Berlin at the end of 1924 wasno longer the desperate city that had chased out an emperor. Money had been good for a year now. New buildings were rising across the city. The enthusiastic fusion of art, revolution, and unreason had passed. An expressionist stage play even had a character say, “Let us hear no more about war, revolution and the salvation of the world!” The euphoria had gone, yet there was still optimism; there was art; there was achievement.
Einstein toured a monument to all three when he traveled to Potsdam, in Berlin’s suburbs, to attend the opening of a new building known as “the Einstein Tower.” It was a squat (five-story), concrete, lighthouse-styled structure with a solar observatory on top and an eye-catching entrance at the bottom. Einstein traveled with the architect, Erich Mendelsohn, a war veteran who had starting making drafts of an astronomy center while he huddled in the trenches. Mendelsohn led Einstein through the building, showing off both its design and its equipment. From the windows Einstein could see the woods, rivers, and fountains of Potsdam. The tower’s supporters liked the way it combined form and fun. Its opponents disliked its modernity and lack of German flavor. Einstein kept his counsel. The builders had sponsored it to test Einstein’s predictions about gravitational effects on light frequency. Einstein’s tour might have persuaded him—correctly,
as history would show—that the tower would be of only minor help in proving his theory. After tromping through the building in silence, he and Mendelsohn went to a meeting of the building committee. There, finally, Einstein whispered one modern adjective in Mendelsohn’s ear, “Organic.”
Modernity had become such a part of the new Germany that the quantum crisis, too, made its way into popular discussion. Science always has its buffs, amateurs who know what the disputes are, and by January 1925 German life had become orderly enough for science fans to notice a debate over nature’s lawfulness. The question even made its way into the Leipzig Illustrated News, a stylish newspaper filled with photographs. Into its gaudy pages, between the pictures of carnivals and catastrophes, Wilhelm Wien, the one-time leader of physics’ anti-Berlin movement, provided a story about the quantum debate. “The notion that nature is comprehensible,” Wien wrote, “is identical with the conviction that all natural processes can be reduced to … invariably valid natural laws.” BKS, Wien said, was trying to sidestep fixed law and replace it with a statistical foundation, and he insisted that statistics not based on a deeper law “will never be recognized by physics as something final.”
Wien had gone to the heart of Einstein’s alarm. When BKS broke with energy conservation, it broke with the idea that things happen for a natural reason. Statistical thinking was not new in physics. Einstein was a master of it. His explanation of Brownian motion was a perfect illustration of how to use statistics to take a meaningful step. Pollen floating on water is knocked to and fro by molecules that cannot be seen or measured directly, but by using statistical reasoning we can determine the number of molecules present. Einstein’s statistics assumed there were underlying reasons for an action, but BKS statistics looked for no such deeper meaning. Indeed, its rejection of energy conservation did away with deeper causes. That was why Einstein told his friend Max Born he found it “intolerable” that an electron would be granted “free will” to act when and how it “chose.”
Unexplained actions, of course, were a commonplace in physics. They were what kept physics incomplete and assured physicists of future employment. As long as there were unexplained phenomena
there would be a need for scientists to try to understand them. Without that search for explanations, Archimedes could get back in his tub. Bohr was particularly adept at generating unexplained physics. When Rutherford first read Bohr’s proposal for how the atom worked, he objected that there was no apparent reason for either limiting electrons to particular orbits or for an electron’s choosing one orbit over another. Bohr shrugged off that concern. He did not look for reasons, although he did not insist there were none to be found. In the years since Bohr’s theory first appeared, physicists working on the atom had grown used to the fact that there were as yet no reasons behind the basic rules. Bohr’s closest disciples, too, were looking for equations that worked, not reasons that explained why. Einstein, of course, wondered why. Questions bubbled persistently on his brain’s back burner.
Skreek-skreek-skreek. What fundamentals underlie atomic behavior?
Chatter about the Pirandello play at Max Reinhardt’s theater. Why are electron orbits fixed the way they are?
Feel dread over the news that an extremist, nationalist politician has been put in the cabinet. What determines the moment that an atom emits light?
More pressing, however, was the question of whether to explain the Compton effect with light quanta or BKS? Two teams of experimenters were especially important in trying to resolve the issue. In Berlin, Hans Geiger and Walther Bothe, the same pair that had performed Einstein’s would-be experimentum crucis, went to work testing the rival theories. According to the light quanta theory, light should scatter at the same instant that an electron recoils from the collision. BKS expected that on close examination the two events would not show such a firmly fixed association. In America too, Arthur Compton was still at work. He was preparing an experiment with a colleague to test energy conservation. They planned to use a device called a cloud chamber that let physicists observe free electrons. If energy was conserved, the electrons should move at predictable angles. If Einstein’s light quanta idea was wrong, the angles would be random.
Einstein showed no doubts about how these experiments would come out. Max Born even wrote to Bohr in January 1925, months before the results were reported, that Einstein was feeling “trium-
phant.” Others were more troubled. Wolfgang Pauli wrote a friend, “Physics is very muddled again at the moment; it is much too hard for me anyway, and I wish I were a movie comedian or something like that and had never heard anything about physics.” In the Netherlands, Ehrenfest was moderately optimistic. He loved Einstein and was utterly charmed by Bohr, so he tended to be neutral in their disputes. Yet he wrote Einstein, “If Bothe and Geiger find … a correlation [between electron and the scattered light quantum] it will be a triumph of Einstein over Bohr. This time, as an exception [to the usual neutrality] I firmly believe you are right, and I would therefore be happy if the correlation were to be demonstrated.”
