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

by Why Are We Here (pdf)

  spending a sabbatical year. After Higgs’s talk at Harvard, where

  Sheldon Glashow was now a professor, Glashow apparently

  complimented him on having invented a “nice model” and moved

  on. Such was the fixation on the strong interaction that Glashow

  didn’t realize then that this might be the key to resolving the issues

  in the weak interaction theory he had published five years earlier.

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  P a r t T h r e e

  R E V E L A T I O N

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  C h a p t e r 1 7

  T H E W R O N G P L A C E AT T H E

  R I G H T T I M E

  Be not deceived: evil communications corrupt good manners.

  —1 CORINTHIANS 15:33

  All of the six authors of the papers that describe what is most

  commonly called the Higgs mechanism (though after the recent

  Nobel Prize that Higgs shared with Englert, some are now calling it

  the BEH mechanism, for Brout, Englert, and Higgs) suspected and

  hoped that their work would help in understanding the strong force

  in nuclei. In their papers, any discussions of possible experimental

  probes of their ideas referenced the strong interaction—and in

  particular Sakurai’s proposal of heavy vector mesons mediating this

  force. They hoped that a theory of the strong interaction that

  explained nuclear masses and short-range strong nuclear forces was

  around the corner.

  Besides the general fascination with the strong nuclear force in

  nuclear physics, I suspect physicists tried to apply their new ideas to

  this theory for another reason. Given the range and strength of this

  force, the masses of new Yang-Mills-like particles that would be

  necessary to mediate the strong interaction would be comparable to

  the masses of protons and neutrons themselves and also of the other

  new particles being discovered in accelerators. Since experimental

  confirmation is the highest honor that theorists can achieve, it was

  natural to focus on understanding physics at these accessible energy

  scales, where new ideas, and new particles, could be quickly tested

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  and explored in existing machines—with fame, if not fortune,

  around the corner. By contrast, as Schwinger had shown, any theory

  involving new particles associated with the weak force would require

  them to have masses several orders of magnitude larger than those

  available at accelerators at the time. This was clearly a problem to be

  considered at a later time, or so most physicists thought.

  One of the many people who were fascinated by the physics of

  the strong interaction was the young theorist Steven Weinberg.

  There is poetry here as well. Weinberg grew up in New York City

  and attended the Bronx High School of Science, from which he

  graduated in 1950. One of his high school classmates was Sheldon

  Glashow, and the two of them moved together to study at Cornell

  University, living together in a temporary dorm there in their first

  semester before going their separate ways. While Glashow went to

  Harvard for graduate school, Weinberg moved on to Copenhagen—

  where Glashow would spend time as a postdoc—before arriving at

  Princeton to complete his PhD. Both of them were on the faculty at

  Berkeley in the early 1960s, leaving in the same year, 1966, for

  Harvard, where Glashow took up a professorship and Weinberg took

  a visiting position while on leave from Berkeley. Weinberg then

  moved to MIT in 1967, only to return to Harvard in 1973 to take the

  same chair and office that had been vacated by Julian Schwinger,

  Glashow’s former supervisor. (When Weinberg moved into the

  office, he found in the closet a pair of shoes that Schwinger had left,

  clearly as a challenge to the younger scientist to try to fill them. He

  did.) When Weinberg left Harvard in 1982, Glashow then moved to

  occupy the same chair and office, but no shoes were left in the closet.

  The lives of these two scientists were intertwined perhaps as

  closely as those of any other scientists in recent times, yet they form

  an interesting contrast. Glashow’s brilliance is combined with an

  almost childlike enthusiasm for science. His strength lies in his

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  creativity and his understanding of the experimental landscape and

  not so much in his detailed calculational abilities. By contrast,

  Weinberg is perhaps the most scholarly and serious (about physics)

  physicist I have ever known. While he has a wonderful ironic sense

  of humor, he never undertakes any physics project lightly, without

  the intent of mastering the relevant field. His physics textbooks are

  masterpieces, and his popular writing is lucid and full of wisdom. An

  avid reader of ancient history, Weinberg fully communicates the

  historical perspective not only on what he is doing, but on the whole

  physics enterprise.

  Weinberg’s approach to physics is like that of a steamroller.

  When I was at Harvard, we postdocs used to call Weinberg “Big

  Steve.” When he was working on a problem, the best thing you could

  do was get out of the way, or you would be rolled over by the

  immense power of his intellect and energy. Earlier, before I moved

  to Harvard and was still at MIT, a friend of mine at the time,

  Lawrence Hall, was a graduate student at Harvard. Lawrence was

  ahead of me in his work, graduating before me. He told me that he

  was only able to complete the work that became his thesis with

  Weinberg because Weinberg had just won the Nobel Prize in 1979,

  and the ensuing hubbub forced him to slow down enough so that

  Lawrence could complete his calculations before Weinberg beat him

  to the punch.

