At 85, Hans Bethe helped solve the solar neutrino puzzle

Person: Hans Bethe
Lived: 1906–2005
Field: Theoretical physics
From: German-born American

The story

Neutrinos are ghostly particles that pass through almost everything. The nuclear reactions in the Sun's core, first explained by Hans Bethe in 1938 and 1939, produce enormous numbers of them. In the late 1960s the Homestake experiment of Raymond Davis Jr. and John Bahcall became the first to measure the flow of neutrinos from the Sun, and it found far fewer than the standard model of the Sun predicted. Later detectors confirmed the shortfall, finding between a half and two thirds of the expected neutrinos missing. For decades this solar neutrino problem suggested that either astronomers misunderstood the Sun or physicists misunderstood the neutrino.

In his mid-eighties Bethe, the man who had first explained how the Sun makes its energy, turned to the puzzle. At 85 he wrote an important article supporting the mechanism proposed by Stanislav Mikheyev, Alexei Smirnov and Lincoln Wolfenstein, known as the MSW effect. In this picture electron neutrinos produced in the Sun are converted into other types of neutrino as they travel out through solar matter, so detectors that see only electron neutrinos miss many of them.

Bethe argued that this required physics beyond the Standard Model of particle physics. The Standard Model assumed that neutrinos have no mass, in which case they could not change from one type into another; the MSW effect required that they do.

Bethe hoped that the Sudbury Neutrino Observatory in Ontario, Canada, would confirm the idea by his ninetieth birthday. SNO used a large quantity of heavy water, which allowed it to measure electron neutrinos separately and all types of neutrino together. The call came in June 2001, when Bethe was nearly 95. Electron neutrinos made up only about 34 per cent of the total, in line with prediction: the missing neutrinos had been found, transformed on their way to Earth.

The discovery reshaped particle physics. Davis shared the 2002 Nobel Prize in Physics for detecting solar neutrinos, and in 2015 Arthur McDonald of SNO and Takaaki Kajita of Super-Kamiokande received the Nobel Prize for evidence that neutrinos change type, which shows that they have mass.

Why it matters

The discovery that neutrinos have mass was one of the major advances in particle physics of recent decades, and Bethe helped point the way.

About Hans Bethe

Refugee physicist who explained how the stars shine, led theory at Los Alamos, cracked the Lamb shift and spent half a century campaigning against the nuclear arms race; Nobel Prize 1967.

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Hans Bethe

1906–2005 · Theoretical physics

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