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quantum mechanics, and then to Leiden, Holland, where he met with 
the most famous physicists of the day—Born, Heisenberg, Pauli, Lo-
rentz, and Einstein, to name a few—before returning to Italy to teach. 
In 192s, Wolfgang Pauli proposed the `exclusion principle," which dis-
closed that two electrons could not occupy exactly the same quantum 
state at the same time and place, and which laid the basis of all of atomic 
physics. Within a year, Fermi applied this idea to systems of many such 
identical particles that, like electrons, have two possible values of spin, 
angular momentum, which we call spin up, and spin down. He thus 
established the modern form of the field called statistical mechanics, 
which is at the basis of almost all materials science, semiconductors, 
and those areas of physics that led to the creation of modern electronic 
components such as computers. 
As I earlier emphasized, there is no intuitive way to picture a point 
particle as spinning around some axis. It is simply one of the ways that 
quantum mechanics evades our notions of common sense. Electrons are 
called spin 34 particles because the magnitude of their spin angular mo-
mentum turns out to be half as big as the lowest value of angular momen-
tum associated with the orbital motion of electrons in atoms. Any spin 1/2
particle such as an electron is called a fermion, named in Fermi's honor. 
At the tender age of twenty-six Fermi was elected to a new chair in 
theoretical physics at the University of Rome and thereafter led a vi-
brant group of students, including several subsequent Nobel laureates, 
as they explored atomic and then nuclear physics. 
In 1933, Fermi was motivated by another proposal of Pauli's, that for 
the new particle produced in the decay of neutrons, which Fermi labeled 
a neutrino. But naming the new particle was just an aside. Fermi had 
much bigger fish to fry, and he produced a theory for neutron decay 
that revealed the possible existence of a new fundamental force in na-
ture, the first new force known to science beyond electromagnetism and 
gravity—which was in its own way inspired by thinking about light. Al-
though it wasn't obvious at the time, this was to be the first of two new 
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forces associated with atomic nuclei, which together with electromag-
netism and gravity, comprise all the forces known to operate in nature, 
from the smallest subatomic scales to the motion of galaxies. 
When Fermi submitted his proposal to the journal Nature, the edi-
tor turned it down because it was "too remote from physical reality to 
be of interest to readers." For many of us who have since had papers 
rejected by equally high-handed editors at that journal, it is comfort-
ing to know that Fermi's paper, one of the most important proposals in 
twentieth-century physics, also didn't make the cut. 
This inappropriate rejection was undoubtedly frustrating to Fermi, 
but it did have a useful side effect. Fermi decided instead to return to 
experimental physics, and in short order he began to experiment with 
the neutrons discovered by Chadwick two years earlier. Within several 
months Fermi had developed a powerful radioactive source of neutrons 
and found that he was able to induce radioactive decays in otherwise 
stable atoms by bombarding them with neutrons. Bombarding ura-
nium and thorium with neutrons, he also witnessed nuclear decays and 
thought he had created new elements. In fact, he had actually caused 
the nuclei to split, or fission, into lighter nuclei, which were later found 
to also emit more neutrons than they absorbed in the process—as other 
scientists discovered in 1939. 
Fermi's segue into experiment turned out to be good for him. Four 
years later, in 1938, at the age of thirty-seven, he was awarded the Nobel 
Prize for introducing artificial radioactivity, creating new radioactive 
elements by neutron bombardment. Yet by 1938 the Nazis had begun 
to establish their racial laws in Germany, and Italy had followed suit, so 
Fermi's Jewish wife, Laura, was endangered. So, after receiving the prize 
in Stockholm, Fermi and his family didn't return to Italy but moved to 
New York City, where he accepted a position at Columbia. 
When Fermi learned the news about nuclear fission in 1939 in New York, 
following a lecture by Niels Bohr at Princeton, Fermi amended his earlier 
Nobel acceptance speech to clarify his earlier error and in short order re-
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produced the German results. Before long, he and his collaborators realized 
that this produced the possibility of a chain reaction. Neutrons could bom-
bard uranium, causing it to fission and release energy, and to release more 
neutrons that could bombard more uranium atoms and so on. 