  One of the great fortunes of my life was to have the opportunity

  to work closely with both Glashow and Weinberg during the early

  and formative years of my own career. After Glashow helped rescue

  me from the black hole of mathematical physics, he became my

  collaborator at Harvard and for years later. Weinberg taught me

  much of what I know about particle theory. At MIT one doesn’t

  have to take courses, just pass exams, so I only took one or two

  physics courses at MIT while working toward my PhD. But one of

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  the perks of being at MIT was that I could take classes at Harvard. I

  took or sat in on every graduate class that Weinberg taught during

  my graduate career, from quantum field theory onward. Glashow

  and Weinberg formed complementary role models for my own

  career. At my best I’ve tried to emulate aspects I learned from each

  of them, while recognizing that most often my “best” wasn’t much in

  comparison.

  Weinberg had, and has, a broad and abiding interest in the details

  of quantum field theory, and like many others during the early

  1960s, he tried to focus on how one might understand the nature of

  the strong interaction using ideas of symmetry that, in large part due

  to the work of Gell-Mann, so dominated the field at the time.

  Weinberg too was thinking about the possible application of ideas

  of symmetry breaking to understanding nuclear masses, based on


  Nambu’s work, and like Higgs, Weinberg was quite disappointed by

  Goldstone’s result that massless particles would always accompany

  such physics. So Weinberg decided, as he almost always did when he

  was interested in some physics idea, that he needed to prove it to

  himself. Thus his subsequent paper with Goldstone and Salam

  provided several independent proofs of the theorem in the context

  of strongly interacting particles and fields. Weinberg was so

  despondent about possible explanations of the strong interaction

  using spontaneous symmetry breaking that he added an epigraph to

  the draft of the paper that echoed Lear’s response to Cordelia:

  “Nothing will come of nothing: speak again.” (My book A Universe

  from Nothing makes plain why I am not a big fan of this quote.

  Quantum mechanics blurs the distinction between something and

  nothing.)

  Weinberg subsequently learned about Higgs’s (and others’) result

  that one could get rid of unwanted massless Goldstone bosons that

  occur through symmetry breaking if the symmetry being broken was

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  a gauge symmetry—where in this case the massless Goldstone

  bosons would disappear and otherwise massless gauge bosons would

  become massive—but Weinberg wasn’t particularly impressed,

  viewing it as many other physicists did, as an interesting technicality.

  Moreover, in the early 1960s the idea that the pion resembled in

  many ways a Goldstone boson was useful in deriving some

  approximate formulas for certain strong interaction reaction rates.

  Thus, the notion of getting rid of Goldstone bosons in the strong

  interaction became less attractive. Weinberg spent several years

  during this period exploring these ideas. He worked out a theory

  whereby some symmetries that were thought to be associated with

  the strong interaction might become broken spontaneously, and

  various strongly interacting vector gauge particles that convey the

  strong interaction might get masses via the Higgs mechanism. The

  problem was he couldn’t get agreement with observations without

  spoiling the initial gauge symmetry that would protect the theory.

  The only way he could avoid this and preserve the initial gauge

  symmetry he needed was if some vector particles became massive,

  and others remained massless. But this disagreed with experiment.

  Then one day in 1967 while driving in to MIT, he saw the light,

  literally and metaphorically. (I have driven with Steve in Boston, and

  while I have lived to talk about it, I have seen how when he is

  thinking about physics, all awareness of large masses such as other

  cars disappears.) Weinberg suddenly realized that maybe he, and

  everyone else, was applying the right ideas of spontaneous symmetry

  breaking, but to the wrong problem! Another example in nature

  could involve two different vector bosons, one type massless and one

  type massive. The massless vector boson could be the photon, and

  the massive one (or ones) could be the massive mediator(s) of the

  weak interaction that had been speculated by Schwinger a decade

  earlier.

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  If this was true, then the weak and electromagnetic interactions

  could be described by a unified set of gauge theories—one

  corresponding to the electromagnetic interaction (remaining

  unbroken) and one corresponding to the weak interaction, with a

  broken-gauge symmetry resulting in several massive mediators for

  that interaction.

  In this case the world we live in would be precisely like a

  superconductor.

