In sports, as in science, there are two very different types of heroes. There are the giants who lead the their teams and their sport, winning championships and accolades, for years, and whose fame lives on for decades: the Michael Jordans, the Peles, the Lou Gherigs, the Joe Montanas. And then there are the unlikely heroes, the ones who just happen to have a really good day at a really opportune time; the substitute player who comes on the field for an injured teammate and scores the winning goal in a championship; the fellow who never hits a home run except on the day it counts; the mediocre receiver who turns a short pass into a long touchdown during the Super Bowl. We celebrate both types, in awe of the great ones, and in amused pleasure at the inspiring stories of the unlikely ones.
In science we have giants like Newton, Darwin, Boyle, Galileo… The last few decades of particle physics brought us a few, such as Richard Feynman and Ken Wilson, and others we’ll meet today. Many of these giants received Nobel Prizes. But then we have the gentlemen behind what is commonly known as the Higgs particle — the little ripple in the Higgs field, a special field whose presence and properties assure that many of the elementary particles of nature have mass, and without which ordinary matter, and we ourselves, could not exist. Following discovery of this particle last year, and confirmation that it is indeed a Higgs particle, two of them, Francois Englert and Peter Higgs, have been awarded the 2013 Nobel Prize in physics. Had he lived to see the day, Robert Brout would have been the third.
My articles Why The Higgs Particle Matters and The Higgs FAQ 2.0; the particles of nature and what they would be like if the Higgs field were turned off; link to video of my public talk entitled The Quest for the Higgs Boson; post about why Higgs et al. didn’t win the 2012 Nobel prize, and about how physicists became convinced since then that the newly discovered particle is really a Higgs particle;
The paper written by Brout and Englert; the two papers written by Higgs; the paper written by Gerald Guralnik, Tom Kibble and Carl Hagen; these tiny little documents, a grand total of five and one half printed pages — these were game-winning singles in the bottom of the 9th, soft goals scored with a minute to play, Hail-Mary passes by backup quarterbacks — crucial turning-point papers written by people you would not necessarily have expected to find at the center of things. Brout, Englert, Higgs, Guralnik, Kibble and Hagen are (or rather, in Brout’s case, sadly, were) very fine scientists, intelligent and creative and clever, and their papers, written in 1964 when they were young men, are imperfect but pretty gems. They were lucky: very smart but not extraordinary physicists who just happened to write the right paper at the right time. In each case, they did so
- …not by studying the real world but by studying simplified, imaginary worlds, in which it was easier to learn the general lessons they wanted to extract…
- ...with a motivation based on a real-world question — the origin of mass for hadrons — for which, it turns out, their ideas do not apply… [Hadrons, including protons and neutrons, are today known to be particles made from quarks and gluons; but that wasn't known in the early 1960s.]
- …and with no suspicion that their results would instead be applied to a different real-world question: the possibility that the weak nuclear force is associated with photon-like particles, called W and Z, that have a mass. [A photon is a particle of light, has spin=1, and, unlike the W and Z particles, has no mass.]
History in general, and history of science in particular, is always vastly more complex than the simple stories we tell ourselves and our descendants. Making history understandable in a few pages always requires erasing complexities and subtleties that are crucial for making sense of the past. Today, all across the press, there are articles explaining incorrectly what Higgs and the others did and why they did it and what it meant at the time and what it means now. I am afraid I have a few over-simplified articles of my own. But today I’d like to give you a little sense of the complexities, to the extent that I, who wasn’t even alive at the time, can understand them. And also, I want to convey a few important lessons that I think the Hi(gg)story can teach both experts and non-experts. Here are a couple to think about as you read:
1. It is important for theoretical physicists, and others who make mathematical equations that might describe the world, to study and learn from imaginary worlds, especially simple ones. That is because
- 1a. one can often infer general lessons more easily from simple worlds than from the (often more complicated) real one, and
- 1b. sometimes an aspect of an imaginary world will turn out to be more real than you expected!
2. One must not assume that research motivated by a particular goal depends upon the achievement of that goal; even if the original goal proves illusory, the results of the research may prove useful or even essential in a completely different arena.
My summary today is based on a reading of the papers themselves, on comments by John Iliopoulos, and on a conversation with Englert, and on reading and hearing Higgs’ own description of the episode.
The story is incompletely but perhaps usefully illustrated in the figure below, which shows a cartoon of how four important scientific stories of the late 1950s and early 1960s came together. They are:
- How do superconductors (materials that carry electricity without generating heat) really work?
- How does the proton get its mass, and why are pions (the lightest hadrons) so much lighter than protons?
- Why do hadrons behave the way they do; specifically, as suggested by J.J. Sakurai (who died rather young, and after whom a famous prize is named), why are there photon-like hadrons, called rho mesons, that have mass?
- How does the weak nuclear force work? Specifically, as suggested by Schwinger and developed further by his student Glashow, might it involve photon-like particles (now called W and Z) with mass?
These four questions converged on a question of principle: “how can mass be given to particles?”, and the first, third and fourth were all related to the specific question of “how can mass be given to photon-like particles?’’ This is where the story really begins. [Almost everyone in the story is a giant with a Nobel Prize, indicated with a parenthetic (NPyear).]
My best attempt at a cartoon history…
In 1962, Philip Anderson (NP1977), an expert on (among other things) superconductors, responded to suggestions and questions of Julian Schwinger (NP1965) on the topic of photon-like particles with mass, pointing out that a photon actually gets a mass inside a superconductor, due to what we today would identify as a sort of “Higgs-type’’ field made from pairs of electrons. And he speculated, without showing it mathematically, that very similar ideas could apply to empty space, where Einstein’s relativity principles hold true, and that this could allow elementary photon-like particles in empty space to have mass, if in fact there were a kind of Higgs-type field in empty space.
