Bohr had an easier time in Munich, where the head of theoretical physics was the forty-six-year-old Arnold Sommerfeld. Although he had spent a number of years in Göttingen, Sommerfeld retained a youthful zest for innovation and novelty. He had been one of the first to embrace Einstein’s special theory of relativity, when other physicists of his generation were struggling to accept that space and time had changed. When the Bohr atom arrived on the scene, he had written promptly to tell Bohr that although he couldn’t yet dismiss a certain skepticism about the model, its ability to yield quantitative results was “unquestionably a great achievement.” In Munich, Sommerfeld received Bohr warmly, and encouraged his students to turn to the new physics.
It was now August 1914, a fateful month. Niels and Harald Bohr left Germany to hike for a while in the Tyrolean Alps. In the newspapers they read urgent, panicky accounts of impending war and learned that summer vacationers across Europe were streaming homeward across a tense continent. The Bohrs got on a train and found themselves back in Germany just half an hour after the declaration of war with Russia. Reaching Berlin, they encountered screaming crowds, breathless for the fighting to begin. “It is the custom in Germany,” Bohr observed drily, “to find such enthusiasms as soon as something military is concerned.” After another anxious train journey to the northern coast, they boarded a ferry to Denmark and safety.
Just as Bohr had made his debut in the German physics world, the war closed off most contacts for years. In the meantime, he was trying to find himself a better situation in Copenhagen. He had no laboratory and, burdened with teaching physics to medical students, hardly any time for research. Worse, he had no colleagues with whom he could thrash out his ideas. He started agitating for the university to open an institute of theoretical physics, but with war in the offing the Danish government could put no high priority on such a plan. Instead, Bohr gratefully accepted an offer from Rutherford to return to Manchester. But now Rutherford took up war research (he devised methods to detect submarines by the noise they made underwater), and Bohr was left largely to fend for himself.
Throughout his life Bohr’s ideal working method was to involve himself in a continuous, open-ended discussion, a permanently convened informal seminar with colleagues. He thought out loud, threw out ideas, commented and criticized, jumped forward, digressed, stopped and pondered. His two years in Manchester were personally happy for him and his young wife (the industrial city was less charming than Cambridge, she said, but the people were warmer) but scientifically lonely.
Despite the war, science went on. Sequestered in Germany, Sommerfeld took up Bohr’s atom in earnest. Papers and journals trickled back and forth across the trenches. Ideas could still travel. Even indirectly, Bohr could strike sparks in others.
The original Bohr atom explained, really, one thing alone. It accounted for the Balmer series of lines in hydrogen. But there were other lines, other atoms, and even the Balmer lines were not as simple as Bohr at first thought. The American physicist Albert Michelson, using a spectrometer of exceptionally high quality, had discovered in 1892 that individual lines, examined closely, often resolved into doublets—that is, two lines set closely together, corresponding to spectral excitations at two very slightly different frequencies.
It occurred to Bohr that this splitting of spectral lines might arise if electron orbits could be elliptical as well as circular. This happens because the electrons are moving so fast that certain effects of Einstein’s relativity become important. In Newtonian mechanics, an infinite family of orbits can exist, all with the same energy but with varying degrees of ellipticity. Each family has one circular orbit, which has zero ellipticity. But relativity makes the energy of all these orbits slightly different, depending on how elliptical they are.
So Bohr imagined that if an atom could contain an elliptical orbit partnered with each circular one, it would then have two slightly different transition energies, depending on which orbit an electron jumped into or out of. And that would cause spectroscopic lines to split into two. But at this point Bohr, alone in Manchester, got stuck. Why would there only be one elliptical orbit, and what would determine its ellipticity? Some new rule was needed, and Bohr couldn’t see it.
