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times the current flowing through an average family house. 
10. The strands of cable needed to make up the magnetic coils in the 
LHC span about 270,000 kilometers, or about six times the circum-
ference of the Earth. If all the filaments in the strands were unrav-
eled, they would stretch to the Sun and back more than five times. 
11. The total energy in each beam is about the same as that of a four-
hundred-ton train traveling at 150 km/hr. This is enough energy 
to melt five hundred kilograms of copper. The energy stored in the 
superconducting magnets is thirty times higher than this. 
12. Even with the superconducting magnets—which make power 
consumption in the machine manageable—when the machine is 
running, it uses about the same power as the total consumption 
of all of the households in Geneva. 
So much for the machine itself. To analyze the collisions at the LHC, 
a variety of large detectors have been built. Each of the four currently 
operating detectors has the size of a significant office building and the 
complexity of a major laboratory. To have the opportunity to go under-
ground and see the detectors is to feel like Gulliver in Brobdingnag. The 
scale of absolutely every component is immense. Here is a photo of the 
CMS detector, the smaller of the two largest detectors at the LHC: 
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If you are actually at the detector, it is hard to even grasp the full 
picture, as can be seen in the more up-close-and-personal view: 
The complexity of the machines is almost unfathomable. For a theo-
rist such as me, it is hard to imagine how any single group of physicists 
can keep track of the device, much less design and build it to the exact-
ing specifications required. 
Each of the two largest detectors, ATLAS and CMS, was built by a 
collaboration of over two thousand scientists. More than ten thousand 
scientists and engineers from over a hundred countries participated in 
building the machine and detectors. Consider the smaller of the two 
detectors, CMS. It is more than twenty meters long, fifteen meters high, 
and fifteen meters wide. Some 32,soo tons of iron are in the detector, 
more than in the Eiffel Tower. The two halves of the detector are sepa-
rated by a few meters when it is being worked on. Even though they are 
not on wheels, if the two halves were apart when the large magnetic field 
of the detector was turned on, they would be dragged together. 
Each detector is separated into millions of components, with trackers 
that can measure particle trajectories to an accuracy of ten-millionths 
of a meter, with calorimeters, which detect to a high accuracy energy 
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deposited in the detectors, and with devices for measuring the speed 
of particles by measuring the radiation they emit as they traverse the 
detector. In each collision hundreds or thousands of individual particles 
may be produced, and the detector must keep track of almost all of them 
to reconstruct each event. 
Physicist Victor Weisskopf was the fourth director general of CERN, 
between 1961 and 1966, and he likened the great accelerators of that time 
to the Gothic cathedrals of medieval Europe. In thinking of CERN and 
the LHC, the comparison is particularly interesting. 
The Gothic cathedrals stretched the technology of the time, requir-
ing new building techniques and new tools to be created. Hundreds or 
thousands of master craftsmen from dozens of countries built them 
over many decades. Their scale dwarfed that of any buildings that had 
previously been created. And they were built for no more practical rea-
son than to celebrate the glory of God. 
The LHC is the most complicated machine ever built, requiring new 
building techniques and new tools to be created. Thousands of PhD sci-
entists and engineers from hundreds of countries speaking dozens of lan-
guages, and hailing from a background of at least an equal number of 
religions, were required to build the accelerator and the detectors that 
monitor it—taking almost two decades to complete the task. Its scale 
dwarfs that of all machines constructed before it. And it was built for no 
more practical reason than to celebrate and explore the beauty of nature. 
Seen in this perspective, the cathedrals and the collider are both 
monuments to what may be best about human civilization—the ability 
and the will to imagine and construct objects of a scale and complexity 
that requires the cooperation of countless individuals, from around the 
globe if necessary, for the purpose of turning our awe and wonder at 
the workings of the cosmos into something concrete that may improve 
the human condition. Colliders and cathedrals are both works of in-
comparable grandeur that celebrate the human experience in different 
realms. Nevertheless, I think the LHC wins, and its successful construc-
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tion over two decades demonstrates that the twenty-first century is not 
yet devoid of culture and imagination. 
Which brings me finally to the road to July 4, zon. 
By 2011 the LHC was cruising along, as one of the CERN officials 
put it. The amount of data taken by October of that year was already 4 
million times higher than during the first run in zoio, and thirty times 
higher than had been obtained by the beginning of 2O11. 