In all its essential elements, he had the right idea. But since he didn’t put math behind his speculation, not everyone believed him. In fact, in 1964 Walter Gilbert (NP1980 for chemistry, due to work relevant in molecular biology — how’s that for a twist?) even gave a proof that Anderson’s idea couldn’t work in empty space!
But Higgs immediately responded, arguing that Gilbert’s proof had an important loophole, and that photon-like particles could indeed get a mass in empty space.
Meanwhile, about a month earlier than Higgs, and not specifically responding to Anderson and Gilbert, Brout and Englert wrote a paper showing how to get mass for photon-like particles in empty space. They showed this in several types of imaginary worlds, using techniques that were different from Higgs’ and were correct though perhaps not entirely complete.
A second paper by Higgs, written before he was aware of Brout and Englert’s work, gave a simple example, again in an imaginary world, that made all of this much easier to understand… though his example wasn’t perhaps entirely convincing, because he didn’t show much detail. His paper was followed by important theoretical clarifications from Guralnik, Hagen and Kibble that assured that the Brout-Englert and Higgs papers were actually right. The combination of these papers settled the issue, from our modern perspective.
And in the middle of this, as an afterthought added to his second paper only after it was rejected by a journal, Higgs was the first person to mention something that was, for him and the others, almost beside the point — that in the Anderson-Brout-Englert-Higgs-Guralnik-Hagen-Kibble story for how photon-like particles get a mass, there will also generally be a spin-zero particle with a mass: a ripple in the Higgs-type field, which today we call a Higgs-type particle. Not that he said very much! He noted that spin-one (i.e. photon-like) and spin-zero particles would come in unusual combinations. (You have to be an expert to even figure out why that counts as predicting a Higgs-type particle!) Also he wrote the equation that describes how and why the Higgs-type particle arises, and noted how to calculate the particle’s mass from other quantities. But that was it. There was nothing about how the particle would behave, or how to discover it in the imaginary worlds that he was considering; direct application to experiment, even in an imaginary world, wasn’t his priority in these papers.
In his second paper, Higgs considers a simple imaginary world with just a photon-like particle and a Higgs-type field. Equation 2b is the first place the Higgs-type particle explicitly appears in the context of giving photon-like particles a mass (equation 2c). From Physical Review Letters, Volume 13, page 508
About the “Higgs-type” particle, Anderson says nothing; Brout and Englert say nothing; Guralnik et al. say something very brief that’s irrelevant in any imaginable real-world application. Why the silence? Perhaps because it was too obvious to be worth mentioning? When what you’re doing is pointing out something really “important’’ — that photon-like particles can have a mass after all — the spin-zero particle’s existence is so obvious but so irrelevant to your goal that it hardly deserves comment. And that’s indeed why Higgs added it only as an afterthought, to make the paper a bit less abstract and a bit easier for a journal to publish. None of them could have imagined the hoopla and public excitement that, five decades later, would surround the attempt to discover a particle of this type, whose specific form in the real world none of them wrote down.
In the minds of these authors, any near-term application of their ideas would probably be to hadrons, perhaps specifically Sakurai’s theory of hadrons, which in 1960 predicted the “rho mesons”, which are photon-like hadrons with mass, and had been discovered in 1961. Anderson, Brout-Englert and Higgs specifically mention hadrons at certain moments. But none of them actually considered the real hadrons of nature, as they were just trying to make points of principle; and in any case, the ideas that they developed did not apply to hadrons at all. (Well, actually, that’s not quite true, but the connection is too roundabout to discuss here.) Sakurai’s ideas had an element of truth, but fundamentally led to a dead end. The rho mesons get their mass in another way.
Meanwhile, none of these people wrote down anything resembling the Higgs field which we know today — the one that is crucial for our very existence — so they certainly didn’t directly predict the Higgs particle that was discovered in 2012. It was Steven Weinberg (NP1979) in 1967, and Abdus Salam (NP1979) in 1968, who did that. (And it was Weinberg who stuck Higgs’ name on the field and particle, so that everyone else was forgotten.) These giants combined
- the ideas of Higgs and the others about how to give mass to photon-like particles using a Higgs-type field, with its Higgs-type particle as a consequence…
- …with the 1960 work of Sheldon Glashow (NP1979), Schwinger’s student, who like Schwinger proposed the weak nuclear force was due to photon-like particles with mass,…
- …and with the 1960-1961 work of Murray Gell-Man (NP1969) and Maurice Levy and of Yoichiro Nambu (NP2008) and Giovanni Jona-Lasinio, who showed how proton-like or electron-like particles could get mass from what we’d now call Higgs-type fields.
This combination gave the first modern quantum field theory of particle physics: a set of equations that describe the weak nuclear and electromagnetic forces, and show how the Higgs field can give the W and Z particles and the electron their masses. It is the primitive core of what today we call the Standard Model of particle physics. Not that anyone took this theory seriously, even Weinberg. Most people thought quantum field theories of this type were mathematically inconsistent — until in 1971 Gerard ‘t Hooft (NP1999) proved they were consistent after all.
The Hi(gg)story is populated with giants. I’m afraid my attempt to tell the story has giant holes to match. But as far as the Higgs particle that was discovered last year at the Large Hadron Collider, the unlikely heroes of the story are the relatively ordinary scientists who slipped in between the giants and actually scored the goals.