Dreams of Earth and Sky

by Freeman Dyson

  Delbrück preserved the script of the performance but never published it. The German text is still unpublished. Thirty years after the performance, Gamow borrowed the script from Delbrück and translated it into English with the help of his wife Barbara. The English version was finally published, with illustrations by Gamow, in his book Thirty Years That Shook Physics.† Gamow was by that time firmly established in America as a writer of popular books about science and as the founding father of big bang cosmology.

  Einstein has a minor role in the play, as a king with a retinue of trained fleas who cause considerable annoyance to the other characters. The fleas are Einstein’s unified field theories, which in 1932 were already becoming an obsession. His distrust of quantum mechanics, and his addiction to unified field theories, had the effect of cutting him off from his old friends. Delbrück was holding up the mirror to Einstein, to show him how he looked to the younger generation. But Einstein was not looking into the mirror. He was not in the audience.

  Gino Segrè, a professor of physics and astronomy at the University of Pennsylvania, has used the Copenhagen performance in 1932 as the centerpiece for his book Faust in Copenhagen: A Struggle for the Soul of Physics.‡ The book is a history of the quantum revolution that started with a daring proposal by Max Planck in 1900. Planck suggested that light and heat radiation are emitted in little packets that he called quanta, the energy of each quantum being proportional to the frequency of the radiation. The revolution gathered strength in 1905 when Einstein described light as consisting of little quantum particles that maintain their separate existence not only when they are emitted but also while they are traveling from place to place. The next big step forward came in 1913 when Bohr described atoms as miniature solar systems, with electrons traveling in orbits around the nucleus like planets orbiting the sun, and the energies of the orbits taking discrete values limited by quantum conditions. All through the years from 1900 to 1923, physicists were suffering from schizophrenia. They had been educated to believe that the laws of classical physics could explain everything, but the new quantum effects were confirmed by experiments and were obviously inconsistent with the classical laws.

  The real quantum revolution started in 1923 when the French physicist Louis de Broglie proposed dropping the classical laws altogether and representing all material objects by waves. The Austrian Erwin Schrödinger found the wave equation that converted Broglie’s vision of matter waves into a coherent theory. The years from 1925 to 1928 were the era of Knabenphysik, or “Boy Physics.” The radical new ideas of quantum mechanics emerged in rapid succession from the brains of twenty-five-year-old boys, in particular from the brains of Heisenberg, Pauli, and Dirac, while the older generation, including Bohr and Einstein and Schrödinger and Ehrenfest, struggled to keep up with them.

  By 1932, when the Faust spoof was performed, the revolution was over. Quantum mechanics was firmly established. Dirac had announced the end of the revolution in 1929 with his customary clarity: “The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known.” One of the main themes of Delbrück’s script was the fact that the boy geniuses who invented quantum mechanics in 1925 were in 1932 already growing old. At the end of the play, Dirac makes another clear statement:

  … Old age is a cold fever

  That every physicist suffers with!

  When one is past thirty,

  He is as good as dead!

  Heisenberg adds a fiercer tone to Dirac’s lament: “It would be best to give them an early death.” Finally, Pauli, who in real life was never at a loss for a word, ends the play with a sad confession: “Pauli has here nothing more to say!”

  The play ends, and Pauli’s reign as Mephistopheles is over. Delbrück is proclaiming to his twenty-five-year-old friends in the audience that the thirty-year-old wunderkinder in the front row are fading, and it is now time for the twenty-five-year-olds to take over the leadership of the revolution. It becomes clear at the end that Delbrück’s sharpest satire is not directed against Ehrenfest but against the thirty-year-old geniuses who have too soon become elder statesmen.

  In his subtitle, Segrè calls the Copenhagen performance “A Struggle for the Soul of Physics.” The subtitle is not an accurate description, either of the performance or of the book. The performance was hardly concerned with physics at all. It was concerned with a remarkable group of human beings who had worked together for many years and achieved an amazing success. The performance celebrated their success by turning it into a comedy, using the pompous language of Goethe to make fun of their personal idiosyncrasies. It is a portrait of the group, seen in the distorting mirror of Delbrück’s wit. Segrè’s book is concerned with physics, but not with a struggle for the soul. It gives a lively account of the quantum revolution, interspersed with extracts from Goethe and Delbrück to add personal color to the narrative. Only at the end is there a brief passage describing the struggles for the soul of physics that began ten years later and had little to do with the quantum revolution.

  There were two separate struggles for the soul of physics. One began when physics was applied on a gigantic scale to the production of nuclear weapons in World War II. Another began when physics after the war was increasingly dominated by big particle accelerators with large teams of scientists and engineers to operate them. Neither of these struggles was foreseen by Delbrück or by anyone else in 1932. The chief worry of the physicists in 1932 was the danger that they might run out of ideas. They did not worry about being taken over by the military or by heavy industry. They did not worry about losing their souls. Delbrück saw Faust as a convenient source of literary quotations, not as a moral dilemma for physicists. The idea that physicists working on nuclear energy were making a Faustian bargain with the devil came later, after the discovery of fission in 1938. The earliest such bargains were made by Heisenberg in Berlin in 1939, and by Bohr and many others at Los Alamos in 1943. Neither Heisenberg nor Bohr ever expressed remorse for the bargains that they made. Both of them remained firm believers in the promise of nuclear energy as a boon to all mankind.

