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world. So too it was realized that light could help probe for a possible 
electroweak unification. 
The first great success of quantum electrodynamics was the correct 
prediction of the spectrum of hydrogen, and eventually other atoms. But 
if electrons also feel the weak force, then this will provide a small addi-
tional force between electrons and nuclei that should alter—if slightly—
the characteristics of their atomic orbits. For the most part these are 
unobservable because electromagnetic effects swamp weak effects. But 
weak interactions violate parity, so the same weak-electromagnetic 
neutral current interference that was being explored using polarized 
electron beams can produce novel effects in atoms that would vanish if 
electromagnetism was the only force involved. 
In particular, for heavy atoms, the Weinberg-Salam theory predicted 
that if polarized light was transmitted through a gas of atoms, then the 
direction of the polarization of the light would be rotated by about a 
millionth of a degree, due to parity-violating neutral current effects in 
the atoms through which the light passed. 
In 1977 the results of two independent atomic physics experiments, 
in Seattle and Oxford, were published in back-to-back articles in Physi-
cal Review Letters. The results were dismaying. No such optical rotation 
was seen at a level ten times smaller than that predicted by the elec-
troweak theory. Had only one experiment reported the result, it would 
have been more equivocal. But the same result from two independent 
experiments using independent techniques made it appear definitive. 
The theory appeared to be ruled out. 
Nevertheless, the SLAC experiment, which had begun three years 
earlier, was well under way, and since all of the experimental prepara-
tion had begun, the experiment was approved to begin to take data in 
early 1978. Because of the earlier null results from the atomic physics ex-
periments, the Stanford collaboration added several bells and whistles 
to the experiment so that if they saw no effect, they could guarantee that 
they could have seen such an effect were it there. 
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Within two months the experiment began to show clear signs of par-
ity violation, and by June 1978 the scientists announced a nonzero re-
sult, in agreement with the predictions of the Glashow-Weinberg-Salam 
model, based on measured neutrino neutral current scattering, which 
measured the strength of the Z interaction. 
Still, questions remained, especially given the apparent disagree-
ment with the Seattle/Oxford results. At a talk at Caltech on the subject, 
Richard Feynman, characteristically, homed in on a key outstanding ex-
perimental question and asked whether the SLAC experimentalists had 
checked that the detector responded equally well to both left-handed 
and right-handed electrons. They hadn't, but for theoretical reasons 
they had had no reason to expect the detectors to behave differently for 
the different polarizations. (Feynman would famously get to the heart 
of another complex problem eight years later after the tragic Challenger 
explosion, when he simply demonstrated the failure of an O-ring seal to 
the investigating commission and to the public watching the televised 
proceedings.) 
Over the fall the SLAC experiment refined their efforts to rule out 
both this concern and others that had been raised, and by the fall they 
reported a definitive result in agreement with the Glashow-Weinberg-
Salam prediction, with an uncertainty of less than io percent. Elec-
troweak unification was vindicated! 
To date, I don't know if anyone has a good explanation of why the 
original atomic physics results were wrong (later experiments agreed 
with the Glashow-Weinberg-Salam theory) except that the experiments, 
and the theoretical interpretation of the experiments, are hard. 
But a mere year later, in October 1979, Sheldon Glashow, Abdus 
Salam, and Steven Weinberg were awarded the Nobel Prize for their 
electroweak theory, now validated by experiment, that unified two of 
the four forces of nature based on a single fundamental symmetry, 
gauge invariance. If the gauge symmetry hadn't been broken, hidden 
from view, the weak and electromagnetic interactions would look iden-
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tical. But then all of the particles that make us up wouldn't have mass, 
and we wouldn't be here to notice.... 
This is not the end of our story, however. Two out of four is still 
only two out of four. The strong interaction, which had motivated much 
of the work that led to electroweak unification, had continued to stub-
bornly resist all attempts at explanation even as the electroweak theory 
took shape. No explanation of the strong nuclear force via spontane-
ously broken gauge symmetries met the test of experiment. 
