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about the laws of physics appears to change with time—results in the 
conservation of energy in the physical universe. 
In quantum electrodynamics, one fundamental symmetry is in the 
nature of electric charges. What we call "positive" and "negative" are 
clearly arbitrary. We could change every positive charge in the universe 
to negative, and vice versa, and the universe would look and behave pre-
cisely the same. 
Imagine, for example, that the world is one giant chessboard, with 
black and white squares. Nothing about the game of chess would be 
changed if I changed black into white, and white into black. The white 
pieces would become black pieces and vice versa, and otherwise the 
board would look identical. 
Now, precisely because of this symmetry of nature, the electric 
charge is conserved: no positive or negative charge can spontaneously 
appear in any process, even due to quantum mechanics, without an 
equal and opposite charge appearing at the same time. For this rea-
son, virtual particles are only produced spontaneously in empty space 
in combination with antiparticles. It is also why lightning storms occur 
on Earth. Electric charges build up on Earth's surface because storm 
clouds build up large negative charges at their base. The only way to get 
rid of this charge is to have large currents flow from the ground upward 
into the sky. 
The conservation of charge resulting from this symmetry can be un-
derstood using my chessboard analogy. That every white square must be 
located next to a black square means that whenever I switch black and 
white, the board ultimately looks the same. If I had two black squares 
in a row, which would mean the board had some net "blackness," then 
"black" and "white" would no longer be equivalent arbitrary labels. Black 
would be physically different from white. In short, the symmetry be-
tween black and white on the board would be violated. 
Bear with me now, because I am about to introduce a concept that 
is much more subtle, but much more important. It's so important that 
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essentially all of modern physical theory is based on it. But it's so subtle 
that without using mathematics, it is hard to describe. It is so subtle that 
its ramifications are still being unraveled today, more than a hundred 
years since it was first suggested. So, don't be surprised if it takes one or 
two readings to fully get your head around the idea. It has taken physi-
cists much of the past century to get their heads around it. 
This symmetry is called gauge symmetry for an obscure historical 
reason I shall describe a bit later. But the strange name is irrelevant. It is 
what the symmetry implies that is important: 
Gauge symmetry in electromagnetism says that I can actually 
change my definition of what a positive charge is locally at each 
point of space without changing the fundamental laws associated 
with electric charge, as long as I also somehow introduce some 
quantity that helps keep track of this change of definition from point 
to point. This quantity turns out to be the electromagnetic field. 
Let's try to parse this using my chessboard analogy. The global sym-
metry I described before changes black to white everywhere, so when 
the chessboard is turned by 180 degrees, it looks the same as it did be-
fore and the game of chess is clearly not affected. 
Now, imagine instead that I change black to white in one square, 
and I don't change white to black in the neighboring square. Then the 
board will have two adjacent white squares. This board, with two ad-
jacent white squares, clearly won't look the same as it did before. The 
game cannot be played as it was before. 
But hold on for a moment. What if I have a guidebook that tells me 
what game pieces should do every time they encounter adjacent squares 
where one color has been changed but not the next. Then the rules of 
the game can remain the same, as long as I consult the guidebook each 
time I move. This guidebook therefore allows the game to proceed as if 
nothing were changed. 
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In mathematics, a quantity that ascribes some rule associated with 
each point on a surface like a chessboard is called a function. In physics, 
a function defined at every point in our physical space is called a field, 
such as, for example, the electromagnetic field, which describes how 
strong electric and magnetic forces are at each point in space. 
Now here's the kicker. The properties that must characterize the form 
of the necessary function (which allows us to change our definition of 
electric charge from place to place without changing the underlying phys-
ics governing the interaction of electric charges) are precisely those that 
characterize the form of the rules governing electromagnetic fields. 
Put another way, the requirement that the laws of nature remain in-
variant under a gauge transformation—namely some transformation 
that locally changes what I call positive or negative charge—identically 
requires the existence of an electromagnetic field that is governed by 
precisely by Maxwell's equations. Gauge invariance, as it is called, com-
pletely determines the nature of electromagnetism. 