  The main question that the book raises is whether the quantum revolution of the 1920s was a unique event in the history of science, or whether it may some day recur. The generation of physicists who lived through it were mostly convinced that they would live to see it repeated. The experience of living through the crisis affected them so deeply that they could not easily return to less adventurous ways of thinking. They saw that the quantum revolution was incomplete and left many important mysteries unresolved. They could not give up the hope that they could solve the remaining mysteries with a second explosion of new ideas.

  Many of the leaders of the first revolution, like Einstein, spent the rest of their lives pursuing various radical ideas that led nowhere. Each of them imagined that his own personal vision would be the key that would open the door to the second revolution. Their radical ideas were all different, but had in common the lack of any experimental support. The first revolution had been guided and tested by numerous experiments in atomic physics. The later radical ideas were not only untested but untestable. They did not make predictions that were precise enough to be proved right or wrong. Einstein had his unified field theories, bringing together the equations of classical electromagnetism and gravitation. Heisenberg had a nonlinear quantum field theory, which he promoted with great publicity and little success. Even Dirac, who was generally the most levelheaded of the group, spent some years pursuing a crazy version of quantum mechanics in which probabilities were allowed to be greater than one or less than zero. All these efforts failed, and the second revolution did not happen.

  The only one of the older generation of revolutionaries who did not succumb to fantasies of a second revolution was Bohr. He remained until the end of his life actively engaged in supporting and encouraging successive generations of young scientists. He did not, like Einstein, retreat into an ivory tower to
pursue his own ideas in isolation. When I was a young scientist at the Institute for Advanced Study in Princeton, I had occasion to observe at first hand the contrasting styles of Bohr and Einstein. That was in the early 1950s, when Bohr came to the institute among a crowd of younger visitors. He attended our seminars and took part in our arguments. He was interested in everything that we were doing. He enjoyed watching the science of particle physics unfold with frequent discoveries of unexpected particles and interactions. He was confident that the quantum revolution of the 1920s had provided a firm basis for understanding the new discoveries. He did not see any need for a second revolution.

  At the same time, Einstein was working by himself in a nearby office, trying out one set of unified field equations after another. He never came to our seminars and never showed any interest in our activities. For us and for Bohr, the central problem of physics was to understand and explain the new particles. For Einstein, the new particles were uninteresting. He did not allow them to distract him from his chosen path. They never appeared in any of his equations.

  Einstein and Bohr continued to move along divergent trajectories. Einstein was driven by a divine discontent that led him to reject the first quantum revolution and strive to create a second revolution out of pure thought. Bohr was driven by pride in the successes of the first revolution, which led him to continue exploring the details of nuclear and particle physics and enjoying the friendship of new generations of young scientists who came to work with him. The younger generations were faced with a choice between two alternatives. Should they follow Bohr and be content with a lifetime of solid but unrevolutionary research in the established fields of physics? Or should they follow Einstein and spend their lives in a lonely attempt to start a new revolution without any experimental guidance? They were caught in a trap, forced to choose between two paths, one leading to conservative mediocrity and the other to radical irrelevance. Physics was a trap, because the first revolution had already happened, and the only way to attempt another revolution was to jump into a hyperspace of pure speculation.

  Delbrück and Gamow, the two progenitors of the Copenhagen Faust, found an escape from the trap. The way of escape was to move out from physics into other fields where revolutions had not yet happened. In other fields, revolutions were overdue, and it was still possible to start one without losing touch with reality. It was possible to be radical without being irrelevant. Gamow jumped from physics into cosmology, Delbrück from physics into biology, and both of them started revolutions.

  Gamow revolutionized cosmology with his theory that the expansion of the universe started with a hot big bang. He proposed that the early universe was an explosively hot and dense mixture of particles and radiation, and his theory was testable because a relic of the early high-temperature radiation could still be detected. He predicted that a uniform sea of microwave radiation should still be pervading the universe today, with wavelengths increased and temperatures diminished by a factor of a thousand since the time when the universe was an opaque primeval fireball. According to his theory, this microwave background radiation should be barely intense enough to be detectable with sensitive radio telescopes. Three years before Gamow died, the cosmic microwave radiation was discovered by Arno Penzias and Robert Wilson, and the hot big bang cosmology became generally accepted as a true picture of the early universe.