Thus, even as scientist-philosophers of the twentieth century had 
stumbled—often by a convoluted and dimly lit path—outside our cave 
of shadows to glimpse the otherwise hidden reality beneath the surface, 
one more force relevant to understanding the fundamental structure of 
matter was conspicuously missing from the beautiful emerging tapestry 
of nature. 
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Chapter 19 
FREE AT LAST 
Let my people go. 
-EXODUS 9:1 
The long road that led to electroweak unification was a 
tour de force of intellectual perseverance and ingenuity. But it was also 
a detour de force. Almost all of the major ideas introduced by Yang, 
Mills, Yukawa, Higgs, and others that led to this theory were developed 
in the apparently unsuccessful struggle to understand the strongest 
force in nature, the strong nuclear force. Recall that this force, and the 
strongly interacting particles that manifested it, had so bedeviled physi-
cists that in the 196os many of them had given up hope of ever explain-
ing it via the techniques of quantum field theory that had so successfully 
now described both electromagnetism and the weak interaction. 
There had been one success, centered on Gell-Mann and Zweig's 
proposal that all the strongly interacting particles that had been ob-
served, including the proton and the neutron, could be understood as 
being made up of more fundamental objects, which, as I have described, 
Gell-Mann called quarks. All the known strongly interacting particles, 
and at the time undiscovered particles, could be classified assuming 
they were made of quarks. Moreover, the symmetry arguments that led 
231 
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Gell-Mann in particular to come up with his model served as the basis 
for making some sense of the otherwise confusing data associated with 
the reactions of strongly interacting matter. 
Nevertheless, Gell-Mann had allowed that his scheme might merely 
be a mathematical construct, useful for classification, and that quarks 
might not represent real particles. After all, no free quarks had ever 
been observed in accelerators or cosmic-ray experiments. He was also 
probably influenced by the popular idea that quantum field theory, and 
hence the notion of elementary particles themselves, broke down on 
nuclear scales. Even as late as 1972 Gell-Mann stated, "Let us end by 
emphasizing our main point, that it may well be possible to construct an 
explicit theory of hadrons, based on quarks and some kind of glue.. . . 
Since the entities we start with are fictitious, there is no need for any 
conflict with the bootstrap ... point of view." 
Viewed in this context, the effort to describe the strong interaction by 
a Yang-Mills gauge quantum field theory, with real gauge particles medi-
ating the force, would be misplaced. It also seemed impossible. The strong 
force appeared to operate only on nuclear scales, so if it was to be de-
scribed by a gauge theory, the photonlike particles that would convey the 
force would have to be heavy. But there was also no evidence of a Higgs 
mechanism, with massive strongly interacting Higgs-like particles, which 
experiments could have easily detected. Compounding this, the force was 
simply so strong that even if it was described by a gauge theory, then all of 
the quantum field theory techniques developed for deriving predictions—
which worked so well for the other forces—would have broken down if 
applied to the strong force. This is why Gell-Mann in his quote referred to 
the "bootstrap"—the Zen-like idea that no particles were truly fundamen-
tal. The sound of no hands clapping, if you will. 
Whenever theory faces an impasse like this, it sure helps to have ex-
periment as a guide, and that is exactly what happened, in 1968. A series 
of pivotal experiments, performed by Henry Kendall, Jerry Friedman, 
and Richard Taylor, using the newly built SLAC accelerator to scatter 
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high-energy electrons off protons and neutrons, revealed something re-
markable. Protons and neutrons did appear to have some substructure, 
but it was strange. The collisions had properties no one had expected. 
Was the signal due to quarks? 
Theorists were quick to come to the rescue. lames Bjorken demon-
strated that the phenomena observed by the experimentalists, called 
scaling, could be understood if protons and neutrons were composed of 
virtually noninteracting pointlike particles. Feynman then interpreted 
these objects as real particles, which he dubbed partons, and suggested 
they could be identified with Gell-Mann's quarks. 
