|Fig. 1: Fermi's Los Alamos ID badge photo. (Source: Wikimedia Commons)|
Born in Rome, Italy, on September 29, 1901, Enrico Fermi was one of the most influential physicists of the 20th century. In Fig. 1 one can see his photon taken for his Los Alamos ID. He made significant contributions to a wide variety of fields such as quantum theory, nuclear physics, and particle physics. The sheer breadth and importance of Fermi's work speaks to his unique intellectual disposition: he was both a skilled theorist and an experimentalist. He had a natural propensity for mathematics and physics from an early age. In his early teens he would design and build electric motors and use the money saved from his allowance to buy used physics books from markets.  His talents were recognized by a family friend, Adolfo Amidei, a university trained engineer who would feed Fermi the books he needed to satisfy his intellectual curiosity. In 1918 Fermi graduated from high school and decided to apply to the Scuola Normale Superiore, an elite institution operating within the University of Pisa that accepted only 40 students based on the results of a difficult entrance exam. During Fermi's year, there was an essay question titled, "Characteristics of Sound", which Fermi responded to by using Fourier analysis to solve the partial differential equation governing a vibrating rod.  Needless to say, he impressed the examiners and matriculated the following fall. While at the Scuola Normale Superiore Fermi taught himself tensor calculus, the basics of quantum mechanics, atomic physics, and the theory of general relativity. 
In 1920 Fermi was admitted into the Physics department of the Scuola Normale Superiore, which at the time had only three students. Fermi decided to write his thesis on X-ray diffraction experiments, which he would carry out in university laboratory. He also submitted papers for publication on the implications of general relativity on electrostatic theory.  After receiving his degree of Doctor of Philosophy in physics from the University of Pisa along with his diploma from the Scuola Normale Superiore in 1922, Fermi completed postdocs in Germany and Holland.
During his postdoc in Holland at the University of Leiden, Fermi became heavily involved in researching quantum mechanics, and was particularly influenced by Austrian physicist Wolfgang Pauli's 1925 paper detailing his famous exclusion principle. Pauli's exclusion principle asserts that two identical particles with half integer spin can not occupy the same quantum state.  Spin is a type of angular momentum intrinsic to particles, and particles with half integer spin can only be observed to have half integer values of this angular momentum, i.e. 1/2, 3/2 , 5/2 (up to a multiplicative factor).  Without getting too bogged down in the nuances of quantum mechanics, a quantum state is a mathematical object which encodes information about the system at hand such as the probability distributions governing the particles position and momentum. For example, electrons are particles with half integer spin (specifically +1/2 or -1/2) and they obey the Pauli exclusion principle.  Fermi took this principle and wrote a paper where he applied it to the nature of ideal gases. His statistical formulation of how particles behave in systems of many identical particles that obey the Pauli exclusion principle became known as Fermi-Dirac statistics, as a similar theory would be put forth by British physicist Paul Dirac shortly thereafter.  Fermi-Dirac statistics could be used to understand the nature of matter, from the workings of neutron stars to the properties of metals that allow them to conduct electricity.  It is no wonder that particles that have half integer spin and obey Fermi-Dirac statistics are known as fermions. Fermi's paper was groundbreaking to say the least, and it helped him earn his first professorship at the Sapienza University of Rome in 1926.
Fermi's tenure at the Sapienza University of Rome was a productive period for him. He tackled the problem of beta decay, which is the process of a nucleus emitting a beta particle like an electron or positron. A particular challenge facing scientists regarding beta decay was that most experimental results showed slight deviations from the conservation of energy.  To account for this, Wolfgang Pauli came up with a new particle called the neutrino, a particle with very little mass and no charge.  Fermi took Pauli's idea once again and expanded upon it, elaborating on the theoretical emission of neutrinos during beta decay. At the same time Fermi was also investigating new results found by the Joliot-Curies on induced radioactivity. The Joliot-Curies had bombarded atoms with alpha particles and made them radioactive. Fermi postulated he could do the same with neutrons and carried out a series of experiments that proved his suspicions.
At the same time he noticed that he would have much higher rates of radioactivity depending on the conditions of his experiment.  For example, samples on wooden tables were far more likely to become radioactive than the same samples on marble tables.  After further investigation, Fermi deduced that it was because the neutrons were colliding with Hydrogen atoms, and that these interactions slowed them down sufficiently to make the samples they collided with more radioactive.  For example, consider a pool ball colliding with another pool ball at rest. The pool ball loses a lot of its energy and gives it to the other one as both ricochet off in different directions. In contrast, if the pool ball collides with the side of the pool table or another massive entity, it will lose less energy and be deflected with more of its original kinetic energy. The mass of a Hydrogen nucleus is close to that of a neutron, so neutrons would lose more energy colliding with them. As it happened to be, these slow neutrons were more effective in causing radioactivity. Fermi received the Nobel Prize in Physics in 1938 for his discovery of these two phenomena, as he had opened up an entire new field of neutron physics.  After attending the ceremony, Fermi left for the United States due to the increasingly anti-semitic laws that were being implemented in Italy.
When Fermi arrived in America in 1939, he took up a position in Columbia University, where he would become embroiled in researching nuclear fission. Inspired by Fermi's work, two German scientists, Otto Hahn and Fritz Strassmann, conducted their own neutron bombardment experiments on uranium and discovered that barium was produced. Instead of interpreting this result as a consequence of radioactivity, they interpreted it as fission, and the news spread across the Atlantic quickly.  Fission was of the utmost interest to governments across the world because it held immense potential for energy production. One gram of U-235 undergoing fission produces the same amount of energy as 3 tons of coal or 700 gallons of fuel oil.  Furthermore, this energy production could be used for bombs, and this was particularly important as fission was discovered in Germany on the eve of the second world war. Fermi and his colleagues confirmed these groundbreaking results with a fission experiment of their own. A Hungarian scientist named Leo Szilard had realized that one could take this nuclear process and make it self sustaining. Szilard immigrated to the United States and began working with Fermi at the University of Chicago in 1941.  Creating a self sustaining reaction was going to be extremely difficult because of the rarity of the elements in questions. Fermi had calculated that approximately 2.5 neutrons were created per fission reaction, which is enough to create a self-sustaining reaction, but the problem was that fission was difficult to induce consistently.  The substance best suited for their purposes, U-235 (a natural isotope of uranium), is only found in 0.7% of uranium samples.  They would need a large amount of uranium and protocols that would prevent the slow neutrons from getting absorbed by the abundant amounts of U-238 - the other isotope of uranium - found in their samples. Despite the challenges facing them, Fermi and Szilard were determined to press forward, and their apparatus - the Chicago Pile-1 nuclear reactor - successfully demonstrated a chain reaction on December 2, 1942, at the rate of 0.5 watts for 4.5 minutes.  This discovery made the development of the atomic bomb feasible and cemented Fermi's position as the father of the atomic age. He would later join the Manhattan project to work on the atomic bomb but return to primarily to teaching and research after the second world war.  He passed away in 1954 due to stomach cancer. 
© Sean Afshar. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 D. Cooper, Enrico Fermi: And the Revolutions of Modern Physics (Oxford University Press, 1999).
 E. Segre, Enrico Fermi, Physicist (University of Chicago Press, 1970).
 D. J. Griffiths, Introduction to Quantum Mechanics, 2nd Edition (Pearson Prentice Hall, 2004).