- Segrè, Gino and Bettina Hoerlin.
The Pope of Physics.
New York: Henry Holt, 2016.
ISBN 978-1-62779-005-5.
-
By the start of the 20th century, the field of physics had
bifurcated into theoretical and experimental specialties. While
theorists and experimenters were acquainted with the same
fundamentals and collaborated, with theorists suggesting
phenomena to be explored in experiments and experimenters
providing hard data upon which theorists could build their
models, rarely did one individual do breakthrough work in both
theory and experiment. One outstanding exception was Enrico
Fermi, whose numerous achievements seemed to jump effortlessly
between theory and experiment.
Fermi was born in 1901 to a middle class family in Rome,
the youngest of three children born in consecutive years. As was
common at the time, Enrico and
his brother Giulio were sent to be wet-nursed and raised by
a farm family outside Rome and only returned to live with
their parents when two and a half years old. His father was
a division head in the state railway and his mother taught
elementary school. Neither
parent had attended university, but hoped all of their children
would have the opportunity. All were enrolled in schools
which concentrated on the traditional curriculum of Latin,
Greek, and literature in those languages and Italian. Fermi
was attracted to mathematics and science, but little
instruction was available to him in those fields.
At age thirteen, the young Fermi made the acquaintance of Adolfo
Amidei, an engineer who worked with his father. Amidei began to
loan the lad mathematics and science books, which Fermi
devoured—often working out solutions to problems which Amidei
was unable to solve. Within a year, studying entirely on his
own, he had mastered geometry and calculus. In 1915, Fermi
bought a used
book, Elementorum Physicæ
Mathematica, at a flea market in Rome. Published in 1830
and written entirely in Latin, it was a 900 page compendium
covering mathematical physics of that era. By that time, he was
completely fluent in the language and the mathematics used in
the abundant equations, and worked his way through the entire
text. As the authors note, “Not only was Fermi the only
twentieth-century physics genius to be entirely self-taught, he
surely must be the only one whose first acquaintance with the
subject was through a book in Latin.”
At sixteen, Fermi skipped the final year of high school, concluding
it had nothing more to teach him, and with Amidei's encouragement,
sat for a competitive examination for a place at the
elite Sculoa Normale Superiore, which provided a complete
scholarship including room and board to the winners. He
ranked first in all of the examinations and left home to
study in Pisa. Despite his talent for and knowledge of
mathematics, he chose physics as his major—he had always
been fascinated by mechanisms and experiments, and looked
forward to working with them in his career. Italy, at the
time a leader in mathematics, was a backwater in
physics. The university in Pisa had only one physics
professor who, besides having already retired from research,
had knowledge in the field not much greater than Fermi's own.
Once again, this time within the walls of a university,
Fermi would teach himself, taking advantage of the university's
well-equipped library. He taught himself German and English in
addition to Italian and French (in which he was already fluent)
in order to read scientific publications. The library subscribed
to the German
journal Zeitschrift für
Physik, one of the most prestigious sources for
contemporary research, and Fermi was probably the
only person to read it there. In 1922, after completing a thesis on
X-rays and having already published three scientific papers, two
on X-rays and one on general relativity (introducing what are
now called Fermi
coordinates, the first of many topics in physics which would bear
his name), he received his doctorate in physics,
magna cum laude. Just
twenty-one, he had his academic credential, published work
to his name, and the attention of prominent researchers
aware of his talent. What he lacked was the prospect of
a job in his chosen field.
Returning to Rome, Fermi came to the attention of Orso Mario
Corbino, a physics professor and politician who had become a
Senator of the Kingdom and appointed minister of public
education. Corbino's ambition was to see Italy enter the top
rank of physics research, and saw in Fermi the kind of talent
needed to achieve this goal. He arranged a scholarship so Fermi
could study physics in one the centres of research in northern
Europe. Fermi chose Göttingen, Germany, a hotbed of work
in the emerging field of quantum mechanics. Fermi was neither
particularly happy nor notably productive during his eight
months there, but was impressed with the German style of
research and the intellectual ferment of the large community of
German physicists. Henceforth, he published almost all of his
research in either German or English, with a parallel paper
submitted to an Italian journal. A second fellowship allowed
him to spend 1924 in the Netherlands, working with Paul
Ehrenfest's group at Leiden, deepening his knowledge of
statistical and quantum mechanics.
Finally, upon returning to Italy, Corbino and his colleague
Antonio Garbasso found Fermi a post as a lecturer in physics
in Florence. The position paid poorly and had little prestige,
but at least it was a step onto the academic ladder, and
Fermi was happy to accept it. There, Fermi and his colleague
Franco Rasetti did experimental work measuring the spectra of
atoms under the influence of radio frequency fields. Their
work was published in prestigious journals such as
Nature and Zeitschrift für
Physik.