At this point in the collection of data that physicists had been wait-
ing forty years for, rumors began to fly in the community. Many of these 
came from the experimenters themselves. I have a part-time position at 
Australian National University in Canberra, and the International Con-
ference on High Energy Physics was going to be in held in Melbourne in 
July of 2012. Melbourne has a big LHC contingent, and when visiting, I 
kept hearing how a greater and greater possible mass range for the Higgs 
particle had been ruled out by the experiments already. 
Many experimentalists relish being able to prove theorists wrong. So 
it was in this case. One experimentalist had excitedly told me less than 
six months before the meeting that the entire Higgs mass region had 
been ruled out except for a narrow range between no and 130 times 
the mass of the proton. She expected that by July they would be able to 
rule out that region too. As one who was skeptical of the Higgs, I wasn't 
unhappy to hear this. In fact, I was getting a paper ready to explain why 
the Higgs might not exist. 
On April s, the situation got more interesting as the LHC center-of-
mass beam energy was increased slightly, to eight thousand times the 
rest energy of the proton. This translated into an increased potential for 
new particle discovery. By mid-June it was announced that the leaders 
of the two main experiments, along with the director general of CERN, 
would not be traveling to Melbourne for the meeting, but would be pre-
senting results remotely from a televised conference on the morning of 
July 4 in the main colloquium room at CERN—the same room where 
Rubbia had announced the discovery of the W particles. 
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On July 4 I was at a physics meeting in Aspen, Colorado. Because 
of the significance of the impending announcement, the physics com-
munity there had set up a live remote presentation screen—so that at 
1:00 ■ 
we could all sit and watch history unfold. About fifteen of us 
showed up in the dark at the Aspen Center for Physics, mostly physi-
cists, but also a few journalists, including Dennis Overbye from the New 
York Times, who knew he was going to have a late night writing. As it 
turned out, so would I. The Times had asked me for an essay for the fol-
lowing week's Science Times section if things worked out as expected. 
Then the show began, and in the next forty-five minutes or so spokes-
people presented data from both of the two large detectors that compel-
lingly demonstrated the existence of a new elementary particle with mass 
of about 126 times the mass of the proton. After the initial catastrophe in 
2009, the LHC had functioned impeccably—as had both the detectors. 
I and many of my colleagues were amazed during the early months by 
the immaculately dean results the detectors displayed regarding known 
background processes. So we were not surprised that when something 
new appeared, these detectors could find it, in spite of the unbelievably 
complicated environment that the detectors were functioning in. 
But more than this, the particle was discovered by looking precisely 
at the decay channels that had been predicted for a Standard Model 
Higgs particle. The relative decays into photons (via intermediate top 
quarks or W's) versus particles such as electrons (via intermediate Z 
bosons) agreed more or less with what was predicted, as did the pro-
duction rate of the new particle in the proton-proton collisions. Of the 
billions and billions of collisions analyzed by the two detector collabo-
rations up to that point, about fifty potential Higgs candidates had been 
discovered. Many tests needed to be performed to get a more definitive 
identification, but if it walked like a Higgs and quacked like a Higgs, 
it probably was a Higgs. The evidence was good enough that Francois 
Englert and Peter Higgs were awarded the Nobel Prize in October of 
2013, the first year possible after the claimed discovery. 
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In February 2O13, the LHC shut down so the machine could be up-
graded so that it could finally run at its originally designed energy and 
luminosity. By the final weeks before turnoff, the CERN mass-storage 
systems had stored more than one hundred petabytes of data, more info 
than in ioo million CDs. New results continued to roll in from data 
that had not yet been analyzed before the first announcement (includ-
ing tantalizing hints of a possible new and unexpected heavy particle, 
six times heavier than the Higgs, hints that disappeared just as this 
book was being sent off to press). 
For a real discovery, the more data you have, the better it looks, 
whereas anomalous results tend to disappear over time. This time 
things looked good, almost embarrassingly so. If one compared five dif-
ferent predicted decay channels into photons, Z particles, W particles, 
tau particles (the heaviest known cousin of the electron), and particles 
containing b quarks, to observation, the predictions of the Standard 
Model Higgs, with no extra accessories, agreed strikingly well. 