This picture had a big problem, however. If all strongly interact-
ing particles were composed of quarks, then quarks should surely be 
strongly interacting themselves. Why should they appear to be almost 
free inside protons and neutrons and not be interacting strongly with 
each other? 
Moreover, in 196s, Nambu, Moo-Young Han, and Oscar Greenberg 
had convincingly argued that, if strongly interacting particles were 
composed of quarks and if they were fermions, like electrons, then Gell-
Mann's classification of known particles by various combinations of 
quarks would only be consistent if quarks possessed some new kind of 
internal charge, a new Yang-Mills gauge charge. This would imply that 
they interacted strongly via a new set of gauge bosons, which were then 
called gluons. But where were the gluons, and where were the quarks, 
and why was there no evidence of quarks interacting strongly inside 
protons and neutrons if they were really to be identified with Feynman's 
partons? 
In yet another problem with quarks, protons and neutrons have weak 
interactions, and if these particles were made up of quarks, then the 
quarks would also have to have weak interactions in addition to strong 
interactions. Gell-Mann had identified three different types of quarks as 
comprising all known strongly interacting particles at the time. Mesons 
could be comprised of quark-antiquark pairs. Protons and neutrons 
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could be made up of three fractionally charged quarks, which Gell-
Mann called up (u) and down (d) quarks. The proton would be made of 
two up quarks and one down quark, while the neutron would be made 
of two down quarks and one up quark. In addition to these two types 
of quarks, one additional type of quark, a heavier version of the down 
quark, was required to make up exotic new elementary particles. Gell-
Mann called this the strange (s) quark, and particles containing s quarks 
were dubbed to possess "strangeness." 
When neutral currents were first proposed as part of the weak inter-
action, this created a problem. If quarks interacted with the Z particles, 
then u, d, and s quarks could remain u, d, and s quarks before and after 
the neutral current interaction, just as electrons remained electrons be-
fore and after the interaction. However, because the d and s quarks had 
precisely the same electric and isotopic spin charges, nothing would pre-
vent an s quark from converting into a d quark when it interacted with 
a Z particle. This would allow particles containing s quarks to decay 
into particles containing d quarks. But no such "strangeness-changing 
decays" were observed, with high sensitivity in experiments. Something 
was wrong. 
This absence of "strangeness-changing neutral currents" was ex-
plained brilliantly, at least in principle, by Sheldon Glashow, along with 
collaborators John Iliopoulos and Luciano Maiani, in 1970. They took 
the quark model seriously and suggested that if a fourth quark, dubbed 
a charm (c) quark, existed, which had the same charge as the u quark, 
then a remarkable mathematical cancellation could occur in the calcu-
lated transformation rate for an s quark into a d quark, and strangeness-
changing neutral currents would be suppressed, in agreement with 
experiments. 
Moreover, this scheme began to suggest a nice symmetry between 
quarks and particles such as electrons and muons, all of which could 
exist in pairs associated with the weak force. The electron would be 
paired with its own neutrino, as would the muon. The up and down 
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quarks would form one pair, and the charm and the strange quark an-
other pair. W particles interacting with one particle in each pair would 
turn it into the other particle in the pair. 
None of these arguments addressed the central problems of the 
strong interaction between quarks, however. Why had no one ever ob-
served a quark? And, if the strong interaction was described by a gauge 
theory with gluons as the gauge particles, how come no one had ever 
observed a gluon? And if the gluons were massless, how come the strong 
force was short-range? 
These problems continued to suggest to some that quantum field 
theory was the wrong approach for understanding the strong force. 
Freeman Dyson, who had played such an important role in the develop-
ment of the first successful quantum field theory, quantum electrody-
namics, asserted, when describing the strong interaction, "The correct 
theory will not be found in the next hundred years." 
One of those who were convinced that quantum field theory was 
doomed was a brilliant young theorist, David Gross. Trained under 
Geoffrey Chew, the inventor of the bootstrap picture of nuclear democ-
racy, in which elementary particles were an illusion masking a structure 
in which only symmetries and not particles were real, Gross was well 
primed to try to kill quantum field theory for good. 
