When science was disarrayed, Einstein was disarrayed, but the Compton effect had restored sense to science. Quanta worked as Einstein had imagined and quantum radiation was no longer quite so terrible a blank. It was difficult, mysterious, solvable. Einstein had known the prophet’s struggle before—in his desperate days in Bern as an unemployable physicist his statistical analyses drew no support until he used the technique to understand Brownian motion; then again, even after other physicists hailed him as a wonder he still could not persuade most physicists to take the contradictions between relativity and gravity seriously.

By December 1923, whatever uncertainty Einstein might have felt about the relation between quanta and space-time had passed. It was time to put distractions of fame and life’s other detours aside. Germany, Zionism, and love persisted, but he seemed determined to keep his eye on science. He made a public appearance at the Prussian Academy to discuss before the assembled savants an approach to the quantum crisis based on space and time. He reminded his audience that wave theory still explained much about light. Now that his colleagues were all nodding their heads in agreement that the Compton effect proved light’s particle nature, Einstein was at the front insisting that light waves still worked too. A replacement for both waves and

particles was necessary and, he said, he was looking for that replacement in mathematics where, he expected, space and time, particle and wave could be joined in a new law just as Maxwell had joined electricity and magnetism. Einstein was looking, he said, for a quantum field that would be based on his earlier work on relativity.

Einstein’s return to the public’s eye reflected a recovered civic boldness. In Munich, the month before Einstein presented his paper, the unknown Adolph Hitler had joined with the war hero General Ludendorff to seize the Bavarian government. Hitler’s action combined audacity and absurdity in an amalgam that would characterize his behavior straight through to his suicide. General Gastov von Kahr, Bavaria’s prime minister, was scheduled to make a speech at a beer hall. Rumor had it that Kahr planned to announce the restoration of the Bavarian monarchy, something unacceptable to both democrats and national extremists. Hitler’s brown-shirted hoodlums surrounded the beer hall and seized General Kahr. Hitler then proclaimed a new government with himself as its leader. He dreamed that the masses would rally to his side, but that nonsense came to naught. Hitler’s action, however, did cause much excitement and rumor. Nobody was sure at first how large Hitler’s plot was or how widespread the coup was. Sensible people tended to suppose that Hitler’s action could not really have been as crazy and hopeless as it looked. Planck was especially worried for Einstein, fearing that he might be seized or arrested as part of a wider plot.

Nobody could see it clearly at the time, but Hitler’s absurd theater at a Munich beer hall marked postwar Germany’s rock bottom. Brutality was no longer in charge. Then, the next month, the government replaced the ravaged currency with a new money called the Rentenmark. One Rentenmark was worth a trillion (sic!) old paper marks. Just producing all those ruinous trillions had taxed Germany’s production capacity. Hundreds of paper mills and thousands of printing presses, including even newspaper presses, had been diverted from useful activities to supporting runaway inflation. Would the new money work? Nobody could be certain. When Einstein made his presentation on quanta and space-time, people were just beginning to hope that a new, more law-based Germany might have been born.

In the former enemy countries, many ordinary people still wished Germany ill. The war was five years gone, but the hatreds were alive and active. Scientists like to picture themselves as above the emotional moods that hold the masses, but of course they are not. German physicists were still scorned by non-German scientists. The Solvay Conference brouhaha had resumed. Another meeting was planned for 1924. Einstein’s instincts said attend, especially when there was something like the Compton effect to explore, but again Einstein was to be the only German participant, and again his fellow German physicists insisted that he should not attend. Before Compton’s bullet, when the world was rich in distractions, Einstein might have easily skipped Solvay. But now it was clear that light quanta worked excellently in a scheme of space-time. He was ready to concentrate on science and forget politics. Others, however, took nationality seriously, and Einstein admitted to Madame Curie that “the disinclination of Belgians and French to meet Germans [is] not psychologically incomprehensible to me,” but it was complicated. In Germany his fellow scientists expected him to be a good German and to refuse any honors that were forbidden other Germans. Meanwhile many rightist German politicians and agitators were eager to put Einstein among the first Jews to be driven off.

Whenever one side was ready to forget the war, there was somebody on the other side who remembered. Abram Joffe, a Russian physicist, came to Germany to renew old ties. During the century’s early years he had studied in Munich and had known Philipp Lenard. Joffe looked up his old companion, but when he arrived at the University of Heidelberg, Lenard sent the porter to say he was too busy to speak with “the enemies of his fatherland.”

This continuing bitterness was a great help to Bohr and his institution in neutral Copenhagen. Bright young German students, welcome almost nowhere else, could come there and mix with the up and coming stars of other countries. In December, while Einstein was speaking before the Prussian Academy, Bohr and Kramers were having excited discussions with a recently arrived American. His name was John Slater and he had given Bohr an idea about how to defeat the Compton effect.

