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
͟͜͞
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
͟͞͝
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
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 24
