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Case of the Missing Particles
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 3
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
First Steps: 1920-1956
1920: Theory of Sunshine
British astrophysicist Sir Arthur Eddington proposes that the Sun generates heat and light by
"burning" hydrogen into helium. According to Eddington, every time that 4 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.
1
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
that no theorist should ever do."
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 that Pauli "invented". Each time 4 hydrogen nuclei
change into a helium nucleus, 2 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 actually exists.
Grand Experiment: 1964-1968
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
John 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 1 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
Deep in the Homestake gold mine in Lead, South Dakota and 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.
2
1968: Davis's initial results
The much-touted experiment appears a failure. Davis announces that he has detected only about ⅓ 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".
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.
Decades of Doubt: 1965-1985
In the 2 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 problem:
1969: A possible explanation
Working in the Soviet Union , Physicists Vladimir Gribov and Bruno Pontecorvo 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
In 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 missing neutrinos?
3
More than 4,800 feet
underground in the Homestake
Mine, Ray Davis and John
Bahcall pose by the tank.
Mystery Solved: 1998-2002
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 3 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 3 decades earlier. Ray Davis's experimental work is vindicated as well because SNO finds
that only about ⅓ of the solar neutrinos that reach Earth are still in the same state that Davis could
measure. Roughly ⅔change type -- or oscillate -- during the journey.
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.
4
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.
Ray
Davis's
wife
Anna, 5 grown children,
and 11 grandchildren were
with him in Stockholm as
he accepted the Nobel
Prize.
http://www.pbs.org/wgbh/nova/transcripts/3306_neutrino.html
The Ghost Particle
(Program Transcript)
[Narrator]: Right now, something very strange is happening to you: a swarm of ghosts is flowing
straight through your body.
[Boris Kayser] (Fermi National Accelerator Laboratory): There's something like a hundred trillion of
them streaming through each of us, every second... every second a hundred trillion.
[Henry Sobel] (University of California, Irvine): Most of them pass through without doing anything, so
this was ghostly or poltergeist-like.
[Narrator]: The ghosts are "neutrinos" -- tiny particles that fill the Universe. Without them, the stars
would not shine and the Earth would be a dead and frozen world. They make existence
possible and contain the secret of our past.
[Boris Kayser]: Not only have they solved several mysteries, but they are also our parents.
[Narrator]: Capturing these ghosts is one of the greatest challenges scientists have ever faced.
[John Bahcall] (Institute for Advanced Study): When you think about it, it's almost unbelievable what
we were doing. And I'm glad we didn't think about it too carefully when we were doing
it.
[Narrator]: A scientific detective story on the trail of the elusive ghost particle...
5
[a child]: They're here!
[Narrator]:: ...right now on NOVA.
Google is proud to support NOVA in the search for knowledge: Google.
[Narrator]: What is the World made of? It's the most ancient scientific question. Today, scientists
believe they have discovered the recipe for matter. It's called the "Standard Model". And
it says that everything is made from just 12 basic ingredients, 12 fundamental particles.
But the Standard Model fails to explain where all these particles came from in the first
place. And so for years, scientists have searched for a clue that might explain the great
mystery of the origin of matter. Today, they believe they may have found that clue. And
it's thanks to two of America's greatest scientists.
For decades, Ray Davis and John Bahcall struggled to convince their colleagues that
they had uncovered a basic flaw in the understanding of matter. It all began 40 years ago
with a daring underground experiment. Ray Davis had tunneled deep into the Earth to
build a trap for the most elusive thing in the Universe. It was an experiment which few
thought could ever succeed.
[Boris Kayser]: He set out to do something which sounds totally impossible.
[Narrator]: And it produced a result which no one believed.
[Andrew Davis] (University of Chicago/Ray Davis's Son): Well, no, there's got to be something wrong
with that experiment. That can't be right.
[Narrator]: Everyone was convinced that the 2 scientists had made an embarrassing mistake.
[John Bahcall]: It was a personal shock, a very painful one. We learned from that, but it was a painful
shock.
[Narrator]: But Davis and Bahcall refused to give up. Today, the experiment which no one believed
has led to an astonishing discovery which is causing scientists to re-think their
fundamental theory of what the Universe is made of and where it all came from. And at
the heart of the story lies the thing which Davis and Bahcall hunted for over 40 years -- a
tiny particle called the neutrino. It is one of the 12 fundamental building blocks of
matter. And yet from the day it was born, the neutrino has been an enigma.
That day was the 4th of December, 1930. The great Austrian physicist Wolfgang
Pauli was getting ready for a party. But he found the time to write a letter -- one of the
most famous in the history of Science.
It was addressed to colleagues attending a conference on a subject that was causing
great puzzlement among physicists -- the phenomenon of radioactive decay.
[Wolfgang Pauli]: Dear radioactive ladies and gentlemen -- Unfortunately I am unable to come to
Tübingen personally since I am indispensable here because of a ball to be held
in Zurich.
[Narrator]: In the early decades of the 20th Century, physics had taken the first steps in understanding
what the Universe was made of. Matter, they knew, consisted of atoms. But they had
6
found that atoms were made of still tinier particles. Protons, with a positive electrical
charge clustered in the atom's nucleus, which was surrounded by a cloud of negatively
charged electrons. Protons and electrons appeared to be the two ultimate building blocks of
all matter. But there was something strange about the way these particles behaved.
[Boris Kayser]: Pauli had to deal with a very, very puzzling situation. On the level of atomic nuclei
and particles smaller than that, many things don't live forever. They disintegrate or
they "decay", as we say.
[Narrator]: It was almost as if some atomic nuclei had too much energy, making them unstable. They
would suddenly spit out a particle -- often an electron -- leaving behind a new nucleus
with less energy. That was strange enough. But what was even odder was what was
happening to the energy.
[Boris Kayser]: There is a very, very well-established principle in physics called the principle of
Conservation of Energy. It says that you don't get more energy than you had
before, and you don't have less energy. You don't lose energy that you had before -energy does not disappear.
[Narrator]: But that's just what seemed to be happening. The energy the nucleus lost when it decayed
should all have been taken up by the electron. There was nowhere else for it to go. But it
seemed the electron did not carry away as much energy as it should.
[Boris Kayser]: In fact, what they saw was that in different decays -- always with the same original
nucleus, always with the same final one -- the electron had differing amounts of
energy, typically not all the energy that was released. Energy was somehow
disappearing.
[Narrator]: But disappearing energy was simply not acceptable to Pauli. The energy lost by the nucleus
had to be going somewhere. It was time to be bold.
[Wolfgang Pauli]: I have had an idea for a desperate remedy in order to save the validity of the
energy law.
[Narrator]: Pauli's idea was that there had to be a third particle involved in radioactive decay -- a new
kind of particle which no one had ever seen but which was carrying away the missing
energy.
[Boris Kayser]: He proposed in addition to the little particles that were known at that time, there was
another one that would be emitted in radioactive decay along with the electron. Pauli
suggested that this new particle was very elusive, hard to detect, and this is why
people have never seen it. But the particle would take up whatever energy the
electron didn't, thus resurrecting and saving the principle of Conservation of Energy.
He did it, I'm sure, with great hesitation. But he did it. It was a very bold move.
[Narrator]: But there was a problem. Pauli's hypothetical particle -- dubbed the neutrino, "the little
neutral one" -- had no electric charge. So it would not feel the electrical forces of
attraction and repulsion which are what make solid matter solid. To a neutrino, solid
objects would seem like empty space. It would pass through them without causing a
ripple. And that went for scientific instruments, too.
7
Neutrinos were the "ghosts" of the particle world. Even if they really existed, scientists
saw no way to detect them.
