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discovery of Yukawa's meson, this discovery was not without its own 
problems. In 1940 the decay of a meson to an electron, predicted by Yu-
kawa, was observed in cosmic-ray tracks. However, over the years 1943 
to 1947 it became clear that the particles Anderson and Neddermeyer 
had discovered interacted much more weakly with nuclei than Yukawa's 
particle should have. 
Something was wrong. 
Three of Yukawa's Japanese colleagues suggested that mesons were of 
two different sorts, and that a Yukawa-type meson might decay into yet 
another, different and more weakly interacting meson. But their articles 
were in Japanese and didn't appear in English until after the war, by which 
time a similar proposal had been made by the US physicist Robert Marshak. 
This delay proved fortuitous. New techniques were being developed 
to observe the tracks of cosmic rays in photographic emulsions, and a 
series of brave researchers dragged their equipment up to high eleva-
tions to search for possible new signals. Many cosmic rays interact and 
disappear before reaching sea level, so this group and others interested 
in exploring this wondrous new source of particles coming from the 
heavens had no choice but to seek higher elevations. Here cosmic rays 
would have traversed less distance in the atmosphere and might be 
more easily detected. 
The former Italian mountain guide turned physicist Giuseppe Oc-
chialini had been invited from Brazil to join a British team working on 
the A-bomb during the war. As a foreign national, he couldn't work on 
the project, so instead he joined the cosmic-ray physics group at Bris-
tol. Occhialini's mountain training proved useful as he dragged photo-
graphic emulsions up to the Pic du Midi at twenty-eight hundred meters 
in France. Today you can travel to the observatory on top of this peak by 
cable car, and it is a terrifyingly exciting ride. But in 1946 Occhialini had 
to climb to the top, risking his health in the effort to discover signals of 
exotic new physics. 
And he and his team did discover exotic new physics. As Cecil Pow-
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ell, Occhialini's collaborator at Bristol (and future Nobel laureate, while 
Occhialini, who had done the climbing, did without), put it, they saw 
"a whole new world. It was as if, suddenly, we had broken into a walled 
orchard, where protected trees flourished and all kinds of exotic fruits 
had ripened in great profusion! 
Less poetically, perhaps, what they discovered were two examples 
in which an initial meson stopped in the emulsion and gave rise to a 
second meson, just as had been suggested by the theorists. Many more 
events were observed with emulsions taken to an elevation almost twice 
as high as Pic du Midi. In October of 1947, in the journal Nature, Pow-
ell, Occhialini, and Powell's student Cesare Lattes published a paper in 
which they named the initial meson the pion—which seemed to inter-
act with the nuclear strength appropriate to Yukawa's meson—and the 
subsequent meson the muon. 
It seemed at long last that Yukawa's meson had been discovered. As 
for its "partner" the muon, which had been confused with Yukawa's 
meson, it was nothing of the sort. Not spinless, it instead had the same 
spin as the electron and the proton. And its interactions with matter 
were nowhere near strong enough to play a role in nuclear binding. The 
muon turned out to be simply a heavy, if unstable, copy of the electron, 
which is what motivated Rabi's question "Who ordered that?" 
So, the particle that made Yukawa famous wasn't the particle he 
predicted after all. His idea became famous because the original ex-
perimental result had been misinterpreted. Fortunately, the Nobel 
committee waited until the 1947 discovery of the pion before awarding 
Yukawa their prize in 1949. 
But, given the track record of errors and mislabeling, it is natural to 
wonder if the pion was in fact the particle Yukawa had predicted. The 
answer is both yes and no. Exchange of charged pions between protons 
and neutrons is indeed one accurate way of trying to estimate the strong 
nuclear force holding nuclei together. But in addition to charged pions-
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the mesons that Yukawa had predicted—there are neutral pions as well. 
Who ordered those? 
Moreover, the theory that Yukawa wrote down to describe the strong 
force, like Fermi's theory to describe neutron decay, was not fully math-
ematically consistent, as Yukawa had conceded when he proposed it. 