Recall that even as late as 196s, when Richard Feynman received 
his Nobel Prize, it was still felt that the procedure he and others had 
developed for getting rid of infinities in quantum field theory was a 
trick—that something was fundamentally wrong at small scales with 
the picture that quantum field theory presented. 
Russian physicist Lev Landau had shown in the 195os that the elec-
tric charge on an electron depends on the scale at which you measure 
it. Virtual particles pop out of empty space, and electrons and all other 
elementary particles are surrounded by a cloud of virtual particle-
antiparticle pairs. These pairs screen the charge, just as a charge in a di-
electric material gets screened. Positively charged virtual particles tend 
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THE GREATEST STORY EVER TOLD-SO FAR 
to closely surround the negative charge, and so at a distance the physical 
effects of the initial negative charge are reduced. 
This meant, according to Landau, that the closer you get to an elec-
tron, the larger its actual charge will appear. If we measure the electron 
charge to be some specific value at large distances, as we do, that would 
mean that the "bare" charge on the electron—namely the charge on the 
fundamental particle considered without all the infinite dressing by 
particle-antiparticle pairs surrounding it on ever-smaller scales—would 
have to be infinite. Clearly something was rotten with this picture. 
Gross was influenced not only by his supervisor, but also by the pre-
vailing sentiments of the time, mostly arguments by Gell-Mann, who 
dominated theoretical particle physics in the late fifties and early sixties. 
Gell-Mann advocated using algebraic relations that arise from thinking 
about field theories, then keeping the relations and throwing away the 
field theory. In a particularly Gell-Mann-esque description, he stated, 
"We may compare this process to a method sometimes employed in 
French cuisine: a piece of pheasant meat is cooked between two slices of 
veal, which are then discarded." 
Thus one could abstract out properties of quarks that might be useful 
for predictions, but then ignore the actual possible existence of quarks. 
However, Gross began to be disenchanted by just using ideas associated 
with global symmetries and algebras and longed to explore dynamics 
that might actually describe the physical processes that were occurring 
inside strongly interacting particles. Gross and his collaborator Cur-
tis Callan built upon earlier work by James Bjorken to show that the 
charged particle apparently located inside protons and neutrons had to 
have spin 1/2, identical to that of electrons. Later, with other collabora-
tors, Gross showed that a similar analysis of neutrino scattering off pro-
tons and neutrons as measured at CERN revealed that the components 
looked just like the quarks that Gell-Mann had proposed. 
If it quacks like a duck and walks like a duck, it is probably a duck. 
Thus, for Gross, and others, the reality of quarks was now convincing. 
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But as convinced as many such as Gross were by the reality of quarks, 
they were equally convinced that this implied that field theory could not 
possibly be the correct way to describe the strong interaction. The re-
sults of the experiment required the constituents to be essentially non-
interacting, not strongly interacting. 
In 1969 Gross's colleagues at Princeton Curtis Callan and Kurt Sy-
manzik rediscovered a set of equations explored by Landau, and then 
Gell-Mann and Francis Low, that described how quantities in quantum 
field theory might evolve with scale. If the partons inferred by the SLAC 
experiments had any interactions at all—as quarks must have—then 
measurable departures from the scaling that Bjorken had derived would 
occur, and the results that Gross and his collaborators had also derived 
when comparing theory and the SLAC experiments would also have to 
be modified. 
Over the next two years, with the results of 't Hooft and Veltman, 
and the growing success of the predictions of the theory of the weak 
and electromagnetic interactions, more people began to turn their at-
tention once again to quantum field theory. Gross decided to prove in 
great generality that no sensible quantum field theory could possibly re-
produce the experimental results about the nature of protons and neu-
trons observed at SLAC. Thus he hoped to kill this whole approach to 
attempting to understand the strong interaction. First, he would prove 
that the only way to explain the SLAC results was if somehow, at short 
distances, the strength of the quantum field interactions would have 
to go to zero, i.e., the fields would essentially become noninteracting at 
short distances. Then, after that, he would show that no quantum field 
theory had this property. 