[Jonas Schultz] (University of California, Irvine): They concluded that it was a practical impossibility - that no one would ever see these neutrinos. And I think that's what put people off
for many years from even trying.
[Narrator]: But according to Pauli, neutrinos were produced when atomic nuclei decayed. And
eventually physicists discovered how to make atoms decay at will.
[Janet Conrad] (Columbia University): What happens when a nuclear bomb goes off is that there's a
chain reaction of decays. So many decays happen all at once. And if Pauli was right
and neutrinos are produced in each decay, then an intense pulse of these neutrinos
should come out when the bomb goes off.
[Narrator]: Fred Reines was a young researcher working on America's nuclear deterrent. But he really
wanted to do fundamental physics. And then he realized that the atomic weapons
program was the perfect place to hunt the elusive neutrino.
[Jonas Schultz] He sat in an office for a long time, staring at a blank pad, trying to think of an idea.
And he hit on the idea of looking for the neutrino. For him, it was intolerable that the
neutrino could exist and not be seen. And he had to resolve that problem.
[Narrator]: Reines realized that nuclear bombs were not a very practical source of neutrinos. But
nuclear reactors also harness radioactive decay to make power or fuel.
[Henry Sobel]: In a reactor, you get elements being produced. And when they decay, they give off
neutrinos. So you get lots and lots of neutrinos. It's an enormous number -- 10 with
13 zeros after it (i.e., 1013) -- per second, going through every little square centimeter
of your detector nearby the reactor.
[Narrator]: With such an intense source of neutrinos, perhaps Reines and his colleagues could finally
detect the "ghost particle". They christened their enterprise "Project Poltergeist".
The name was apt. Without an electric charge, the neutrino was invisible to all
scientific instruments. So how to detect it?
Reines realized that just as radioactive decay produced neutrinos, so neutrinos could
sometimes produce radioactive decay. If a neutrino collided with a nucleus, there was a
very slight chance that it might destabilize it and cause it to decay.
In Reines' experiment, the sign would be a distinctive double pulse of energy -- a signal
that would enable him to detect the invisible particle by proxy.
[Henry Sobel]: There was a particular signature of this detection. You saw a pulse … and then you saw
another pulse afterwards within a certain specified period of time. And that very
characteristic signature enabled you to pull it out from the background.
[Narrator]: It was a matter of watching an oscilloscope, waiting for that double pulse.
8
On June 14, 1956, Reines and his colleagues announced that they had detected -- for the
first time -- the particle which Pauli had theorized 26 years earlier!
[Boris Kayser]: They sent Pauli a telegram informing him of this discovery. And Pauli was very, very
happy, saying something like, "All things come to him who knows how to wait."
[Janet Conrad]: So Pauli was right. And what he had discovered wasn't actually just an esoteric bit of
information. It turns out that this new particle is absolutely crucial to the way the
Universe works because the process which ignites stars involves the neutrino.
[Narrator]: Deep inside every star, scientists knew there must be a source of energy. They suspected
that that source was nuclear fusion, a process in which small atomic nuclei fused together
to form bigger ones. Just as neutrinos were produced in nuclear decay, so they were also
emitted in nuclear fusion. And it was this idea that made neutrino hunters out of Ray
Davis and John Bahcall.
40 years ago, physicist Ray Davis was already a renowned designer of experiments, a
man who got the evidence scientists needed to test their theories. Theorist John Bahcall
was just beginning his career. He was drawn to astrophysics -- the science of stars and
galaxies. What brought them together was a shared desire to understand what made stars
shine. And they believed that neutrinos would allow them to do this.
[John Bahcall]: For me and Ray, it was a great challenge to see if we could look inside of a star in the
same way that your doctor can look inside your body with ultrasound or with X-rays.
We wanted to do the same thing with neutrinos: use neutrinos; look right inside the
Sun; and really see what the nuclear reactions are doing in the very interior.
[Narrator]: Deep inside the Sun's core, the crushing pressure forced Hydrogen nuclei to fuse together to
form Helium and heavier elements, in the process releasing the energy that fuelled the
Sun. Or at least that was the theory. Scientists had no direct evidence that this was
happening.
[Boris Kayser]: Now, nuclear fusion would produce not only energy making the Sun shine, but also
neutrinos. And lots of them. By looking at the surface of the Sun, you don't learn the
details of what's going on deep inside. But by looking at the neutrinos from the Sun,
you can.
[Narrator]: Lacking electric charge, neutrinos traveled from the Sun's core, unhindered, all the way to
the Earth. They were "cosmic messengers". Find them and you would have proof that
nuclear fusion really was the source of the Sun's energy.
So Davis asked Bahcall to work out exactly how many neutrinos the Sun made. It
meant creating the first detailed mathematical model of the fusion reactions inside the core.
It produced an astonishing result.
[Narrator]: We believed that the Sun should be emitting a huge number of neutrinos all the time. Every
second through my thumbnail and your thumbnail, about a hundred billion of these solar
neutrinos would be passing through. Every second, a hundred billion solar neutrinos
through your thumbnail every second of every day of every year of your life. And you
never notice it.
9
[Ray Davis]: What can you do?
[Narrator]: For Ray Davis, the challenge was clear. Confirm that John's fusion model of the Sun
correctly predicted the number of solar neutrinos. In 1965, Ray Davis embarked on one
of the most difficult experiments in the history of Science: to count the neutrinos
coming from the Sun.
It meant building a laboratory deep underground -- in a goldmine in South Dakota -- to
shelter it from confusing background radiation from space. The heart of the experiment
was Ray's neutrino trap: 600 tons of cleaning fluid, a liquid full of Chlorine atoms. It was
the ability of neutrinos to occasionally provoke the decay of one of these Chlorine nuclei
that was the key.
[Boris Kayser]: When a neutrino strikes a Chlorine atom and does anything at all, it will convert the
Chlorine into Argon. And this particular form of Argon will be radioactive. Ray
Davis thought you could use the radioactivity of the Argon atoms to give themselves
away.
[Narrator]: The idea was the more neutrinos flowed through the tank, the more Argon atoms they would
make. So by counting the Argon atoms, Ray would be indirectly counting the neutrinos.
But it was here that the immense difficulty of the experiment became apparent.
Trillions of neutrinos went through the tank every second. But they interacted so rarely
that John Bahcall calculated just 10 Argon atoms would be made each week. Finding them
seemed a ludicrously impossible task.
[Boris Kayser]: Davis was claiming that he could take a tank consisting of 350 zillion atoms of
Chlorine and other stuff and extract from it only 10 Argon atoms. It's worse than a
"needle in a haystack".
[Narrator]: Nevertheless, every few weeks Ray would bubble Helium through the cleaning fluid to
sweep out the Argon atoms that had accumulated. He then brought them back to his New
York laboratory to be counted.
[Anna Davis] (Ray Davis's wife): I used to joke that he traveled all the way across the Country with a
little tube full of nothing. Which was not strictly true, of course. It turned out to be a
very important piece of nothing.
[Narrator]: But as the first results began to come through, it was immediately clear that something was
wrong. John had expected 10 Argon atoms per week. But Ray only counted 3. Most of
the neutrinos were missing.
[John Bahcall]: Right from the beginning, it was apparent that Ray was measuring fewer neutrino
events than I had predicted. Only about a third. And that was a very serious problem.
[Andrew Davis]: My father and I would always talk whenever I'd come home. And I mean it was
certainly very perplexing that the number was low.
[Narrator]: It looked like Ray's daring experiment simply wasn't working.
10
[Andrew Davis]: I remember even people coming and saying, "Well, you know there's got to be
something wrong with that experiment. That can't be right."
[Narrator]: The skepticism was understandable.