For a man counted among the great theorists of physics, Bohr had remarkably little ability in the higher realms of mathematics. His papers are not festooned with equations. Instead, he sets out broad concepts and assumptions and tries to draw out quantitative conclusions as simply as possible. Through most of Bohr’s career, it was only with the help of a string of mathematically gifted assistants that he was able to turn his remarkable physical insights into quantitative arguments. This way of working fed into the somewhat mystical status Bohr gradually attained. He seemed to be able to discern where the answer to some problem lay, even though he couldn’t see exactly how to get there. Many years later Werner Heisenberg wrote of a conversation in which, he said, “Bohr confirmed to me…that he had not worked out the complex atomic models by classical mechanics; they had come to him intuitively, rather, on the basis of experience, as pictures.”
Unable to work out fully his idea of elliptical orbits, Bohr published a sketchy outline of the suggestion. This paper found its way to Munich, where it came before the highly trained and resourceful mind of Arnold Sommerfeld. Educated in the best German tradition, equipped with a mastery of mathematical techniques and their application to mechanics, electromagnetic theory, and much else, Sommerfeld was just the man to make the next move.
Incorporating Bohr’s idea into a sophisticated analysis of the orbital mechanics of the atom, Sommerfeld cooked up a plausible argument to explain why the ellipticity of electron orbits must be restricted to certain values. Ellipticity, like the sizes of the orbits themselves, was “quantized.”
Other spectroscopic puzzles yielded to similar reasoning. When atoms are placed in electric or magnetic fields, their spectral lines split into doublets, triplets, and more complicated combinations. These are known as the Stark and Zeeman effects, after their respective discoverers. They came about, Sommerfeld and others now proposed, because electron orbits must lie at some angle relative to these externally imposed fields, and depending on the angle, the orbit energy would change slightly. Here again, not just any old angle was permitted. Orientation too was quantized into a set of allowed dispositions.
In this more complicated system, three so-called quantum numbers were needed to specify any particular electron orbit. The first indicated the orbit’s size, the second its ellipticity, the third its orientation. Electron jumps among these various orbits could account for a host of spectroscopic subtleties.
Bohr was thrilled to see the capabilities of his atom expanded so far and so fast. “I do not believe I have ever read anything with more joy than your beautiful work,” he wrote to Sommerfeld. So important were Sommerfeld’s augmentations that many physicists began to speak of the Bohr-Sommerfeld atom.
These were the triumphal years of what became known as the old quantum theory. It was a funny business, no doubt. The mechanics of the orbits followed entirely from old physics—the electrons obeying Newtonian rules (with occasional modifications from Einstein), controlled by an inverse square law of attraction between electrons and nucleus. But then the quantum limitations came in. Of the infinite range of possible orbits, only certain shapes and sizes and alignments were in fact permitted. These quantum rules had a certain logical consistency, but at bottom they were arbitrary, imposed by fiat.
Conceptually, this awkward hybrid of old and new made little sense. Where did the quantum rules come from? How, as Rutherford had asked, did an electron decide when to jump and where to jump to? Were these jumps in fact triggered in some unknown way, or were they—as Einstein feared—truly spontaneous and ultimately unpredictable?
To these strange, unprecedented questions, no one had the remotest inkling of an answer. But for the time being, no matter! The Bohr-Sommerfeld atom splendidly explained all manner of hitherto impenetrable sp
ectroscopic mysteries. It did its job inexplicably well, undeservedly well.
The rise of the Bohr-Sommerfeld atom marked not only a maturing of quantum theory but also a historic displacement of the geographical center of theoretical physics from Great Britain to continental Europe, and especially to Germany. The atomic nucleus was a sterling product of the British Empire, conceived by Rutherford, a New Zealander, after work in Canada and England. The primitive Bohr atom could likewise claim a substantial British pedigree, since it derived in large part from Bohr’s contact with Rutherford and Darwin. But in the war years, while Bohr stayed in Manchester, his ideas took root in Germany, and that was where the old quantum theory of the atom came to fruition.