Yet Germany was still the capital of quantum physics. In troubled Bavaria, Arnold Sommerfeld turned out promising students one after another. “You have produced so much young talent,” Einstein marveled to Sommerfeld, “like stamping them out of the ground.” And in Göttingen, where Max Born had landed a star position, student physicists received their final polish. Einstein in Berlin with the friendly Maxes—Planck and von Laue—found this tension over Germany profoundly distracting. He was trying simply to concentrate on science and draw new insights from the physics of hυ, but the German question persisted.

Einstein wrote to Lorentz, “Sommerfeld believes it is not right for me to take part in the Solvay Congress because my German colleagues are excluded. In my opinion it is not right to bring politics into science matters, nor should individuals be held responsible for the government of the country to which they belong. If I took part in the congress I would by implication be an accomplice to an action which I considered most strongly to be distressingly unjust.” He closed by pleading with Lorentz, “I would be grateful if you would send me no further invitations to the Councils. I hope not to be put in a position where I am obliged to refuse an invitation, as such a gesture might hinder any progress toward reestablishing amicable collaborations between physicists of all nationalities.”

He sent Madame Curie a splendidly aristocratic yelp, “It is unworthy of cultured men to treat one another in this type of superficial way, as though they were members of the common herd being led by mass suggestion.” He returned to his equations, working in his upstairs study with Newton’s portrait on the wall. He mainly sat quietly with a pen and paper, playing with images and contemplating what they meant, trying out his notions with mathematical symbols.

Thinking about almost any physics principle, Einstein told Count Kessler that year, almost always led to progress because, without exception, scientific propositions are wrong. Every generalization about nature leads to contradictions. Einstein, sitting quietly in his chair, lost to the world, following thoughts that were inaccessible to all others, visualized ideas, looked for contradictions between rules and facts. When he found a discrepancy he looked for ways to restate the rule so

that the facts survived while the contradictions were resolved. When Einstein wrote equations on paper he was either testing an idea—does a+b work in this case?—or, trying to resolve a problem he had noticed,—why doesn’t a+b work this time?

Niels Bohr, about 200 miles northwest of Berlin, thought many classical descriptions had it pretty much right. Instead of tinkering with equations, he looked for an analogy that applied classical principles to novel observations.

Einstein’s challenge was that, after years of looking, he still lacked a successful understanding that linked waves and particles or predicted the direction that light quanta will travel when emitted “spontaneously.”

Bohr’s trouble was that quanta did not fit into any analogies he knew.

Einstein planned to keep looking for another new idea; Bohr’s plan was to resist the particle evidence as long as possible.

But the accursed German question kept getting in Einstein’s way. He finally ended one recurring hullabaloo by ending the ambiguity of his situation. On February 7, 1924, Einstein issued a statement that 10 years earlier, when he joined the Prussian Academy, he had acquired the rights of Prussian citizenship. It was a nice way to put it—talking about rights rather than duties. After rejecting Solvay and embracing Prussia, his commitment to democratic Germany again looked fixed.

Two months later Einstein published a newspaper article in the Berliner Tageblatt on “The Compton Experiment” in which he described the crisis over radiation’s particle and wave properties. At the same time, Bohr published a paper, jointly attributed to Kramers and the American student Slater, denying that the crisis had to persist. Slater had given Bohr the idea he had lacked the previous summer. Bohr now thought he saw a way to account for Compton’s data without using light quanta.

Compton’s interpretation of his data had assumed that all energy and momentum is conserved. No energy is destroyed; none is created. The same held for momentum. Bohr now thought there was a way around that idea. In Copenhagen, during an introductory bull session when Slater talked about what was on his mind, he spoke vaguely of a

virtual field guiding the light quanta. It was the old, tantalizing but unfruitful idea of the ghost whip guiding wave-particles, but Bohr and Kramers suddenly saw a way to adapt this image to their needs. Instead of guiding (as Slater imagined), the field could communicate. Instead of concerning quanta (as Slater proposed), the field could concern itself with electrons. Slater protested, but Bohr and Kramers talked to him. They talked some more. They talked long and hard. Slater was a bright young American, a whiz kid who knew himself to be lucky to be in Copenhagen. Niels Bohr even took him seriously. On the other hand, weren’t they changing his idea rather drastically? Bohr talked some more and in the end Slater agreed to add his name to a paper that transformed his notion that a field guided the light quanta into a field that provided instantaneous communication between electrons. The paper with this idea was titled “The Quantum Theory of Radiation” but was more commonly know by its authors’ last names, Bohr-Kramers-Slater. Even more commonly it was referred to by the authors’ initials, BKS.

