The Greatest Story Ever Told—So Far

by Lawrence M. Krauss

  What has all this concern over strong nuclear forces to do with neutron 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 particle could be absorbed by protons in the nucleus, producing a force of attraction whose magnitude Yukawa was able to calculate using equations that were extrapolated from, you guessed it, electromagnetism.

  The analogy with electromagnetism could not be exact, however, because 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 torpedoes, full speed ahead.

  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 reabsorbed, 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 represents 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:

  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 intermediate 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 describing neutron decay—at least for predicting the details of neutron 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 1936, less than two years after Yukawa’s prediction, 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 hundred 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 discovery of Yukawa’s meson, this discovery was not without its own problems. In 1940 the decay of a meson to an electron, predicted by Yukawa, was observed in cosmic-ray tracks. However, over the years 1943 to 1947 it became clear that the particles Anderson and Neddermeyer had discovered interacted much more weakly with nuclei than Yukawa’s particle should have.

  Something was wrong.

  Three of Yukawa’s Japanese colleagues suggested that mesons were of two different sorts, and that a Yukawa-type meson might decay into yet another, different and more weakly interacting meson. But their articles were in Japanese and didn’t appear in English until after the war, by which time a similar proposal had been made by the US physicist Robert Marshak.

  This delay proved fortuitous. New techniques were being developed to observe the tracks of cosmic rays in photographic emulsions, and a series of brave researchers dragged their equipment up to high elevations to search for possible new signals. Many cosmic rays interact and disappear before reaching sea level, so this group and others interested in exploring this wondrous new source of particles coming from the heavens had no choice but to seek higher elevations. Here cosmic rays would have traversed less distance in the atmosphere and might be more easily detected.

  The former Italian mountain guide turned physicist Giuseppe Occhialini had been invited from Brazil to join a British team working on the A-bomb during the war. As a foreign national, he couldn’t work on the project, so instead he joined the cosmic-ray physics group at Bristol. Occhialini’s mountain training proved useful as he dragged photographic emulsions up to the Pic du Midi at twenty-eight hundred meters in France. Today you can travel to the observatory on top of this peak by cable car, and it is a terrifyingly exciting ride. But in 1946 Occhialini had to climb to the top, risking his health in the effort to discover signals of exotic new physics.

  And he and his team did discover exotic new physics. As Cecil Powell, Occhialini’s collaborator at Bristol (and future Nobel laureate, while Occhialini, who had done the climbing, did without), put it, they saw “a whole new world. It was as if, suddenly, we had broken into a walled orchard, where protected trees flourished and all kinds of exotic fruits had ripened in great profusion.”

  Less poetically, perhaps, what they discovered were two examples in which an initial meson stopped in the emulsion and gave rise to a second meson, just as had been suggested by the theorists. Many more events were observed with emulsions taken to an elevation almost twice as high as Pic du Midi. In October of 1947, in the journal Nature, Powell, Occhialini, and Powell’s student Cesare Lattes published a paper in which they named the initial meson the pion—which seemed to interact with the nuclear strength appropriate to Yukawa’s meson—and the subsequent meson the muon.

  It seemed at long last that Yukawa’s meson had been discovered. As for its “partner” the muon, which had been confused with Yukawa’s meson, it was nothing of the sort. Not spinless, it instead had the same spin as the electron and the proton. And its interactions with matter were nowhere near strong enough to play a role in nuclear binding. The muon turned out to be simply a heavy, if unstable, copy of the electron, which is what motivated Rabi’s question “Who ordered that?”

  So, the particle that made Yukawa famous wasn’t
the particle he predicted after all. His idea became famous because the original experimental result had been misinterpreted. Fortunately, the Nobel committee waited until the 1947 discovery of the pion before awarding Yukawa their prize in 1949.