Niels Bohr all his life remained devoted to Rutherford, whom he had first met soon after his father died and whom he described as “almost like a second father.” Over the years he continued to let Rutherford know how his work on the atom was going, telling him at the beginning of 1918, “I am at present myself most optimistic as regards the future of the theory.” Rutherford always responded encouragingly, but at heart he was a practical man, an experimenter. He told his Cambridge colleagues that the quantum theorists “play games with their symbols, but we, in the Cavendish, turn out the real solid facts of Nature.” Rutherford liked to say, in his booming manner, that any physicist worth his salt ought to be able to explain his researches to a barmaid, otherwise what was the point? Bohr had trouble enough explaining his physics to his fellow physicists. But as long as he could still get his ideas across to Rutherford, perhaps he could feel he was on safe ground.
In 1916, with plans for his own institute meeting official approval (and having turned down offers to stay on in Manchester or move to Berkeley, California), Bohr returned to his beloved Copenhagen. There he would found an institute to build quantum theory. But that would take time, and while Bohr wrestled with bureaucracy as well as research, it was Sommerfeld and his students in Munich who took the lead.
In Great Britain, meanwhile, theory went on hiatus. It may have been that the British tradition in mathematical physics, like the empire itself, was overburdened and exhausted. The previous era’s giants were gone. The resounding achievements of nineteenth-century Britain, in electromagnetism, optics, acoustics, fluid dynamics, and so on, were a hard act to follow. Some remnant of a Victorian ethos held sway, a spirit of bluff practicality, heartiness, mens sana in corpore sano. Theory, in the classical style, ought not to stray too far from common sense. The new ideas of quantum theory—like new art, new music—seemed dangerously avant-garde, unconnected to the plainspoken theories that had worked very well thus far. Experimental physics, especially nuclear physics, flourished in Britain under the powerful command of Rutherford, who in 1919 took over the reins of the Cavendish Laboratory from J. J. Thomson. But theory—deep theory, modern theory—subsided.
Germany, meanwhile, was by no means a blank slate. In both theory and experiment German physicists had built a solid reputation. There had been in the German-speaking world, moreover, a bruising battle over the meaning of theory—a debate that most British scientists professed to find amusing, the sort of thing morbidly philosophical Germans might indulge in but not straightforward Anglo-Saxons. Ludwig Boltzmann, a firm believer in the reality of atoms, had clashed with his fellow Austrian the physicist-philosopher Ernst Mach, who was cheerleader in chief for the ideology of positivism. To Mach, theory harbored no deep meaning about the fundamental structure of the physical world. A theory was merely a set of mathematical relationships linking tangible phenomena. The atom, therefore, was at best a convenient fiction, at worst an unverifiable hypothesis.
The atomists had won that battle. Boltzmann’s struggles brought him sympathizers and allies among the pure mathematicians, who were intrigued to see physics making incisive use of principles and theorems that had seemed to belong only to them. German theorists, by the early twentieth century, had become mathematically venturesome in a way that their British counterparts were mostly not.
And then there was the First World War, the war to end all wars. At first, it all went very satisfactorily for the Germans, who imagined that German culture and civilization were about to eclipse tired Anglo-Saxon ways. But that expectation imploded in 1918, when the German authorities crumbled and surrendered almost before their people knew that anything was amiss.
In October 1914, when prospects looked glorious, Max Planck had been one of ninety-three distinguished German intellectuals to put their names to an “Appeal to the Cultured Peoples of the World.” This lamentable manifesto, published in newspapers across the country, announced the virtue of the German cause, the many superiorities of German civilization, and the tender respect held by Germans for the cultural achievements of lesser nations. What prompted this declaration had been the destruction by German troops of the historic library in Louvain, Belgium. Planck and his fellow intellectuals denied that cultured, civilized Germans could have committed such an outrage, denied reports that Belgian towns and villages had been destroyed, denied, really, that Germany was anything more than an unwilling, put-upon victim of the carnage now spreading across Europe.