[Boris Kayser]: He set out to do something which sounds totally impossible. If you have a shot of the
size of the tank of cleaning fluid that he used … and then you mention how many
atoms are in one of those tanks and the fact that he extracts 10 or 3-or-4 and he counts
them correctly. Oh, yeah? Give me a break!
[Narrator]: Most of Ray's colleagues were convinced that the missing neutrinos revealed not a problem
with their theories, but a problem with his experiment.
[William Fowler] (1969): We think if Ray improves the sensitivity of his equipment, he'll find the
neutrinos all right.
[Narrator]: And, in every other respect, physicists' theories of matter had indeed made great progress.
By the 1970s, they had finally figured out the basic recipe of the Universe: the Standard
Model of particle physics. It seemed that everything was made from just 4 basic
ingredients, 4 fundamental particles -- one of which was the neutrino. But the Standard
Model also said that each of these particles came in 3 different "flavors". So, in fact,
there were 3 different kinds of neutrino.
{magician}: I actually have a neutrino... there you go. The problem is that once you look at one
very carefully. Sometimes it behaves as if there are two. The 3 neutrinos... Can you
open your hand again?
[Narrator]: According to the Standard Model, the 3 neutrinos had bizarre properties. Not only did they
have no electric charge, but they also had no mass either which meant they would flit
invisibly through the Universe at the speed-of-light.
As a recipe for matter, the Standard Model was a tremendous achievement. No matter
what experiment scientists performed, the Standard Model correctly predicted the result …
except, that is, for Ray's missing neutrinos.
{magician}: They're very slippery … very difficult to detect.
[Narrator]: All through the '80s, Ray continued to improve his detector. And year-after-year, the results
were the same. He could only find ⅓of the neutrinos John Bahcall had predicted. He
became sure there was nothing wrong with the experiment.
[Ray Davis] (1976): We have lived with it a long time and thought of all possible tests. We feel that our
result is valid. And we realize it's -- as John Bahcall calls it -- "a socially
unacceptable result."
[Narrator]: Inevitably, the focus of scientific skepticism began to shift.
[Janet Conrad]: If Ray Davis was right, then that would mean that John Bahcall must be wrong. And
so he was under a lot of pressure to try to explain why it is that his theory must be
right in the face of this experiment.
11
[John Bahcall]: Almost every theoretical physicist believes that we astrophysicists have just messed it
up and it's our fault. We never understood what was happening in the center of the
Sun no matter how much we pretended to do so.
[Janet Conrad]: I saw John stand up to give a talk. He began and started his very well-reasoned
arguments. And it wasn't 5 minutes into the talk before somebody stood up and
started arguing with him. I was very impressed at how well John handled this, even
when this person in the audience suddenly broke into Hebrew and John was arguing
back in Hebrew.
[Narrator]: Despite the barrage of criticism, John Bahcall was confident there was nothing wrong with
his model. He continued to insist that the Sun must be producing far more neutrinos than
Ray was detecting.
[John Bahcall]: And it didn't matter how convinced I was that they were wrong. Every year for 30
years, I had to demonstrate scientifically that yes, the expectation from the Sun was
robust and therefore, you should take the discrepancy seriously.
[Narrator]: It became more and more puzzling. Nobody could see what was wrong with John's theory
or find fault with Ray's experiment.
[John Bahcall]: This is the one I love -- this is you swimming.
[Narrator]: But at least they finally had everyone's attention. Their neutrino anomaly had become the
biggest mystery in particle physics.
[Patrick Moore] ("The Sky at Night", BBC, 1983): Everything indicates that this apparatus is accurate
and can tell us how many neutrinos are coming from the Sun. But observation and
theory don't agree. They're simply aren't enough neutrinos. And that's causing a
great many raised eyebrows.
[John Bahcall]: I think we need a new experiment in order to decide who is right and who is wrong.
[Narrator]: In Kamioka, Japan, there was another experiment. But it wasn't to study the Sun. It wasn't
even designed to look at neutrinos. And at first, it only seemed to deepen the mystery.
In 1983, the Japanese started looking for a rare kind of nuclear decay. They had built
an experiment called "Kamiokande" deep inside a mountain to shield it from radiation from
space. But there was one thing the mountain couldn't shield them from: neutrinos!
[Janet Conrad]: Neutrinos can be produced by all kind of nuclear interactions. And one that is
particularly interesting is one where the particles that are flying through the
Universe. The high-energy particles called "cosmic rays" hit our atmosphere. And
when they collide with the atmosphere, a spray of particles comes out and that
includes neutrinos. And those neutrinos are called atmospheric neutrinos.
[Narrator]: These atmospheric neutrinos were a nuisance, easily confused with what they were really
looking for. But then they noticed something strange about the atmospheric neutrinos.
They were detecting far fewer of them than they expected.
12
[Kunio Inoue] (Tohoku University): They found that atmospheric neutrino is not coming as we
expected. Surprisingly, we found that neutrinos coming from the atmosphere is
smaller than the expectation. We called it an "atmospheric neutrino anomaly".
[Janet Conrad]: The atmospheric neutrino anomaly which the Kamiokande scientists were seeing looks
in many ways similar to the solar neutrino deficit. They were looking for a certain
type of neutrino, and they see a lot less than was expected. And you can't blame
this one on John Bahcall, okay? This has nothing to do with the Sun. And so once
this happens, the scientists start to think, "Hmm, maybe something's going on here."
[Narrator]: Physicists went back to basics. The Standard Model said there were 3 different types of
neutrino. The first were electron neutrinos, the type the Sun produced and the only
neutrinos Ray's experiment was designed to detect. But there were also muon neutrinos
and tau neutrinos. Could this be the key to the problem?
[Dave Wark] (Imperial College London): Very suggestive, of course, that Ray's experiment sees ⅓ of
what John thought it should. There are 3 flavors of neutrinos. So it's not a great leap of
the imagination that those 2 numbers might be connected.
[Narrator]: Still, the connection was not obvious. True, Davis could only detect 1 of the 3 neutrino
flavors. That was the only flavor the Sun could create. But that was not all there was to
it.
[Dave Wark]: There was a theoretical proposal that neutrinos might change from one type of flavor into
another type of flavor. This is called neutrino oscillations. You emit the neutrino as
one particular flavor. But later on when you detect it, it might be another.
[Narrator]: In this theory, neutrinos would be continuously changing from type-to-type as they traveled
through space. What started as an electron neutrino would later look like a muon
neutrino … still later a tau neutrino … and then an electron neutrino again.
[Dave Wark]: And in fact, it can change back-and-forth and back-and-forth and back-and-forth. That's
why it's called a neutrino oscillation. And this is sort of like a pendulum.
[Narrator]: Was this why Ray saw only a third of the neutrinos that John predicted the Sun was
making? In the time it took them to travel from the Sun's core to the Earth, had electron
neutrinos oscillated into muon and tau neutrinos which his experiment couldn't detect?
That would explain everything. But there was just one problem.
[Dave Wark]: The problem with that is that in the Standard Model, neutrinos are mass-less. And massless neutrinos can't do this. They can't change from one type of neutrino to the other.
[Narrator]: It all had to do with time. For anything to change, time must pass. But the Standard Model
said the neutrino was a mass-less particle traveling at the speed-of-light. And according
to Einstein, if you are traveling at the speed-of-light, time stands still so nothing can
change.
[Boris Kayser]: When a particle moves fast, its clocks -- its internal timing mechanism -- slows down.
And as it approaches the speed-of-light, the clock slows down until it's not moving at
all. A particle which is mass-less is moving at the speed-of-light, so it has no sense of
what time it is.
13
[Narrator]: Without mass, a neutrino would be frozen in time traveling at the speed-of-light, but unable
to change.