Recall that Landau had shown that quantum electrodynamics, the 
prototypical consistent quantum field theory, has precisely the opposite 
behavior. The strength of electric charges becomes larger as the scale 
at which you probe particles (such as electrons) gets smaller due to the 
cloud of virtual particles and antiparticles surrounding them. 
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Early in 1973 Gross and his collaborator Giorgio Parisi had completed 
the first part of the proof, namely that scaling as observed at SLAC im-
plied the strong interactions of the proton's constituents must go to zero 
at small-distance scales if the strong nuclear force was to be described 
by any fundamental quantum field theory. 
Next, Gross attempted to show that no field theories actually had this 
behavior—the strength of interactions going to zero at small-distance 
scales—which he dubbed asymptotic freedom. With help from Har-
vard's Sidney Coleman, who was visiting Princeton at the time, Gross 
was able to complete this proof for all sensible quantum field theories, 
except for Yang-Mills-type gauge theories. 
Gross now took on a new graduate student, twenty-one-year-old 
Frank Wilczek, who had come to Princeton from the University of Chi-
cago planning to study mathematics, but who switched to physics after 
taking Gross's graduate class in field theory. 
Gross was either lucky or astute because he served as the graduate 
supervisor of probably the two most remarkable intellects among physi-
cists in my generation, Wilczek and Edward Witten, who helped lead 
the string theory revolution in the 198os and '9os and who is the only 
physicist ever to win the prestigious Fields Medal, the highest award 
given to mathematicians. Wilczek is probably one of the few true phys-
ics polymaths. Frank and I became frequent collaborators and friends 
in the early 1980s, and he is not only one of the most creative physicists 
I have ever worked with, he also has an encyclopedic knowledge of the 
field. He has read almost every physics text ever written, and he has 
assimilated the information. In the intervening years, he has made nu-
merous fundamental contributions not only to particle physics, but to 
cosmology and also the physics of materials. 
Gross assigned Wilczek to explore with him the one remaining 
loophole in Gross's previous proof—determining how the strength of 
the interaction in Yang-Mills theories changed as one went to shorter-
distance scales—to prove that these theories too could not exhibit 
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239 
asymptotic freedom. They decided to directly and explicitly calculate 
the behavior of the interactions in the theories at shorter and shorter-
distance scales. 
This was a formidable task. Since that time tools have been developed 
for doing the calculation as a homework problem in a graduate course. 
Moreover, things are always easier to calculate when you know what 
the answer will be, as we now do. After several hectic months, with 
numerous false starts and numerical errors, in February of 1973 they 
completed their calculations and discovered, to Gross's great surprise, 
that in fact Yang-Mills theories are asymptotically free-the interaction 
strength in these theories does approach zero as interacting particles 
get closer together. As Gross later put it, in his Nobel address, "For me 
the discovery of asymptotic freedom was totally unexpected. Like an 
atheist who has just received a message from a burning bush, I became 
an immediate true believer! 
Sidney Coleman had assigned his own graduate student David 
Politzer to do a similar calculation, and his independent result agreed 
with Gross and Wilczek's and was obtained at about the same time. 
That the results agreed gave both groups greater confidence in them. 
Not only can Yang-Mills theories be asymptotically free, they are the 
only field theories that are. This led Gross and Wilczek to suggest, in the 
opening of their landmark paper, that because of this uniqueness, and 
because asymptotic freedom seemed to be required for any theory of 
the strong interaction given the 1968 SLAC experimental results, per-
haps a Yang-Mills theory could explain the strong interaction. 
Which Yang-Mills theory was the right one needed to be deter-
mined, and also why the massless gauge particles that are the hallmark 
of Yang-Mills theories had not been seen. And related to this, perhaps 
the most important long-standing question remained: Where were the 
quarks? 
But before I address these questions, you might be wondering why 
Yang-Mills theories have such a different behavior from their sim-
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pier cousin quantum electrodynamics, where Landau had shown the 
strength of the interaction between electric charges gets larger on 
small-distance scales. 