From the angular distribution and energies of the decay products, 
with a new larger sample of Higgs candidates, the LHC detectors were 
able to explore whether the particle was indeed a scalar particle, which 
would make it the first fundamental scalar ever observed in nature. 
On March 26, 2ois, the ATLAS detector at CERN released results that 
showed with greater than 99 percent confidence that the new particle 
was a spin o particle, with precisely the proper parity assignment to be 
a Higgs scalar. Nature had shown that it does not abhor scalar fields like 
the Higgs, as I for one had thought. The existence of such a fundamental 
scalar changes a great deal about what may be possible in nature, and 
people, including me, began to consider scenarios we would never be-
fore have considered. 
In September 2015, about a month before the first draft of this book 
was written, the two large detectors ATLAS and CMS combined their 
data from 2011 and 2012 and presented for the first time a unified com-
parison of theory and experiment. The result—involving a mammoth 
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computational effort to take into account separate systematic effects in 
each experiment, involving a total of forty-two hundred parameters—
showed with a residual uncertainty of about 10 percent that the new 
particle had all the properties predicted for the Standard Model Higgs. 
This simple conclusion may seem almost anticlimactic, following as 
it does a half century of directed effort by thousands of individuals—
the theorists who developed the Standard Model and the others who 
performed the incredibly complex calculations needed to compare pre-
dictions with experiments, to determine background rates, and so on, 
and the thousands of experimental physicists who had built and tested 
and operated the most complex machine ever constructed. Their story 
was marked by incredible heights of intellectual bravery, years of confu-
sion, bad luck and serendipity, rivalries and passion, and above all the 
persistence of a community focused on a single goal—to understand 
nature at her most fundamental scales. Like any human drama, it also 
included its share of envy, stubbornness, and vanity, but more impor-
tant, it involved a unique community built completely independent of 
ethnicity, language, religion, or gender. It is a story that carries with it 
all the drama of the best epic tales and reflects the best of what science 
can offer to modern civilization. 
That nature would be so kind as to actually use the ideas that a small 
collection of individuals wrote down on paper, inspired by abstract 
ideas of symmetry and using the complex mathematics of quantum field 
theory, will always seem to me nothing short of remarkable. It is hard 
to express the mixture of exhilaration and terror that comes from the 
realization that nature might actually work the way you are proposing 
it does when putting the final touches on a paper, possibly late at night, 
alone in your study. I suppose it may resemble the reaction Plato de-
scribed that his poor philosophers might have as they are dragged out 
into the sunlight away from the cave for the first time. 
To have discovered that nature really follows the simple and elegant 
rules intuited by the twentieth- and twenty-first-century versions of Pla-
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to's philosophers is both shocking and reassuring. It hints that the will-
ingness of scientists to build an intellectual house of cards that could 
come tumbling down at the slightest experimental tremor was not mis-
placed. It gives us courage to continue to suppose, as Einstein had once 
expressed his amazement about, that the universe on its grandest scale 
is fathomable after all. 
After witnessing the announcement of the Higgs discovery on July 4, 
2012, I wrote the following: 
The apparent discovery of the Higgs may not result in a better toaster 
or a faster car. But it provides a remarkable celebration of the hu-
man mind's capacity to uncover nature's secrets, and of the technol-
ogy we have built to control them. Hidden in what seems like empty 
space—indeed, like nothing which is getting more interesting all the 
time—are the very elements that allow for our existence. 
By demonstrating this, last week's discovery will change our view 
of ourselves and our place in the universe. Surely that is the hall-
mark of great music, great literature, great art... and great science. 
It is too early yet to judge or even fully anticipate what changes in our 
picture of reality will result from the Higgs discovery at the LHC, or the 
discoveries that may follow. Yet fortune does favor the prepared mind, 
and it is at once the responsibility and the joy of theorists such as me to 
ponder just that. 
While nature may have appeared to be kind to us this time, perhaps 
it was too kind. The epic saga I have described here may yet provide a 
dramatic new challenge for physics and for physicists, and an explicit re-
minder that nature doesn't exist to make us comfortable. Because while 
we may have found what we expected, no one really expected to find 
just that and nothing else.... 
