Lawrence Krauss - The Greatest Story Ever Told--So Far

by Why Are We Here (pdf)

  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