The field in BKS was called a “virtual” field, meaning it did not really exist. A real field, a magnetic field for example, includes the energy needed to swing a compass needle. The BKS virtual field had no detectable energy. Yet the BKS field did have some real effects. The field’s electrons all communicated instantly with one another. Some electrons would absorb energy, others would emit energy. These emissions might have appeared to Compton as scattering, but they were just random radiation emissions. Energy was not being conserved moment by moment through every action. It was conserved over time and across the whole virtual field. BKS was extremely vague about how this happened. The paper had exactly one equation, and that one was Bohr’s famous quantum lines equation from 1913. Mathematics did nothing to further or clarify the BKS argument.

Einstein was not impressed and did not see BKS as a threat to his own work. More distracting was the contradiction people saw between his Zionism and his commitment to Germany. Nationalists considered his Zionism as barring his claims to be German while many Jews believed that assimilation into German culture prevented any interest in Zionism. Einstein handled the questions the same way he

did the particle-wave contradiction, by underlining both. He was thoroughly secular, with no sympathy for religious tradition, but he joined Berlin’s New Synagogue to reaffirm his solidarity with Germany’s Jews. Thread by thread he was getting his life back in order so that there would be no distractions from his physics.

Even when he went out for an evening, physics held his thoughts. Janos Plesch, a fashionable doctor who treated and socialized with Einstein, described how his famous patient talked and laughed with Abram Joffe, the same Russian physicist whom Lenard had refused to see. The two were puzzling over the imponderables of physics on the quantum level and laughed gleefully at various absurd solutions. Einstein was back to having fun with his work.

The BKS paper, however, was not so much fun and Einstein did not hide his distaste. “What does Einstein think?” Bohr asked an up-and-coming student in Göttingen named Wolfgang Pauli.

The reply came back, “Completely negative.”

Einstein wrote to Max Born and his wife, “I should not want to be forced into abandoning strict causality without defending it more strongly than I have so far. I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will, not only its moment to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming-house, than a physicist. Certainly my attempts to give tangible form to the quanta have foundered again and again, but I am far from giving up hope. And even if it never works there is always that consolation that this lack of success is entirely mine.”

Bohr had stung Einstein in BKS by using one of Einstein’s great ideas. In 1917 Einstein had shown how to deduce the quantum’s existence by assuming the conservation of energy and momentum. Einstein had always been troubled by that paper because it offered no way to predict when or in what direction a light quantum would be emitted. That, for Einstein, was an issue yet to be resolved. Now Bohr, citing Einstein, was using random quanta emissions as an argument against the conservation that Einstein had assumed in the first place.

Einstein was still grumbling about BKS’s irrationality in June when he visited the university at Göttingen. It was a memorable visit for

Werner Heisenberg, who had become Max Born’s assistant. During Einstein’s visit, Heisenberg was introduced to the great man and invited to join him in a private stroll. Heisenberg was even more mathematically oriented than Einstein and so seemed unlikely to be much impressed by the mathematically empty BKS theory. He had told his good friend Pauli, “I do not see [BKS] as an essential progress.” But then he went to Copenhagen, where Bohr gave him the full treatment, grinding Heisenberg into a BKS enthusiast. On his walk he told Einstein as much.

Einstein had a long list of objections. He had thought about this ghost field idea many times over many years and concluded it was not the answer. Energy conservation had been central to whatever success the study of radiation and thermodynamics had had. First, it had been critical to establishing the conditions that led to Planck’s discovery of the quantum. Then it had been the fundamental assumption in Einstein’s own argument that light quanta were real. Nature seemed strictly to obey the conservation laws. Now Bohr was proposing a virtual field with action at a distance. All right, but why should we assume that nature was ready to abandon conservation when it allowed actions at a distance?

We can only assume that Heisenberg had little to say in defense. He was bolder, brassier than the average German student, but he probably was not cavalier enough to challenge the world’s most famous scientist while he defended a cornerstone of physics.

Einstein also disliked BKS because he had no wish to abandon causality. For him causality meant lawfulness. Nature does not do just anything it damn pleases. Certainly Einstein was not yet ready to abandon the assumption that had supported all his previous efforts in the field.

Again, Heisenberg was probably silent, or in un-hunh mode. He too had been trained according to conservation and causal assumptions. What could he say?

Einstein had technical arguments as well. If it were not for the laws of conservation, engineers could build perpetual motion machines. Einstein proceeded to describe a BKS perpetual motion machine: place a sealed box containing radiation in empty space,

where outside radiation cannot get at it. The box would be knocked about by the internal radiation, and this knocking would increase, becoming ever more ferocious as the radiation inside the box continued. Without conservation to counter the increase, the box would bang about with forever increasing violence.

