectroscopic capability when cost-saving efforts threatened these. When HST finally flew, though the effort to bring its science center to Princeton had failed (in favor of a proposal from Johns Hopkins University), Bahcall became head of the key-project team studying quasar absorption lines, which produced something like fifteen papers over the next decade. Neta Bahcall commuted by train between Princeton and Baltimore to serve as head of the science program selection office and chief of the general observer support branch at Space Telescope Science Institute for about six years under director Riccardo Giacconi.

Since 1960, the astronomical community has produced decadal reports recording past progress and priori-tizing its equipment needs for the next ten years. The published products have inevitably been named for the scientists who chaired the panels: the (Albert) Whitford Report for the 1960s, the (Jesse) Greenstein report for the 1970s (which did not really advocate an optical telescope in space), and the (George) Field report for the 1980s. In about 1988, geophysicist Frank Press, then president of the U.S. National Academy of Sciences, suggested that the next one should be the Bahcall Report for the 1990s. With great interest but some misgivings, Bahcall took this on.

The process involved a larger number of astronomers than had ever been directly concerned before (or since), with fifteen disciplinary panels of twenty members each, as well as the main panel of fifteen chaired directly by Bahcall. There were also numerous town meetings and presentations to groups of astronomers. The final recommendations prioritized ground- and space-based initiatives in a single list, and most of the highest-ranked programs had been accomplished or were underway by the time the next panel reported, including U.S.-owned 8-meter telescopes in Hawaii and Chile (operational from the late 1990s), the Space Infrared Telescope Facility (launched in August 2003 and then renamed the Spitzer Space Telescope), and a large array for millimeter radio astronomy, construction of which began in the Atacama Desert of Chile in 2005. A characteristic moment in Bah-call chairing had him responding to two panel members who had disagreed at length, and somewhat obscurely, with “I agree with you both; now let’s go on.”

Prizes and Recognition. Over the years, Bahcall received the Warner Prize (for young astronomers, in 1970), the Heineman (a mid-career award, in 1995, the same year Davis won the Tinsley prize), and the Russell Lecturership (a lifetime award, in 1999) from the American Astonomical Society. Additional major recognition included membership in the U.S. National Academy of Sciences and the Academia Europaea, the Hans Bethe Prize of the American Physical Society, the U.S. National Medal of Science (1998), the Benjamin Franklin Medal and the Fermi Award of the Department of Energy (both shared with Davis in 2003), the million-dollar Dan David Prize (2003) from Israel, and the Comstock Prize of the U.S. National Academy of Sciences (2004). When the 2002 Nobel Prize in Physics was announced as going half to Raymond Davis and Masatoshi Koshiba for the development of neutrino astrophysics and half to Riccardo Giacconi (for contributions to the establishment of x-ray astronomy), there was considerable feeling in the physics community that it should have been a three-way award, entirely in neutrino astrophysics, with Bahcall as the third winner. His public stance was that he was proud to have been considered.

BIBLIOGRAPHY

WORKS BY BAHCALL

“Beta Decay in Stellar Interiors.” Physical Review126, no. 3 (1962): 1143–1149.

“Solar Neutrinos. 1. Theoretical.” Physical Review Letters12, no. 11 (1964): 300–302.

With R. A. Wolf. “An Observational Test of Theories of Neutron-Star Cooling.” Astrophysical Journal142, no. 3 (1965): 1254–1256.

With Edwin E. Salpeter. “On the Interaction of Radiation from Distant Sources with the Intervening Medium.” Astrophysical Journal142, no. 4 (1965): 1677–1681.

With Maarten Schmidt. “Does the Fine-Structure Constant Vary with Cosmic Time?” Physical Review Letters19, no. 22 (1967): 1294–1295.

With Giora Shaviv. “Solar Models and Neutrino Fluxes.” Astrophysical Journal153 (1968): 113–125.

“A Systematic Method for Identifying Absorption Lines as Applied to PKS 0237–23” Astrophysical Journal153 (1968): 679–688.

With P. James E. Peebles. “Statistical Tests for the Origin of Absorption Lines Observed in Quasi-Stellar Sources.”

Astrophysical Journal156 (1969): L7–L10.

With Ron D. Ekers. “On the Possibility of Detecting Redshifted 21-cm Absorption Lines in the Spectra of Quasi-Stellar Sources.” Astrophysical Journal157 (1969): 1055–1064.

With Maarten Schmidt and James E. Gunn. “Are Some Quasi-Stellar Objects Associated with Clusters of Galaxies?”Astrophysical Journal157 (1969): L77–L79.

With Neta Bahcall and Geoffrey Burbidge. “Relative Correlation of Large- and Small-Redshift Quasi-Stellar Objects with Clusters of Galaxies.” Astrophysical Journal166 (1971): L77–L80.

With Neta Bahcall. “The Period and Light Curve of HZ Herculis.” Astrophysical Journal178, no. 1 (1972): L1–L4.

With Richard Hills. “The Hubble Diagram for the Brightest Quasars.” Astrophysical Journal179, no. 3 (1973): 699–703.

With Paul C. Joss and Roger Lynds. “On the Temperature of the Microwave Background Radiation at a Large Redshift.”Astrophysical Journal182, no. 3 (1973): L95–L98.

With Jeremiah Ostriker. “Massive Black Holes in Globular Clusters?” Nature 256, no. 5512 (1975): 23–24.

With Henry Primakoff. “Neutrino-Antineutrino Oscillations.” Physical Review D18, no. 9 (1978): 3463–3466.