The key is somewhat subtle and lies in the nature of the massless 
gauge particles in Yang-Mills theory. Unlike photons in QED, which 
have no electric charge, the gluons that were predicted to mediate the 
strong interaction possess Yang-Mills charges, and therefore gluons in-
teract with each other. But because Yang-Mills theories are more com-
plicated than QED, the charges on gluons are also more complicated 
than the simple electric charges on electrons. Each gluon not only looks 
like a charged particle, but also like a little charged magnet. 
If you bring a small magnet near some iron, the iron gets magnetized 
and you end up with a more powerful magnet. Something similar hap-
pens with Yang-Mills theories. If I have some particle with a Yang-Mills 
charge, say, a quark, then quarks and antiquarks can pop out of the vac-
uum around the charge and screen it, as happens in electromagnetism. 
But gluons can also pop out of the vacuum, and since they act like little 
magnets, they tend to align themselves along the direction of the field 
produced by the original quark. This increases the strength of the field, 
which in turn induces more gluons to pop out of the vacuum, which 
further increases the field, and so on. 
As a result, the deeper into the virtual gluon cloud you penetrate—
i.e., the closer you get to the quark—the weaker the field will look. Ulti-
mately, as you bring two quarks closer together, the interaction will get 
so weak that they will begin to act as if they are not interacting at all, the 
characteristic of asymptotic freedom. 
I used gluons and quarks as labels here, but the discovery of asymp-
totic freedom did not point uniquely to any specific Yang-Mills theory. 
However, Gross and Wilczek recognized the natural candidate was the 
Yang-Mills theory that Greenberg and others had posited was neces-
sary for Gell-Mann's quark hypothesis to explain the observed nature 
of elementary particles. In this theory each quark carries one of three 
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241 
different types of charges, which are labeled, for lack of better names, by 
colors, say, red, green, or blue. Because of this nomenclature Gell-Mann 
coined a name for this Yang-Mills theory: quantum chromodynamics 
(QCD), the quantum theory of colored charges, in analogy to quantum 
electrodynamics, the quantum theory of electric charges. 
Gross and Wilczek posited, based on the observational arguments in 
favor of such a symmetry associated with quarks, that quantum chro-
modynamics was the correct gauge theory of the strong interaction of 
quarks. 
The remarkable idea of asymptotic freedom got an equally remark-
able experimental boost within a year or so of these theoretical develop-
ments. Experiments at SLAC and at another accelerator in Brookhaven, 
Long Island, made the striking and unexpected discovery of a new mas-
sive elementary particle that appeared as if it might be made up of a new 
quark—indeed, the so-called charmed quark that had been predicted by 
Glashow and friends four years earlier. 
But this new discovery was peculiar, because the new particle lived 
far longer than one might imagine based on the measured lifetime of 
unstable lighter strongly interacting particles. As the experimentalists 
who discovered this new particle said, observing it was like wandering 
in the jungle and finding a new species of humans who lived not up to 
one hundred, but up to ten thousand years. 
Had the discovery been made even five years earlier, it would have 
seemed inexplicable. But in this case, fortune favored the prepared 
mind. Tom Appelquist and David Politzer, both at Harvard at the time, 
quickly realized that if asymptotic freedom was indeed a property of the 
strong interaction, then one could show that the interactions governing 
more massive quarks would be less strong than the interactions govern-
ing the lighter, more familiar quarks. Interactions that are less strong 
would mean particles decay less quickly. What would otherwise have 
been a mystery was in this case a verification of the new idea of asymp-
totic freedom. Everything seemed to be fitting into place. 
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Except for one pretty bt:g. thing. If the theory of quantum chromo-
dynamics was a theory of the interactions of quarks and gluons, where 
were the quarks and gluons? How come none had ever been seen in an 
experiment? 
Asymptotic freedom provides a key clue. If the strength of the strong 
interaction gets weaker the closer one gets to a quark, then conversely it 
should get stronger and stronger the farther one is away from the quark. 