[Boris Kayser]: Neutrino oscillation is a time-dependent phenomenon. It requires a neutrino clock that
requires that the neutrino travels slower than light and that the neutrino have a mass.
[Dave Wark]: And so as an explanation for the Davis experiment, it's not very attractive because if you
don't believe neutrinos have mass, then they can't oscillate. Then, you know, whether
there's a factor of 3 here or 3 there, it doesn't matter. It can't be the explanation.
[Narrator]: But then scientists made a discovery that completely transformed all their ideas about the
neutrino. Back in Japan, they had completed a vastly scaled-up version of the
Kamiokande experiment called "Super-Kamiokande".
[Masatoshi Koshiba] (University of Tokyo): This is really marvelous opportunity. So I decided that we
go ahead, we change the detector, improve it, to make our detector really
capable of new type of neutrino observation.
[Narrator]: Super-Kamiokande was truly colossal: a 40-meter high tank holding 50,000 tons of ultrapure water, surrounded by 11,000 photo-multiplier tubes. Super-Kamiokande could still
only detect 2 of the 3 neutrino flavors. But because it was so big, the scientists could tell
what direction the neutrinos were coming from.
[Henry Sobel]: A neutrino comes into your detector and produces a charged particle. The direction that
the charged particle goes pretty much matches the initial direction of the neutrino. So
by reconstructing the track of the charged particle, you can tell where the neutrino
came from. You can make a plot, and you could say, "How many neutrinos do I have
coming from there? From there? From there? And from there?" You make a plot
on the sky.
[Narrator]: And when they plotted where the neutrinos were coming from, the Kamioka team made an
astonishing discovery.
[Janet Conrad]: Neutrinos are produced in the atmosphere above you. Neutrinos are also produced in
the atmosphere on the other side of the Earth "below" you. And those neutrinos can
travel through the 13,000 kilometers up to your detector. And so the SuperKamiokande experimenters expected to see neutrinos coming in from above and
neutrinos coming in from below in equal numbers.
[Narrator]: But that's not what they found.
[Kunio Inoue]: Neutrino flux coming from above and coming from below should be the same. But
what we have observed was that neutrinos coming from below is about half of that
coming from above.
[Henry Sobel]: The number of neutrinos that are coming down and going through a small distance in
getting to us is about what you'd expect. But the number of neutrinos that are coming
up though the Earth -- which are going through tens of thousands of kilometers -- are
fewer of them than you'd expect.
14
[Narrator]: For the charge-less neutrino, the solid rock of the Earth is just empty space. So the only
difference between the atmospheric neutrinos coming from above and those from below
was the time they had been traveling before they reached the detector. Which meant that
contrary to all theory, neutrinos must have a sense of time.
[Dave Wark]: Just that fact that the neutrinos coming down from above still get here but the neutrinos
coming up from below don't, tells you that neutrinos have mass because they tell you a
neutrino knows how far it's gone. And the only way it can know how far it's gone is if
its clock isn't stopped. Which means it can't be traveling at the speed-of-light. Which
means it must have a mass.
[Narrator]: It was a bombshell. The Standard Model had gotten neutrinos completely wrong. They did
have mass and they could change flavor. So had the missing solar neutrinos been there
all along, just changed into the 2 flavors Ray's experiment could not detect?
There was only one way to know for sure. All eyes turned to a nickel mine in Sudbury,
Ontario. Here, 2 kilometers below ground, a team of British, Canadian, and American
scientists were building a new kind of solar neutrino detector -- one sensitive to all 3
flavors.
It was the deepest such experiment ever built. And it also had to be the cleanest
because the confusing background radiation came not just from space but from the rocks
themselves.
[Dave Wark]: If we got an amount of dust like that into our detector, it would just ruin it. It would
destroy its sensitivity to the neutrinos by blocking them out with other signals. And so
we have to build the detector with fantastic levels of cleanliness. We just have to get
rid of all of this stuff.
[Narrator]: All these precautions made the Sudbury Neutrino Observatory -- "SNO" for short -- one of
the least radioactive places in the Universe.
[Dave Wark]: When the SNO detector was finished, the exact center of the SNO detector has the lowest
level of radiation of any point in the Solar System.
[Narrator]: After 9 year's construction, SNO started taking data in November, 1999, looking for proof
that neutrinos could change flavor.
[Steve Biller] (Oxford University): When I joined the experiment, I was betting there was no neutrino
oscillations. It just seemed too bizarre.
[Narrator]: At the heart of the detector was an acrylic sphere containing 1,000 tons of "heavy water" -- a
substance which neutrinos could interact with in 2 distinct ways. One reaction -- like
Ray's tank of cleaning fluid -- was sensitive only to electron neutrinos. This would be a
direct check on the accuracy of Ray's experiment.
[Dave Wark]: But there's a different reaction which doesn't care what kind of neutrino it is. So it
allows you to see all of the neutrinos. Now, measuring that reaction allows you to
check John directly. If you see the number of neutrinos that John predicts, then he
really does know how the Sun works.
15
[Narrator]: For 40 years, John Bahcall's predictions of the number of neutrinos coming from the Sun
had flown in the face of his friend's experiment. Was that now at last about to change?
Over 19 months, 10 billion trillion neutrinos passed silently through the SNO detector.
Just 2,000 of them reacted with the heavy water.
[John Bahcall]: Almost every theoretical physicist believes the astrophysicists have just messed it up.
[Andrew Davis]: There's no other really likely explanation than that one of those 2 guys was wrong.
[Patrick Moore]: There simply aren't enough neutrinos.
eyebrows.
And that's causing a great many raised
[Ray Davis] (1976): ..."socially unacceptable result."
[Narrator]: In June, 2001, the SNO team announced their estimate of the total neutrino flux from the
Sun, taking -- for the first time -- all 3 flavors of neutrino into account.
[Steve Biller]: It was almost too good to be true. The Sun works as we expect, which is good. And
there is this funny business that neutrinos coming from the Sun that arrive at the Earth
are not all electron neutrinos -- that they have somehow changed in their nature.
[Narrator]: For decades the question had been, "Who was right -- Ray or John?" The answer was,
"They were both right. It was the Standard Model that was wrong."
[John Bahcall]: I was called right after the announcement was made by someone from the New York
Times and asked how I felt. And without thinking, I said, "I feel like dancing, I'm so
happy!" And the one thing that my kids kept sending each other e-mails about all
week was, "Did you see where it said in the New York Times that Dad felt like
dancing?" They kept making fun of me about that, but I was deliriously happy.
It was like for 3 decades, people had been pointing at this guy and saying, "This is
the guy that wrongly calculated the flux of neutrinos from the Sun". And suddenly that
wasn't so. And it was like a person who had been sentenced for some heinous crime,
and then a DNA test is made, and it's found that he isn't guilty. And that's exactly the
way I felt.
[Narrator]: The 30-year struggle to resolve the problem of the missing neutrinos has revealed weird
properties that defy the predictions of the Standard Model. They hint at a new theory of
matter -- one even more profound.
[Janet Conrad]: Scientists have searched for so long -- my whole scientific career -- to find a problem
with the Standard Model. It has been very resilient. And that is why it is so
exciting to suddenly come up with this new information that neutrinos have mass,
because that doesn't fit within our theory. And so it's like opening a door. And of
course when you open a door, behind that you find a lot more doors.
[Narrator]: Today, neutrino physicists like Janet Conrad are building a new generation of neutrino
experiments.
16
[Janet Conrad]: I strongly suspect that the neutrino has more up its sleeve than we have observed so
far.
[Narrator]: Do neutrinos have other strange properties? Could there be more than 3 flavors of neutrino?