Soon after, Fermi gave a lecture to the US Navy warning of the po-
tential significance of this result, but few took him seriously. Later that 
year, Einstein's famous letter made its way to President Roosevelt and 
changed the course of history. 
Fermi had recognized the potential dangers inherent in releasing the 
energy of the atomic nucleus even earlier. A year after getting his doctor-
ate, in 1923, he wrote the appendix for a book on relativity and talked of 
the potential of E = me, writing at the time, "It does not seem possible, at 
least in the near future, to find a way to release these dreadful amounts of 
energy—which is all to the good because the first effect of an explosion of 
such a dreadful amount of energy would be to smash into smithereens the 
physicist who had the misfortune to find a way to do it." 
That idea must have been on his mind in 1941 when, as part of the 
newly established Manhattan Project, Fermi was assigned the task of 
creating a controlled chain reaction—namely creating a nuclear reactor. 
While those in charge were understandably worried about doing this in 
an urban area, Fermi was confident enough to convince the leader of the 
project to allow him to build it at the University of Chicago. On Decem-
ber z, 1942, the reactor went critical, and Chicago survived. 
Two and a half years later, Fermi was on hand in New Mexico to 
observe the first nuclear explosion, the Trinity test. Typical of Fermi, 
while the others stood in awe and horror, he conducted an impromptu 
experiment to estimate the bomb's strength by dropping several strips 
of paper when the blast wave came by, to see how far they were carried. 
Fermi's constant experimental approach to physics is one of the rea-
sons I cherish his memory. He always found a simple, easy way to reach 
the correct answer. Even though he had great mathematical skill, he dis-
liked complication, and he realized that he could get an approximate 
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answer that was "good enough" in a short time, while getting the exact 
answer might take months or years. He refined his abilities and helped 
his students do so by inventing what we now call Fermi Problems, which 
he is also said to have assigned at lunchtime each day to the team working 
for him. My favorite problem, which I always assign to my introductory-
physics students, is `How many piano tuners are there in Chicago? Try 
it. If you get between one hundred and five hundred, you did well. 
Fermi won the Nobel Prize for his experimental work, but his theo-
retical legacy for physics may be far greater. True to form, the `theory" 
he proposed in his famously rejected paper on neutron decay was re-
markably simple, yet it did the job. It wasn't a full theory at all, and at 
the time it would have been premature to develop one. Instead he made 
the simplest possible assumption. He imagined some new kind of inter-
action between particles that took place at a single point. The four par-
ticles were a neutron, a proton, an electron, and the new particle Pauli 
and Fermi named the neutrino. 
The starting point of Fermi's thinking involved light, as did almost all of 
modern physics, and in this case the modern quantum theory of light inter-
acting with matter. Recall that Feynman developed a pictorial framework 
to think about fundamental processes in space and time, when he argued 
that antimatter should exist. The space-time picture of an electron emitting 
a photon is reproduced here, but with the electron replaced by a proton, p: 
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Fermi imagined the decay of a neutron in a similar fashion, but in-
stead of the neutron emitting a photon and remaining the same particle, 
the neutron, n, would emit a pair of particles—an electron, e, and a neu-
trino, v, and would be converted into a proton, p: 
In electromagnetism the strength of the interaction between charged 
particles and photons (determining the probability of emitting a photon 
at the point shown in the first figure on the previous page) is proportional 
to the charge of the particle. Since the charge is what allows particles to 
interact, or "couple" to the electromagnetic field, we call the magnitude 
of the fundamental quantum of charge—the charge on a single electron 
or proton—the "coupling constant" of electromagnetism. 
In Fermi's interaction the numerical quantity that appears at the in-
teraction point in the figure where a neutron converts into a proton de-
termines the probability of such a conversion. The value of this quantity 
is determined by experiment, and we now call it the Fermi constant. 