Imagine, then, what happens if I have a quark and an antiquark that are 
bound together by the strong interaction and I try to pull them apart. 
As I try to pull them apart, I need more and more energy because the 
strength of the attraction between them grows with distance. Eventually 
so much energy becomes stored in the fields surrounding the quarks that 
it becomes energetically favorable instead for a new quark-antiquark pair 
to pop out of the vacuum and then for each to become bound to one of the 
original particles. The process is shown schematically below. 
quark 
anti-quark 
 a 
1 + 00 
40-0 40-0 
It would be like stretching a rubber band. Eventually the band will 
snap into two pieces instead of stretching forever. Each piece in this case 
would then represent a new bound quark-antiquark pair. 
What would this mean for experiments? Well, if I accelerate a par-
ticle such as an electron and it collides with a quark inside a proton, it 
will kick the quark out of the proton. But as the quark begins to exit the 
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proton, the interactions of the quark with the remaining quarks will in-
crease, and it will eventually be energetically favored for virtual quark-
antiquark pairs to pop out of the vacuum and bind to both the ejected 
quark and the other quarks as well. This means that one will create a 
shower of strongly interacting particles, such as protons or neutrons or 
pions or so on, moving along the direction of the original ejected quark, 
and similarly a shower of strongly interacting particles recoiling in the 
direction of motion of the original remaining quarks left over from the 
proton. One will never see the quarks themselves. 
Similarly, if a particle collides with a quark, in recoiling sometimes 
the quark will emit a gluon before it binds with an antiquark popping 
out of the vacuum. Then since gluons interact with each other as well as 
with quarks, the new gluon might emit more gluons. The gluons in turn 
will be surrounded by new quarks that pop out of the vacuum, creating 
new strongly interacting particles moving along the direction of each 
original gluon. In this case one would expect in some cases to see not a 
single shower moving in the direction of the original quark, but several 
showers, corresponding to each new gluon that is emitted along the way. 
Because quantum chromodynamics is a specific, well-defined the-
ory, one can predict the rate at which quarks will emit gluons, and the 
rate at which one would see a single shower, or jet as it is called, kicked 
out when an electron collides with a proton or neutron, and the rate 
at which one would see two showers, and so on. Eventually, when ac-
celerators became powerful enough to observe all these processes, the 
observed rates agreed well with the predictions of the theory. 
There is every reason to believe that this picture of free quarks and 
gluons quickly getting bound to new quarks and antiquarks so that one 
would never observe a free quark or gluon is valid. This is called Con-
finement because quarks and gluons are always confined inside strongly 
interacting particles such as protons and neutrons and can never break 
free from them without getting confined in newly created strongly in-
teracting particles. 
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Since the actual process by which the quarks get confined occurs as 
the forces become stronger and stronger when the quark moves farther 
and farther away from its original companions, the standard calcula-
tions of quantum field theory, which are valid when the interactions are 
not too strong, break down. So this picture, validated by experiment, 
cannot be fully confirmed by tractable calculations at the moment. 
Will we ever derive the necessary mathematical tools to analyti-
cally demonstrate from first principles that confinement is indeed 
a mathematical property of quantum chromodynamics? This is the 
million-dollar question, literally. The Clay Mathematics Institute has 
announced a million-dollar prize for a rigorous mathematical proof 
that quantum chromodynamics does not allow free quarks or gluons to 
be produced. While no claimants to the prize have yet come forward, 
we nevertheless have strong indirect support of this idea, coming not 
only from experimental observations, but also from numerical simula-
tions that closely approximate the complicated interactions in quantum 
chromodynamics. This is heartening, if not definitive. We still have to 
confirm that it is some property of the theory and not of the computer 
simulation. However, for physicists, if not mathematicians, this seems 
pretty convincing. 
