Bahcall , John Noris

(1934–) American physicist

Bahcall was born in Shreveport, Louisiana, and educated at the universities of California, Chicago, and Harvard, where he obtained his PhD in 1961. He immediately moved to the faculty of the California Institute of Technology, and remained there until 1971 when he was appointed to the Institute of Advanced Studies, Princeton.

In the 1960s Bahcall began to consider the emission of neutrinos from the Sun. One of the apparent early triumphs of nuclear physics was the light it threw on the internal workings of the Sun. Theorists such as Hans Bethe had proposed the existence of a number of cyclic fusion reactions producing vast amounts of energy, heavier elements, and a certain number of neutrinos. As neutrinos have a low probability of interacting with other particles, some solar neutrinos should be received at the Earth's surface. Bahcall calculated that one event per second for every 10³⁶ target atoms, one solar neutrino unit (SNU), should be detectable. The matter was put to the test by Ray Davis who used a detector consisting of a tank of 100,000 gallons of cleaning fluid in a one-mile-deep mine.

Bahcall predicted that Davis would observe a flux of about 8 SNU. In fact, from 1967 onwards, Davis recorded a flux of about 2 SNU. The burden was placed upon the theorists to account for the anomalous results, or to revise the theory in ways that would make the results acceptable. This is the solar neutrino problem.

A number of options have been considered by Bahcall. Perhaps, the Sun is passing through a quiet phase and over long periods of time neutrino output will agree with theory. The issue will eventually be resolved when the abundance of radioactive technetium, produced by interactions deep in the Earth between neutrinos and molybdenum, has been accurately measured.

Or, perhaps, our solar model is wrong. This, Bahcall points out, leads nowhere. Alternative solar models agree with the standard model in the rate of neutrino production. Other theorists have challenged generally accepted physical principles. Thus, Bahcall considered the possibility that neutrinos could decay before they reach the Earth. As there seems to be nothing a neutrino can decay into, the suggestion has been dropped.

As a number of new and more sensitive detectors are being built, theorists seem inclined to await fresh data before judging between competing theories. Until then, Bahcall notes, physicists will dismiss the problem as a matter of astronomy while astronomers will attribute the anomaly to the failings of physics.