  But, given the track record of errors and mislabeling, it is natural to wonder if the pion was in fact the particle Yukawa had predicted. The answer is both yes and no. Exchange of charged pions between protons and neutrons is indeed one accurate way of trying to estimate the strong nuclear force holding nuclei together. But in addition to charged pions—the mesons that Yukawa had predicted—there are neutral pions as well. Who ordered those?

  Moreover, the theory that Yukawa wrote down to describe the strong force, like Fermi’s theory to describe neutron decay, was not fully mathematically consistent, as Yukawa had conceded when he proposed it. There was, at the time, no correct relativistic theory involving the exchange of massive particles. Something was still amiss, and a series of surprising experimental discoveries, combined with prescient theoretical ideas that were unfortunately applied to the wrong theories, helped lead to more than a decade of confusion before the fog lifted and light appeared at the end of the tunnel. Or perhaps at the mouth of the cave.

  Chapter 12

  * * *

  MARCH OF THE TITANS

  The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid.

  —ISAIAH 11:6

  The relationship between theoretical insight and experimental discovery is one of the most interesting aspects of the progress of science. Physics is at its heart, like all of science, an empirical discipline. Yet at times brief bursts of theoretical insight change everything. Certainly Einstein’s insights into space and time in the first two decades of the twentieth century are good examples, and the remarkable theoretical progress associated with the development of quantum mechanics by Schrödinger, Heisenberg, Pauli, Dirac, and others in the 1920s is another.

  Less heralded is another period, from 1954 to 1974, which, while not as revolutionary, will, when sufficient time has passed, be regarded as one of the most fruitful and productive theoretical physics eras in the twentieth century. These two decades took us, not without turmoil, from chaos to order, from confusion to confidence, and from ugliness to beauty. It’s a wild ride, with a few detours that might seem to come from left field, but bear with me. If you find it a tad uncomfortable, then recall what I said in the introduction about science and comfort. By putting yourself in the frame of mind of those involved in the quest, whose frustration eventually led to insights, the significance of the insights can be truly appreciated.

  This tumultuous period followed one in which experimental bombshells had produced widespread confusion, making nature “curiouser and curiouser,” as Lewis Carroll might have put it. The discoveries of the positron and quickly thereafter the neutron were just the beginning. Neutron decay, nuclear reactions, muons, pions, and a host of new elementary particles that followed made it appear as if fundamental physics was hopelessly complicated. The simple picture of a universe in which electromagnetism and gravity alone governed the interactions of matter made from protons and electrons disappeared into the dustbin of history. Some physicists at the time, like some on the political right today, yearned for the (often misremembered) simplicity of the good old days.

  This newfound complexity drove some, by the 1960s, to imagine that nothing was fundamental. In a Zen-like picture, they imagined that all elementary particles were made from all other elementary particles, and that even the notion of fundamental forces might be an illusion.

  Nevertheless, percolating in the background were theoretical ideas that would draw back the dark curtains of ignorance and confusion, revealing an underlying structure to nature that is as remarkable as it is strangely simple, and one in which light would once again play a key role.

  It all began with two theoretical developments, one profound and unheralded and another relatively straightforward but brilliant and immediately feted. Remarkably, the same man was involved in both.

  Born in 1922 to a mathematician father, Chen-Ning Yang was educated in China, moving in 1938 from Beijing to Kunming to avoid the Japanese invasion of China. He graduated four years later from National Southwestern Associated University and remained there for another two years. There he met another student who had been forced to relocate to Kunming, Tsung-Dao Lee. While they only had a marginal acquaintance with the United States, in 1946 both of them received scholarships set up by the US government, with funds received from China to allow talented Chinese students to pursue graduate study in America. Yang had a master’s degree and therefore had greater freedom to pursue a PhD, and went with Fermi from Columbia to the University of Chicago. Lee had less choice, as he did not have a master’s degree, but the only US university where he could work directly toward a PhD was also the University of Chicago. Yang did his PhD under the supervision of Edward Teller and worked directly with Fermi as his assistant for only a year after graduation, while Lee did his PhD with Fermi directly.