Relative to electromagnetism, the numerical value of this quantity is 
small because the neutron takes a long time to decay—compared, for 
example, to the rate at which electromagnetic transitions take place in 
atoms. As a result, Fermi's interaction, describing a new force in nature, 
became known as the weak interaction. 
One of the things that made Fermi's proposal so remarkable was 
that it was the first time in physics that anyone had proposed that par-
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ticks other than photons could be spontaneously created in the quan-
tum world. (In this case the electron and the neutrino are created at the 
same time as the neutron converts into a proton.) This both inspired 
and became the prototype for much of the subsequent exploration of 
the quantum character of the fundamental forces in nature. 
Moreover, it didn't just make postdictions about nature. It made 
predictions precisely because a single mathematical form for the inter-
action that caused neutron decay could also predict a host of other phe-
nomena, which were later observed. 
Even more important, this interaction, with precisely the same 
strength, governs similar decays of other particles in nature. For example, 
in 2936 Carl Anderson, the discoverer of the positron, discovered another 
new particle in cosmic rays—the first of what would be so many that par-
ticle physicists would wonder whether the progression would ever end. 
When informed of this discovery, the atomic physicist and later Nobel 
laureate I. I. Rabi is said to have exclaimed, "Who ordered that?" 
We now know that this particle, called the muon and characterized 
by the Greek letter µ, is essentially an exact copy of the electron, only 
about two hundred times heavier. Because it is heavier, it can decay, 
emitting an electron and a neutrino in an interaction that looks identi-
cal to neutron decay, except the muon converts into another type of 
neutrino (called the muon neutrino) instead of a proton. Remarkably, if 
we use the same Fermi constant for the strength of this interaction, we 
derive exactly the right lifetime for the muon. 
Clearly a new fundamental force is at work here, universal in nature, 
with some similarities to electromagnetism, and some important differ-
ences. First, the interaction is much weaker. Second, unlike electromag-
netism, the interaction appears to operate over only a small range—in 
Fermi's model at a single point. Neutrons don't turn into protons in 
one place and cause electrons to turn into neutrinos somewhere else, 
whereas the interaction between electrons and photons allows electrons 
to exchange virtual photons and be repelled by each other even at a 
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great distance. Third, the interaction changes one type of particle into 
another. Electromagnetism involves the creation and absorption of pho-
tons—the quanta of light—but the charged particles that interact with 
them preserve their identity before and after the interaction. Gravity 
too is long-range, and when a ball falls toward the Earth, it remains a 
ball. But the weak interaction causes neutrons to decay into protons, 
muons into neutrinos, and so on. 
Clearly something about the weak interaction is different, but you may 
wonder if it is worth worrying about. Neutron decay is interesting, but hap-
pily the properties of nuclei protect us from it so that stable atoms can exist. 
Thus it seems to have little impact on everyday lives. Unlike gravity and 
electromagnetism, we don't sense it. If the weak interaction were of little 
other importance, then its anomalous nature could be easily overlooked. 
However, the weak interaction, at least as much as gravity and elec-
tromagnetism, is directly responsible for our existence. In 1939, Hans 
Bethe, who would soon help lead the effort to build the atomic bomb, re-
alized that the interactions that broke apart heavy nuclei as the source of 
the explosive power of the bomb could, under different circumstances, 
be utilized to build larger nuclei from smaller nuclei. This could release 
even more energy than was released in the A-bomb. 
Up until that time the energy source of the Sun was a mystery. It 
was well established that the temperature in the solar core could not 
exceed a few tens of millions of degrees—which may seem extreme, but 
the energies available to the colliding nuclei at those temperatures had 
already been achieved in the lab. Moreover, the Sun could not involve 
simple burning, like a candle. 