This presents us with an interesting philosophical question. Which 
is more fundamental, the symmetry or the physical equations that man-
ifest the symmetry? In the former case, where this gauge symmetry of 
nature requires the existence of photons, light, and all the equations and 
phenomena first discovered by Maxwell and Faraday, then God's appar-
ent command "Let there be light" becomes identical with the command 
"Let electromagnetism have a gauge symmetry." It is less catchy, per-
haps, but nevertheless true. 
Alternatively, one could say that the theory is what it is, and the dis-
covery of a mathematical symmetry in the underlying equations is a 
happy accident. 
The difference between these two viewpoints seems primarily se-
mantic, which is why it might interest philosophers. But nature does 
provide some guidance. If quantum electrodynamics were the only 
theory in nature that respected such a symmetry, the latter view might 
seem more reasonable. 
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But every known theory describing nature at a fundamental scale 
reflects some type of gauge symmetry. As a result, physicists now tend 
to think of symmetries of nature as fundamental, and the theories that 
then describe nature as being restricted in form to respect these sym-
metries, which in turn then reflect some key underlying mathematical 
features of the physical universe. 
Whatever one might think of regarding this epistemological issue, 
what matters in the end to physicists is that the discovery and applica-
tion of this mathematical symmetry, gauge symmetry, has allowed us to 
discover more about the nature of reality at its smallest scales than any 
other idea in science. As a result, all attempts to go beyond our current 
understanding of the four forces of nature, electromagnetism, the two 
forces associated with atomic nuclei, the strong and weak forces, which 
we shall meet shortly, and gravity—including the attempt to create a 
quantum theory of gravity—are built on the mathematical underpin-
nings of gauge symmetry. 
• 
• 
• 
That gauge symmetry has such a strange name has little to do with 
quantum electrodynamics and is an anachronism, related to a property 
of Einstein's General Theory of Relativity, which, like all other funda-
mental theories, also possesses gauge symmetry. Einstein showed that 
we are free to choose any local coordinate system we want to describe 
the space around us, but the function, or field, that tells us how to con-
nect these coordinate systems from point to point is related to the un-
derlying curvature of space, determined by the energy and momentum 
of material in space. The coupling of this field, which we recognize as 
the gravitational field, to matter, is precisely determined by the invari-
ance of the geometry of space under the choice of different coordinate 
systems. 
The mathematician Hermann Weyl was inspired by this symmetry of 
General Relativity to suggest that the form of electromagnetism might 
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also reflect an underlying symmetry associated with physical changes in 
length scales. He called these different "gauges," inspired by the various 
track gauges of railroads. (Einstein, and Sheldon on The Big Bang The-
ory, aren't the only physicists who have been inspired by trains.) While 
Weyl's guess turned out to be incorrect, the symmetry that does apply 
to electromagnetism became known as gauge symmetry. 
Whatever the etymology of the name, gauge symmetry has become 
the most important symmetry we know of in nature. From a quantum 
perspective—in the quantum theory of electromagnetism, quantum 
electrodynamics—the existence of gauge symmetry becomes even more 
important. It is the essential feature that ensures that QED is sensible. 
If you think about the nature of symmetry, then it begins to make 
sense that such a symmetry might ensure that quantum electrodynam-
ics makes sense. Symmetries tell us, for example, that different parts 
of the natural world are related, and that certain quantities remain the 
same under various types of transformations. A square looks the same 
when we rotate it ninety degrees because the sides are all the same 
length and the angles at each corner are the same. So, symmetry can 
tell us that different mathematical quantities that result from physical 
calculations, such as the effects of many virtual particles, and many vir-
tual antiparticles, for example, can have the same magnitude. They may 
also have opposite signs so that they might cancel exactly. The existence 
of this symmetry is what can require such exact cancellations. 
