The Greatest Story Ever Told—So Far

by Lawrence M. Krauss

  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, beginning 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 proposed 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 electron, 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 lectures 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 Gürsey, 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 Italian 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.

  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 remarkable figures from that equally remarkable period in physics, most influenced 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 physical models, and the complexity of machinery now used to test them, are separately too sophisticated to allow any single individual today, no matter 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 possibilities 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 impressed 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 mechanics 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 exploring 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 quantum mechanics, and then to Leiden, Holland, where he met with the most famous physicists of the day—Born, Heisenberg, Pauli, Lorentz, and Einstein, to name a few—before returning to Italy to teach. In 1925, Wolfgang Pauli proposed the “exclusion principle,” which disclosed 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 ½ particles because the magnitude of their spin angular momentum turns out to be half as big as the lowest value of angular momentum associated with the orbital motion of electrons in atoms. Any spin ½ 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 vibrant 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 pro
duced a theory for neutron decay that revealed the possible existence of a new fundamental force in nature, the first new force known to science beyond electromagnetism and gravity—which was in its own way inspired by thinking about light. Although it wasn’t obvious at the time, this was to be the first of two new forces associated with atomic nuclei, which together with electromagnetism 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 editor 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 comforting 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 uranium 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 reproduced the German results. Before long, he and his collaborators realized that this produced the possibility of a chain reaction. Neutrons could bombard 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 potential 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 doctorate, in 1923, he wrote the appendix for a book on relativity and talked of the potential of E = mc2, 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 December 2, 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 reasons I cherish his memory. He always found a simple, easy way to reach the correct answer. Even though he had great mathematical skill, he disliked complication, and he realized that he could get an approximate 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 theoretical legacy for physics may be far greater. True to form, the “theory” he proposed in his famously rejected paper on neutron decay was remarkably 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 interaction between particles that took place at a single point. The four particles 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 interacting 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:

  Fermi imagined the decay of a neutron in a similar fashion, but instead 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 neutrino, ν, 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 interaction point in the figure where a neutron converts into a proton determines 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 particles other than photons could be spontaneously created in the quantum 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 interaction that caused neutron decay could also predict a host of other phenomena, 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 1936 Carl Anderson, the discoverer of the positron, discovered another new particle in cosmic rays—the first of what would be so many that particle 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 identical 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 differences. First, the interaction is much weaker. Second, unlike electromagnetism, 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 great distance. Third, the interaction changes one type of particle into another. Electromagnetism involves the creation and absorption of photons—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.