In 1925, Fermi took up the problem of reconciling the field of
statistical
mechanics with the discovery by Wolfgang Pauli of the
exclusion
principle, a purely quantum mechanical phenomenon
which restricts certain kinds of identical particles from occupying
the same state at the same time. Fermi's paper, published in 1926,
resolved the problem, creating what is now called
Fermi-Dirac
statistics (British physicist
Paul Dirac
independently discovered the phenomenon, but Fermi published first)
for the particles now called
fermions,
which include all of the fundamental particles that make up
matter. (Forces are carried by other particles called
bosons, which go beyond
the scope of this discussion.)
This paper immediately elevated the twenty-five year old Fermi
to the top tier of theoretical physicists. It provided the
foundation for understanding of the behaviour of electrons
in solids, and thus the semiconductor technology upon which all
our modern computing and communications equipment is based.
Finally, Fermi won what he had aspired to: a physics professorship
in Rome. In 1928, he married Laura Capon, whom he had
first met in 1924. The daughter of an admiral in the World War
I Italian navy, she was a member of one of the many secular and
assimilated Jewish families in Rome. She was less than
impressed on first encountering Fermi:
He shook hands and gave me a friendly grin. You could call
it nothing but a grin, for his lips were exceedingly thin
and fleshless, and among his upper teeth a baby tooth too
lingered on, conspicuous in its incongruity. But his eyes
were cheerful and amused.
Both Laura and Enrico shared the ability to see things precisely
as they were, then see beyond that to what they could become.
In Rome, Fermi became head of the mathematical physics
department at the Sapienza University of Rome, which his mentor,
Corbino, saw as Italy's best hope to become a world leader in
the field. He helped Fermi recruit promising
physicists, all young and ambitious. They gave each other
nicknames: ecclesiastical in nature, befitting their location
in Rome. Fermi was dubbed
Il Papa (The Pope), not
only due to his leadership and seniority, but because he had
already developed a reputation for infallibility: when he made
a calculation or expressed his opinion on a technical topic, he
was rarely if ever wrong. Meanwhile, Mussolini was increasing
his grip on the country. In 1929, he announced the appointment
of the first thirty members of the Royal Italian Academy, with
Fermi among the laureates. In return for a lifetime stipend
which would put an end to his financial worries, he would have
to join the Fascist party. He joined. He did not take the
Academy seriously and thought its comic opera uniforms absurd,
but appreciated the money.
By the 1930s, one of the major mysteries in physics was
beta decay.
When a radioactive nucleus decayed, it could emit one or more
kinds of radiation: alpha, beta, or gamma. Alpha particles had been
identified as the nuclei of helium, beta particles as electrons, and
gamma rays as photons: like light, but with a much shorter wavelength
and correspondingly higher energy. When a given nucleus decayed by
alpha or gamma, the emission always had the same energy: you could
calculate the energy carried off by the particle emitted and compare
it to the nucleus before and after, and everything added up according
to Einstein's equation of E=mc². But something
appeared to be seriously wrong with beta (electron) decay. Given
a large collection of identical nuclei, the electrons emitted
flew out with energies all over the map: from very low to an
upper limit. This appeared to violate one of the most fundamental
principles of physics: the conservation of energy. If the nucleus
after plus the electron (including its kinetic energy) didn't add up
to the energy of the nucleus before, where did the energy go? Few
physicists were ready to abandon conservation of energy, but, after all,
theory must ultimately conform to experiment, and if a multitude of
precision measurements said that energy wasn't conserved in beta decay,
maybe it really wasn't.
Fermi thought otherwise. In 1933, he proposed
a theory
of beta decay
in which the emission of a beta particle (electron) from a nucleus
was accompanied by emission of a particle he called a
neutrino, which had been proposed earlier by Pauli. In
one leap, Fermi introduced a third force, alongside gravity and
electromagnetism, which could transform one particle into another,
plus a new particle: without mass or charge, and hence extraordinarily
difficult to detect, which nonetheless was responsible for carrying
away the missing energy in beta decay. But Fermi did not just propose
this mechanism in words: he presented a detailed mathematical
theory of beta decay which made predictions for experiments which
had yet to be performed. He submitted the theory in a paper to
Nature in 1934. The editors rejected it, saying “it
contained abstract speculations too remote from physical reality to be
of interest to the reader.” This was quickly recognised and is
now acknowledged as one of the most epic face-plants of peer review
in theoretical physics. Fermi's theory rapidly became accepted as
the correct model for beta decay. In 1956, the
neutrino
(actually, antineutrino) was detected with precisely the properties
predicted by Fermi. This theory remained the standard explanation
for beta decay until it was extended in the 1970s by the
theory of the
electroweak
interaction, which is valid at higher energies than were
available to experimenters in Fermi's lifetime.