It had been established as early as the eighteenth century that an 
object with the mass of the Sun could only burn with its observed 
brightness for perhaps ten thousand years if it were just something like 
a burning lump of coal. While that meshed nicely with Bishop Ussher's 
estimates for the age of the universe as inferred from the Bible's tale of 
creation, geologists and biologists had already established by the mid-
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nineteenth century that Earth itself was far older. With no apparent new 
energy source, the longevity and brightness of the Sun was inexplicable. 
Enter Hans Bethe. Another of the incredibly talented and prolific 
theoretical physicists coming out of Germany in the first half of the 
twentieth century, Bethe was also another doctoral student of Arnold 
Sommerfeld's and also went on to win the Nobel Prize. Bethe began his 
career in chemistry because the introductory physics instruction at his 
university was poor—a common problem. (I also dropped physics in my 
first year for the same reason, but happily the physics department at my 
university let me take a more advanced course the following year.) Bethe 
switched to physics before moving on to graduate studies and emigrated 
to the United States to escape the Nazis. 
A consummate physicist, Bethe could work through detailed calcula-
tions to solve a wide variety of problems on the blackboard, beginning 
at the upper left of the board and ending at the lower right with almost 
no ensures. Bethe strongly influenced Richard Feynman, who used to 
marvel at Bethe's patient methodological approach to problems. Feyn-
man himself often jumped from the beginning of a problem to the end 
and worked out the steps in between afterward. Bethe's solid technical 
prowess and Feynman's brilliant insights combined well when they both 
worked at Los Alamos on the atomic bomb. They would go down the 
hallway with Feynman loudly countering the patient but persistent Bethe, 
and their colleagues labeled them "the Battleship and the Torpedo Boat." 
Bethe was legendary when I was a young physicist because even into 
his nineties he was still writing important physics articles. He was also 
happy to talk to anyone about physics. When I gave a visiting lecture at 
Cornell—where Bethe spent most of his professional career—I felt im-
mensely honored when he walked into my office to ask me questions and 
then listened intently to me, as if I actually had something to offer him. 
He was also physically robust. A physicist friend of mine told me of a time 
he too visited CornelL One weekend he decided to be ambitious and climb 
one of the many steep hiking trails near the campus. He was proud of himself 
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for huffing and puffing his way almost to the top until he spied Bethe, then 
in his late eighties, happily making his way down the trail from the summit 
While I always liked and admired Bethe, in researching material for this 
book I found two additional happy personal connections that were satis-
fying enough for me to relate them here. First, I found out that I am in a 
sense his intellectual grandson, as my undergraduate physics honors the-
sis adviser, M. K. Sundaresan, was one of his doctoral students. Second, 
I discovered that Bethe, who had little patience for grand claims made of 
fundamental results that were carried out without any real motivation or 
evidence, once wrote a hoax paper while a postdoc poking fun at a paper he 
deemed ridiculous by the famous physicist Sir Arthur Stanley Eddington. 
Eddington claimed to "derive" a fundamental constant of electromagne-
tism using some fundamental principles, but Bethe correctly viewed the 
claim as nothing other than misguided numerology. Learning this made 
me feel better about a hoax paper I wrote when I was an assistant profes-
sor at Yale, responding to what I thought was an inappropriate paper, pub-
lished in a distinguished physics journal, that claimed to discover a new 
force in nature (which indeed later turned out to be false). At the time that 
Bethe wrote his paper, the physics world took itself a little more seriously, 
and Bethe and his colleagues were forced to issue an apology. By the time I 
wrote mine, the only negative reaction I got was from my department chair, 
who was worried that the Physical Review might actually publish my article. 
When he was in his early thirties, Bethe had already established 
himself as a master physicist with his name attached to a host of re-
sults, from the Bethe formula, describing the passage of charged par-
ticles through matter, to the Bethe ansatz, a method to obtain exact 
solutions for certain quantum problems in many-body physics. A series 
of reviews he cowrote on the state of the nascent field of nuclear phys-
ics in 1936 remained authoritative for some time and became known as 
Bethe's Bible. (Unlike the conventional Bible, it made testable predic-
tions, and it was eventually replaced as scientific progress was made.) 
