Illustrating that the time was ripe for this discovery, within a month or so another team, Gerald Guralnik, C. R. Hagen, and Tom Kibble, also published a paper including many of the same ideas.
You may wonder why we call it the Higgs boson and not the Higgs-Brout-Englert-Guralnik-Hagen-Kibble boson. Besides the obvious answer that this label doesn’t trip lightly off the tongue, of all the papers the only one to explicitly predict an accompanying massive scalar boson in massive gauge theories with spontaneous symmetry breaking was Higgs’s paper. And, interestingly, Higgs only included the extra remark because the original version of his paper without that remark had been rejected.
One last bit of poetry. A couple of years after the original paper was published, Higgs completed a longer paper and was invited (in 1966) to speak at several locations in the USA, where he was 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.
Part Three
* * *
REVELATION
Chapter 17
* * *
THE WRONG PLACE AT THE RIGHT TIME
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 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 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 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 bos
ons that occur through symmetry breaking if the symmetry being broken was 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.
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 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—and 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 stumble upon a year later, was precisely the model proposed six years earlier by his old high school friend Sheldon Glashow when he responded to Schwinger’s challenge to find a symmetry that might unify the weak and electromagnetic interactions. No other choice could mathematically reproduce what we see in the world today. Glashow’s model had been largely ignored in the interim because no mechanism was then known to give the weak bosons masses. But now such a mechanism existed, the Higgs mechanism.
Weinberg and Glashow, whose lives had crisscrossed since they were children, would later share the Nobel Prize,
along with Salam, for completely independent discoveries of the greatest unification in physical theory since Maxwell had unified electricity and magnetism and Einstein had unified space and time.
Chapter 18
* * *
THE FOG LIFTS
Their voice goes out through all the earth, and their words to the end of the world.
—PSALM 19:4
You might expect that physicists around the world would have thrown parties with fireworks when Weinberg’s paper came out. But for the next three years following publication of Weinberg’s theory, not a single physicist, not even Weinberg himself, would find cause to reference the paper—now one of the most highly cited papers in all of particle physics. If a great discovery about nature had been made, no one had yet noticed.
After all, Maxwell’s unification made the beautiful prediction that light was an electromagnetic wave whose speed could be calculated from first principles, and lo and behold, the prediction was equal to the measured speed of light. Einstein’s unification of space and time predicted that clocks would slow for moving observers, and lo and behold, they do, and in just the way he predicted. In 1967 the Glashow-Weinberg-Salam unification of the weak and electromagnetic interactions predicted three new vector bosons that were almost one hundred times heavier than any particle that had been yet detected. It also predicted new interactions between electrons and neutrinos and matter due to the newly predicted Z particle that had not only not been seen, but a number of experiments suggested did not exist. It also required the existence of a new and as yet unobserved massive fundamental scalar boson, the Higgs particle, when no fundamental scalar particles were yet known to exist in nature. And finally, as a quantum theory, no one knew if it made sense.
The Greatest Story Ever Told—So Far Page 20