There was, at the time, no correct relativistic theory involving the ex-
change of massive particles. Something was still amiss, and a series of 
surprising experimental discoveries, combined with prescient theoreti-
cal ideas that were unfortunately applied to the wrong theories, helped 
lead to more than a decade of confusion before the fog lifted and light 
appeared at the end of the tunnel. Or perhaps at the mouth of the cave. 
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Chapter 12 
MARCH OF THE TITANS 
The wolf also shall dwell with the lamb, and the 
leopard shall lie down with the kid. 
-ISAIAH 11:6 
The relationship between theoretical insight and experi-
mental discovery is one of the most interesting aspects of the progress 
of science. Physics is at its heart, like all of science, an empirical disci-
pline. Yet at times brief bursts of theoretical insight change everything. 
Certainly Einstein's insights into space and time in the first two decades 
of the twentieth century are good examples, and the remarkable theo-
retical progress associated with the development of quantum mechan-
ics by Schrodinger, Heisenberg, Pauli, Dirac, and others in the 1920s is 
another. 
Less heralded is another period, from 19s4 to 1974, which, while not 
as revolutionary, will, when sufficient time has passed, be regarded as 
one of the most fruitful and productive theoretical physics eras in the 
twentieth century. These two decades took us, not without turmoil, 
from chaos to order, from confusion to confidence, and from ugliness 
to beauty. It's a wild ride, with a few detours that might seem to come 
from left field, but bear with me. If you find it a tad uncomfortable, then 
151 
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recall what I said in the introduction about science and comfort. By put-
ting yourself in the frame of mind of those involved in the quest, whose 
frustration eventually led to insights, the significance of the insights can 
be truly appreciated. 
This tumultuous period followed one in which experimental bomb-
shells had produced widespread confusion, making nature "curiouser 
and curiouser," as Lewis Carroll might have put it. The discoveries of 
the positron and quickly thereafter the neutron were just the begin-
ning. Neutron decay, nuclear reactions, muons, pions, and a host of new 
elementary particles that followed made it appear as if fundamental 
physics was hopelessly complicated. The simple picture of a universe in 
which electromagnetism and gravity alone governed the interactions of 
matter made from protons and electrons disappeared into the dustbin 
of history. Some physicists at the time, like some on the political right 
today, yearned for the (often misremembered) simplicity of the good old 
days. 
This newfound complexity drove some, by the 1960s, to imagine that 
nothing was fundamental. In a Zen-like picture, they imagined that all 
elementary particles were made from all other elementary particles, and 
that even the notion of fundamental forces might be an illusion. 
Nevertheless, percolating in the background were theoretical ideas 
that would draw back the dark curtains of ignorance and confusion, 
revealing an underlying structure to nature that is as remarkable as it 
is strangely simple, and one in which light would once again play a key 
role. 
It all began with two theoretical developments, one profound and 
unheralded and another relatively straightforward but brilliant and im-
mediately feted. Remarkably, the same man was involved in both. 
Born in 1922 to a mathematician father, Chen-Ning Yang was edu-
cated in China, moving in 1938 from Beijing to Kunming to avoid the 
Japanese invasion of China. He graduated four years later from National 
Southwestern Associated University and remained there for another 
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two years. There he met another student who had been forced to re-
locate to Kunming, Tsung-Dao Lee. While they only had a marginal 
acquaintance with the United States, in 1946 both of them received 
scholarships set up by the US government, with funds received from 
China to allow talented Chinese students to pursue graduate study in 
America. Yang had a master's degree and therefore had greater freedom 
to pursue a PhD, and went with Fermi from Columbia to the University 
of Chicago. Lee had less choice, as he did not have a master's degree, but 
the only US university where he could work directly toward a PhD was 
also the University of Chicago. Yang did his PhD under the supervision 
of Edward Teller and worked directly with Fermi as his assistant for 
only a year after graduation, while Lee did his PhD with Fermi directly. 
