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207 
the papers is made clear by their titles. Higgs's paper was called "Broken 
Symmetries and the Masses of Gauge Bosons." The Englert and Brout 
paper was entitled "Broken Symmetry and the Mass of Gauge Vector Me-
sons." It is hard to imagine a closer match without coordinating names. 
As if to add to the remarkable serendipity, twenty years later Higgs 
met Nambu at a conference and learned that Nambu had refereed both 
papers. How much more fitting could it be that the man who first brought 
the ideas of symmetry breaking and superconductivity to particle phys-
ics should referee the papers of the people who would demonstrate just 
how prescient this idea was. And like Nambu, all of these authors were 
fixated on the strong interaction, and on the possibility of figuring out 
how protons, neutrons, and mesons could have large masses. 
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 mas-
sive 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 sabbati-
cal 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 in-
teraction 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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Part Three 
REVELATION 
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Chapter 17 
THE WRONG PLACE AT THE 
RIGHT TIME 
Be not deceived: evil communications corrupt 
good manners. 
-I COR:11THiANS 15:33 
Al 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 pro-
posal 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 nu-
clear 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 
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THE GREATEST STORY EVER TOLD-SO FAR 
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 un-
derstanding physics at these accessible energy scales, where new ideas, 
and new particles, could be quickly tested and explored in existing ma-
chines—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 physi-
cists thought. 
One of the many people who were fascinated by the physics of the 
strong interaction was the young theorist Steven Weinberg. There is po-
etry here as well. Weinberg grew up in New York City and attended 
the Bronx High School of Science, from which he graduated in 195o. 
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 sepa-
rate ways. While Glashow went to Harvard for graduate school, Wein-
berg 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 196os, 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 
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as those of any other scientists in recent times, yet they form an inter-
esting contrast. Glashow's brilliance is combined with an almost child-
like 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 for-
mative 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 
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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 sub-
sequent 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 Corde-
lia: "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 
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 196os the idea that the pion resembled in many 
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215 
ways a Goldstone boson was useful in deriving some approximate for-
mulas 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 bro-
ken spontaneously, and various strongly interacting vector gauge par-
ticles that convey the strong interaction might get masses via the Higgs 
mechanism. The problem was he couldn't get agreement with observa-
tions 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 2967 while driving in to MIT, he saw the light, liter-
ally 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 ap-
plying 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 specu-
lated 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 correspond-
ing 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 
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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 in-
side 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 par-
ticles since the situation there was still confused. So he decided to think 
about particles that interact only via the weak or electromagnetic in-
teraction, 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 envis-
aged, 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 par-
ticles. 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 
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217 
years earlier—was if there was one additional neutral weak boson that 
right- and left-handed electrons could interact with in addition to in-
teracting with photons. This new boson Weinberg dubbed the Z, zero. 
A new field would have to exist in nature that would form a conden-
sate 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 mas-
sive, 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 elec-
trodynamics might also allow this theory to produce finite sensible re-
sults. 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 par-
ticles 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 
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THE GREATEST STORY EVER TOLD-SO FAR 
existence of the massive particles from which we are made, would thus 
arise as an accident of nature—the formation of a specific Higgs conden-
sate 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 com-
pletely 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 acci-
dental 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 Wein-
berg was driven to in 2967, 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 mathemati-
cally 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 com-
pletely independent discoveries of the greatest unification in physical 
theory since Maxwell had unified electricity and magnetism and Ein-
stein had unified space and time. 
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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 mea-
sured 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 2967 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 in-
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teractions 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. 
Is it any wonder that the idea did not immediately catch fire? Never-
theless, within a decade everything would change, resulting in the most 
theoretically productive period for elementary particle physics since the 
discovery of quantum mechanics. While a gauge theory of the weak 
interaction started the ball rolling, what resulted was far greater. 
The first crack in the dike holding back the waters of progress came, fit-
tingly, with the work of Dutch graduate student Gerardus Hooft, in 1971. 
I always remember how to spell his name because a particularly brilliant 
and witty former Harvard colleague, the late Sidney Coleman, used to say 
that if Gerard had monogrammed cuff links, they would need an apostro-
phe on them. Before 1971 many of the greatest theorists in the world had 
tried to figure out whether the infinities that plague most quantum field 
theories would disappear for spontaneously broken gauge theories as they 
do for their unbroken cousins. But the answer eluded them. Remarkably 
this young graduate student, working under the supervision of a seasoned 
pro—Martinus Veltman—found a proof that others had missed. Often 
when presented with a new result, we physicists can work through the 
details and imagine how we might have discovered it ourselves. But many 
of 't Hooft's insights, and there were many—almost all the new ideas in 
the inos derived in one way or another from his theoretical inventions—
seemed to come from some hidden reservoir of intuition. 