Could neutrinos even be the answer to the greatest riddle in physics: "Where did all the
matter in the Universe originally come from?"
The Standard Model says that when the pure energy of the Big Bang condensed into the
stuff of the Universe. For every particle of matter produced, there would have been a
corresponding anti-particle of anti-matter.
[Steve Biller]: The problem with that sort of universe is we wouldn't exist because matter and antimatter would annihilate each other and there'd be nothing left. And so the reason that
we exist is, in fact, because there is an imbalance. That there is much more matter than
anti-matter in the Universe. And although we know this is a fact, we don't fully
understand why this is.
[Janet Conrad]: It may be that the neutrinos -- which have provided the first chink in the Standard
Model -- can provide us the clue to this too. Because when I describe a particle and
an anti-particle, I'm describing them by saying that they have opposite charge. So,
for example, the electron -- which has negative electric charge -- has an anti-particle
partner that has positive electric charge. It's called the positron.
But the neutrino is actually very special. It is the only set of particles in the
Standard Model that don't carry any charge. And so they can escape the definition.
They can be their own anti-matter particle. A neutrino and an anti-neutrino might
actually be different aspects of the same beast.
[Dave Wark]: In the Big Bang, we would have made huge numbers of neutrinos. And if neutrinos have
mass, it is possible that the matter in the Universe today arose because of the decay of
massive neutrinos created in the early Universe. So we may be the grandchildren of
neutrinos. All the matter that makes us up may have arisen purely through the decay
of neutrinos.
[Steve Biller]: So in a bizarre way, it may be that neutrinos tell us why we exist.
[Narrator]: Their hunt for the most elusive thing in the Universe may have brought scientists close to
uncovering the origin of everything around us. And it all began 40 years ago with Ray
Davis' pioneering underground experiment.
[Steve Biller]: Ray Davis is a hero to everybody in this field. This was really the first time somebody
really seriously tried to measure such an impossible thing coming from the Sun.
[Boris Kayser]: Ray Davis's persistence in the face of seemingly wrong experimental results,
contradictions between not only his experiment and theory but also his experiment
and other experiments... He stuck to his guns. And he was right.
[Narrator]: Ray continued to work on his experiment well into his '80s until he was forced to stop by
the onset of Alzheimer's disease. Then one day in October, 2002, Anna Davis got an
early morning phone call.
17
[Anna Davis]: My nephew -- who works for Minnesota Public Radio -- called us at 6:00 in the morning
and said, "Congratulations." And I said, "For what?" He said, "Don't you know? Ray
got a Nobel Prize in Physics."
We gathered all of our 5 children, their spouses and our 11 grandchildren, flew them
all over to Stockholm. And everybody had a wonderful time for 8 days.
[Andrew Davis]: The entourage of Davises was 23 people including the Nobel Prize winner himself.
So it was … it was really special.
[Narrator]: Ray Davis shared the Nobel Prize with the scientist behind the Kamioka experiments -Masatoshi Koshiba.
[Masatoshi Koshiba]: I was happy. I was happy … that's all.
[Narrator]: The awards were a tribute to all the scientists whose work over 40 years had gradually
uncovered the true nature of the neutrino.
[Dave Wark]: In the end, to have put that much of your life into something and have it work -- and not
just work but to work so beautifully! -- that is just the most tremendous feeling a
scientist can have.
Then it just hits you what you've done -- that you've actually learned something
about the Universe that nobody ever knew before. And now you get to tell them.
[Boris Kayser]: We are descended from neutrinos, yeah? We are descended from neutrinos. What a
kick! It's true, we think.
18
"Dancing With Neutrinos"
http://www.pbs.org/wgbh/nova/neutrino/dancing.html
The story of how many neutrinos the sun produces and how many reach the Earth is one of dogged
persistence, the patience of Job, and a tight-knit collaboration between 2 researchers who steadfastly
believed in their findings.
In this interview with astrophysicist John Bahcall (who died in
August 2005 at age 70), read about the career-long quest that he and
Nobel Prize-winning chemist Raymond Davis Jr. launched in the early
1960s and finally completed in 2001. When results announced that
year proved them right, Bahcall -- when asked how he felt -- responded,
"I feel like dancing I'm so happy!"
[NOVA]: This all got started when Ray Davis sent you a letter in the early 1960s asking if you could
calculate the rate of neutrino production in the Sun. What was the motivation for making
such a calculation?
[John Bahcall]: When I got this letter from Ray, the consensus view among scientists who thought
about it at all was that stars like our Sun shine by burning hydrogen into helium and
converting the small amount of extra mass into a lot of extra energy. But that was not
a quantitatively tested theory. It was supported by a lot of circumstantial evidence
but no precise quantitative evidence. For me and for Ray, I think it was a great
challenge to see if we could see directly into the interior of a star, deep inside where
the temperature and the densities are the highest. That's where the nuclear cauldron is
-- that's where the nuclei are burnt and the energy is created.
So for us, it was both an excitement like climbing to the top of the mountain and
really being able to see the whole view clearly. But also it was the challenge of being
able to test quantitatively this consensual theory -- this theory which everybody
thought was right but hadn't been quantitatively tested.
[NOVA]: How did Ray Davis plan to detect solar neutrinos?
[Bahcall]: Well, physicist Bruno Pontecorvo -- when he was working in Canada -- suggested that one
might be able to capture neutrinos from a reactor using chlorine. The neutrinos from the
reactor would convert some of the chlorine atoms to argon atoms, which are radioactive
and could be counted in small quantities in small counters. One could be sensitive to even
something as weakly interacting as a neutrino if one had a huge vat of chlorine.
That was the basic idea that Ray had in mind. But he'd found that he couldn't detect them
from a reactor. That was interpreted correctly as saying that a reactor produces antineutrinos -- not neutrinos. Anti-neutrinos will not convert chlorine into argon. You need
neutrinos like those produced in the Sun to convert chlorine to argon.
Now, the fact that neutrinos are very difficult to detect means that to get one neutrino per
day, you need a tank the size of an Olympic-size swimming pool filled with a huge vat of
cleaning fluid containing chlorine. That very fact is what makes it possible to look right into
19
the center of the Sun with the neutrinos in the same way that your doctor can look inside your
body with ultrasound or X-rays and make a diagnosis of how your body is working. We
wanted to do the same thing with neutrinos -- use neutrinos to look right inside the Sun and
see what the nuclear reactions are doing in the very interior.
[NOVA]: So you can see things with neutrinos you can't see with light?
[Bahcall]: Well, light -- as we all know -- doesn't penetrate anything. If I put my hand in front of my
face, you can't see my face. The light won't go through my hand. It doesn't penetrate any
appreciable amount of material. Neutrinos can go through unimaginable amounts of
material without being affected. There is less than a percent chance that anything would
ever happen to them as they passed through the Sun -- certainly through the Earth.
[NOVA]: So what happened when you started working on this problem?
[Bahcall]: When I got to Caltech and began the calculations of how many neutrinos there should be
from the Sun, I realized that the problem was immensely more complicated than I had
recognized early on because there were many different reactions competing with each
other. The rates of the individual reactions were not well known. All of that had to be
determined at the same time. We had to determine precisely the chemical composition and
temperature and density and pressure in the Sun to high accuracy.
The net result of all those calculations was that we believed that the Sun should be
emitting a huge number of neutrinos all the time. I can
illustrate that. Your thumbnail's about roughly a
square centimeter. Every second, about a hundred
billion of these solar neutrinos according to our
calculations would be passing through your thumbnail
every second of every day of every year of your life.
And you never notice it. They really don't do much.
And that's the reason why Ray needed a huge tank of
chlorine in order to detect what I said should be one neutrino event per day (and which
turned out to be less than that -- much less, a third of a neutrino event per day).