During the 1940s, the University of Chicago was one of the greatest 
centers of theoretical and experimental physics in the country, and its 
graduate students benefited from their exposure to a remarkable set of 
scientists—not only Fermi and Teller, but others including the brilliant 
but unassuming astrophysicist Subrahmanyan Chandrasekhar. When 
he was nineteen, Chandra, as he was often called by colleagues, had 
proved that stars greater than 1.4 times the mass of the Sun must col-
lapse catastrophically at the end of their nuclear-burning lifetime, either 
through what is now known to be a supernova explosion, or directly in 
what is now known as a black hole. While his theory was ridiculed at 
the time, he was awarded the Nobel Prize for that work fifty-three years 
later. 
Chandra was not just a brilliant scientist but, like Fermi, a dedicated 
teacher. Even though he was pursuing research at the Yerkes Observa-
tory in Wisconsin, he drove one hundred miles round-trip each week to 
teach a class to just two registered students, Lee and Yang. Ultimately, 
the entire class, professor included, became Nobel laureates, which is 
probably unique in the history of science. 
Yang moved to the venerable Institute for Advanced Study in Prince-
ton in 1949, where he nurtured his budding collaboration with Lee on 
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a variety of topics. In 1952 Yang was made a permanent member of the 
institute, while Lee moved in 19s3 to nearby Columbia in New York City, 
where he remained for the rest of his career. 
Each of these men made major contributions to physics in a variety 
of areas, but the collaboration that made them famous began with a 
strange experimental result, again coming from cosmic-ray observa-
tions. 
In the same year that Yang moved from Chicago to the IAS, Cecil 
Powell, the discoverer of the pion, discovered a new particle in cosmic 
rays, which he called the tau meson. This particle was observed to decay 
into three pions. Another new particle was discovered shortly thereaf-
ter, called the theta meson, which decayed into two pions. Surprisingly, 
this new particle turned out to have precisely the same mass and life-
time as that tau meson. 
This might not seem that strange. Might they be the same particle, 
simply observed to decay in two different ways? Remember that in quan-
tum mechanics, anything that is not forbidden can happen, and as long 
as the new particle was heavy enough to decay into either two or three 
pions—and the weak force allowed such decays—both should occur. 
But, if it were sensible, the weak force shouldn't have allowed both 
decays. 
Think for a moment about your hands. Your left hand differs from 
your right hand. No simple physical process, short of entering through 
the looking glass, can convert one into the other. No series of move-
ments, up or down, turning around, or jumping up and down, can turn 
one into the other. 
The forces that govern our experience, electromagnetism and gravity, 
are blind to the distinction between left and right. No process moder-
ated by either force can turn something such as your right hand into its 
mirror image. I cannot turn your right hand into your left hand merely 
by shining light on it, for example. 
Put another way, if I shine a light on your right hand and look at it 
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from a distance, the intensity of reflected light will be the same as it 
would be if I did the same thing to your left hand. The light doesn't care 
about left or right when it is reflecting off something. 
Our definition of left and right is imposed by human convention. 
Tomorrow we could decide that left is right and vice versa, and nothing 
would change except our labels. As I write this on an airplane, flying 
economy class, the person to my right may be quite different from the 
person to my left, but again that is just an accident of my circumstances. 
I don't expect that the laws governing the flight of this plane are differ-
ent for the right wing than for the left wing. 
Think about this in the subatomic world. Recall that Enrico Fermi 
found that, given the rules of quantum mechanics, the mathematical 
behavior of groups or pairs of elementary particles depends on whether 
they have spin 54, i.e., are fermions. The behavior of groups of fermions 
is quite different from the behavior of particles such as photons, which 
have a spin value of 1 (or any integer value of spin angular momentum, 
i.e., o, i, 2, 3, etc.). The mathematical "wave function" that describes a 
pair of fermions, for example, is santisymmetric," while one describing 
a pair of photons is "symmetric." This means that if one interchanges 
one particle with another, the wave function describing fermions 
changes sign. But for particles such as photons, the wave function re-
mains the same under such an interchange. 