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Chapter 22 
MORE QUESTIONS THAN 
ANSWERS 
A fool takes no pleasure in understanding, but only 
in expressing his opinion. 
-PROVERBS 18:2 
I n one sense, our story might end here, because we have 
come to the limits of our direct empirical knowledge about the universe 
at its fundamental scales. But no one says we have to stop dreaming, 
even if the dreams are not always pleasant. Before July 2012 particle 
physicists had two nightmares. The first was that the LHC would see 
precisely nothing. For if it did, it would likely be the last large accelerator 
ever built to probe the fundamental makeup of the cosmos. The second 
was that the LHC would discover the Higgs ... period. 
Each time we peel back one layer of reality, other layers beckon. So 
each important new development in science generally leaves us with more 
questions than answers. But it also usually leaves us with at least the out-
line of a road map to help us begin to seek answers to those questions. The 
discovery of the Higgs particle, and with it the validation of the existence 
of an invisible background Higgs field throughout space, was a profound 
validation of the bold scientific developments of the twentieth century. 
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However, the words of Sheldon Glashow continue to ring true: The 
Higgs is like a toilet. It hides all the messy details we would rather not 
speak of. The Higgs field, as elegant as it might be, is within the Stan-
dard Model essentially an ad hoc addition. It is added to the theory to 
do what is required to accurately model the world of our experience. But 
it is not required by the theory. The universe could have happily existed 
with a long-range weak force and massless particles. We would just not 
be here to ask about them. Moreover, the detailed physics of the Higgs 
is, as we have seen, undetermined within the Standard Model alone. The 
Higgs could have been twenty times heavier, or a hundred times lighter. 
Why, then, does the Higgs exist at all? And why does it have the mass 
it does? (Recognizing once again that whenever scientists ask "Why?," 
we really mean "How?") If the Higgs did not exist, the world we see 
would not exist, but surely that is not an explanation. Or is it? Ulti-
mately to understand the underlying physics behind the Higgs is to un-
derstand how we came to exist. When we ask, "Why are we here?," at a 
fundamental level we may as well be asking, "Why is the Higgs here?" 
And the Standard Model gives no answer to this question. 
Some hints do exist, however, coming from a combination of theory 
and experiment. Shortly after the fundamental structure of the Stan-
dard Model became firmly established, in 1974, and well before the 
details were experimentally verified over the next decade, two differ-
ent groups of physicists at Harvard, where both Glashow and Wein-
berg were working, noticed something interesting. Glashow, along with 
Howard Georgi, did what Glashow did best: they looked for patterns 
among the existing particles and forces and sought out new possibilities 
using the mathematics of group theory. 
Remember that in the Standard Model the weak and electromagnetic 
forces are unified at a high-energy scale, but when the symmetry is spon-
taneously broken by the Higgs field condensate, this leaves, at observ-
able scales, two separate and distinct forces—with the weak force being 
short-range and electromagnetism remaining long-range. Georgi and 
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Glashow tried to extend this idea to include the strong force and discov-
ered that all of the known particles and the three nongravitational forces 
could naturally fit within a single fundamental larger-gauge symmetry 
structure. They then speculated that this fundamental symmetry could 
spontaneously break at some ultrahigh energy and short-distance scale 
far beyond the range of current experiments, leaving two separate and 
distinct unbroken gauge symmetries left over—resulting in the separate 
strong and electroweak forces. Subsequently, at a lower energy and larger 
distance scale, the electroweak symmetry would break, separating that 
into the short-range weak and the long-range electromagnetic force. 
They called such a theory, modestly, a Grand Unified Theory (GUT). 
At around the same time, Weinberg and Georgi along with Helen 
Quinn noticed something interesting—following the work of Wilczek, 
Gross, and Politzer. While the strong interaction got weaker as one 
probed it at smaller-distance scales, the electromagnetic and weak in-
teractions got stronger. 
It didn't take a rocket scientist to wonder whether the strength of 
the three different interactions might become identical at some small-
distance scale. When they did the calculations, they found (with the 
accuracy with which the interactions were then measured) that such a 
unification looked possible, but only if the scale of unification was about 
fifteen orders of magnitude in scale smaller than the size of the proton. 
