required both an intense beam and one whose initial polarization
was well determined.
The best place to perform these experiments was at the Stanford
Linear Accelerator, a two-mile-long electron linear accelerator built
in 1962 that was the longest and straightest structure that had ever
been built. In 1970 polarized beams were introduced, but not until
1978 was an experiment designed and run with the sensitivity
required to look for weak-electromagnetic interference in electron
scattering.
While the successful observation of neutral currents in 1974
meant that the Glashow-Weinberg-Salam theory began to have wide
acceptance among theorists, what made the 1978 SLAC experiment
so important was that in 1977 two atomic physics experiments had
reported results that, if correct, convincingly ruled out the theory.
In our story thus far, light has played a crucial role, illuminating
(if you will forgive the pun) our understanding not only of electricity
and magnetism, but space, time, and ultimately the nature of the
quantum 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 additional 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 Glashow-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 Physical Review Letters. The results were dismaying. No
such optical rotation was seen at a level ten times smaller than that
predicted by the electroweak 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
preparation had begun, the experiment was approved to begin to
take data in early 1978. Because of the earlier null results from the
atomic physics experiments, 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.
Within two months the experiment began to show clear signs of
parity violation, and by June 1978 the scientists announced a
nonzero result, 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
disagreement with the Seattle/Oxford results. At a talk at Caltech on
the subject, Richard Feynman, characteristically, homed in on a key
outstanding experimental 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 10
percent. Electroweak 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
͟͞͠
identical. 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 stubbornly resist all attempts at explanation even as the
electroweak theory took shape. No explanation of the strong nuclear
force via spontaneously 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.
͞͠͠
C h a p t e r 1 9
F R E E AT L A S T
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 physicists that in the 1960s many of them had given up
hop
e of ever explaining 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
observed, 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 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 mediating 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 described 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 fundamental. The sound of no hands clapping, if
you will.
Whenever theory faces an impasse like this, it sure helps to have
experiment 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 high-energy electrons off protons and
neutrons, revealed something remarkable. 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. James Bjorken
demonstrated 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 interacting
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 1965, 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 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
interaction, 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 before and after the interaction. However,
because the d and s quarks had precisely the same electric and
isotopic spin charges, nothing would prevent 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
explained 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 calculated 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 p
articles 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
quarks would form one pair, and the charm and the strange quark
another 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
observed 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
development of the first successful quantum field theory, quantum
electrodynamics, 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
democracy, 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 1965, 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 1950s that the
electric 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 dielectric material gets screened. Positively charged
virtual particles tend to closely surround the negative charge, and so
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 26