In 1938, Bethe was induced to attend a conference on "stellar energy 
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generation: though at that time astrophysics was not his chief interest By 
the end of the meeting, he had worked out the nuclear processes by which 
four individual protons (the nuclei of hydrogen atoms) eventually "fuse"—as 
a result of Fermi's weak interaction—to form the nucleus of helium, con-
taining two protons and two neutrons. This fusion releases about a million 
times more energy per atom than is released when coal burns. This allows 
the Sun to last a million times longer than previous estimates would have 
permitted, or about io billion years instead of ten thousand years. Bethe 
later showed that other nuclear reactions help power the Sun, including a 
set that converts carbon to nitrogen and oxygen—the so-called CNO cycle. 
The secret of the Sun—the ultimate birth of light in our solar system—
had been unveiled. Bethe won the Nobel Prize in 1967, and almost forty 
years after that, experiments on neutrinos coming from the Sun confirmed 
Bethe's predictions. Neutrinos were the key experimental observable that 
allowed such confirmation. This is because the whole chain begins with a 
reaction in which two protons collide, and via the weak interaction one of 
them converts into a neutron, allowing the two to fuse into the nucleus of 
heavy hydrogen, called deuterium, and release a neutrino and a positron. 
The positron later interacts in the Sun, but neutrinos, which interact only 
via the weak interaction, travel right out of the Sun, to Earth and beyond. 
Every second of every day, more than 400,000 billion of these neutri-
nos are passing through your body. Their interaction strength is so weak 
that they could traverse on average through ten thousand light-years of solid 
lead before interacting, so most of them travel right through you, and Earth, 
without anyone's noticing. But if not for the weak interaction, they would not 
be produced, the Sun wouldn't shine, and none of us would be here to care. 
So the weak interaction, although extremely weak, nevertheless is 
largely responsible for our existence. Which is one of the reasons why, 
when the Fermi interaction, developed to characterize it, and the neu-
trinos first predicted by it, turned out to both defy common sense, phys-
icists had to stand up and take notice. And they were driven to change 
our notions of reality itself. 
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Chapter 11 
DESPERATE TIMES 
AND DESPERATE MEASURES 
To every thing there is a season, and a time for 
every purpose. 
-ECCLESIASTES 3:1 
The rapid succession of events during the 1930s, from the 
discovery of the neutron to probing the nature of neutron decay, as well 
as the discovery of the neutrino and the consequent discovery of a new 
and universal short-range weak force in nature, left physicists more con-
fused than inspired. The brilliant march that had led to the unification 
of electricity and magnetism, and the unification of quantum mechanics 
and relativity, had been built on exploring the nature of light. Yet it 
wasn't clear how the elegant theoretical edifice of quantum electrody-
namics could guide considerations of a new force. The weak interaction 
is far removed from direct human experience and involves new and ex-
otic elementary particles and nuclear transmutations reminiscent of al-
chemy but, unlike alchemy, testable and reproducible. 
The fundamental confusion lay with the nature of the atomic nu-
cleus itself and the question of what held it together. The discovery of 
the neutron helped resolve the paradox that had earlier seemed to re-
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quire electrons to be confined in the nucleus to counter the charge of 
additional protons necessary to produce correct nuclear masses, but the 
observation of beta decay—which resulted in electrons emerging from 
nuclei—didn't help matters. 
The realization that in beta decay neutrons became protons in the 
nucleus clarified matters, but then another question naturally arose: 
Could this transformation somehow explain the strong binding that 
held protons and neutrons together inside nuclei? 
In spite of the obvious differences between the weak forces and quan-
tum theory of electromagnetism, QED, the remarkable success of QED 
in describing the behavior of atoms and the interactions of electrons 
with light colored physicists' thinking about the new weak force as well. 