  Delbrück started a revolution in biology by choosing the bacteriophage, a simple type of virus that infects bacteria, as the object to be studied in detail. He observed that the revolution in physics had succeeded in large part because the hydrogen atom was chosen as the object of study. The hydrogen atom is the simplest kind of atom, consisting of a single proton and a single electron, and it has the simplest rules of behavior. Its behavior was simple enough to allow accurate comparisons of theory with experiment while the theory was being developed. So Delbrück chose the bacteriophage, or phage for short, as the hydrogen atom of biology. It was the simplest known form of life, and therefore the most likely to be intelligible.

  To study the phage in detail was the most promising way to reach an understanding of life. First in Berlin, then at Vanderbilt University and the California Institute of Technology in the US, Delbrück organized a group of young scientists that he called the Phage Group. They studied phages with the tools of physics as well as the tools of biology. It turned out that the phage was well chosen as a key to some of the mysteries of life, but not to all of them. Life has two main functions: metabolism and replication. Metabolism is the complicated network of chemical processes that enable a living cell to maintain its integrity in a variable environment. Replication is the much simpler process of chemical copying that enables a parent cell to duplicate itself and produce two daughters. The phage is the simplest kind of organism because it has only replication and no metabolism. It is a pure parasite, replicating itself within a bacterium and borrowing the metabolic apparatus of the bacterium to perform its missing metabolic functions. The phage allowed Delbrück to elucidate the basic rules of replication without the complications associated with metabolism. The phage was in fact, as he had surmised at the beginning, a good substitute for the hydrogen atom.

  Bohr’s understanding of quantum mechanics was based on a philosophical principle, which he called complementarity. Two descriptions of nature are said to be complementary when they are both true but cannot both be seen in the same experiment. In quantum mechanics, the wave picture and the particle picture of an electron or a light quantum are complementary. You see the wave picture when you do an experiment with electrons or light reflected from a diffraction grating and observe the diffracted waves. You see the particle picture when you detect the electrons or light quanta in an electronic counter and count them one at a time. Complementarity in quantum mechanics is an established fact. But in 1932 Bohr proposed to extend the idea of complementarity to biology, suggesting that the description of a living creature as an organism and the description of it as a collection of molecules are also complementary. In this context, complementarity would mean that any attempt to observe and localize precisely every molecule in a living creature would result in the death of the organism. The holistic view of a creature as a living organism and the reductionist view of it as a collection of molecules would be both correct but mutually exclusive. Bohr believed strongly in this application of complementarity to the understanding of life. Delbrück believed in it too when he decided to become a biologist.

  It is one of the ironies of history that Delbrück chose to study the phage, which may be the only organism simple enough to be described without invoking complementarity. The life of the phage is pure replication without metabolism. Replication is a chemical process that was completely explained by the double-helix structure of the DNA molecule discovered by Francis Crick and James Watson in 1953. When Crick and Watson discovered the double helix, they loudly claimed to have discovered the basic secret of life. The discovery came as a disappointment to Delbrück. It seemed to make complementarity unnecessary. Delbrück said it was as if the behavior of the hydrogen atom had been completely explained without requiring quantum mechanics. He recognized the importance of the discovery, but sadly concluded that it proved Bohr wrong. Life was, after all, simply and cheaply explained by looking in detail at a molecular model. Deep ideas of complementarity had no place in biology.

  Segrè agrees with this judgment. He says dogmatically, “Bohr’s conjecture was provocative, as it was meant to be, but in the end it turned out to be wrong. DNA and RNA are the answer to life, not complementarity.” In the middle years of the twentieth century, this was the verdict of the majority of scientists. The historic debate over complementarity between Bohr and Einstein was over. Bohr had won in physics. Einstein had won in biology.

  Now, fifty years later, Segrè’s opinion is widely held by physicists, less widely by biologists. I disagree with it profoundly. In my opinion, the double helix is much too simple to be the secret of life. If DNA had been the secret of
life, we should have been able to cure cancer long ago. The double helix explains replication but it does not explain metabolism. Delbrück chose to study the phage because it embodies replication without metabolism, and Crick and Watson chose to study DNA for the same reason. Replication is clean while metabolism is messy. By excluding messiness, they excluded the essence of life. The genomes of human and other creatures have now been completely mapped and the processes of replication have been thoroughly explored, but the mysteries of metabolism still remain mysteries.

  The phage is still the only living creature whose behavior is simple enough to be completely understood and predicted. To understand other kinds of creatures, from fruit flies to humans, we need also a deep understanding of metabolism. The understanding of metabolism will perhaps be the theme of the next revolution in biology. I have already discussed a seminal paper by the biologist Carl Woese with the title “A New Biology for a New Century,” pointing the way toward the next revolution.§ Woese’s new biology is based on the idea that a living creature is a dynamic pattern of organization in the stream of chemical materials and energy that passes through it. Patterns of organization are constantly forming and reforming themselves. If we try to observe and localize every molecule as it passes through an organism, we are likely to destroy the patterns that constitute metabolic life. In Woese’s picture of life, complementarity plays a central role, just as Bohr said it should.