The other remarkable thing about Gerard is how gentle, shy, and un-
assuming he is. For someone who became famous in the field when he 
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was a student, one might have expected some sense of privilege. But 
from the first time I met him—again when I was a lowly graduate stu-
dent—Gerard treated me as an interesting friend, and I am pleased to 
say that relationship has continued. I always try to remember this atti-
tude when I meet young students who may seem shy or intimidated, and 
I try to emulate Gerard's open generosity of spirit. 
His supervisor Tini Veltman, as he is often called, couldn't appear 
more different. Not that Tini isn't fun to talk to. He is. But he always 
made explicitly clear to me the moment we started a discussion that 
whatever I might say, I didn't understand things well enough. I always 
enjoyed the challenge. 
It is important to note that 't Hooft would never have approached the 
problem if Veltman had not been obsessed with it, even as most others 
gave up. The notion that one might ultimately extend the techniques 
that Feynman and others had developed to tame quantum electrody-
namics to try to understand more complex theories such as spontane-
ously broken Yang-Mills theory was simply viewed as naïve by many 
in the field. But Veltman stayed with the project, and he wisely found a 
graduate student who was also a genius to help him. 
It took a while for 't liooft's and Veltman's ideas to sink in and the 
new techniques 't Hooft had developed to become universally adopted, 
but within a year or so physicists agreed that the theory that Weinberg, 
and later Salam, had proposed, made sense. Citations of Weinberg's 
paper suddenly began to grow exponentially. But making sense and 
being right are two different things. Did nature actually use the specific 
theory that Glashow, Weinberg, and Salam had suggested? 
That remained the key open question, and for a while it looked as if 
the answer was no. 
The existence of the new neutral particle, the Z, required by the the-
ory, was a significant addition, beyond the charged particles suggested 
years earlier by Schwinger and others that were required to change 
neutrons into protons and electrons into neutrinos. It meant that there 
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would be a new kind of weak interaction, not just for electrons and neu-
trinos but also for protons and neutrons, mediated by a new neutral-
particle exchange. In this case, as for electromagnetism, the identity of 
the particles interacting would not change. Such interactions became 
known as neutral current interactions, and the obvious way to test the 
theory was to look for them. The best place to look for them was in the 
interactions of the only particles in nature that just feel the weak inter-
action, namely neutrinos. 
You may recall that the prediction of such neutral currents was 
one of the reasons that Glashow's 1961 suggestion never caught on. 
But Glashow's model wasn't a full theory. Particle masses were simply 
put into the equations by hand, and as a result quantum corrections 
couldn't be controlled. However, when Weinberg and Salam proposed 
their model for electroweak unification, all elements that allowed for de-
tailed predictions were there. The mass of the Z particle was predicted, 
and as 't Hooft had shown, one could calculate all quantum corrections 
in a reliable way, just as one did for quantum electrodynamics. 
This was a good thing, and a bad thing because no wiggle room was 
left to argue away any possible disagreements with observation. And in 
1967 there appeared to be such disagreements. No such neutral currents 
had been observed in high-energy collisions of neutrinos with protons, 
with an upper limit being set of about lo percent of the rate observed 
for more familiar charge-changing weak interactions of neutrinos and 
protons, such as neutron decay. Things looked bad, and most physicists 
assumed weak neutral currents didn't exist. 
Weinberg had a vested interest in this quest, and in 1971 he reason-
ably argued that there was still wiggle room. But this view was not 
gernerally held by others in the community. 
In the early 297os, new experiments at the European Organization 
for Nuclear Research (CERN) in Geneva were performed using the pro-
ton accelerator there, which smashed high-energy protons into a long 
target. Most particles produced in the collision would be absorbed in 
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the target, but neutrinos would emerge from the other end—as their in-
teractions are so weak that they could traverse the target without being 
absorbed. The resulting high-energy neutrino beam would then strike 
a detector placed in its path that could record the few events in which 
neutrinos might interact with the detector material. 
A huge new detector was built, named Gargamelle after the giant-
ess mother of Gargantua, from the work of the French writer Rabelais. 
This five-meter-by-two-meter "bubble chamber" vessel was filled with a 
superheated liquid in which trails of bubbles would form when an ener-
getic charged particle traversed it, sort of like seeing the vapor trail high 
in the sky of a plane that is itself not visible. 
Interestingly, when the experimentalists who built Gargamelle met in 
1968 to discuss their plans for neutrino experiments, the idea of search-
ing for neutral currents wasn't even mentioned—an indication of how 
many physicists thought the issue was then settled. Of far more interest 
to them was the possibility of following up on recent exciting experi-
ments at the Stanford Linear Accelerator (SLAC), where high-energy 
electrons had been used as probes to explore the structure of protons. 
Using neutrinos as probes of protons might give cleaner measurements 
because the neutrinos are not charged. 
After the results of 't Hooft and Veltman, however, in 1972., experi-
mentalists began to take the gauge theory description of the weak in-
teraction, and in particular the Glashow-Weinberg-Salam proposal, 
seriously. That meant looking for neutral currents. The Gargamelle 
collaboration had the capability to do this, in principle, even though it 
hadn't been designed for the task. 
