Lawrence Krauss - The Greatest Story Ever Told--So Far
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Yang proposed that this might be a general property for various
elementary particles, which they suggested might come in pairs with
opposite parity. They called this idea “parity doubling.”
Such was the situation in the spring of 1956 when the
International Conference on High Energy Physics, held every year at
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the University of Rochester, took place. In 1956, the entire
community of physicists interested in particle and nuclear physics
could fit in a single university lecture hall, and these physicists,
including all the major players, tended to gather at this annual
meeting. Richard Feynman was sharing a room at the meeting with
Marty Block. Being an experimentalist, Block was not as burdened by
the possible heresy inherent in the suggestion that some force in
nature was not blind to the distinction between left and right, and he
asked Feynman if possibly the weak interaction governing the decays
Powell observed might distinguish left from right. This would allow
a single particle to decay to states of differing parity—meaning the
tau and theta could both be the same particle.
Block didn’t have the temerity to raise this question in the public
session, but Feynman did, even though he privately thought this was
extremely unlikely. Yang replied that he and Lee had thought about
this, but so far nothing had come of the idea. Eugene Wigner, who
would later win a Nobel Prize for elucidating the importance of such
things as parity in atomic and nuclear physics, was also present, and
he too raised the same question about the weak interaction.
But to the victor go the spoils, and speculating about the possible
violation of parity by a new force in nature that might distinguish left
from right was different from demonstrating it. A month later Lee
and Yang were at a café in New York, and they decided to examine
all known experiments involving the weak interaction to see if any of
them could dispel the possibility of parity violation. To their great
surprise, they realized that not a single one definitively resolved the
issue. As Yang later said, “The fact that parity conservation in the
weak interaction was believed for so long without experimental
support was very startling. But what was more startling was the
prospect that a space-time symmetry law which the physicists have
learned so well may be violated. This prospect did not appeal to us.”
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To their credit, Lee and Yang proposed a variety of experiments
that could test the possibility that the weak interaction distinguished
right from left. They suggested considering the beta decay of a
neutron in the nucleus of cobalt-60. Because this radioactive nucleus
has nonzero spin angular momentum—i.e., it behaves as if it is
spinning—it also acts like a little magnet. In an external magnetic
field the nuclei will line up in the direction of the field. If the
electron emitted when a neutron in the nucleus decays preferentially
ends up in one hemisphere instead of another, this would be a sign
of parity violation, because in the mirror the electrons would end up
in the opposite hemisphere.
If this was true, then at a fundamental level, nature would be able
to distinguish right from left. The human-created distinctions
between them (i.e., sinister versus good) would not then be totally
artificial. Thus the world in a mirror could be distinguished from the
real world, or, as Richard Feynman poetically put it later, we could
use this experiment to send a message to tell a Martian what
direction is “left”—say, the hemisphere where more electrons were
observed to emerge—without drawing a picture.
At the time, this was viewed as such a long shot that many in the
physics community were amused, but no one ran out to perform the
experiment. No one, that is, except Lee’s colleague at Columbia the
experimentalist Chien-Shiung Wu, known as Madame Wu.
Even as we bemoan today the paucity of female physicists trained
at American institutions, the situation was much worse in 1956.
After all, women weren’t even admitted as undergraduates at Ivy
League institutions until the late 1960s. Almost thirty years after Wu
arrived from China to study at Berkeley in 1936, she noted in a
Newsweek article about her, “It is shameful that there are so few
women in science. . . . In China there are many, many women in
physics. There is a misconception in America that women scientists
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are all dowdy spinsters. This is the fault of men. In Chinese society, a
woman is valued for what she is, and men encourage her to
accomplishments—yet she remains eternally feminine.”
Be that as it may, Wu was an expert in neutron decay and became
intrigued by the tantalizing possibility of searching for parity
violation in the weak interaction after learning of it from her friends
Lee and Yang. She canceled a European vacation with her husband
and embarked on an experiment in June, one month after Lee and
Yang had first thought of the problem, and by October of that year—
the same month Lee and Yang’s paper appeared in print—she and
several colleagues had assembled the apparatus necessary to do the
experiment. Two days after Christmas of that year they had a result.