This was good news if the unified theory was the one proposed by 
Georgi and Glashow—because if all the particles we observe in nature got 
unified in this new large-gauge group, then new gauge bosons would exist 
that produce transitions between quarks (which make up protons and neu-
trons), and electrons and neutrinos. That would mean protons could decay 
into other lighter particles. As Glashow put it, "Diamonds aren't forever." 
Even then it was known that protons must have an incredibly long 
lifetime. Not just because we still exist almost 14 billion years after the 
Big Bang, but because we all don't die of cancer as children. If protons 
decayed with an average lifetime smaller than about a billion billion 
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years, then enough protons would still decay in our bodies during our 
childhood to produce enough radiation to kill us. Remember that in 
quantum mechanics, processes are probabilistic. If an average proton 
lives a billion billion years, then if one has a billion billion protons, on 
average one will decay each year. A lot more than a billion billion pro-
tons are in our bodies. 
However, with the incredibly small proposed distance scale and 
therefore the incredibly large mass scale associated with spontaneous 
symmetry breaking in Grand Unification, the new gauge bosons would 
get large masses. That would make the interactions they mediate be so 
short-range that they would be unbelievably weak on the scale of pro-
tons and neutrons today. As a result, while protons could decay, they 
might live, in this scenario, perhaps a million billion billion billion years 
before decaying. No problem. 
• 
• 
• 
With the results of Glashow and Georgi, and Georgi, Quinn, and Wein-
berg, the smell of grand synthesis was in the air. After the success of the 
electroweak theory, particle physicists were feeling ambitious and ready 
for further unification. 
How would one know if these ideas were correct, however? There 
was no way to build an accelerator to probe an energy scale a million 
billion times greater than the rest mass energy of protons. Such a ma-
chine would have to have a circumference of the Moon's orbit. Even if 
it was possible, considering the earlier debacle over the SSC, no govern-
ment would ever foot the bill. 
Happily, there was another way, using the kind of probability argu-
ments I just presented that give limits to the proton lifetime. If the new 
Grand Unified Theory predicted a proton lifetime of, say, a thousand 
billion billion billion years, then if one could put a thousand billion bil-
lion billion protons in a single detector, on average one of them would 
decay each year. 
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Where could one find so many protons? Simple: in about three thou-
sand tons of water. 
So all that was required was to get a tank of, say, three thousand 
tons of water, put it in the dark, make sure there were no radioactiv-
ity backgrounds, surround it with sensitive phototubes that can detect 
flashes of light in the detector, and then wait for a year to see a burst 
of light when a proton decayed. As daunting as this may seem, at least 
two large experiments were commissioned and built to do just this, one 
deep underground next to Lake Erie in a salt mine, and one in a mine 
near Kamioka, Japan. The mines were necessary to screen out incom-
ing cosmic rays that would otherwise produce a background that would 
swamp any proton decay signal. 
Both experiments began taking data around 1982-83. Grand Uni-
fication seemed so compelling that the physics community was confi-
dent a signal would soon appear and Grand Unification would mean the 
culmination of a decade of amazing change and discovery in particle 
physics—not to mention another Nobel Prize for Glashow and maybe 
some others. 
Unfortunately, nature was not so kind in this instance. No signals 
were seen in the first year, the second, or the third. The simplest elegant 
model proposed by Glashow and Georgi was soon ruled out. But once 
the Grand Unification bug had caught on, it was not easy to let it go. 
Other proposals were made for unified theories that might cause proton 
decay to be suppressed beyond the limits of the ongoing experiments. 
On February 23, 1987, however, another event occurred that demon-
strates a maxim I have found is almost universal: every time we open 
a new window on the universe, we are surprised. On that day a group 
of astronomers observed, in photographic plates obtained during the 
night, the closest exploding star (a supernova) seen in almost four hun-
dred years. The star, about i6o,000 light-years away, was in the Large 
Magellanic Cloud—a small satellite galaxy of the Milky Way observable 
in the southern hemisphere. 