In this way, one might imagine that in quantum electrodynamics 
the nasty terms that might otherwise give infinite results can cancel 
with other potentially nasty terms, and all the nastiness can disappear. 
And this is precisely what happens in QED. The gauge symmetry en-
sures that any infinities that might otherwise arise in deriving physical 
predictions can be isolated in a few nasty terms that can be shown by 
the symmetry to either disappear or to be decoupled from all physically 
measurable quantities. 
This profoundly important result, proven by decades of work by some 
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of the most creative and talented theoretical physicists in the world, es-
tablished QED as the most precise and preeminent quantum theory of 
the twentieth century. 
Which made it all the more upsetting to discover that, while this 
mathematical beauty indeed allowed a sensible understanding of one 
of nature's fundamental forces—electromagnetism—other nastiness 
began when considering the forces that govern the behavior of atomic 
nuclei. 
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Chapter 9 
DECAY AND RUBBLE 
There is no new thing under the sun. 
-ECCLESIASTES 1:9 
When I first learned that we human beings are radioac-
tive, it shocked me. I was in high school listening to a lecture by the re-
markable polymath and astrophysicist Tommy Gold, who had done 
pioneering work in cosmology, pulsars, and lunar science, and he in-
formed us that the particles that made up most of the mass of our bod-
ies, neutrons, are unstable, with a mean lifetime of about ten minutes. 
Given, I hope, that you have been reading this book for longer than 
ten minutes, this may surprise you too. The resolution of this seeming 
paradox is one of the first and most wonderful of the gorgeous accidents 
of nature that make our existence possible. As we continue to explore 
more deeply the question "Why are we here?," this accident will loom 
large on the horizon. While the neutron may seem far removed from 
light, which has been the centerpiece of our story thus far, we shall see 
that the two are ultimately deeply connected. The decay of neutrons—
responsible for the "beta decay" of unstable nuclei—required physicists 
to move beyond their simple and elegant theories of light and open up 
new fundamental areas of the universe for investigation. 
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But I am getting ahead of myself. 
In 1929, when Dirac first wrote down his theory of electrons and 
radiation, it looked as if it might end up being a theory of almost every-
thing. Aside from electromagnetism, the only other force in town was 
gravity, and Einstein had just made great strides in understanding it. 
Elementary particles consisted of electrons, photons, and protons, to-
gether comprising all the objects that appeared necessary to understand 
atoms, chemistry, life, and the universe. 
The discovery of antiparticles upset the applecart somewhat, but 
since Dirac's theory had effectively predicted them (even if Dirac him-
self had to catch up with the theory), this was more like a speed bump 
on the road to reality than a roadblock or detour. 
Then came 1932. Up to that time, scientists had presumed that atoms 
were composed entirely of protons and electrons. This posed a bit of a 
problem, however, because the masses of atoms didn't quite add up. In 
1911 Rutherford discovered the existence of the atomic nucleus, contain-
ing almost all the mass of atoms in a small region one hundred thou-
sand times smaller than the size of the orbits of the electrons. Following 
that discovery, it became clear that the mass of heavy nuclei was just a 
bit more than twice the mass that could be accounted for if the number 
of protons in the nucleus equaled the number of electrons orbiting the 
nucleus, ensuring that atoms would be electrically neutral. 
The proposed solution to this conundrum was simple. Actually twice 
as many protons were in the nucleus as electrons surrounding it, but 
just the right number of electrons were trapped inside the nucleus, so 
that again the total electric charge of the atom would be equal to zero. 
However, quantum mechanics implied that the electrons couldn't be 
confined within the nucleus. The argument is a bit technical, but it goes 
something like this: If elementary particles have a wavelike character, 
then if one is going to confine them to a small distance, the magnitude 
of their wavelength must be smaller than the confinement scale. But 
the wavelength associated with a particle is, in quantum mechanics, 
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inversely proportional to the momentum carried by the particle, and 
hence also inversely proportional to the energy carried by the particle. 