Heisenberg might have had more to ask and say about arguments like that one, but as Einstein’s physics was perfectly proper he could not have argued for long.

Another of Einstein’s technical objections considered classical optics. Bohr had been so busy hunting for a way to explain Compton scattering that he had ignored the long-studied, ordinary scattering such as what happens when light bounces off a mirror. BKS was miserable at explaining that familiar event. In a world where BKS held true, the phrase “mirror image” would refer to a blurry, off-color resemblance.

Einstein “had a hundred arguments” against BKS, Heisenberg gloomily reported the next day, but the striking fact is that Einstein’s talk did not end the dispute. Heisenberg stayed with BKS and refused to recognize Compton’s scattering data as the chit that authorized light quanta’s reality.

There is in the story of Bohr and Einstein a mysterious element that seems irrecoverable: the force of Bohr’s personality which, like Eden’s fruit, once tasted could not be forgotten. “You can talk about people like Buddha, Jesus, Moses, Confucius,” a student recalled, “but the thing that convinced me that such people existed were the conversations with Bohr.” With Bohr long dead, that personality works today the way sex works in stories read by bright 10-year-olds. We can only gather in some vague way that Bohr’s presence and persuasiveness was powerful, but we cannot really grasp what it was about Bohr that gave him such authority among physicists. It was the kind of aura that Einstein had among nonphysicists—the force of an oracle. Such a thing had not been known among physical scientists since Giordano Bruno and doubtless many people had supposed that a scientific oracle would never be known again.

The public admired Einstein and accepted him as a genius despite their inability to understand him, let alone defend his ideas—perhaps even because of that inability. Physicists, especially young ones, even

more especially young ones like Heisenberg who had met Bohr, were inclined to credit Bohr’s instincts, even though they could not defend them. Physics had become so abstract—and Einstein’s light quanta threatened to make it even more abstract—that Bohr’s confidence that it all worked, even if the working sounded a bit obscure, was persuasive.

Older physicists, like Einstein and Planck, who knew what it was like to have a terribly perplexing mystery click into place, were less given to embracing obscure prognostications. Even when they liked the idea that the conservation laws were merely statistical, older scientists balked at BKS’s philosophical obscurities. Erwin Schrödinger, for example, was a 37-year-old professor of physics in Zürich who had given his inaugural lecture on chance’s role in physical laws. He immediately sent Bohr a congratulatory letter, praising him for daring to resist the quantum revolution with such a “far reaching return to classical theory.” There was just one little obscurity that Schrödinger had trouble with. Exactly what did Bohr mean by the word virtual? To Schrödinger, something that produces physical results is real, not virtual. How poetic, Schrödinger wondered, was Bohr being?

But trying to pin Bohr down on a word’s meaning was like trying to cage a beautiful cloud. He could no more be cross-examined on such a point than the Oracle of Delphi.

Einstein was confident. He wrote to Ehrenfest that Bohr, Kramers, and Slater had sought to abolish free quanta but added that free quanta “would not allow themselves to be dispensed with.” The coming revolution was not to be denied. Einstein had just one other loose thread from his personal life to snip off. He had resolved the ambiguities about his citizenship and ethnicity. He was a German Jew. Now he had only the confusion over his marriage to settle. He sent his secretary, Bette Neumann, a note telling her that he had to seek a happiness in the stars that was denied him on earth. One hopes she saw it for the caddish, self-centered poppycock it was. Where was she supposed to seek happiness? Einstein’s Saint Francis side was again taking control over the Pantagruel. His emotions had no more time for women. He was returning his emotions full-time to science, not to Elsa.

Meanwhile, the physics world was growing restless at this dispute

between Bohr and Einstein. Wolfgang Pauli probably spoke for the majority when he told Bohr, “The available data are not sufficient to decide for or against your view…. Even if it were psychologically possible for me to form a scientific opinion on the grounds of some sort of belief in authority (which is not the case, however, as you know), this would be logically impossible (at least in this case) since here the opinion of two authorities are so very contradictory.”

While ordinary physicists wrestled over who was right, Einstein had moved on ahead. Correspondents around the world—unknown thinkers like Louis de Broglie in Paris and even somebody from Dacca, India named Saryendra Bose—had given him new ideas and he was out ahead again, thinking about parts of physics that nobody else was minding. When Fritz Haber wrote to Einstein, “I was in Copenhagen and spoke with Herr Bohr,” it was news from a battlefield that Einstein considered conquered. “How strange it is,” Haber added, for he did not think the battle over, “that the two of you, in the field where all weaker imaginations and powers of judgment have long ago withered, alone have remained and now stand in such deep opposition.”