Interchanging two particles is the same as reflecting them in the 
mirror. The one on the left now becomes the one on the right. Thus an 
intimate connection exists between such exchanges and what physicists 
call parity, which is the overall property of a system under reflection 
(i.e., interchanging left and right). 
If an elementary particle decays into two other particles, the wave 
function describing the "parity" of the final state (i.e., whether the wave 
function changes sign or not under left-right interchange of the par-
ticles) allows us then to assign a quantity we can call parity to the initial 
particle. In quantum mechanics if the force that governs the decay is 
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blind to left and right, then the decay will not change the parity of the 
quantum state of the system. 
If the wave function of the system is antisymmetric under inter-
change of the particles after the decay, then the system has "negative" 
parity. In this case the wave function describing the initial quantum 
state of the decaying particle must also have negative parity (i.e., it 
would change sign if left and right were interchanged). 
Now, pions, the particles discovered by Powell and hypothesized by 
Yukawa, have negative parity, so that the wave function that describes 
the quantum state of their mirror image would change sign compared to 
the original wave function. The distinction between positive and negative 
parity is kind of like considering first a nice spherical ball, which looks 
identical when reflected in the mirror, and hence has positive parity: 
O 0 
Versus, say, your hand, which changes character (from left to right) 
when reflected in a mirror and could therefore be said to have negative 
parity: 
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These somewhat abstract considerations made the observed data on 
the decays of the new particles that Powell discovered perplexing. Be-
cause a pion has negative parity, two pions would have positive parity, 
since (-0.= 1. A system of three pions, however, would, by the same 
consideration, have negative parity, since (-03 = —1. Therefore if parity 
doesn't change when a particle decays, a single original particle cannot 
decay into two different final states of different parity. 
If the force responsible for the decay behaved like all the other 
known forces at the time, such as electromagnetism or gravity, it would 
be blind to parity (it would not distinguish between right and left), so it 
shouldn't change the original parity of the system after the decay, just 
as shining a light on your right hand will not cause it to look like your 
left hand. 
Since it seemed impossible for a single type of particle to decay some-
times into two, and sometimes into three, pions, the solution seemed 
simple. There must be two different new elementary particles, with op-
posite parity properties. Powell dubbed these the tau particle and theta 
particle—one of which could decay into two pions, and one into three 
pions. 
Observations suggested that the two particles had precisely the same 
masses and lifetimes, which was a bit strange, but Lee and Yang pro-
posed that this might be a general property for various elementary par-
ticles, which they suggested might come in pairs with opposite parity. 
They called this idea "parity doubling." 
Such was the situation in the spring of 190 when the International 
Conference on High Energy Physics, held every year at the University of 
Rochester, took place. In 1956, the entire community of physicists inter-
ested in particle and nuclear physics could fit in a single university lec-
ture 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 
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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 symme-
try law which the physicists have learned so well may be violated. This 
prospect did not appeal to us." 
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-6o. 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 an-
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other, 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 ex-
perimentalist 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 institu-
tions 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 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 femi-
nine." 
Be that as it may, Wu was an expert in neutron decay and became in-
trigued 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 as-
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sembled 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 195os. It 
was also a time when physicists apparently didn't bother to take holi-
days. Despite its being the yuletide, the Friday "Chinese Lunches" orga-
nized 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 ex-
periment. 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 Mar-
cel 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. 
Columbia University called what was probably the first press confer-
ence ever announcing a scientific result. Feynman lost a 55o bet, but 
Wolfgang Pauli was luckier. He had written a letter from Zurich on Jan-
uary is 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. 
Upon hearing the news, Pauli later wrote, "Now that the first 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. 