If electrons were confined to a region the size of an atomic nucleus, the 
energy they would need to possess would be about a million times the 
energy associated with the characteristic energies released by electrons 
as they jump between energy levels in their atomic orbits. 
How could they achieve such energies? They couldn't. For, even if 
electrons were tightly bound to protons within nuclei by electronic 
forces, the binding energy that would be released in this process as they 
"fell" into the nucleus would be more than ten times smaller than the 
energy needed to confine the quantum mechanical electron wave func-
tion to a region contained within the nucleus. 
Here too the numbers just didn't add up. 
Physicists at the time were aware of the problem, but lived with it. 
I suspect that an agnostic approach was deemed prudent, and physi-
cists were willing to suspend disbelief until they knew more, because 
the issues involved the cutting-edge physics of quantum mechanics 
and atomic nuclei. Instead of proposing exotic new theories (there were 
probably some at the margins that I am not aware of), the community 
was eventually driven by experiments to overcome its natural hesitation 
to take the logical next step: to assume nature was more complicated 
than had thus far been revealed. 
In 1930, about the time that Dirac was coming to grips with the pos-
sibility that his antiparticles weren't really protons, a series of experi-
ments provided just the clues that were needed to unravel the nuclear 
paradox. The poetry of the discoveries was rivaled only by the drama in 
the private lives of the researchers. 
Max Planck had helped pioneer the quantum revolution by resolving 
the paradox of the spectrum of radiation emitted by atomic systems. So 
it was fitting that Planck should indirectly help resolve the paradoxical 
makeup of the nucleus. While he didn't himself spearhead the relevant 
research, he recognized the talents of a young student of mathemat-
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ics, physics, chemistry, and music at the University of Berlin, Walther 
Bothe, and in 1912 Planck accepted him as a doctoral student and men-
tored him throughout the rest of his career. 
Bothe was spectacularly lucky to be mentored by Planck and, shortly 
thereafter, by Hans Geiger, of Geiger counter fame. Geiger, in my mind, 
is one of the most talented experimental physicists to have been over-
looked for a Nobel Prize. Geiger had begun his career by doing the 
experiments, with Ernest Marsden, that Ernest Rutherford utilized to 
discover the existence of the atomic nucleus. Geiger had just returned 
from England, where ■ 
worked with Rutherford, to direct a new labo-
ratory in Berlin, and one of his first acts was to hire Bothe as an as-
sistant. There Bothe learned to focus on important experiments, using 
simple approaches that yielded immediate results. 
After an "involuntary vacation" of five years, as a prisoner of war in 
Siberia during the First World War, Bothe returned and built a remark-
able collaboration with Geiger, eventually succeeding him as director 
of the laboratory. During their time together they pioneered the use of 
"coincidence methods" to explore atomic, and eventually nuclear, phys-
ics. Using different detectors located around a target, and using care-
ful timing, they could look for simultaneous events, signaling that the 
source had to be a single atomic or nuclear decay. 
In 1930 Bothe and his assistant Herbert Becker observed something 
completely new and unexpected. While bombarding beryllium nuclei 
with products of nuclear decay called alpha particles (already known to 
be the nuclei of helium), the two observed the emission of a completely 
new form of high-energy radiation. This radiation had two unique fea-
tures. It was more penetrating than the most energetic gamma rays, 
but like gamma rays, the radiation was composed of electrically neutral 
particles so that it did not ionize atoms as it passed through matter. 
News of this surprising discovery made its way to other physics labo-
ratories throughout Europe. Bothe and Becker had initially proposed 
that this radiation was some new sort of gamma ray. In Paris, Irene 
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Joliot-Curie, the daughter of famed physicist Marie Curie, and Irene's 
husband, Frederic, replicated Bothe and Becker's results and explored 
the radiation in more detail. In particular, they found that when it bom-
barded a paraffin target, it knocked out protons with incredible energy. 