About 100 billion solar neutrinos pass through your thumbnail every
second, says Bahcall, and you never notice a thing.
[NOVA]: So despite the extremely small number of expected events, Ray Davis was confident he could
detect them?
[Bahcall]: Yes. I told Ray how many argon atoms should be produced in his tank per day. And Ray
told me that he was sure that he could do that. He invited me back to Brookhaven
[National Laboratory] in order to try to sell the experiment. We had to present our ideas to
the director of the laboratory.
Ray told me, "Never mind your enthusiasm for your understanding of how the Sun
shines. Never mind the models that you've made of how many neutrinos come from the
Sun. Talk to the director only about the nuclear physics because he will say that that's very
20
interesting and new nuclear physics and very clever nuclear physics.
interested at all in the astronomy."
He won't be
I was a young person and very enthusiastic and full of calculations that we'd done about
the Sun. I argued with Ray. But Ray said, "John, I know the director of Brookhaven. He
often says that no astrophysicist can calculate anything with sufficient precision to be of
interest to any particle physicist." He said, "Trust me. Forget about your models of the
Sun. Our only chance of selling the experiment is talking about your nuclear physics and
I'll talk about how to do the experiment."
So I deferred to him. I talked about the nuclear physics, and the director was very
enthusiastic about that. Eventually his wife did an experiment to test those ideas, and Ray
described how he could do the experiment. About 3 weeks later, the director approved the
experiment with no proposal ever having been written. And very shortly afterwards, Ray
began to look for a mine where he could do the experiment.
Seen in 1963, this neutrino detector,
located 2,300 feet down in a
limestone mine in Ohio, established
the techniques Ray Davis used later
in the much larger Homestake mine
detector.
[NOVA]: Why a mine?
[Bahcall]: Ray had to find some place far underground to do his experiment because -- as we've already
discussed -- neutrino interactions are extremely rare. They almost don't happen. So you
want to go someplace where nothing else can interfere with your experiment.
On the surface of the Earth, many things can happen. Particular particles from outer
space called "cosmic rays" can cause events in your tank that could be confused with
neutrino interactions. Deep underground, none of these confusing events from cosmic rays
or other activities on the surface of the Earth occur.
[NOVA]: So what exactly was involved in counting these precious few neutrino events?
[Bahcall]: It was necessary to find an efficient way of getting the few atoms out of this huge tank every
couple of months. Then it was necessary to count them and not confuse the counting of
these few atoms with anything else that was happening. All of that was beyond the current
range of technology at the time that Ray started it. But Ray was absolutely confident that
he could do it. He's just an enormously modest, quiet, unassuming person. And I assumed
that it was as he described it -- just "plumbing".
But I saw it later through the eyes of other experimentalists. I think it was somewhat
miraculous what he did. I didn't realize it at the time because he was the only person that I
talked to about it. For him, it was going to be just a matter of "plumbing" -- plumbing on a
very big scale and using chemistry that was well understood. But he did it with precision
and care and attention to detail and insight into what were all the important processes that I
think probably was well beyond the abilities of anybody else.
21
Then, of course, when his experimental results came out and they were in conflict with
our calculations, he and I would be invited to give theory and experimental talks
everywhere. And it was only many years later that I realized that other people were very
skeptical of Ray's ability to do what he was absolutely confident of and which -- as it
turned out -- he was absolutely right about! He did it with ease and simplicity and elegance
and beauty.
[NOVA]: Yes. From the people we've talked to, Ray seems to command not just enormous respect but
also affection.
[Bahcall]: Many people know what a great scientist Ray is. It's really quite extraordinary what he's
done scientifically. But I -- and I think most of the people who know him well -- admire
him much more for his character than for his scientific abilities, extraordinary as they are.
Ray is the kind of person who treats everyone the same -- with dignity and respect, with
politeness and attention, with generosity and support.
In all the years I have known Ray, he has never differentiated between the janitor that
came into the office to clean the trash can and the most distinguished professor who came in
to ask him a question. He treats everybody with the same friendliness, the same courtesy, the
same gentleness, and the same attentiveness.
"Ray," Bahcall said, "is the kind of person
who treats everyone the same—with dignity
and respect, with politeness and attention,
with generosity and support."
[NOVA]: But as you said, there was at the time considerable skepticism in the scientific community
about your claims.
[Bahcall]: When you think about it, it's almost unbelievable what we were doing. I'm glad we didn't
think about it too carefully when we were doing it because I made calculations on a sheet
of paper and with the computer about how many neutrinos were produced at high
temperatures with nuclear reactions deep inside the Sun and how many would be captured
in a huge tank. And Ray with his huge tank would boil through helium every day to flush
out the few atoms of argon that were in there. Every 2-or-3 months, he would purge
something like hopefully a dozen-or-10 of these argon atoms; separate them from the rest
of the material that he had; and put them in a small glass counter and wait for a few months
to count the 3-or-4 or -- as it turned out -- maybe 1 atom of argon that was in this glass vial
containing gas of argon and carrier gas.
It's an incredible connection between scribbles on a sheet of paper and flashes that you
see when an argon atom decays in a gas contained in a small glass vial deep underground.
But that's the conceptual connection that we made and, surprisingly enough, we convinced
people that the number of atoms in his little glass vial had something to do with the number
of lines that I drew on a sheet of paper.
22
Nearly a mile underground, Ray Davis
takes a swim in the 300,000-gallon tank of
water that surrounded the central chlorine
tank in the Homestake mine. The water
reduced background radiation, which could
interfere with his counting of neutrinos.
[NOVA]: And yet there was a nagging discrepancy between your results and his, right?
[Bahcall]: Well, right from the beginning, it was apparent that Ray was measuring fewer neutrinos
events than I had predicted. He came to Caltech in early 1968 to spend a week with me
while he and I wrote our papers up describing for me a refined calculation -- for him, the
first measurement of the rate in his tank. It was clear that the rate that he was getting was a
factor of 3 smaller than I was predicting. And that was a very serious problem.
There was a famous meeting at Caltech. Just a few physicists -- Dick Feynman, Murray
Gell-Mann, Willie Fowler, Bob Christie, and a couple of others -- in a small meeting room
where Ray presented his results and I presented my calculations of what he should have
measured. There was some discussion of it afterwards, and it was pretty inconclusive. There
was a discrepancy -- it looked like one of us was wrong.
I was very visibly depressed, I guess. Dick Feynman asked me after the meeting if I
would like to go for a walk. We just went for a walk, and he talked to me about
inconsequential things -- personal things -- which was very unusual for him to spend his time
in quite idle conversation. It never happened to me in the many years that I knew him that he
did that before or afterwards. And only toward the end of the walk -- which lasted over an
hour -- he told me, "Look, I saw that after this talk you were depressed. I just wanted to tell
you that I don't think you have any reason to be depressed. We've heard what you did, and
nobody's found anything wrong with your calculations. I don't
know why Davis's result doesn't agree with your calculations. But
you shouldn't be discouraged because maybe you've done
something important. We don't know. I don't know what the
explanation is, but you shouldn't feel discouraged."
After the Caltech meeting, Richard Feynman,
the Nobel laureate in physics, gave John Bahcall reason to hope when all
hope seemed lost.
For me, I think of all of the walks or conversations I have had in my professional life, that
one was the most important because I was a young man without tenure. And while I'd done
many calculations by that time, this was the one that was most visible and people had paid
the most attention to. And it looked like it was wrong. I really was feeling very, very, very
discouraged. And for a person whom I so enormously admired, Dick Feynman -- to tell me
"You haven't done anything that's visibly wrong, maybe you've done something important" -for me that was a huge boost!