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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 19s7, 
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 elec-
tromagnetism interaction as a force between two different electric cur-
rents, each corresponding to the two separate moving electrons that 
interact with each other. The weak interaction could be thought of in 
a somewhat similar way, if in one current a neutron, during the inter-
action, 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 interac-
tion 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 al-
luded, 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 dif-
fers from its mirror image. If the currents in the weak interaction had a 
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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 al-
lowed 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 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 '947, Marshak had been the first to suggest that two dif-
ferent 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 
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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. 
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 in-
teraction. 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 igsos, Gell-Mann would produce many of the most impor-
tant 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 pro-
posal, but that at best Gell-Mann was planning to include the notion in 
one paragraph of a long general paper on the weak interaction. 
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Meanwhile, 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 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 com-
ing 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 spin-
ning. Now, consider the direction of their motion and pretend for a mo-
ment the particle is like a top spinning around that axis. Put your right 
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166 
hand out and let your thumb point in the direction of the particle's mo-
tion. Then curl your other fingers around. If they are curling in the same 
(counterclockwise) direction that the particle/top is spinning about the 
direction of motion, the particle is said to be right-handed. If you put 
your left hand out and do the same thing, a left-handed particle would 
be spinning clockwise to match the direction of your left-curled hand: 
right-handed 
left-handed 
Just as viewing your left hand in a mirror will make it look like a right 
hand, if you see a spinning arrow in the mirror, its direction of motion 
will be flipped, so that if the arrow is moving away from you in the real 
world, it will be moving toward you in the mirror, but the spin will not 
be flipped. Thus, in the mirror a left-handed particle will turn into a 
right-handed particle. (And so, if the poor souls in Plato's cave had had 
mirrors, they might have felt less strange about the shadows of arrows 
flipping direction.) 
This working picture of left-handed particles is not exact, because if 
you think about it, you can also turn a left-handed particle into a right-
handed particle by simply moving faster than the particle. In a frame 
in which a person at rest observes the particle zipping by, it may be 
moving to the left. But if you hop in a rocket and head off to the left and 
pass by that particle, then relative to you, it is moving to the right. As 
a result, only for particles that are massless—and are therefore moving 
at the speed of light—is the above description exact. For, if a particle 
is moving at the speed of light, nothing can move fast enough to pass 
the particle. Mathematically, the definition of left-handed has to take 
this effect into account, but this complication need not concern us any 
more here. 
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Electrons can spin in either direction, but what the V-A interaction 
implies mathematically is that only those moving electrons whose cur-
rents are left-handed can "feel" the weak force and participate in neu-
tron decay. Right-handed currents don't feel the force. 
What is more amazing is that neutrinos only feel the weak force, and 
no other force. As far as we can tell, neutrinos are only left-handed. It 
is not just that only one sort of neutrino current engages in the weak 
interaction. In all the experimental observations so far, there are no 
right-handed neutrinos—perhaps the most explicit demonstration of 
the violation of parity in nature. 
The seeming silliness of this nomenclature was underscored to me 
years ago when I was watching a Star Thek Deep Space Nine episode, 
during which a science officer on the space station discovers something 
wrong with the laws of probability in a gaming casino. She sends a neu-
trino beam through the facility, and the neutrinos are observed to be 
coming out only left-handed. Clearly something was wrong. 
Except that is the way it really is. 
What is wrong with nature? How come, for at least one of the fun-
damental forces, left is different from right? And why should neutrinos 
be so special? The simple answer to these questions is that we don't yet 
know, even though our very existence, which derives from the nature of 
the known forces, ultimately depends on it. That is one reason we are 
trying to find out. The elucidation of a new force led to a new puzzle, and 
like most puzzles in science, it ultimately provided the key that would 
lead physicists down a new path of discovery. Learning that nature 
lacked the left-right symmetry that everyone had assumed was funda-
mental led physicists to reexamine how symmetries are manifested in 
the world, and more important, how they are not. 
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