This observation made it clear that the radiation couldn't be a gamma 
ray. Why? 
The answer is relatively simple. If you throw a piece of popcorn at an 
oncoming truck, you are unlikely to stop the truck or even break a win-
dow. That is because the popcorn, even if you throw it with great energy, 
carries little momentum because the popcorn is light. To stop a truck 
you have to change its momentum by a large amount because, even if it 
is moving slowly, it is heavy. To stop a truck or knock a heavy object off 
the truck, you have to throw a big rock. 
Similarly, to knock out a heavy particle such as a proton from paraf-
fin, a gamma ray, made of massless photons, would have to carry great 
energy (so that the momentum carried by the individual photons was 
large enough to kick out a heavy proton), and not enough energy was 
available, by an order of magnitude at least, in any known nuclear-decay 
processes for this. 
Surprisingly, the Joliot-Curies (they were modern and both adopted 
the same hyphenated last name) were probably loath, like Dirac, to pro-
pose new elementary particles to explain data—since protons, electrons, 
and photons were not only familiar, but sufficient up to that time to ex-
plain everything known, including exotic quantum phenomena associ-
ated with atoms. So, Irene and Fr€deric didn't make the now-obvious 
proposal that maybe a new neutral massive particle was being produced 
in the decays that Bothe and Becker had discovered. Unfortunately, a 
similar timidity caused the Joliot-Curies to fail to claim discovery of the 
positron—in spite of having actually observed it in their experiments 
before Carl Anderson reported his own discovery somewhat later. 
It fell to the physicist James Chadwick to push things further. Chad-
wick clearly had a great nose for physics, but his political acumen was 
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not so sharp. After graduation from the University of Manchester with 
a master's degree in 1913, working with Rutherford, he obtained a fel-
lowship that would allow him to study anywhere. So he went to Berlin 
to work with Geiger. He couldn't have picked a better mentor, and he 
began to do important studies of radioactive decays. Unfortunately, the 
First World War broke out while Chadwick was in Germany, and he 
spent the next four years in an internment camp. 
Eventually he returned to Cambridge, where Rutherford had since 
moved, to complete his PhD under Rutherford's direction. Following this 
Chadwick stayed on to work with Rutherford and help direct the Caven-
dish laboratory there. While he was aware of Bothe and Becker's results 
and even reproduced them, only when one of his students informed him 
of the Joliot-Curies results did Chadwick became convinced, using the 
energy argument I mentioned above, that the radiation that had been 
observed had to result from a new neutral particle—of mass comparable 
to that of the proton—that might reside in atomic nuclei, an idea he and 
Rutherford had been germinating for years. 
Chadwick reproduced and extended the Joliot-Curies' experiments, 
bombarding targets other than paraffin to explore the outgoing protons. 
He confirmed not only that the energetics of the collisions made it im-
possible for the source to be gamma rays, but also that the interaction 
strength of the new particles with nuclei was far greater than would be 
predicted for gamma rays. 
Chadwick didn't dawdle. Within two weeks of beginning his experi-
ments in 1932, he sent a letter to Nature entitled "Possible Existence of 
a Neutron" and followed this up with a more detailed article sent to the 
Royal Society. The neutron, which we now know makes up most of the 
mass of heavier nuclei, and thus most of the mass in our bodies, had 
been discovered. 
For his discovery he was awarded the Nobel Prize in Physics three 
years later, in 1935. In a kind of poetic justice, three of the people whose 
experiments had made Chadwick's results possible—but who missed 
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out on identifying the neutron—were awarded Nobel Prizes for other 
work. Bothe won the Nobel Prize in 1954 for his work on using coinci-
dences between observed events in different detectors to explore the de-
tailed nature of nuclear and atomic phenomena. Both Irene and Frederic 
Joliot-Curie, who barely missed out on two other Nobel Prize—winning 
discoveries, won the Nobel Prize in Chemistry in 193s for their discov-
ery of artificial radioactivity—which was later an essential ingredient in 
the development of both nuclear power and nuclear weapons. Interest-
ingly, only after winning the Nobel Prize was Irene awarded a profes-
sorship in France. With the two Nobel Prizes for her mother, Marie, the 
Curie family garnered a total of five Nobel Prizes, the most that have 
ever been received by a single family. 