After 20 years of Einstein’s insistence that his light quanta hypothesis was sound, it was time to look at everyone’s cards. The bets were in. There could be no more posing, no more bluffing. Ehrenfest’s support, Einstein’s confidence, Pauli’s nervousness could contribute nothing to the outcome. The gambler’s emotion while waiting for the ball to drop into a roulette number was caught by Count Kessler. Over tea that February he told Einstein that he seemed to be an unusually successful scientist.
Einstein replied that he was merely unusually lucky. He knew many thinkers who were just as bright and suited to make major discoveries, but were less fortunate.
Kessler held his ground, saying that Einstein seemed “to have some special sort of feelers to tell him where the solution to a problem lies.”
Einstein usually did not stress luck’s role in science, but Bohr, too, was widely respected for his antennae that smelled out a solution. Bohr had solved perhaps the only problem that Einstein had ever rejected because it looked too hard. Late in his miracle year of 1905, when everything he examined turned to gold, he wrote one of his friends, “There is not always a subtle theme to meditate upon. At least not an exciting one. There is, of course, the theme of spectral lines, but I do not think a simple connection of these phenomena with those already explored exists; so that for the moment the thing does not show much promise.” Eight years later Bohr explained spectral lines by linking quanta and the Rutherford atom; so Einstein knew that special feelers alone were not likely to make him triumphant over Bohr.
For months Berlin was rife with rumors that the Bothe-Geiger experiments supported Einstein, but the results were too important to rush. They had to be studied carefully, checked for accuracy, and checked again. It would be intolerable to declare one side victorious and then have the other side win, so to speak, on appeal. Early in April 1925, however, Geiger sent Bohr a note, warning him that his experiment did not support BKS. The scattered light and bouncing electron acted simultaneously, permitting a cause-and-effect explanation for the event and being much too perfectly coordinated to result by chance.
Bohr replied promptly, thanking Geiger “for the great friendliness with which you have informed me of your important results.” He added, “I was completely prepared [for the news] that our proposed point of view should turn out to be incorrect. The whole thing was more of an expression of an attempt to achieve as great as possible application of classical concepts, rather than a completed theory.” It was good of Bohr to state his motive for the record: to save as much of classical theory as he could. Then he seemed to balk, ending his note to Geiger by saying, “In general I believe that [many] difficulties so far exclude the maintaining of the ordinary space-time description of phenomena, so that in spite of the existence of coupling [between scattering and electron motion], conclusions based on an eventual corpuscular nature of radiation lack a satisfactory basis.”
Bohr always was a wordy fellow, but he seemed to be saying that despite the experimental results he still could not accept Einstein’s light quanta. Nine days later, however, Bohr appeared to have decided against guerilla resistance. He added a P.S. to a letter sent to a friend at Cambridge, “Just this moment I have received a letter from Geiger….”
How dramatic; too bad for our story that we know Bohr had responded to Geiger more than a week earlier.
Bohr continued, “… It seems therefore that there is nothing else to do than give our revolutionary efforts as honourable a funeral as possible.” In the end, Compton’s data and Geiger’s results—not to mention the accuracy of Einstein’s photoelectric law—were just too many facts to deny. Reject that authority and his fellow scientists might have stripped Bohr of his union card.
So BKS was dead. Almost exactly 20 years after publishing his
light-quanta paper, Einstein’s revolutionary idea had become as orthodox as republicanism. Science was just going to have to make room for wavy little chunks of hυ. Compton’s cloud-chamber results came only that summer, so they were anticlimactic, but they too supported Einstein by showing that the angle between scattered X-ray and recoiling electron matched exactly the requirements for conserving energy and momentum.
Pauli, who had been in such despair that he wanted to join Charlie Chaplin, was deeply relieved. Rather tactlessly he wrote to Kramers, “I think it was a magnificent stroke of luck that the theory of Bohr, Kramers and Slater was so rapidly refuted by the beautiful experiments of Geiger and Bothe, as well as the recently published ones of Compton…. Many excellent physicists would have maintained this theory and this ill-fated work … would perhaps for a long time have become an obstacle to progress in theoretical physics! because it moves in a completely false direction…. It can now be taken for granted by every unprejudiced physicist that light quanta are as much (or as little) physically real as electrons.”
Bohr can hardly have been happy when Kramers reported what Pauli had said, but Einstein was delighted and gleefully told Ehrenfest, “We both had no doubts about it.” Yet Einstein was not content. He still could not conceive how to unite a wave with a particle, nor, apart from Compton scattering, could he yet predict when or in what direction a light quantum was apt to move. Other physicists were more sanguine about these mysteries. A promising young American named John van Vleck remarked, “It is not surprising that paradoxical theories are required to explain paradoxical phenomena,” but in Einstein’s eye all phenomena were paradoxical until a coherent theory explained them.
And Bohr had not completely surrendered. From mid-1925 onward he accepted light quanta, but his main feeling was that BKS’s radical rejection of cause and effect had not been revolutionary enough. He had told Geiger that with BKS he had tried to apply “classical concepts”—meaning distinct waves and distinct particles moving through measurable space—as much as possible. Now he threw that conservative ambition out the window. In a paper pub-
lished that summer Bohr added a postscript saying that after BKS’s failure “the generalization of classical electrodynamic theory … will require a sweeping revolution.” Electrodynamic theory was jargon meaning that, with the success of light quanta, Bohr now expected a full revision of ideas about space, time, and relativity.