With Raymond Soneira. “The Universe at Faint Magnitudes. I. Models for the Galaxy and the Predicted Star Counts.”

Astrophysical Journal Supplement Series44, no. 1 (1980): 73–110.

With Raymond Soneira. “The Distribution of Stars to V=16^th Magnitude near the North Galactic Pole: Normalization, Clustering Properties, and Counts in Various Bands.” Astrophysical Journal 246 (1981): 122–135.

With Raymond Soneira. “Predicted Star Counts in Selected Fields and Photometric Bands: Applications to Galactic Structure, the Disk Luminosity Function, and the Detection of a Massive Halo.” Astrophysical Journal Supplement Series47, no. 1 (1981): 357–401.

Oral History Program Interviews conducted by P. Hanle, primarily about Hubble Space Telescope. 3 January 1983, 20 December 1983, 22 March 1984, 29 March 1984. National Air and Space Museum, Washington DC.

With Piet Hut and Scott Tremaine. “Maximum Mass of the Objects that Constitute the Unseen Disk Material.” Astrophysical Journal290 (1985): 15–20.

With Neta Bahcall and Donald P. Schneider. “Multiple Quasars for Multiple Images.” Nature323, no. 6088 (1986): 515–516.

With Sheldon Glashow. “Upper Limit on the Mass of the Electron Neutrino.” Nature326 (1987): 476–477.

With Roger Ulrich. “Solar Models, Neutrino Experiments, and Helioseismology.” Reviews of Modern Physics60, no. 2 (1988): 297–372.

With Buell Jannuzi, Donald P. Schneider, et al. “The Ultraviolet Absorption Spectrum of 3C 273.” Astrophysical Journal377 (1991): L5–L8.

With Dan Maoz, Roger Doxsey, et al. “The Snapshot Survey: A Search for Gravitationally Lensed Quasars with the Hubble Space Telescope 1.” Astrophysical Journal387 (1992): 56–68.

Interview conducted by Robert Smith, primarily about the Decadal Survey. 26 August 1992.

With Jacqueline Bergeron, Alec Boksenberg, et al. “The HST Quasar Absorption Line Key Project I: First Observational Results, Including Lyman-(and Lyman-Limit Systems.” Astrophysical Journal Supplement Series87 (1993): 1–43.

With Sofia Kirhakos and Donald P. Schneider. “HST Images of Nearby Luminous Quasars.” HST Special issue, Astrophysical Journal Letters 435, no. 1, pt. 2 (1 November 1994): L11–L14.

With Sofia Kirhakos and Donald P. Schneider. “HST Images of Twenty Nearby Luminous Quasars.” In Quasar Hosts: Proceedings of the ESO-IAC Conference Held on Tenerife, Spain, 24–27 September 1996. Heidelberg: Springer Verlag, 1997.

With Eli Waxman. “Ultrahigh Energy Cosmic Rays May Come from Clustered Sources.” Astrophysical Journal542 (2000): 542–547.

With Peter Mészáros. “5–10 GeV Neutrinos from Gamma-Ray Burst Fireballs?” Physical Review Letters85 (2000): 1362–1365.

With Vernon Barger and Danny Marfatia. “How Accurately Can One Test CPT Conservation with Reactor and Solar Neutrino Experiments?” Physics Letters B534, nos. 1–4 (2002): 120–123. Available from http://xxx.arxiv.org/abs/hep-ph?papernum=0201211.

With Eli Waxman. “Has the GZK Suppression Been Discovered?” Physics Letters B556, nos. 1–2 (2003): 1–6. Available from http://xxx.arxiv.org/abs/hepph?papernum=0206217.

With Charles L. Steinhardt and David Schlegel. “Does the Fine-Structure Constant Vary with Cosmological Epoch?” Astrophysical Journal600 (2004): 520.

With Hitoshi Murayama and Carlos Pena-Garay. “What Can We Learn from Neutrinoless Double Beta Decay Experiments?” Physical Review D70 (2004): art. no. 033012. Available from http://xxx.arxiv.org/ abs/hepph?papernum=0403167.

With Aldo M. Serenelli and Sarbani Basu. “New Solar Opacities, Abundances, Helioseismology, and Neutrino Fluxes.” Astrophysical Journal621 (2005): L85. Available from http://xxx.arxiv.org abs/astro-ph?papernum=0412441.

With Eric B. Norman. “Improved Limit on Charge Conservation Derived from⁷¹ Ga Solar Neutrino Experiments.” Physical Review D53, no. 7 (1996): 4086–4088.

With Ray J. Weymann, Buell Jannuzi, et al. “The Hubble Space Telescope Quasar Absorption Line Key Project. XIV. The Evolution of Ly(Absorption Lines in the Redshift Interval z= 0–1.5.” Astrophysical Journal506 (1998): 1–18.

OTHER SOURCES

Burbidge, E. Margaret, Geoffrey R. Burbidge, William A. Fowler, and Fred Hoyle. “Synthesis of the Elements in Stars.” Reviews of Modern Physics 29 (1957): 547–650. Known to astrophysicists as B² FH.

Hargittai, Magdolna, and Hargittai, Istvan. “Interview with John Bahcall.” Candid Science IV: Conversations with Famous Physicists. London: Imperial College Press, 2004.

Ostriker, Jeremiah. “John Norris Bahcall 1935–2005.” Nature 437 (1 September 2005): 43.

Virginia Trimble