The mathematical symmetries associated with QED worked beautifully 
to ensure that otherwise worrisome infinities in the calculations arising 
from the exchange of virtual particles vanished when making predic-
tions of physical quantities. Would something similar work to under-
stand the force binding protons and neutrons in nuclei? 
Specifically, if the electromagnetic force was due to the exchange of par-
ticles, then it was reasonable to think that the force that held together the 
nucleus might also be due to the exchange of particles. Werner Heisenberg 
proposed this idea in 1932 around the time the neutron was discovered. If 
neutrons and protons could convert into each other, with the proton ab-
sorbing an electron to become a neutron, then maybe the exchange of elec-
trons between them might somehow produce a binding force? 
A number of well-known problems marred this picture, however. 
First was the problem of "spin." If one assumed, as Heisenberg did, that 
the neutron was essentially made up of a proton and an electron bound 
together, and since both were spin 1/2 particles, then adding them to-
gether in the neutron, it couldn't have spin 1/2 as well, since 1/2 + 1/2 can't 
equal 1/2. Heisenberg argued, in desperation, because those were des-
perate times when it seemed all the conventional rules were breaking 
down, that the "electron" that was transferred between neutrons and 
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protons, and which bound them together in the nucleus, was somehow 
different from a free electron and had no spin at all. 
In retrospect, this picture has another problem. Heisenberg was mo-
tivated to consider electrons binding together neutrons and protons 
because he was thinking about hydrogen molecules. In hydrogen, two 
protons are bound together by sharing electrons that orbit them. The 
problem with using a similar explanation for nuclear binding is one of 
scale. How could neutrons and protons exchange electrons and be bound 
together so tightly that their average distance apart is more than one 
hundred thousand times smaller than the size of hydrogen molecules? 
Here is another way of thinking about this problem that will be use-
ful to return to later. Recall that electromagnetism is a long-range force. 
'IWo electrons on opposite sides of the galaxy experience a repulsion—
albeit extremely small—due to the exchange of virtual photons. The 
quantum theory of electromagnetism makes this possible. Photons 
are massless, and virtual photons can travel arbitrarily far, carrying 
arbitrarily small amounts of energy, before they are absorbed again—
without violating the Heisenberg uncertainty principle. If the photons 
were massive, then this would not be possible. 
Now if a force between neutrons and protons in nuclei arose due 
to the absorption and emission of virtual electrons, say, then the force 
would be short-range because the electrons are massive. How short-
range? Well, it works out to be about one hundred times the size of 
typical nuclei. So, exchanging electrons doesn't work to produce nuclear-
scale forces. As I say, those were desperate times. 
Heisenberg's desperate idea about a strange spinless version of the 
electron was not lost on a young Japanese physicist, the shy twenty-
eight-year-old Hideki Yukawa. Working in 1935 when Japan was just 
beginning to emerge from centuries of isolation, and just before its im-
perial designs ignited the war in the Pacific, Yukawa published the first 
original work in physics to be published by a physicist educated entirely 
in Japan. No one took notice of the paper for at least two years, yet four-
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teen years later he won the Nobel Prize for this work, which had by then 
become noticed, but for the wrong reasons. 
Einstein's visit to Japan in 1922 had cemented Yukawa's growing in-
terest in physics. When Yukawa was still in high school and searching 
for material to help him pass examinations in a second foreign language, 
he found Max Planck's Introduction to Theoretical Physics in German. 
He rejoiced in reading both the German and the physics and was aided 
by his classmate Sin-Itiro Tomonaga, a talented physicist who was his 
colleague both in high school and later at Kyoto University. Tomonaga 
was so talented that he would later share the 1965 Nobel Prize with 
Richard Feynman and Julian Schwinger for demonstrating the math-
ematical consistency of quantum electrodynamics. 