In modern times particle physics experiments might take decades
from design to completion, but that was not the case in the 1950s. It
was also a time when physicists apparently didn’t bother to take
holidays. Despite its being the yuletide, the Friday “Chinese Lunches”
organized by Lee continued, and the first Friday after New Year’s
Day Lee announced that Wu’s group had discovered that not only
was parity violated, but it was violated by the maximum amount
possible in the experiment. The result was so surprising that Wu’s
group continued their work to ensure they weren’t being fooled by
an experimental glitch.
Meanwhile, Leon Lederman and colleagues Dick Garwin and
Marcel Weinrich, also at Columbia, realized that they could check
the result in their experiments on pion and muon decays at
Columbia’s cyclotron. Within a week, both groups, as well as Jerry
Friedman and Val Telegdi in Chicago, independently confirmed the
result with high confidence, and by mid-January 1957 they
submitted their papers to the Physical Review. They changed our
picture of the world forever.
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Columbia University called what was probably the first press
conference ever announcing a scientific result. Feynman lost a $50
bet, but Wolfgang Pauli was luckier. He had written a letter from
Zurich on January 15 to Victor Weisskopf at MIT betting that Wu’s
experiment would not show parity violation, not knowing that the
experiment already had. Pauli exclaimed in the letter, “I refuse to
believe that God is a weak left-hander,” demonstrating an interesting
appreciation for baseball as well. Weisskopf, who by then knew of
the actual result, was too kind to take the bet.
shock is over, I begin to collect myself.” It really was a shock. The
idea that one of the fundamental forces in nature distinguished
between right and left flew in the face of common sense, as well as of
much of the basis of modern physics as it was understood then.
The shock was so great that, for one of the few times in the
history of the Nobel Prizes, Nobel’s will was actually carried out
properly. His will stipulates that the prize should go to the person or
persons in each field whose work that year was the most important.
In October of 1957, almost exactly a year from the publication of Lee
and Yang’s paper, and only ten months after Wu and Lederman
confirmed the notion, the thirty-year-old Lee and the baby-faced
thirty-four-year-old Yang shared the Nobel Prize for their proposal.
Sadly, Madame Wu, known as the Chinese “Madame Curie,” had to
be content with winning the inaugural Wolf Prize in Physics twenty
years later.
Suddenly the weak interaction became more interesting, and also
more confusing. Fermi’s theory, which had sufficed up to that point,
was roughly modeled after electromagnetism. We can think of the
electromagnetism interaction as a force between two different
electric currents, each corresponding to the two separate moving
electrons that interact with each other. The weak interaction could
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be thought of in a somewhat similar way, if in one current a neutron,
during the interaction, converts into a proton, and in the other
current is an outgoing electron and neutrino.
There are two crucial differences, however. In Fermi’s weak
interaction the two different currents interact at a single point rather
than at a distance, and the currents in the weak interaction allow
particles to change from one type to another as they extend through
space.
While electromagnetic interactions are the same in the mirror as
they are in the real world, if parity is violated in the weak interaction,
the “currents” involved would have to have a “handedness,” as Pauli
alluded, as for example a corkscrew or pair of scissors has, so that
their mirror images will not be the same.
Parity violation in weak interactions would then be like the social
rule that we always shake hands with our right hand. In a mirror
world, people would always shake with their left hand. Thus, the real
world differs from its mirror image. If the currents in the weak
interaction had a handedness, then the weak interaction could
distinguish right from left and in a mirror world would be different
from the force in the world in which we live.
A great deal of work and confusion resulted as physicists tried to
figure out precisely what types of new possible interaction could
replace Fermi’s simple current-to-current interaction, in which no
apparent handedness could be attributed to the particles involved.
Relativity allowed a variety of possible generalizations of Fermi’s
interaction, but the results of different experiments led to different,
mutually exclusive mathematical forms for the interaction, so it
appeared impossible that one universal weak interaction could
explain all of them.
Around the time when the first experimental results on neutron
and muon decay had come out suggesting that parity violation was
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as large as it could be, a young graduate student at the University of
Rochester, George Sudarshan, began exploring the confused
situation and came up with what eventually was the correct form of
a universal interaction that could replace Fermi’s form—something
that also required that at least some of the experimental results at
the time were wrong.