Four years later, with the country devastated, the population starving, and flaring socialist revolution provoking reactionary backlash in the anarchic cities, this document became as pathetic as it was shameful. Planck later claimed he had not properly read the appeal when he signed it, but had done so because of the distinguished list of those who had already appended their names. He did, though, during the war, begin to moderate his unthinking embrace of German unity and purpose, and he acknowledged, in response to letters from colleagues elsewhere in Europe, that German soldiers had not always conducted themselves according to the high standards the appeal proclaimed.
Even so, the spirit behind the appeal lived on in a chastened way. Germany might be physically destroyed, but intellectual Germany must endure. The country at war’s end was ruined, economically, politically, and psychologically. During the “turnip winter” of 1916–17 people had starved and frozen, and food continued to be short after the war. Political institutions fell apart. Competing factions ranging from extreme monarchists to outright communists indulged in gang violence and assassination. The rest of the world showed no sympathy. Germany had brought about its own ruin. The onerous Treaty of Versailles imposed huge reparations on an already impoverished country. Germany was made into an international pariah, excluded from the budding League of Nations. In the scientific world, Germans were ostracized, refused entry to international conferences, refused publication in many journals.
Amid this dark turmoil, Planck and others believed, science could stand as a beacon for the future. In the Berliner Tageblatt at the end of 1919 Planck stated his confidence that “as long as German science can continue in the old way, it is unthinkable that Germany can be driven from the ranks of civilized nations.” Planck was like any number of Germans who, having been at first wholeheartedly in favor of the war, later decided it had been an aberration, a disastrous misadventure imposed by rabid militarists on an unwilling populace. Now that it was all over, Planck thought, German pride and honor and tradition could live on in science. The isolation imposed by the outside world made German scientists all the more determined to save their profession and, with it, some fraction of their country’s honor.
That year, 1919, saw the sudden rise to international fame of Germany’s greatest theorist, Albert Einstein, whose general theory of relativity received much ballyhooed confirmation in observations, by the British astronomer Arthur Eddington, of the bending of light by the sun’s gravity. But Einstein’s Germanness was a delicate matter. Born in southwestern Germany and educated for a time in Munich, young Albert had rebelled against the intellectual rigidity and military overtones of his schooling, and at the age of fifteen had fled to Milan, Italy, where his father had gone to establish an electrical business. Later Einstein enrolled at the Swiss Polytechnic in Zurich and moved smartly to obtain Swiss citize
nship, renouncing his German passport. By the end of the war, however, his fame had brought him appointment at the center of German science, as a professor in Berlin. Germany, for the time being, proudly claimed him.
In politics as well as science, Einstein was his own man and floated above crass considerations of nationality or chauvinism. He loathed German militarism, but did not approve of the post-war scientific isolation of Germany. It would only prolong hostility and ill feeling, he thought, and he was mostly right. Though he had no love for certain overly patriotic German scientists—Johannes Stark, discoverer of the Stark effect, was soon to take a leading role in denouncing the “Jewish science” of relativity and, later, quantum theory—Einstein stayed away from a number of international meetings on the grounds that all Germans were disbarred, regardless of their politics, attitudes to the war, and current efforts to restore comity.
Einstein’s growing worldwide celebrity thrust his political views as well as his authorship of relativity into the public arena. Other of his scientific achievements have tended as a result to be sometimes eclipsed. In the rise of quantum theory, Einstein’s crucial role was in turning Planck’s mysterious little allotments of energy into physically meaningful units of electromagnetic radiation. In his miraculous year of 1905, two of Einstein’s four legendary papers established special relativity (the short second paper included the world’s most famous scientific equation, E = mc2). Another, as we know, dealt with Brownian motion. The fourth concerned what he took to calling “light quanta.” Einstein argued for taking Planck’s argument about little packets of energy at face value: treat the energy packets as if they were bona fide discrete little objects, employ the standard statistical methods developed by Boltzmann and others, and many of the established properties of electromagnetic radiation pop right out. If that failed to convince, he had another argument. By asserting that light was made up of little packets of energy, he was easily able to explain previously puzzling details of the photoelectric effect, in which light striking certain metals generates a small voltage.
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