After his discovery Chadwick set out to measure the mass of the 
neutron. His first estimate, in 1933, suggested a mass of slightly less than 
the sum of the masses of a proton and an electron. This reinforced the 
idea that perhaps the neutron was a bound state of these two particles, 
and the mass difference, using Einstein's relation E = mca, was due to the 
energy lost in binding them together. However, after several conflicting 
measurements by other groups, further analysis a year later by Chad-
wick using a nuclear reaction induced by gamma rays—which allowed 
all energies to be measured with great precision—definitely indicated 
that the neutron was heavier than the sum of the proton and electron 
masses, even if barely so, with the mass difference being less than 0.1 
percent. 
It is said that `close only matters when tossing horseshoes or hand 
grenades, but the closeness in mass between the proton and the neutron 
matters a great deal. It is one of the key reasons we exist today. 
Henri Becquerel discovered radioactivity in uranium in 1896, and 
only three years later Ernest Rutherford discerned that radioactivity oc-
curred in two different types, which he labeled alpha and beta rays. A 
year later gamma rays were discovered, and Rutherford confirmed them 
as a new form of radiation in 1903, when he gave them their name. Bec-
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querel determined in 1900 that the "rays" in beta decay were actually 
electrons, which we now know arise from the decay of the neutron. 
In beta decay a neutron splits into a proton and an electron, which, 
as I describe below, would not be possible if the neutron weren't slightly 
heavier than protons. What is surprising about this neutron decay is not 
that it occurs, but that it takes so long. Normally the decay of unstable 
elementary particles occurs in millionths or billionths of a second. Iso-
lated neutrons live, on average, more than ten minutes. 
One of the chief reasons that neutrons live so long is that the mass of 
the neutron is only slightly more than the sum of the masses of a proton 
plus an electron. Thus, there is only barely enough energy available, via 
the neutron's rest mass, to allow it to decay into these particles and still 
conserve energy. (The other reason is that a neutron doesn't decay into 
only a proton plus an electron. It decays into three particles ... stay tuned!) 
While ten minutes may be an eternity on atomic timescales, it is 
pretty short compared to a human life or the lifetime of atoms on Earth. 
Returning to the puzzle I mentioned at the beginning of this chapter, 
what gives? How can we be largely made up of neutrons if they decay 
before the first commercial break in a thirty-minute TV show? 
The answer again lies in the extreme closeness of the neutron and 
proton masses. A free neutron decays in ten minutes or so. But consider 
a neutron bound inside an atomic nucleus. Being bound means that it 
takes energy to kick it out of the nucleus. But that means that it loses 
energy when it gets bound to the nucleus in the first place. But, Einstein 
told us that the total energy of a massive particle is proportional to its 
mass, via E = me. That means that, if the neutron loses energy when it 
gets bound in a nucleus, its mass gets smaller. But since its mass when it 
is isolated is just a smidgen more than the sum of the masses of a proton 
and an electron, when it loses mass, it no longer has sufficient energy to 
decay into a proton and an electron. If it were to decay into a proton, it 
would have to either release enough energy to also eject the proton from 
the nucleus, which, given standard nuclear-binding energies, it would 
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not have, or else release enough energy to allow the new proton to re-
main in a new stable nucleus. Since the new nucleus would be that of a 
different element, adding one additional positive charge to the nucleus 
also generally requires more energy than the minute amount available 
when a neutron decays. As a result, the neutron and most atomic nuclei 
containing neutrons remain stable. 