Most of the high-energy neutrinos in the beam would interact with 
protons in the target by turning into muons, the heavier partners of 
electrons. The muons would exit the target, producing a long charged-
particle track all the way to the edge of the detector. The protons would 
be converted into neutrons, which would themselves not produce a 
track but would collide with nuclei, producing a short shower of charged 
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particles that would leave tracks. Thus, the experiment was designed to 
detect muon tracks, as well as accompanying charged-particle showers, 
both arising as separate signals of a single weak interaction. 
However, sometimes a neutrino would interact with material out-
side the detector, producing a neutron that might recoil back into the 
detector and then interact there. Such events would consist of a single 
strongly interacting shower of particles due to the colliding neutron, 
with no accompanying muon track. 
When Gargamelle began to search for neutral current events, such 
isolated charged-particle showers without an accompanying muon be-
came just the signal the scientists needed to focus on. In neutral current 
events a neutrino that interacts with a neutron or proton in the detec-
tor doesn't convert into a charged muon, but simply bounces off and 
escapes the detector unobserved. All that would be observable would be 
the recoiling nuclear shower—the same signature produced by the more 
standard neutrino interactions outside the detector that produce neu-
trons that recoil back into the detector and produce a nuclear shower. 
The challenge, then, if the experiment was to definitively detect neu-
tral current events, was to distinguish neutrino-induced events from 
such neutron-induced events. (This same problem has provided the 
chief challenge to experimentalists looking for any weakly interacting 
particles, including the presumed dark matter particles that are being 
searched for in underground detectors around the world today.) 
The observation of a single recoil electron, with no other charged-
particle tracks in the detector, was observed in early 1973. This could 
have arisen from the less frequent predicted neutral current collisions of 
neutrinos with electrons instead of protons or neutrons. But generally a 
single event is not enough to definitively claim a new discovery in par-
ticle physics. However, it did give hope, and by March of 1973 a careful 
analysis of neutron backgrounds and observed isolated particle show-
ers appeared to provide evidence that weak neutral current interactions 
actually exist. Nevertheless, not until July of 1973 did the researchers at 
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CERN complete a sufficient number of checks to be confident enough 
to claim a detection of neutral currents, which they did at a conference 
in Bonn in August. 
The story might have ended there, but unfortunately, shortly after 
this, another collaboration searching for neutral currents rechecked 
their apparatus and found that a previous signal for neutral currents 
had disappeared. This produced significant confusion and skepticism 
in the physics community, where once again neutral currents seemed 
suspect. Ultimately the Gargamelle collaboration returned to the draw-
ing board, tested the detector using a proton beam directly, and took a 
great deal more data. At a conference almost a year later, in June 2974, 
the Gargamelle collaboration presented overwhelming confirmation of 
the signal. Meanwhile the competing collaboration had found the cause 
of its error and confirmed the Gargamelle result. Glashow, Weinberg, 
and Salam were vindicated. 
Neutral currents had arrived, and a remarkable unification of the 
weak and the electromagnetic interactions appeared to be at hand. But 
two loose ends still remained to be cleared out. 
The existence of neutral currents in neutrino scattering validated 
the notion that the Z particle existed, but this didn't guarantee that the 
weak interaction was identical to that proposed by Glashow, Weinberg, 
and Salam, where the weak and the electromagnetic interactions were 
unified. To explore this required an experiment using a particle that 
participated in both the weak and the electromagnetic interaction. The 
electron was ideal for this purpose because these are the only two inter-
actions it experiences. 
When electrons interact with other charges by their electromag-
netic attraction, left-handed electrons and right-handed electrons 
behave identically. However, the Weinberg-Glashow-Salam theory re-
quired that weak interactions occur differently for left-handed versus 
right-handed particles. This implied that careful measurements of the 
scattering of polarized electrons—electrons prepared initially in left- or 
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THE GREATEST STORY EVER TOLD-SO FAR 
right-handed states using magnetic fields—off various targets should re-
veal a violation of left-right symmetry, but not as extreme an asymmetry 
as that observed in neutrino scattering—because the neutrino is purely 
left-handed. The degree of violation in electron scattering, if it existed, 
would then reflect the extent to which the weak interaction and electro-
magnetism were mixed together in a unified theory. 
The idea of testing for such interference using electron scattering had 
actually been suggested as early as 19S8 by the remarkable Soviet physi-
cist Yakov B. Zel'dovich. But it would take twenty years for sufficiently 
sensitive experiments to actually take place. And as for the neutral cur-
rent discovery, the road to success was full of potholes and wrong turns 
along the way. 
One of the reasons it took so long to test this idea is that the weak 
interaction is weak. Because the dominant interaction of electrons with 
matter is electromagnetic, the left-right asymmetry predicted due to a 
possible exchange of a Z particle was small, smaller than one part in ten 
thousand. To test for such a small asymmetry 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 Weinberg-Glashow-Salam theory began to have wide accep-
tance among theorists, what made the 1978 SLAC experiment so im-
portant 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 
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