[NOVA]: But there were plenty of scientists who did think there was something wrong with your
model of the Sun.
23
[Bahcall]: Well, initially very few people paid any attention to this discrepancy. But the discrepancy
persisted. ... And every year for 30 years, I had to look at different processes that people
would imaginatively suggest that might play a role in the Sun. And it didn't matter how
convinced I was that they were wrong. I had to demonstrate scientifically that these
processes were not important in order to convince people. That yes, the expectation from
the Sun was robust and therefore you should take the discrepancy seriously. It took, I
would guess, 3½ decades before I convinced everybody.
[NOVA]: When did things start to change?
[Bahcall]: Well, we had information beginning in the late 1980s -- around 1988 -- that measurements
made on the surface of the Sun about how the Sun vibrates were giving us information
about the interior of the Sun. And the first indications were that the measurements were in
agreement with our predictions using our model of the Sun. So that was very encouraging
to me, and I began sort of speaking my mind. Until that time, I'd been quite reserved -- at
least for me. I would state the facts as I knew them, but I never tried to make very strong
claims. But once this evidence from the surface of the Sun seemed to confirm our
predictions based on how the Sun vibrated in its interior, I began being much bolder in my
calculations.
Evidence collected in the late 1980s
from the surface of the sun appeared to
confirm Bahcall's findings, but years
would still pass before full vindication
would come.
I remember a meeting in Toledo, Spain -- I think it was in 1991 -- where based on these
measurements of what we call "helioseismology", I said it was time for us to declare the solar
neutrino problem solved. It was time for the astronomers to declare a victory -- that it was
clear that our models were in sufficient agreement with the Sun that that could not be the
source of the discrepancy.
And that was a big mistake on my part because the summary speaker at that conference
was a very eloquent, humorous speaker who also had the ability to make very beautiful and
very humorous drawings. He made several caricatures of me which he showed in the
viewgraphs in the summary. And he had the whole auditorium -- including me -- laughing at
the bravado, the hubris of this guy claiming that he could say something about particle
physics based on this complicated Sun. I tapered down my comments for a few years based
on that rather humiliating personal attack. It wasn't a scientific attack. But it was a very,
very effective attack.
[NOVA]: Meanwhile, other experiments started to find a similar neutrino deficit.
[Bahcall]: The first experiment that was done after Ray's experiment was done by the JapaneseAmerican collaboration called "Kamiokande", which converted a water detector designed
to see the decay of the proton into a very sensitive detector of neutrinos from the Sun and
from supernovae. Just in time, they made the conversion so that they could see the
neutrinos from Supernova 1987A. That was a very spectacular achievement.
24
[NOVA]: And their results supported Ray's?
[Bahcall]: Yes. When their first results came out, I was absolutely thrilled because they got a result
which showed that the flux was definitely less than what I had predicted. And that was a
confirmation of Ray's result. My feeling was aha! we've eliminated the possibility of
experimental results being wrong, and I'm confident in my theory. I think we're onto
something good.
"I think we're onto something good,"
Bahcall said when results from the
Kamiokande experiment (seen here)
seemed to confirm Ray Davis's
measurements.
In fact, 2 years later one of my idols and heroes -- Hans Bethe -- and I used the first
results from the Kamiokande experiment together with Ray's results. A very, very basic
result from our solar models to argue very strongly that either one of the 2 experiments was
wrong -or- we needed new physics, that it couldn't be something wrong with my solar
models. Hans and I (Hans is the guy who first worked out, in 1939, the nuclear reactions that
we think make the stars shine) compared the results from the chlorine experiment and the
Kamiokande experiment and showed that on very general grounds, either one of the
experiments had to be wrong -- which didn't seem likely by that time -- or there had to be
some new physics. And the latter took me off the hook. I was no longer the person who had
done the wrong calculation.
[NOVA]: What about the Sudbury Neutrino Observatory? What role did it play in finally putting the
solar neutrino question to bed?
[Bahcall]: The SNO experiments had been in development for, I guess, almost a decade and a half.
They were designed to finally solve this problem clearly so that it wasn't a matter of
stacking up one argument against another argument against another argument -- to prove
that it had to be new physics -- but to demonstrate it all within 1-or-2 clear experiments.
The SNO experiment could look at a very particular high-energy branch of neutrinos,
which only is about a 1/100th of a percent of all the neutrinos I think come from the Sun.
They could find how many of those neutrinos were in a form called "electron-type
neutrinos".
[NOVA]: So neutrinos come in different forms?
[Bahcall]: Neutrinos can come in different "flavors". For ice cream we have, for example, chocolate
(which is what I prefer), vanilla, and strawberry. Neutrinos can come in the flavor of
electron type (associated with electrons) or with other types of particles called "muons" and
"taus". [StealthSkater note: a Table of all the particles predicted by the Standard
Model is included in "The Elegant Universe" archive at doc pdf URL-doc URL-pdf ]
What the Sudbury Neutrino Observatory was uniquely able to do was observe neutrinos
in the electron type only. They were able to determine how many neutrinos of the electron
type got to us at Earth in this huge tank of heavy water. And then they made use of the result
25
from a much larger tank of water called Super-Kamiokande in Japan, which measured
primarily electron-type neutrinos but had a little sensitivity to muon and tau neutrinos.
So we had 2 measurements: one of just the electron type from SNO, and one from
mostly the electron type but a little from muon and tau neutrinos from the JapaneseAmerican experiment. And combining those 2 data points, the SNO people together with the
Japanese-American collaboration could work out 2 things: how many neutrinos of the
electron type got here, and how many neutrinos of all types got here. And it's the one which
is neutrinos of all types that's really exciting.
Because what I can calculate for the Sun is the neutrinos of all types that start in the
interior of the Sun. You want to know what's the total number of neutrinos reaching the
Earth. That's what the Sudbury Neutrino Observatory was able to measure together with the
Japanese-American experiment in 2001. And their answer was bang! on our prediction. I
mean so close that it was embarrassingly close.
The Sudbury Neutrino Observatory in
Sudbury, Ontario finally solved the
discrepancy
between
Davis's
measurements and Bahcall's calculations
-- decades after the discrepancy first
came to light.
[NOVA]: How did it make you feel?
[Bahcall]: For me personally, it was the most exciting time after the understanding of how to increase
the rate of the neutrino capture in Ray's tank. It was just enormously exciting for me. In
fact, I was called right after the announcement was made by someone from The New York
Times and asked how I felt. Without thinking, I said "I feel like dancing I'm so happy!"
The one thing my kids kept sending each other e-mails about all week was, "Did you see
where it said in The New York Times that Dad felt like dancing?" They kept making fun of
me about that. But I was deliriously happy.
For 3 decades, people had been pointing at this guy and saying this is the guy who
wrongly calculated the flux of neutrinos from the Sun. And suddenly that wasn't so. It was
like a person who had been sentenced for some heinous crime, and then a DNA test is made
and it's found that he isn't guilty. That's exactly the way I felt.
John Bahcall (left)
visits Ray Davis at his
home in 2003, a year after
Davis won the Nobel Prize
in Physics for his efforts in
detecting solar neutrinos.
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Awesome Detectors
http://www.pbs.org/wgbh/nova/neutrino/detectors.html
Scientists estimate that many hundreds of billions of neutrinos will have harmlessly sped through
your body by the time you finish reading this sentence. But despite their abundance, there's only a 10
percent chance over the course of your entire lifetime that even one of these invisible particles will ever
(again, harmlessly) interact with any other particle in your body.