  During the 1940s, the University of Chicago was one of the greatest centers of theoretical and experimental physics in the country, and its graduate students benefited from their exposure to a remarkable set of scientists—not only Fermi and Teller, but others including the brilliant but unassuming astrophysicist Subrahmanyan Chandrasekhar. When he was nineteen, Chandra, as he was often called by colleagues, had proved that stars greater than 1.4 times the mass of the Sun must collapse catastrophically at the end of their nuclear-burning lifetime, either through what is now known to be a supernova explosion, or directly in what is now known as a black hole. While his theory was ridiculed at the time, he was awarded the Nobel Prize for that work fifty-three years later.

  Chandra was not just a brilliant scientist but, like Fermi, a dedicated teacher. Even though he was pursuing research at the Yerkes Observatory in Wisconsin, he drove one hundred miles round-trip each week to teach a class to just two registered students, Lee and Yang. Ultimately, the entire class, professor included, became Nobel laureates, which is probably unique in the history of science.

  Yang moved to the venerable Institute for Advanced Study in Princeton in 1949, where he nurtured his budding collaboration with Lee on a variety of topics. In 1952 Yang was made a permanent member of the institute, while Lee moved in 1953 to nearby Columbia in New York City, where he remained for the rest of his career.

  Each of these men made major contributions to physics in a variety of areas, but the collaboration that made them famous began with a strange experimental result, again coming from cosmic-ray observations.

  In the same year that Yang moved from Chicago to the IAS, Cecil Powell, the discoverer of the pion, discovered a new particle in cosmic rays, which he called the tau meson. This particle was observed to decay into three pions. Another new particle was discovered shortly thereafter, called the theta meson, which decayed into two pions. Surprisingly, this new particle turned out to have precisely the same mass and lifetime as that tau meson.

  This might not seem that strange. Might they be the same particle, simply observed to decay in two different ways? Remember that in quantum mechanics, anything that is not forbidden can happen, and as long as the new particle was heavy enough to decay into either two or three pions—and the weak force allowed such decays—both should occur.

  But, if it were sensible, the weak force shouldn’t have allowed both decays.

  Think for a moment about your hands. Your left hand differs from your right hand. No simple physical process, short of entering through the looking glass, can convert one into the other. No series of movements, up or down, turning around, or jumping up and down, can turn one into the other.

  The forces that govern our experience, electromagnetism and gravity, are blind to the distinction between left and right. No process moderated by either force can turn something such as your right hand into its mirror image. I cannot turn your right hand into your left hand merely
by shining light on it, for example.

  Put another way, if I shine a light on your right hand and look at it from a distance, the intensity of reflected light will be the same as it would be if I did the same thing to your left hand. The light doesn’t care about left or right when it is reflecting off something.

  Our definition of left and right is imposed by human convention. Tomorrow we could decide that left is right and vice versa, and nothing would change except our labels. As I write this on an airplane, flying economy class, the person to my right may be quite different from the person to my left, but again that is just an accident of my circumstances. I don’t expect that the laws governing the flight of this plane are different for the right wing than for the left wing.

  Think about this in the subatomic world. Recall that Enrico Fermi found that, given the rules of quantum mechanics, the mathematical behavior of groups or pairs of elementary particles depends on whether they have spin ½, i.e., are fermions. The behavior of groups of fermions is quite different from the behavior of particles such as photons, which have a spin value of 1 (or any integer value of spin angular momentum, i.e., 0, 1, 2, 3, etc.). The mathematical “wave function” that describes a pair of fermions, for example, is “antisymmetric,” while one describing a pair of photons is “symmetric.” This means that if one interchanges one particle with another, the wave function describing fermions changes sign. But for particles such as photons, the wave function remains the same under such an interchange.

  Interchanging two particles is the same as reflecting them in the mirror. The one on the left now becomes the one on the right. Thus an intimate connection exists between such exchanges and what physicists call parity, which is the overall property of a system under reflection (i.e., interchanging left and right).