  The weak interaction would be weak because of the simple

  accident that the ground state of fields in our current universe breaks

  the gauge symmetry that would otherwise govern the weak

  interaction symmetry. The photonlike gauge particles would get

  large masses, and as Schwinger had expected, the weak interaction

  would become so short-range that it would almost die off even on

  the length scale of protons and neutrons. This would also explain

  why neutron decay would happen so slowly.

  The massive particles mediating the weak interaction would

  appear to us just as photons would appear to hypothetical physicists

  living inside a superconductor. So too the distinction between

  electromagnetism and the weak interaction would be just as illusory

  as the distinction that physicists on the ice crystals on that

  windowpane would make between forces along the direction of their

  icicle versus those perpendicular to that direction. It would be a

  simple accident that one gauge symmetry gets broken in the world

  of our experience, and the other doesn’t.

  Weinberg wanted to avoid thinking about strongly interacting

  particles since the situation there was still confused. So he decided to

  think about particles that interact only via the weak or

  electromagnetic interaction, namely electrons and neutrinos. Since

  the weak interaction turns electrons into neutrinos, he had to

  imagine a set of charged vector photonlike particles that would

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  produce such a transformation. These are nothing other than the

  charged vector bosons that Schwinger envisaged, conventionally

  called W plus and W minus bosons.

  Since only left-handed electrons and neutrinos get mixed

  together by the weak interaction, one type of gauge symmetry would

  have to govern just the interactions of left-handed particles with the

  W particles. But since both left-handed electrons and right-handed

  electrons interact with photons, the gauge symmetry of

  electromagnetism would somehow have to be incorporated in this

  unified model in such a way that left-handed electrons could interact

  with both photons and the new charged W bosons—while right-

  handed electrons would interact only with photons and not the W

  particles.

  Mathematically, the only way to do this—as Sheldon Glashow

  had discovered when he was thinking about electroweak unification

  six years earlier—was if there was one additional neutral weak boson

  that right- and left-handed electrons could interact with in addition

  to interacting with photons. This new boson Weinberg dubbed the

  Z, zero.

  A new field would have to exist in nature that would form a

  condensate in empty space to spontaneously break the symmetries

  governing the weak interaction. The elementary particle associated

  with this field would be the massive Higgs, while the remaining

  would-be Goldstone bosons would now be eaten by the W and Z

  bosons to make them massive, by the mechanism that Higgs first

  proposed. This would leave only the photon left over as a massless

  gauge boson.

  But there’s more. By virtue of the gauge symmetry he introduced,

  Weinberg’s new Higgs particle would also interact with electrons,

  and when the condensate formed, the effect would be to give

  electrons a mass as well as the W and Z particles. Thus, not only

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  would this model explain the masses of the gauge particles that

  mediate the weak force—an
d therefore determine the strength of

  that force—but the same Higgs field would also give electrons mass.

  All the ingredients necessary for the unification of the weak and

  electromagnetic interaction were present in this model. Moreover,

  by starting with a Yang-Mills gauge theory with massless gauge

  bosons before symmetry breaking, there was hope that the same

  remarkable symmetry properties of gauge theories first exploited in

  quantum electrodynamics might also allow this theory to produce

  finite sensible results. While a fundamental theory with massive

  photonlike particles clearly had pathologies, the hope was that if the

  masses only resulted after symmetry breaking, these pathologies

  might not appear. But it was just a hope at the time.

  Clearly in a realistic model the Higgs particle would couple to

  other particles engaged in the weak interaction, beyond the electron.

  In the absence of a Higgs condensate all these particles, protons, or

  the particles that made them up, and muons, etc., all of them would

  be exactly massless. Every facet that is responsible for our existence,

  indeed the very existence of the massive particles from which we are

  made, would thus arise as an accident of nature—the formation of a

  specific Higgs condensate in our universe. The particular features

  that make our world what it is—the galaxies, stars, planets, people,

  and the interactions among all of these—would be quite different if

  the condensate had never formed.

  Or if it had formed differently.

  Just as the world experienced by imaginary physicists on the ice

  crystal on that windowpane on a cold winter morning would have

  been completely different if the crystal had lined up in a different

  direction, so too the features of our world that allow our existence

  depend crucially on the nature of the Higgs condensate. What might

  seem so special about the features of the particles and fields that

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  make up the world we live in would thus be no more special,

  planned, or significant than would be the accidental orientation of

  the spine of that ice crystal, even if it might appear to have special

  significance to beings living on the crystal.

  And one last bit of poetry. The unique Yang-Mills model that

  Weinberg was driven to in 1967, which Abdus Salam would also