The entire stability of the nuclei that make up everything we see, in-
cluding most of the atoms in our body, is an accidental consequence of 
the fact that the neutron and proton differ in mass by only o.i percent, 
so that a small shift in the mass of the former, when embedded in nuclei, 
means it can no longer decay into the latter. That is what I learned from 
Tommy Gold. 
It still amazes me when I think about it. The existence of complex 
matter, the periodic table, everything we see, from distant stars to the 
keyboard I am typing this on—hinges on such a remarkable coinci-
dence. Why? Is it an accident, or do the laws of physics require it for 
some unknown reason? Questions such as these drive us physicists to 
search deeper for possible answers. 
The discovery of the neutron, and the subsequent observation of its 
decay, introduced more than one new particle into the subatomic zoo. 
It suggested that perhaps two of the most fundamental properties of 
nature—the conservation of energy and the conservation of momen-
tum—might break down on the microscopic-distance scales of nuclei. 
Almost twenty years before discovering the neutron, James Chadwick 
had observed something strange about beta rays, well before he or anyone 
else knew that they originated from decaying neutrons. The spectrum 
of energy carried by electrons emitted in neutron decay is continuous, 
going from essentially zero energy up to a maximum energy, which de-
pends on the energy available after the neutron has decayed—for a free 
neutron this maximum energy is the energy difference between the mass 
of the neutron and the sum of the masses of the proton and electron. 
There is a problem with this, however. It is easiest to see the problem 
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if we imagine for the moment that the proton and the electron have 
equal masses. Then, if the proton carries off more energy than the elec-
tron after the decay, it would be moving faster than the electron. But if 
they have the same mass, then the proton would also have more mo-
mentum than the electron. But if the neutron decays at rest, then its 
momentum before the decay would be zero, so the momentum of the 
outgoing proton would have to cancel that of the outgoing electron. But 
that is impossible unless they have equal momenta, going in opposite 
directions. So the magnitude of the proton's momentum could never be 
greater than that of the electron. In short, there is only one value for the 
energy and the momentum of the two particles after the decay if they 
have equal masses. 
The same reasoning, though mathematically a bit more involved, 
applies even if the proton and electron have different masses. If they 
are the only two particles produced in the decay of the neutron, their 
speeds, and hence their energy and momenta, would be required to each 
have unique, fixed values that depend on the ratio of their respective 
masses. 
As a result, if electrons from beta decay of neutrons come off with a 
range of different energies, this would violate the conservation of energy 
and momentum. But, as I subtly suggested above, this is only true if the 
electron and proton are the only particles produced as products of the 
neutron decay. 
Again, in 1930, only a few years before the discovery of the neutron, 
the remarkable Austrian theoretical physicist Wolfgang Pauli wrote a 
letter to colleagues at the Swiss Federal Institute of Technology, begin-
ning with the immortal header "Dear radioactive ladies and gentlemen," 
in which he outlined a proposal to resolve this problem, which he also 
said he didn't "feel secure enough to publish anything about! He pro-
posed that a new electrically neutral elementary particle existed, which 
he called a neutron, and that in addition to the electron and the proton 
this new neutral particle was produced in beta decay so that the elec-
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tron, proton, and this particle together could share the energy available 
in the decay, allowing a continuous spectrum. 
Pauli, who later won the Nobel Prize for his "exclusion principle" in 
quantum mechanics, was no fool. In fact, he had no patience for fools. 
He was famous for supposedly rushing up to the blackboard during lec-
tures and removing the chalk from the speaker's hand if he felt nonsense 
was being spouted. He could be scathingly critical of theories he didn't 
like, and his worst criticism was reserved for any idea that was so vague, 
as he put it, "it isn't even wrong." (A dear old colleague of mine when I 
taught at Yale, the distinguished mathematical physicist Feza Gursey, 
once responded to a reporter who asked what was the significance of 
an announcement of some overhyped idea proposed by some scientists 
seeking publicity by saying, "It means Pauli must be dead.") 
