How can you look inside the sun to see how it shines? In the mid-1960s, Ray
Davis and John Bahcall thought they had a way. Drawing on advances made by
other physicists earlier in the century, they intended to use notoriously
elusive particles called neutrinos to verify ideas about the sun's inner
workings. Theorist Bahcall calculated the number of neutrinos they expected to
find, and experimentalist Davis tried to catch them. But for more than three
decades, their results didn't jibe. In the chronology below, follow the case of
the missing neutrinos, which ultimately led not only to a triumph for Davis and
Bahcall but also to a surprising breakthrough in particle
physics.—Susan K. Lewis
1920: Theory of sunshine
astrophysicist Sir Arthur Eddington proposes that the sun generates heat and
light by "burning" hydrogen into helium. According to Eddington, every time
four hydrogen atoms fuse to become a single atom of helium at the sun's core, a
tiny bit of mass is converted into energy, just as Einstein indicated was
possible in his famous equation E = mc2.
1930: Neutrino "invented"
Austrian physicist Wolfgang Pauli conjures up the notion of a novel subatomic
particle to solve a puzzle about the apparent non-conservation of energy in
radioactive beta decays. A few years later, Italian physicist Enrico Fermi dubs
the particle, which has no electrical charge, the neutrino, or "little
neutral one." But there is no conclusive evidence that the particle exists, and
most scientists think it may be impossible to ever detect.
1939: Theory of sunshine refined
In his landmark paper "Energy Production in Stars," Hans Bethe lays out details
of how hydrogen is fused into helium in stars like the sun. His work leads to
an understanding that the fusion process releases not only energy but also the
particles Pauli "invented." Each time four hydrogen nuclei change into a helium
nucleus, two neutrinos are emitted.
1956: Neutrino detected
In an endeavor dubbed "Project Poltergeist" conducted at the Savannah River
nuclear reactor, Frederick Reines and Clyde Cowan prove that the neutrino
1964: Davis and Bahcall launch test
Ray Davis and John Bahcall propose that a study of neutrinos emitted from the
sun can show that nuclear fusion—the "burning" of hydrogen nuclei to
helium nuclei—is indeed the source of the sun's energy.
1964: Bahcall predicts number of neutrinos
Bahcall creates the first detailed mathematical model of fusion reactions in
the sun's interior. As Bahcall later notes, he has to take account of "a
smorgasbord of nuclear reactions at energies where measurements are difficult."
He draws upon Hans Bethe's work, including Bethe's estimate of the sun's core
temperature. There are countless pitfalls in devising the model. Just a one
percent error in the temperature figure alone means a 30 percent error in the
predicted number of neutrinos. And the projected number is astounding: about a
hundred billion solar neutrinos pass through your thumbnail every second,
according to Bahcall's model.
1965-1966: Davis builds experiment
in the Homestake gold mine in Lead, South Dakota, sheltered from confusing
background radiation, Ray Davis oversees construction of a giant neutrino trap:
a tank of cleaning fluid roughly as big as an Olympic-size swimming pool. The
cleaning fluid is mostly chlorine, which occasionally turns into a radioactive
isotope of argon when struck by solar neutrinos. Bahcall has calculated that
roughly 10 atoms of argon will be produced each week, and Davis is confident he
can extract and measure them.
1968: Davis's initial results
much-touted experiment appears a failure. Davis announces that he has detected
only about one third as many radioactive argon atoms as Bahcall predicted.
Scientists call the discrepancy "The Solar Neutrino Problem." The press calls
it "The Mystery of the Missing Neutrinos."
Decades of Doubt
In the two decades following their disappointing results, Davis fine-tunes his
solar neutrino detector, and Bahcall refines and checks his calculations.
Hundreds of other physicists, chemists, and astronomers also examine Bahcall
and Davis's work. No one can find significant fault with either the apparatus
or the calculations. Yet along the way there are hints of a solution to the
1969: A possible explanation
Vladimir Gribov and Bruno Pontecorvo, working in the Soviet Union, suggest that
Davis and Bahcall's missing neutrinos can be explained by "neutrino
oscillations": perhaps, as they travel to Earth, some of the neutrinos made
inside the sun oscillate, or change, into types of neutrinos that Davis's
apparatus can't detect. It's been known since mid-century that different types
of neutrinos exist. But few physicists take stock in Gribov and Pontecorvo's
idea. According to the Standard Model, the cornerstone of modern particle
physics, neutrino types are distinct and can never change one into another.
1978 and 1985: Pursuing a bold notion
Building on Gribov and Pontecorvo's radical solution, Lincoln Wolfenstein in
1978 and Stanislav Mikheyev and Alexei Smirnov in 1985 show how electron
neutrinos created at the sun's core might switch quantum states as they
interact with other matter in the sun and travel outward to the surface.
1985: More missing particles
an experiment called Kamiokande, sited in the Kamioka Mozumi mine in Japan,
Masatoshi Koshiba and colleagues detect far fewer atmospheric
neutrinos—neutrinos produced by the collision of cosmic rays with Earth's
atmosphere—than they expect to see. While atmospheric neutrinos are a
different type from those produced by the sun, the so-called "atmospheric
neutrino anomaly" is similar to the solar neutrino problem. Where are the
1998: Answer to riddle of atmospheric neutrinos
A scaled-up version of Kamiokande called Super-Kamiokande reports on more than
500 days of data collecting. The detector is so big that it can tell what
direction atmospheric neutrinos are coming from, and it has picked up far fewer
neutrinos traveling from the other side of the Earth than from the sky directly
above Japan. There is evidence that many of the atmospheric neutrinos from the
other side of the Earth have changed into a different type of neutrino during
their journey. This confirmation of neutrino oscillation carries a profound
implication: the Standard Model of particle physics must be modified.
2001-2002: Proof of solar neutrino oscillation
The Sudbury Neutrino Observatory (SNO), the first neutrino detector that can
pick up all three known types of neutrinos, resolves conclusively that, in the
case of the missing solar neutrinos, the neutrinos are not, in fact, missing.
SNO finds that the total number of neutrinos from the sun is remarkably close
to what John Bahcall predicted three decades earlier. Ray Davis's experimental
work is vindicated as well, because SNO finds that only about a third of the
solar neutrinos that reach Earth are still in the same state that Davis could
measure. Roughly two-thirds change type—or oscillate—during the
2002: Nobel Prize recognizes achievement
The Nobel Prize in Physics is awarded to Ray Davis and Masatoshi Koshiba, a
leader of the Kamiokande group. The Nobel citation praises them "for pioneering
contributions to astrophysics, in particular for the detection of cosmic
neutrinos." The award is also a tribute to their colleagues and the many
dedicated scientists whose work led to a fundamental shift in particle physics.
Wolfgang Pauli lecturing in 1929. The next
year, when he devised the notion of the neutrino, he allegedly said to a
friend, "I have done something very bad today by proposing a particle that
cannot be detected; it is something no theorist should ever do."
When this photo inside the
Homestake Mine tank was taken in 1966, Davis's mammoth neutrino trap was about
half built. From the finished tank, holding 100,000 gallons of cleaning fluid,
Davis hoped to isolate 10 atoms of argon each week.
More than 4,800 feet
underground in the Homestake Mine, Ray Davis and John Bahcall pose by the
Ray Davis continued
working on the solar neutrino experiment until well into his 80s. Here, a 1999
portrait of Dr. Davis at the age of 85.
Ray Davis's wife Anna, five grown
children, and 11 grandchildren were with him in Stockholm as he accepted the