Perhaps soured on theoretical work by the initial rejection of his
paper on beta decay, Fermi turned to experimental exploration of
the nucleus, using the newly-discovered particle, the neutron. Unlike
alpha particles emitted by the decay of heavy elements like
uranium and radium, neutrons had no electrical charge and could
penetrate the nucleus of an atom without being repelled. Fermi saw
this as the ideal probe to examine the nucleus, and began to use
neutron sources to bombard a variety of elements to observe the
results. One experiment directed neutrons at a target of silver
and observed the creation of isotopes of silver when the
neutrons were absorbed by the silver nuclei. But something very odd
was happening: the results of the experiment seemed to differ when
it was run on a laboratory bench with a marble top compared to one
of wood. What was going on? Many people might have dismissed
the anomaly, but Fermi had to know. He hypothesised that the
probability a neutron would interact with a nucleus depended upon its
speed (or, equivalently, energy): a slower neutron would effectively
have more time to interact than one which whizzed through more
rapidly. Neutrons which were reflected by the wood table top were
“moderated” and had a greater probability of interacting
with the silver target.
Fermi quickly tested this supposition by using paraffin wax and
water as neutron moderators and measuring the dramatically increased
probability of interaction (or as we would say today,
neutron capture
cross section) when neutrons were slowed down. This is fundamental
to the design of nuclear reactors today. It was for this work that
Fermi won the
Nobel
Prize in Physics for 1938.
By 1938, conditions for Italy's Jewish population had seriously
deteriorated. Laura Fermi, despite her father's distinguished
service as an admiral in the Italian navy, was now classified as
a Jew, and therefore subject to travel restrictions, as were
their two children. The Fermis went to their local Catholic
parish, where they were (re-)married in a Catholic ceremony and
their children baptised. With that paperwork done, the Fermi
family could apply for passports and permits to travel to
Stockholm to receive the Nobel prize. The Fermis locked their
apartment, took a taxi, and boarded the train. Unbeknownst to
the fascist authorities, they had no intention of returning.
Fermi had arranged an appointment at Columbia University in
New York. His Nobel Prize award was US$45,000 (US$789,000 today).
If he returned to Italy with the sum, he would have been forced to
convert it to lire and then only be able to take the equivalent of
US$50 out of the country on subsequent trips. Professor Fermi
may not have been much interested in politics, but he could do arithmetic.
The family went from Stockholm to Southampton, and then on an
ocean liner to New York, with nothing other than their luggage,
prize money, and, most importantly, freedom.
In his neutron experiments back in Rome, there had been curious
results he and his colleagues never explained. When bombarding
nuclei of uranium, the heaviest element then known, with neutrons
moderated by paraffin wax, they had observed radioactive results
which didn't make any sense. They expected to create new elements,
heavier than uranium, but what they saw didn't agree with the
expectations for such elements. Another mystery…in those
heady days of nuclear physics, there was one wherever you looked.
At just about the time Fermi's ship was arriving in New York, news
arrived from Germany about what his group had observed, but not
understood, four years before. Slow neutrons, which Fermi's
group had pioneered, were able to split, or fission
the nucleus of uranium into two lighter elements, releasing
not only a large amount of energy, but additional neutrons
which might be able to propagate the process into a
“chain reaction”, producing either a large amount
of energy or, perhaps, an enormous explosion.
As one of the foremost researchers in neutron physics, it was
immediately apparent to Fermi that his new life in America was
about to take a direction he'd never anticipated. By 1941,
he was conducting experiments at Columbia with the goal of evaluating
the feasibility of creating a self-sustaining nuclear reaction
with natural uranium, using graphite as a moderator. In 1942, he
was leading a project at the University of Chicago to build
the first nuclear reactor. On December 2nd, 1942,
Chicago Pile-1
went critical, producing all of half a watt of
power. But the experiment proved that a nuclear chain reaction
could be initiated and controlled, and it paved the way for
both civil nuclear power and
plutonium
production for nuclear weapons. At the time he achieved one
of the first major milestones of the Manhattan Project, Fermi's
classification as an “enemy alien” had been removed
only two months before. He and Laura Fermi did not become
naturalised U.S. citizens until July of 1944.