Pauli realized that proposing a new elementary particle that hadn't 
been observed was speculative in the extreme, and he argued in his 
letter that such a particle was unlikely both because it had never been 
seen and would therefore have to interact weakly with matter, and also 
because it would have to be very light to be produced along with an 
electron, given that the energies available in beta decay were so small 
compared to the proton's mass. 
The first problem that arose with his idea was the name he chose. 
After Chadwick's 1932 experimental discovery of the particle we now 
call the neutron, appropriate for a neutral cousin of the proton with 
comparable mass, Pauli's hypothesized particle needed another name. 
The brilliant Italian physicist and colleague of Pauli's—Enrico Fermi—
came up with a solution in 1934, changing its name to neutrino, an Ital-
ian pun for "little neutron." 
It would take twenty-six years for Pauli's neutrino to be discovered, 
enough time for the little particle, and its heavier cousin, the neutron, to 
force physicists to totally revamp their views on the forces that govern 
the cosmos, the nature of light, and even the nature of empty space. 
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Chapter 10 
FROM HERE TO INFINITY: 
SHEDDING LIGHT ON THE SUN 
I have fought a good fight, I have finished my 
course, I have kept the faith. 
-2 TIMOTHY 4:7 
The physicist Enrico Fermi is largely unsung in the public's 
eyes, but he remains one of the greatest twentieth-century physicists. 
He, together with Richard Feynman, more than any of the other remark-
able figures from that equally remarkable period in physics, most influ-
enced my own attitude and approach to the field, as well as my own 
understanding of it. I only wish I were as talented as either of them. 
Born in 1901, Fermi died at the age of fifty-three of cancer, perhaps 
brought on by his work on radioactivity. In 1954, when he died, he was 
nine years younger than I am as I write this. But in his short life he pushed 
forward the frontiers of both experimental and theoretical physics in a 
way that no one has since repeated, and no one is ever likely to do again. 
The complexity of the array of theoretical tools now used to develop phys-
ical models, and the complexity of machinery now used to test them, are 
separately too sophisticated to allow any single individual today, no mat-
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126 
THE GREATEST STORY EVER TOLD-SO FAR 
ter how talented, to remain on the vanguard of both endeavors at the level 
Fermi achieved in his time. 
In 1918, when Fermi graduated from high school in Rome, the possi-
bilities open to a brilliant young scientific mind were far less constrained. 
Quantum mechanics had just been born, new ideas were everywhere, 
and the rigorous mathematics necessary to deal with these ideas had not 
yet been developed or applied. Experimental physics had yet to enter the 
domain of "big science"; experiments could be performed by individual 
researchers in makeshift laboratories, and they could be completed in 
weeks instead of months. 
Fermi applied to the prestigious Scuola Normale Superiore in Pisa, 
which required an essay as part of the entrance exam. The theme that 
year was "specific characteristics of sounds." Fermi submitted an "essay" 
that included solving partial differential equations for a vibrating rod 
and applying a technique called Fourier analysis. Even today, these 
mathematical techniques are not normally encountered until maybe 
the third year of an undergraduate degree, and for some students not 
until graduate school. But as a seventeen-year-old, Fermi sufficiently im-
pressed the examiners to receive first place in the exam. 
At the university, Fermi first majored in mathematics but switched 
to physics and largely taught himself General Relativity—which Einstein 
had only developed a few years earlier—as well as quantum mechan-
ics and atomic physics, which were then emerging fields of research. 
Within three years of arriving at the university he published theoretical 
papers in major physics journals on subjects from General Relativity to 
electromagnetism. At the age of twenty-one, four years after beginning 
his university studies, he received his doctoral degree for a thesis ex-
ploring the applications of probability to X-ray diffraction. At the time 
a thesis on purely theoretical issues was not acceptable for a physics 
doctorate in Italy, so this encouraged Fermi to ensure his competence 
in the laboratory as well as with pen and paper. 
Fermi moved to Germany, the center of the emerging research on 
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