The rest of the story is a bit tragic. At the Rochester conference
three months after the parity-violation discovery, and a year after
Lee and Yang had presented their first thoughts on parity doubling,
Sudarshan asked to present his results. But because he was a
graduate student, he wasn’t allowed. His supervisor, Robert Marshak,
who had suggested the research problem to Sudarshan, was by then
preoccupied with another problem in nuclear physics and chose to
present a talk on that subject at the meeting. Another faculty
member, who was asked to mention Sudarshan’s work, also forgot.
So all of the discussion at the meeting on the possible form of the
weak interaction ended up leading nowhere.
Earlier, in 1947, Marshak had been the first to suggest that two
different mesons were discovered in Cecil Powell’s experiments—
with one being the particle proposed by Yukawa, and the other being
the particle now called a muon. Marshak was also the originator of
the Rochester conferences and probably felt it would show
favoritism to allow his own student to speak. In addition, since
Sudarshan’s idea required at least some of the experimental data to
be wrong, Marshak may have decided it was premature to present it
at the meeting.
That summer Marshak was working at the RAND Corporation in
Los Angeles and invited Sudarshan and another student to join him.
The two most renowned particle theorists in the world then,
Feynman and Murray Gell-Mann, were at Caltech, and each had
become obsessed with unraveling the form of the weak interaction.
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Feynman had missed out on the discovery of parity violation by
not following his own line of questioning, but had since realized that
his work on quantum electrodynamics could shed light on the weak
interaction. He desperately wanted to do this because he felt his
work on QED was simply a bit of technical wizardry and far less
noble than unearthing the form of the law governing another of the
fundamental interactions in nature. But Feynman’s proposal for the
form of the weak interaction also appeared to disagree with
experiments at the time.
Over the 1950s, Gell-Mann would produce many of the most
important and lasting ideas in particle physics from that time. He
was one of two physicists to propose that protons and neutrons were
made of more fundamental particles, which he called quarks. He had
his own reasons for thinking about parity and the weak interaction.
Much of his success was based on focusing on new mathematical
symmetries in nature, and he had used these ideas to come up with a
new possible form for the weak interaction as well, but again his idea
conflicted with experiment.
While they were in LA, Marshak arranged for Sudarshan to have
lunch with Gell-Mann to talk about their ideas. They also met with
an eminent experimentalist, Felix Boehm, whose experiments, he
said, were now consistent with their ideas. Sudarshan and Marshak
learned from Gell-Mann that his ideas were consistent with
Sudarshan’s proposal, but that at best Gell-Mann was planning to
include the notion in one paragraph of a long general paper on the
weak interaction.
Mean
while, Marshak and Sudarshan prepared a paper on their
idea, and Marshak decided to save it for a presentation at an
international conference in Italy in the fall. Learning of the new
experimental data from Boehm, Feynman decided—rather excitedly
—that his ideas were correct and began to write a paper on the
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subject. Gell-Mann, who was competitive in the extreme, decided he
too should write up a paper since Feynman was writing one.
Eventually their department chairman convinced them they needed
to write their paper together, which they did, and it became famous.
Although the paper had an acknowledgment to Sudarshan and
Marshak for discussions, their paper appeared later in the conference
proceedings and could not compete for the attention of the
community.
Later, in 1963, Feynman, who tried to be generous with ideas,
publicly stated, “The . . . theory that was discovered by Sudarshan
and Marshak, publicized by Feynman and Gell-Mann . . .” But it was
too little, too late. It would have been hard in the best of times to
compete in the limelight with Feynman and Gell-Mann, and
Sudarshan had to live for years with the knowledge that the
universal form of the weak interaction, which two of the world’s
physics heroes had discovered, was first proposed—and with more
confidence—by him.
Sudarshan’s theory, as elucidated beautifully in Feynman and
Gell-Mann’s paper, became known as the V-A theory of the weak
interaction. The reason for the name is technical and will make
more sense in coming chapters, but the fundamental idea is simple,
though it sounds both ridiculous and meaningless: the currents in
the Fermi theory must be “left-handed.”
To understand this terminology, recall that in quantum
mechanics elementary particles such as electrons, protons, and
neutrinos have spin angular momentum—they behave as if they are
spinning even though classically a point particle without extension
can’t be pictured as spinning. Now, consider the direction of their
motion and pretend for a moment the particle is like a top spinning
around that axis. Put your right hand out and let your thumb point
in the direction of the particle’s motion. Then curl your other fingers
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around. If they are curling in the same (counterclockwise) direction