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If our ideas about exploding stars are correct, most of the energy re-
leased should be in the form of neutrinos, despite that the visible light re-
leased is so great that supernovas are the brightest cosmic fireworks in the 
sky when they explode (at a rate of about one explosion per hundred years 
per galaxy). Rough estimates then suggested that the huge IMB (Irvine-
Michigan-Brookhaven) and Kamiokande water detectors should see about 
twenty neutrino events. When the IMB and Kamiokande experimental-
ists went back and reviewed their data for that day, lo and behold IMB 
displayed eight candidate events in a ten-second interval, and Kamiokande 
displayed eleven such events. In the world of neutrino physics, this was 
a flood of data. The field of neutrino astrophysics had suddenly reached 
maturity. These nineteen events produced perhaps nineteen hundred pa-
pers by physicists, such as me, who realized that they provided an unprec-
edented window into the core of an exploding star, and a laboratory not 
just for astrophysics but also for the physics of neutrinos themselves. 
Spurred on by the realization that large proton-decay detectors might 
serve a dual purpose as new astrophysical neutrino detectors, several 
groups began to build a new generation of such dual-purpose detectors. 
The largest one in the world was again built in the Kamioka mine and 
was called Super-Kamiokande, and with good reason. This mammoth 
fifty-thousand-ton tank of water, surrounded by ir.,800 phototubes, was 
operated in a working mine, yet the experiment was maintained with 
the purity of a laboratory clean room. This was absolutely necessary 
because in a detector of this size one had to worry not only about exter-
nal cosmic rays, but also about internal radioactive contaminants in the 
water that could swamp any signals being searched for. 
Meanwhile, interest in a related astrophysical neutrino signature also 
reached a new high during this period. The Sun produces neutrinos due 
to the nuclear reactions in its core that power it, and over twenty years, 
using a huge underground detector, Ray Davis had detected solar neutri-
nos, but had consistently found an event rate about a factor of three below 
what was predicted using the best models of the Sun. A new type of solar 
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neutrino detector was built inside a deep mine in Sudbury, Canada, which 
became known as the Sudbury Neutrino Observatory (5/4O). 
Super-Kamiokande has now been operating almost continuously, 
through various upgrades, for more than twenty years. No proton-de-
cay signals have been seen, and no new supernovas observed. However, 
the precision observations of neutrinos at this huge detector, combined 
with complementary observations at SNO, definitely established that 
the solar neutrino deficit observed by Ray Davis is real, and moreover 
that it is not due to astrophysical effects in the Sun but rather due to 
the properties of neutrinos. At least one of the three known types of 
neutrinos is not massless—although it has a small mass indeed, perhaps 
a hundred million times smaller than the mass of the next-lightest par-
ticle in nature, the electron. Since the Standard Model does not accom-
modate neutrinos' masses, this was the first definitive observation that 
some new physics, beyond the Standard Model and beyond the Higgs, 
must be operating in nature. 
Soon after this, observations of higher-energy neutrinos that reg-
ularly bombard Earth as high-energy cosmic-ray protons hit the at-
mosphere and produce a downward shower of particles, including 
neutrinos, demonstrated that yet a second neutrino has mass. This mass 
is somewhat larger, but still far smaller than the mass of the electron. 
For these results team leaders at 5140 and Kamiokande were awarded 
the 2015 Nobel Prize in Physics—a week before I wrote the first draft 
of these words. To date these tantalizing hints of new physics are not 
explained by current theories. 
The absence of proton decay, while disappointing, turned out to be 
not totally unexpected. Since Grand Unification was first proposed, 
the physics landscape had shifted slightly. More precise measurements 
of the actual strengths of the three nongravitational interactions—
combined with more sophisticated calculations of the change in the 
strength of these interactions with distance—demonstrated that if the 
particles of the Standard Model are the only ones existing in nature, 
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the strength of the three forces will not unify at a single scale. In order 
for Grand Unification to take place, some new physics at energy scales 
beyond those that have been observed thus far must exist. The pres-
ence of new particles would not only change the rate at which the three 
known interactions change with scale so that they might unify at a sin-
gle scale of energy, it would also tend to drive up the Grand Unification 
scale and thus suppress the rate of proton decay—leading to predicted 
lifetimes in excess of a million billion billion billion years. 