Because of the rarity of collisions between neutrinos and matter -- events that are necessary to
perceive or study these spectral particles -- neutrinos play an expert game of hard-to-get with physicists,
who have designed giant, extremely sensitive detectors to seek them out. In this slide show, take a tour
of some of the most intriguing neutrino experiments around the Globe and find out what tantalizing
results keep the experts on the trail of the ghost particle. -- Lexi Krock
Homestake
This experiment -- now ended -- was the first to detect
neutrinos from the Sun in the early 1970s.
The
Homestake detector seen here-- pioneered by Nobel
laureate in physics Raymond Davis Jr. -- consisted of a
tank of 615 tons of perchloroethylene (a dry-cleaning
fluid). The tank was situated in the Homestake gold mine
in South Dakota.
On very rare occasions -- about twice every 3 days -- a
neutrino would interact with a nucleus of chlorine in the
liquid and produce a nucleus of radioactive argon. Davis
developed techniques to extract the few atoms of argon created each month and count them by
monitoring their radioactivity. He found fewer neutrinos than expected -- the famous "solar neutrino
problem" which was resolved conclusively in 2001-2002 by the Sudbury Neutrino Observatory.
KAMLAND
An international team of physicists
completed construction on the KAMLAND
detector -- short for Kamioka Liquidscintillator Anti-Neutrino Detector -- in
1997 on the Japanese island of Honshu.
KAMLAND detects anti-neutrinos, the
antimatter opposites of neutrinos which
signal the latter's presence. The detector
uses a telescope made of 1,000 tons of
mineral oil and benzene in a stainless steel
tank ⅔of a mile below the Earth's surface
to measure antineutrinos issuing from
nuclear power reactors and natural nuclear
reactions.
27
In July 2005, KAMLAND scientists measured the Earth's total radioactivity for the first time. Their
findings will allow them to better understand what keeps the Planet warm, the volcanoes active, the
continents drifting, the magnetic field churning -- all things that enable Life. Until this discovery,
geologists relied on the reverberations from earthquakes to estimate the Planet's radioactivity.
ANTARES
The aim of this experiment -- whose
acronym stands for the rather ungainly
Astronomy with a Neutrino Telescope and
Abyss environmental RESearch -- is to answer
questions about the composition of deep space
by detecting neutrinos on the seafloor.
Scheduled for completion in 2006, ANTARES
will use water 8,200 feet below the surface of
the Mediterranean off the south coast of France
to detect the particles called "muons", which
are produced when neutrinos from space
interact in Earth's core.
Muons create radiation as they pass
through water. An array of approximately 1,000 photomultiplier tubes on 10 vertical strings spread over
1½ miles of seafloor sense and measure them. This image shows part of the ANTARES during
installation in the Mediterranean.
ANITA
If it's successful, the ambitious and innovative
ANITA neutrino detector will be the first device to
identify so-called high-energy neutrinos created by
collisions between cosmic rays and cosmic
microwave photons in space. Studying neutrinos
from these sources offers an unprecedented
opportunity to learn about exotic objects at the edge
of the Universe such as black holes.
Beginning in 2006, ANITA will be a balloonborne radio detector experiment circling the
Antarctic continent at 115,000 feet during
approximately 18-day missions. It will scan the vast
expanses of ice for telltale pulses of radio emission
generated by neutrino collisions.
28
MiniBooNE
Like many detectors, this experiment at the Fermi
National Accelerator in Batavia, Illinois investigates
the oscillation of neutrinos from one type to another.
Since 2003, it has observed neutrinos created from
protons in Fermilab's particle booster -- part of the
system that the Lab normally employs to accelerate
protons to higher energies for other experiments.
MiniBooNE is a 40-foot-in-diameter spherical
steel tank filled with 800 tons of mineral oil and lined
with 1,280 phototubes (some of which are being
adjusted in this image) that produce a flash of light
when charged particles travel through them. Analyses
of these light flashes are already providing tantalizing information about the nonzero status of neutrino
mass.
MINOS
MINOS -- or Main Injector Neutrino Oscillation
Search -- is a 2-detector experiment at Fermilab that
began studying neutrino oscillations in 2003. It uses a
beam of neutrinos that first pass through a detector at
Fermilab -- the inside of which is seen in this image -and then through one hundreds of miles away deep
within the Soudan Iron Mine in northern Minnesota.
The distance between the 2 detectors maximizes the
probability that the neutrinos will have revealing
interactions over the course of their journey.
An international collaboration of particle physicists
at Fermilab uses MINOS to investigate the puzzle of
neutrino mass. The 98-foot-long detector consists of 486 massive octagonal planes, lined up like the
slices of a loaf of bread. Each plane is made of a sheet of steel covered on one side with a layer of
plastic that emits light when struck by a charged particle. MINOS will help researchers answer some of
the fundamental questions of particle physics -- especially how particles acquire mass.
29
Super-Kamiokande
This detector began operating in 1996, more
than half-a-mile underground in a zinc mine in
Kamioka, Japan.
Japanese and American
scientists erected a huge tank of water 138 feet
tall to hunt for neutrinos. The walls, ceiling, and
floor of the 12.5-million-gallon tank are lined
with 11,242 light-sensitive phototubes. These
pick up and measure bluish streaks of light called
"Cherenkov radiation", which is left behind as
neutrinos travel through the water.
Super-Kamiokande detects neutrinos that
nuclear interactions in the Sun and atmosphere
produce.
In 2001 after several promising
discoveries related to potential neutrino mass, the
Super-Kamiokande was crippled when several thousand of its light detectors exploded. Repairs on the
detector should be completed in 2007.
SNO
The
Sudbury
Neutrino
Observatory is a collaborative effort
among physicists from Canada, the
U.K., and the U.S. Using 1,000 tons
of so-called heavy water and almost
10,000 photon detectors, they
measure the flux, energy, and
direction of solar neutrinos which
originate in the Sun. Located 6,800
feet underground in an active Ontario
nickel mine, SNO can also detect the
other 2 types of neutrinos -- "muon"
neutrinos and "tau" neutrinos.
In 2001 -- just 2 years after the
observatory opened -- physicists at
SNO solved the 30-year-old mystery
of the "missing solar neutrinos".
They found that the answer lies not with the Sun -- where many physicists had suspected that solar
neutrinos undergo changes -- but with the journey they take from the core of the Sun to the Earth.
30
AMANDA
The South Pole is an inhospitable
place to build and operate a telescope.
But crystal-clear ice is an excellent
medium for observing neutrinos as they
pass through the Earth. Since 1999,
AMANDA -- the Antarctic Muon And
Neutrino Detector Array (seen here
during its assembly) -- has used the
Antarctic ice to seek out neutrinos. When
the particles interact in the ice, they can
produce muons (charged particles that are
like electrons but heavier). The muons
create faint flashes of light as they pass
through the ice some 1.2 miles below the
surface where they are sensed by
AMANDA's hundreds of light-sensitive
phototubes supported on 19 tethers frozen in the ice.
AMANDA's goal is to conduct neutrino astronomy -- identifying and characterizing extra-solar
sources of neutrinos which could provide important clues in the search for dark matter.
IceCube
When it's completed in 2009, IceCube
-- an international neutrino experiment
involving more than 20 research
institutions -- will become the largest
particle detector ever built.
Setting
IceCube's 4,200 optical modules deep
within the South Pole -- where the
detector joins its predecessor AMANDA
-- will require drilling 70 holes 1½ miles
deep each using a novel hot-water drill
(part of which is seen here).
The detector's goal will be to
investigate the still-mysterious sources of
cosmic rays. IceCube's telescope will use
the Antarctic ice to look for the
signatures of "cosmic neutrinos" -elusive particles produced in violent cosmic events such as colliding galaxies, black holes, quasars, and
other phenomena occurring at the margins of the Universe.
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