That Yukawa, who had been a student in Japan at a time when many 
of his instructors did not yet fully understand the emerging field of 
quantum mechanics, came upon a possible solution to the nuclear-force 
problem that had been overlooked by Heisenberg, Pauli, and even Fermi 
was remarkable. I suspect that part of the problem was a phenomenon 
that has occurred several times in the twentieth century and perhaps 
before, and perhaps after. When the paradoxes and complexities asso-
ciated with some physical process begin to seem overwhelming, it is 
tempting to assume that some new revolution, similar to relativity or 
quantum mechanics, will require such a dramatic shift in thinking that 
it doesn't make sense to push forward with existing techniques. 
Fermi, unlike Heisenberg or Pauli, was not looking for a wholesale 
revolution. He was willing to propose, as he called it, a "tentative theory" 
of neutron decay that got rid of electrons in the nucleus by allowing 
them to be spontaneously created during beta decay. He proposed a 
model that worked, which he knew was just a model and not a complete 
theory, but it did allow one to do calculations and make predictions. 
That was the essence of Fermi's practical style. 
Yukawa had followed these developments, translated Heisenberg's 
paper on nuclei along with an introduction, and published it in Japan, so 
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the problems of Heisenberg's proposal were already clear to him. Then 
in 1934 Yukawa read Fermi's theory of neutron decay, which catalyzed a 
new idea in Yukawa's mind. Perhaps the nuclear force binding protons 
and neutrons was due not to the exchange of virtual electrons between 
them, but to the exchange of both the electron and the neutrino that 
were created when neutrons changed to protons. 
Another problem immediately arose, however. Neutron decay is a 
result of what would become known as the weak interaction, and the 
force responsible for it is weak. Plugging in values for the possible force 
that might result between protons and neutrons by the exchange of an 
electron-neutrino pair made it clear that this force would be far too 
weak to bind them. 
Yukawa then allowed himself to do what none of the others had 
done. He questioned why the nuclear force, if it, like QED, results from 
the exchange of virtual particles, had to be due to the exchange of one or 
more of the particles already known or assumed to exist. Remembering 
how loath physicists such as Dirac and Pauli had been to propose new 
particles, even when they were correct, you can perhaps appreciate how 
radical Yukawa's idea was. As Yukawa later described it: 
At this period the atomic nucleus was inconsistency itself, quite 
inexplicable. And why?—because our concept of elementary 
particle was too narrow. There was no such word in Japanese and 
we used the English word—it meant proton and electron. From 
somewhere had come a divine message forbidding us to think 
about any other particle. To think outside of these limits (except 
for the photon) was to be arrogant, not to fear the wrath of the 
gods. It was because the concept that matter continues forever 
had been a tradition since the times of Democritus and Epicurus. 
To think about creation of particles other than photons was 
suspect, and there was a strong inhibition of such thoughts that 
was almost unconscious. 
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One of my good physics friends has said that the only time he was 
able to do complicated calculations was after the birth of each of his 
children, when he couldn't sleep anyway, so he stayed up and worked. 
Thus in October of 1934, just after the birth of his second child and 
unable to sleep, Yukawa realized that if the range of the strong nuclear 
force was to be restricted to the size of a nucleus, then any exchanged 
particle must be far more massive than the electron. The next morning 
he estimated the mass to be two hundred times the electron mass. It 
would have to carry an electric charge if it was to be exchanged between 
neutrons and protons, and it could have no spin, so as not to change the 
proton's or neutron's spin when it was absorbed or emitted. 
What has all this concern over strong nuclear forces to do with neu-
tron decay, the subject that started this chapter and ended the last? you 
may ask. In the 1930s, just as it went against the grain to imagine new 
particles, so too inventing new forces seemed unnecessary at best and 
heretical at worst. Physicists were convinced that all the processes that 
occurred in the nucleus, strong or weak, must be connected. 