As these developments were taking place, theorists were driven by new 
mathematical tools to explore a possible new type of symmetry in nature, 
which became known as supersymmetry. This fundamental symmetry is 
different from any previous known symmetry, in that it connects the two 
different types of particles in nature, fermions (particles with half-integer 
spins) and bosons (particles with integer spins). The upshot of this (many 
other books, including some by me, explore this idea in detail) is that 
if this symmetry exists in nature, then for every known particle in the 
Standard Model at least one corresponding new elementary particle must 
exist. For every known boson there must exist a new fermion. For every 
known fermion there must exist a new boson. 
Since we haven't seen these particles, this symmetry cannot be mani-
fest in the world at the level we experience it, and it must be broken, 
meaning the new particles will all get masses that could be heavy enough 
so that they haven't been seen in any accelerator constructed thus far. 
What could be so attractive about a symmetry that suddenly doubles 
all the particles in nature without any evidence of any of the new par-
ticles? In large part the seduction lay in the very fact of Grand Unifica-
tion. Because if a Grand Unified Theory exists at a mass scale of fifteen 
to sixteen orders of magnitude higher energy than the rest mass of the 
proton, this is also about thirteen orders of magnitude higher than the 
scale of electroweak symmetry breaking. The big question is why and 
how such a huge difference in scales can exist for the fundamental laws 
of nature. In particular, if the Standard Model Higgs is the true last 
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remnant of the Standard Model, then the question arises, Why is the 
energy scale of Higgs symmetry breaking thirteen orders of magni-
tude smaller-scale than the scale of symmetry breaking associated with 
whatever new field must be introduced to break the GUT symmetry 
into its separate component forces? 
The problem is a little more severe than it appears. Scalar particles 
such as the Higgs have several new quantum mechanical properties that 
are unlike those of fermions or spin i particles such as gauge particles. 
When one considers the effects of virtual particles, including particles of 
arbitrarily large mass, such as the gauge particles of a presumed Grand 
Unified Theory, these tend to drive up the mass and symmetry-breaking 
scale of the Higgs so that it essentially becomes close to, or identical to, 
the heavy GUT scale. This generates a problem that has become known 
as the naturalness problem. It is technically unnatural to have a huge 
hierarchy between the scale at which the electroweak symmetry is bro-
ken by the Higgs particle and the scale at which the GUT symmetry is 
broken by whatever new heavy field scalar breaks that symmetry. 
The brilliant mathematical physicist Edward Witten argued in an in-
fluential paper in 1981 that supersymmetry had a special property. It could 
tame the effect that virtual particles of arbitrarily high mass and energy 
have on the properties of the world at the scales we can currently probe. 
Because virtual fermions and virtual bosons of the same mass produce 
quantum corrections that are identical except for a sign, if every boson is 
accompanied by a fermion of equal mass, then the quantum effects of the 
virtual particles will cancel out. This means that the effects of virtual par-
ticles of arbitrarily high mass and energy on the physical properties of the 
universe on scales we can measure would now be completely removed. 
If, however, supersymmetry is itself broken, then the quantum cor-
rections will not quite cancel out. Instead they would yield contribu-
tions to masses that are the same order as the supersymmetry-breaking 
scale. If it was comparable to the scale of the electroweak symmetry 
breaking, then it would explain why the Higgs mass scale is what it is. 
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And it also means we should expect to begin to observe a lot of new par-
ticles—the supersymmetric partners of ordinary matter—at the scale 
currently being probed at the LHC. 
This would solve the naturalness problem because it would protect the 
Higgs boson masses from possible quantum corrections that could drive 
them up to be as large as the energy scale associated with Grand Unifica-
tion. Supersymmetry could allow a "natural" large hierarchy in energy 
(and mass) separating the electroweak scale from the Grand Unified scale. 
That supersymmetry could in principle solve the hierarchy problem, 
as it has become known, greatly increased its stock with physicists. It 
caused theorists to begin to explore realistic models that incorporated 
supersymmetry breaking and to explore the other physical consequences 
of this idea. When they did so, the stock price of supersymmetry went 
through the roof. For if one included the possibility of spontaneously 
broken supersymmetry into calculations of how the three nongravita-
tional forces change with distance, then suddenly the strength of the 
three forces would naturally converge at a single, very small-distance 
scale. Grand Unification became viable again! 