Such was the breakneck pace of the Manhattan Project that even
before the critical test of the Chicago pile, the DuPont company
was already at work planning for the industrial scale production
of plutonium at a facility which would eventually be built at the
Hanford site near Richland, Washington. Fermi played a
part in the design and commissioning of the
X-10
Graphite Reactor in Oak Ridge, Tennessee, which served
as a pathfinder and began operation in November, 1943, operating
at a power level which was increased over time to 4 megawatts.
This reactor produced the first substantial quantities of
plutonium for experimental use, revealing the plutonium-240
contamination problem which necessitated the use of implosion
for the plutonium bomb. Concurrently, he contributed to the
design of the
B Reactor
at Hanford, which went critical in September 1944, running at
250 megawatts, that produced the plutonium for the Trinity
test and the Fat Man bomb dropped on Nagasaki.
During the war years, Fermi divided his time among the
Chicago research group, Oak Ridge, Hanford, and the bomb design
and production group at Los Alamos. As General Leslie Groves,
head of Manhattan Project, had forbidden the top atomic
scientists from travelling by air, “Henry Farmer”,
his wartime alias, spent much of his time riding the rails,
accompanied by a bodyguard. As plutonium production ramped up,
he increasingly spent his time with the weapon designers at
Los Alamos, where Oppenheimer appointed him associate
director and put him in charge of “Division F” (for
Fermi), which acted as a consultant to all of the other
divisions of the laboratory.
Fermi believed that while scientists could make major
contributions to the war effort, how their work and the weapons
they created were used were decisions which should be made by
statesmen and military leaders. When appointed in May 1945 to
the Interim Committee charged with determining how the fission
bomb was to be employed, he largely confined his contributions
to technical issues such as weapons effects. He joined
Oppenheimer, Compton, and Lawrence in the final recommendation
that “we can propose no technical demonstration likely
to bring an end to the war; we see no acceptable alternative
to direct military use.”
On July 16, 1945, Fermi witnessed the Trinity test explosion
in New Mexico at a distance of ten miles from the shot tower.
A few seconds after the blast, he began to tear little pieces of
paper from from a sheet and drop them toward the ground. When the
shock wave arrived, he paced out the distance it had blown
them and rapidly computed the yield of the bomb as around ten
kilotons of TNT. Nobody familiar with Fermi's reputation for
making off-the-cuff estimates of physical phenomena was surprised
that his calculation, done within a minute of the explosion,
agreed within the margin of error with the actual yield of
20 kilotons, determined much later.
After the war, Fermi wanted nothing more than to return to
his research. He opposed the continuation of wartime secrecy
to postwar nuclear research, but, unlike some other prominent
atomic scientists, did not involve himself in public
debates over nuclear weapons and energy policy. When he
returned to Chicago, he was asked by a funding agency
simply how much money he needed. From his experience at
Los Alamos he wanted both a particle accelerator and a big
computer. By 1952, he had both, and began to
produce results in
scattering
experiments which hinted at the
new physics which would be uncovered throughout the 1950s
and '60s. He continued to spend time at Los Alamos, and
between 1951 and 1953 worked two months a year there,
contributing to the hydrogen bomb project and analysis of
Soviet atomic tests.
Everybody who encountered Fermi remarked upon his talents
as an explainer and teacher. Seven of his students: six from
Chicago and one from Rome, would go on to win Nobel Prizes
in physics, in both theory and experiment. He became famous
for posing “Fermi
problems”, often at lunch, exercising the ability to make
and justify order of magnitude estimates of difficult questions.
When Freeman Dyson met with Fermi to present a theory he and
his graduate students had developed to explain the scattering
results Fermi had published, Fermi asked him how many free
parameters Dyson had used in his model. Upon being told
the number was four, he said, “I remember my old friend
Johnny von Neumann used to say, with four parameters I can
fit an elephant, and with five I can make him wiggle his
trunk.” Chastened, Dyson soon concluded his model was
a blind alley.
After returning from a trip to Europe in the fall of 1954,
Fermi, who had enjoyed robust good health all his life, began
to suffer from problems with digestion. Exploratory surgery
found metastatic stomach cancer, for which no treatment was
possible at the time. He died at home on November 28, 1954,
two months past his fifty-third birthday. He had made a Fermi
calculation of how long to rent the hospital bed in which he
died: the rental expired two days after he did.
There was speculation that Fermi's life may have been shortened
by his work with radiation, but there is no evidence of this.
He was never exposed to unusual amounts of radiation in his
work, and none of his colleagues, who did the same work at his
side, experienced any medical problems.
This is a masterful biography of one of the singular figures in
twentieth century science. The breadth of his interests and
achievements is reflected in the list of
things
named after Enrico Fermi. Given the hyper-specialisation of
modern science, it is improbable we will ever again see his like.
July 2017 