One final bit of direct evidence that QCD is correct came from a 
realm where exact calculations can be done. Because quarks are not 
completely free at short distances, I earlier mentioned that there should 
be calculable corrections to exotic scaling phenomena observed in the 
high-energy collisions of electrons off protons and neutrons, as origi-
nally observed at SLAC. Perfect scaling would require completely 
noninteracting particles. The corrections that one could calculate in 
quantum chromodynamics would only be observable in experiments 
that were far more sensitive than those originally performed at SLAC. It 
took the development of new, higher-energy accelerators to probe them. 
After thirty years or so, enough evidence was in so that comparison of 
theoretical predictions and experiment agreed at the i percent level, and 
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Free at Last 
245 
quantum chromodynamics as the theory of the strong interaction was 
finally verified in a precise and detailed way. 
Gross, Wilczek, and Politzer were finally awarded the Nobel Prize in 
2004 for their discovery of asymptotic freedom. The experimentalists 
who had first discovered scaling at SLAC, which was the key observa-
tion that set theorists off in the right direction, were awarded the Nobel 
Prize much earlier, in 1990. And the experimentalists who discovered 
the charmed quark in 1974 won the Nobel Prize two years later, in 1976. 
But the biggest prize of all, as Richard Feynman has said, is not the 
recognition by a medal or a cash award, or even the praise one gets from 
colleagues or the public, but the prize of actually learning something 
new about nature. 
• 
• 
• 
In this sense the 197os were perhaps the richest decade in the twentieth 
century, if not in the entire history of physics. In 1970 we understood 
only one force in nature completely as a quantum theory, namely quan-
tum electrodynamics. By 1979 we had developed and experimentally 
verified perhaps the greatest theoretical edifice yet created by human 
minds, the Standard Model of particle physics, describing precisely 
three of the four known forces in nature. The effort spanned the entire 
history of modern science, from Galileo's investigations of the nature of 
moving bodies, through Newton's discovery of the laws of motion, 
through the experimental and theoretical investigations of the nature of 
electromagnetism, through Einstein's unification of space and time, 
through the discoveries of the nucleus, quantum mechanics, protons, 
neutrons, and the discovery of the weak and strong forces themselves. 
But the most remarkable characteristic of all in this long march 
toward the light is how different the fundamental nature of reality is 
from the shadows of reality that we experience every day, and in par-
ticular how the fundamental quantities that appear to govern our exis-
tence are not fundamental at all. 
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THE GREATEST STORY EVER TOLD-SO FAR 
Making up the heart of observed matter are particles that had never 
been directly observed and, if we are correct, will never be directly 
observable—quarks and gluons. The properties of forces that govern the 
interactions of these particles—and also the particles that have formed the 
basis of modern experimental physics for more than a century, electrons—
are also, on a fundamental level, completely different from the properties 
we directly observe and on which we depend for our existence. The strong 
interaction between protons and neutrons is only a long-distance rem-
nant of the underlying force between quarks, whose fundamental proper-
ties are masked by the complicated interactions within the nucleus. The 
weak interaction and the electromagnetic interaction, which could not be 
more different on the surface—one is short-range, while the other is long-
range, and one appears thousands of times weaker than the other—are in 
fact intimately related and reflect different facets of a single whole. 
That whole is hidden from us because of the accident of nature we call 
spontaneous symmetry breaking, which distinguishes the two weak and 
electromagnetic interactions in the world of our experience and hides 
their true nature. More than that, the properties of the particles that 
produce the characteristics of the beautiful world we observe around us 
are only possible because, after the accident of spontaneous symmetry 
breaking, just one particle in nature—the photon—remains massless. 
If symmetry breaking had never occurred so that underlying symme-
tries of the forces governing matter were manifest—which in turn would 
mean that the particles conveying the weak force would also be mass-
less, as would most of the particles that make us up—essentially nothing 
we see in the universe today, from galaxies to stars, to planets, to people, 
to birds and bees, to scientists and politicians, would ever have formed. 
Moreover, we have learned that even these particles that make us up 
are not all that exist in nature. The observed particles combine in simple 
groupings, or families. The up and down quarks make up protons and 
neutrons. Along with them one finds the electron, and its partner, the 
electron neutrino. Then, for reasons we still don't understand, there is 
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