Models in which supersymmetry is broken have another attractive 
feature. It was pointed out, well before the top quark was discovered, 
that if the top quark was heavy, then through its interactions with other 
supersymmetric partners, it could produce quantum corrections to the 
Higgs particle properties that would cause the Higgs field to condense 
at its currently measured energy scale if Grand Unification occurred at a 
much higher, superheavy scale. In short, the energy scale of electroweak 
symmetry breaking could be generated naturally within a theory in 
which Grand Unification occurs at a much higher energy scale. When 
the top quark was discovered and indeed was heavy, this added to the 
attractiveness of the possibility that supersymmetry breaking might be 
responsible for the observed energy scale of the weak interaction. 
All of this comes at a cost, however. For the theory to work, there 
must be two Higgs bosons, not just one. Moreover, one would expect to 
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begin to see the new supersymmetric particles if one built an accelera-
tor such as the LHC, which could probe for new physics near the elec-
troweak scale. Finally, in what looked for a while like a rather damning 
constraint, the lightest Higgs in the theory could not be too heavy or the 
mechanism wouldn't work. 
As searches for the Higgs continued without yielding any results, ac-
celerators began to push closer and closer to the theoretical upper limit 
on the mass of the lightest Higgs boson in supersymmetric theories. The 
value was something like 135 times the mass of the proton, with details 
to some extent depending on the model. If the Higgs could have been 
ruled out up to that scale, it would have suggested all the hype about 
supersymmetry was just that. 
Well, things turned out differently. The Higgs that was observed at 
the LHC has a mass about izs times the mass of the proton. Perhaps a 
grand synthesis was within reach. 
The answer at present is ... not so clear. The signatures of new super-
symmetric partners of ordinary particles should be so striking at the LHC, 
if they exist, that many of us thought that the LHC had a much greater 
chance of discovering supersymmetry than it did of discovering the Higgs. 
It didn't turn out that way. Following three years of LHC runs, there are no 
signs whatsoever. The situation is already beginning to look uncomfortable. 
The lower limits that can now be placed on the masses of supersymmetric 
partners of ordinary matter are getting higher. If they get too high, then the 
supersymmetry-breaking scale would no longer be close to the electroweak 
scale, and many of the attractive features of supersymmetry breaking for 
resolving the hierarchy problem would go away. 
But the situation is not yet hopeless, and the LHC has been turned 
on again, this time at higher energy. It could be that, in the year between 
the time I write these words and the book going into its tenth printing, 
supersymmetric particles will be discovered. 
If they are, this will have another important consequence. One of the 
bigger mysteries in cosmology is the nature of the dark matter that ap-
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pears to dominate the mass of all galaxies we can see. As I have briefly 
alluded to earlier, there is so much of it that it cannot be made of the 
same particles as normal matter. If it were, for example, the predictions 
of the abundance of light elements such as helium produced in the Big 
Bang would no longer agree with observation. Thus physicists are rea-
sonably certain that the dark matter is made of a new type of elementary 
particle. But what type? 
Well, the lightest supersymmetric partner of ordinary matter is, in 
most models, absolutely stable and has many of the properties of neu-
trinos. It would be weakly interacting and electrically neutral, so that it 
wouldn't absorb or emit light. Moreover, calculations that I and others 
performed more than thirty years ago showed that the remnant abun-
dance today of the lightest supersymmetric particle left over after the 
Big Bang would naturally be in the range so that it could be the dark 
matter dominating the mass of galaxies. 
In that case our galaxy would have a halo of dark matter particles 
whizzing throughout it, including through the room in which you are 
reading this. As a number of us also realized some time ago, this means 
that if one designs sensitive detectors and puts them underground, not 
unlike, at least in spirit, the neutrino detectors that already exist under-
ground, one might directly detect these dark matter particles. Around 
the world a half dozen beautiful experiments are now going on to do just 
that. So far nothing has been seen, however. 
So, we are in potentially the best of times or the worst of times. A race 
is going on between the detectors at the LHC and the underground direct 
dark matter detectors to see who might discover the nature of dark mat-
ter first. If either group reports a detection, it will herald the opening up 
of a whole new world of discovery, leading potentially to an understand-
ing of Grand Unification itself. And if no discovery is made in the coming 
years, we might rule out the notion of a simple supersymmetric origin of 
dark matter—and in turn rule out the whole notion of supersymmetry 
as a solution of the hierarchy problem. In that case we would have to go 
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