Yukawa envisaged a clever way to do this, connecting ideas of both 
Fermi and Heisenberg, and also generalizing ideas from the successful 
quantum theory of electromagnetism. If instead of emitting a photon, 
neutrons in the nucleus emitted a new, heavy, spinless charged particle, 
which Yukawa originally called a mesotron—until Heisenberg corrected 
Yukawa's Greek and the name was shortened to meson—then that par-
ticle could be absorbed by protons in the nucleus, producing a force of 
attraction whose magnitude Yukawa was able to calculate using equa-
tions that were extrapolated from, you guessed it, electromagnetism. 
The analogy with electromagnetism could not be exact, however, be-
cause the meson is massive and the photon is massless. Yukawa took the 
attitude that Fermi might have, if he had thought of it. Yes, the theory 
wasn't complete, but Yukawa was willing to ignore the other aspects of 
electromagnetism that this theory couldn't reproduce. Damn the torpe-
does, full speed ahead. 
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Desperate Times and Desperate Measures 
145 
Yukawa ingeniously—and ultimately incorrectly—connected this 
strong force to observed neutron decay by suggesting that mesons 
might not always simply be exchanged between neutrons and protons in 
the nucleus. A small fraction of the mesons emitted by neutrons might 
decay en route into an electron and neutrino before they could be re-
absorbed, causing neutron decay. In this case, the neutron decay would 
not be described by something like the figure below and on the left, 
where the decay and the emission of the other particles all occur at a 
single point. It would appear like the figure on the right, where the decay 
gets spread out and a new particle, shown by the dashed line (which rep-
resents Yukawa's meson), travels a short distance after emission before 
decaying into the electron and neutrino. With the new intermediate 
particle, the weak interaction mediating neutron decay begins to look 
more like the electromagnetic interaction between charged particles: 
A 
Yukawa had proposed a new intermediate particle, a heavy meson, 
which made neutron decay look similar to the earlier picture of photon 
exchange in electromagnetism—which had motivated his thinking in 
the first place—but with significant differences. In this case the inter-
mediate particle was both massive and electrically charged, and also 
unlike the photon it had no spin angular momentum. 
Nevertheless, Yukawa was able to show that for a heavy meson his 
theory would be indistinguishable from Fermi's point interaction de-
scribing neutron decay—at least for predicting the details of neutron 
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146 
THE GREATEST STORY EVER TOLD-SO FAR 
decay. In addition, Yukawa's theory offered the possibility of reducing all 
of the strange properties of the nucleus—from beta decay of neutrons 
inside the nucleus to the strength of the interaction binding together 
protons and neutrons—to merely understanding the properties of a 
single new interaction, due to the exchange of a new particle, his meson. 
However, if this new heavy meson existed, where was it? Why hadn't 
it yet been seen in cosmic rays? Because of this, and also because Yukawa 
was an unknown entity working in a location far from all the action, 
no real attention was paid to his proposal to explain both the strong 
interaction between nucleons and the weaker one that appeared to be 
responsible for neutron decay. Nevertheless, his proposal, unlike those 
of Heisenberg and others (including Fermi), was simpler and made more 
sense. 
All of this changed in 2936, less than two years after Yukawa's predic-
tion, when Carl Anderson, the discoverer of the positron, together with 
collaborator Seth Neddermeyer, discovered what appeared to be a new 
set of particles in cosmic rays. The characteristics of the tracks of these 
new particles in cloud chambers implied that they produced too little 
radiation in traversing matter to be protons or electrons. They were also 
more massive than electrons and appeared to be sometimes negative 
and sometimes positive. Before long the new particles were determined 
to have a mass in the range that Yukawa had predicted—about two hun-
dred times the mass of the electron. 
It is remarkable how quickly the rest of the world caught on. Yukawa 
published a short note to point out that his theory predicted just such 
particles. Within weeks the major physicists in Europe began exploring 
his model and incorporating his ideas in their work. In 1938, in the last 
major conference before the Second World War interrupted essentially 
all international collaborations in science, of the eight main speakers, 
three dealt with Yukawa's theory—citing a name they would have been 
unfamiliar with a year or two before. 
While much of the rest of the physics world celebrated the apparent 
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