Although it was springtime, Niels Bohr arrived at the Einstein apartment in the guise and spirit of Father Christmas. He carried gifts and good will from Denmark’s untroubled countryside, a land overflowing with butter, cheeses, hams, the splendid fats and sugars of a wealthy world. The gifts were rendered almost tragic when spread on the spare table of Berlin’s most celebrated man. The Einstein household was a particularly sad place that April. In March Einstein’s mother had died miserably in the apartment, suffering from stomach cancer. Einstein had seemed to many people, including himself, as a distant brain, unattached emotionally to anyone in the world, but he wept when his mother died. It is hard to suppose that any man as confident as Einstein did not enjoy a beaming mother’s presence when it counted. And even in the months just before her death, Einstein liked to brag to her. When he heard the first hint that the Eddington expedition supported his theory, he immediately sent a note to his mother boasting that he was proven right. Bohr’s smorgasbord from “Neutralia” (as Einstein called it) was an unexpected pleasure in a house of mourning.
Both Einstein and Bohr claimed to have been instantly attracted to the other. In part, this mutual admiration was the inevitable result of their work, but they also had personal reasons to take a liking. In
particular, neither respected pomp and titles. When Bohr became a professor he was, as custom demanded, presented to his majesty, Christian X, king of Denmark. Later the king would be widely admired for his refusal to accept the anti-Jewish legislation of his Nazi occupiers, and even before his meeting with Bohr the king had advanced Denmark beyond the United States by giving women the vote. Still, he was a formal man with a military bearing and no instinct for spontaneity. Bohr, dressed in morning coat, was an informal man who had done well to remember to keep his white gloves on when he shook the king’s hand. His majesty told Bohr he was pleased to meet Denmark’s great soccer player. The correct response was, “Thank you, sire,” followed by a step back. Bohr, however, replied along the lines of, Thanks, king, but it is my brother who was the soccer star. Christian X was startled and tried again. He said once more how pleased he was to meet Denmark’s great soccer player, and Bohr once again corrected his monarch. With that second contradiction, the king abruptly announced, “The audience is over,” and it was. No wonder Einstein felt a natural attraction to his Danish guest.
Elsa was delighted by the food package and its simple pleasures. Berlin was in a tangle. A transit strike had forced Einstein to guide Bohr on foot to his apartment. The two men hiked for miles from Planck’s home, where Bohr was staying in suburban Dahlem. Einstein showed the way through Berlin’s southwestern district. That’s a wonderful scene to imagine—two absent-minded dreamers feeling their way through the Prussian metropolis while lost in talk of physics. They could have wandered anywhere without noticing and suddenly discovered they had no idea where they had landed or how to find a route back. Those two could not count on boarding a tram without getting lost. Luckily for them, Dahlem’s main route, the Archiv-Strasse, was wide and easy to follow. They could track its curving miles and cross its squares without looking up. Bird-stained statues occasionally looked down on them, but they probably did not return the stare.
Planck’s sufferings offered a deep well for sympathetic gossip. Bohr’s host was 62 and in good health, but he still seemed to prove that old age is a curse. His second daughter, Emma, had died in childbirth, just as her twin sister had. “Planck’s misfortune wrings my heart,”
Einstein told Max Born, “I could not hold back tears when I saw him.”
Both Bohr and Einstein were theorists rather than experimentalists, although Bohr’s work was more guided by experimental results. He depended on lab work to refine his theories. His picture of the atom had been very simple at first, but changes in observed magnetic and electric fields led to many complications. For him theory was a way of describing and anticipating experimental data.
Einstein’s ideas did not work like that. He laid a field’s ground rules and then let others worry about loose ends, although somehow with Einstein’s work loose ends were as rare as middle-aged soccer stars.
Like many people talking and disputing that day in Berlin, they disagreed profoundly on where to look for authority. Einstein preferred logic, Bohr leaned toward analogies. In these tastes neither seemed like the stereotypical, fact-bound scientist. Of course, scientists, like mathematicians, do appreciate logic and some, though hardly all, share a taste for analogies. Only gradually over the coming years would the implications of the difference between Einstein’s mathematical bent and Bohr’s more verbal leanings become apparent.
Bohr was a bit younger than Einstein, six years, and had an even more junior presence. He was shorter, but more gangly, with a long face and thick hair. The pressing issue for him was making sense of atoms and the light they emitted. In his Berlin lecture, Bohr coined the name “correspondence principle” for a technique that allowed him to use classical ideas to solve quantum problems. This rule of thumb let Bohr say, in effect, we have no idea how to solve this puzzle according to quantum rules, but probably older, non-quantum theory is not too far off. Following his analogy that quanta matches classical, he would use old-fashioned calculation methods and come up with answers supported by experiment.
Einstein appreciated the principle’s value. When trying to grasp radically new ideas, old notions provide a stepping stone. His own theories of relativity had shown that in most cases Newton’s old law of gravity works fine and that Newton’s mechanics give wonderfully accurate results at velocities that keep well below the speed of light.
Newton was no longer useful in describing how the universe really works, but in most cases his ideas remain invaluable in calculating what will happen in any particular case. So if Bohr found a way to use well-understood ideas to glimpse what was going on in otherwise badly understood cases, more glory to him.
Most physicists could not understand Bohr’s correspondence principle. Arnold Sommerfeld complained that it worked like a “magic wand”; wave it and Bohr got results that worked but made no sense. The wand’s success made Bohr himself seem like a sorcerer. Try this, he would say, and it worked. Einstein, however, did not judge ideas solely by their success. He was no pragmatist, either in life or in science. He wanted to know why something worked. For him the correspondence principle’s success was a clue, not a solution. The idea that something worked but had no physical meaning told Einstein that the idea was not yet complete. Yet Bohr seemed content to plow forward with his analogy. It was when analogies no longer worked that he became unhappy.
Eventually the two men reached Wilmersdorf, a fast-growing suburb at Berlin’s southern edge. The population was changing so surprisingly that the village would soon build a mosque. The twists and turns from street to street would have demanded some attention, but the two talkers managed to make their way through it.
The radiation mystery that held both men’s attention at the time focused on the light emitted by individual atoms. Just as a steel furnace gives off light, so does a single atom when sufficiently heated. Atoms are not miniature suns, giving off light in all colors, visible and invisible. They emit only a few colors which, when passed through a prism, appear as lines of color instead of a complete rainbow. Atoms also absorb light of specific frequencies, so when sunlight passes through a cloud formed by some element or another, the cloud’s atoms remove some of the colors from the light, making black lines appear in the rainbow. Spectral lines can serve as the atom’s fingerprints or DNA markings. They allow an investigator to know which atoms are present in something too hot or too distant to touch. It is the spectral lines that tell astronomers which elements are found in the stars.
Bohr’s work concentrated on interpreting the atomic spectrum.
Einstein was interested in the spectrum’s light—its light quanta. Trotting along, reaching clumsily for words, Bohr could be desperately inarticulate, but he still made it clear that he did not believe in Einstein’s light quanta. Einstein, meanwhile, could drive his words straight to the heart of the matter, and he used a dry wit to make his case. Years later Bohr recalled Einstein that day using “such picturesque phrases as ‘ghost waves guiding the photons’.” Unfortunately for historical precision, Einstein could not have said exactly that. The term “photon” would not replace “light quanta” in anybody’s vocabulary for many years yet, and Einstein was slow to adopt it even after the word was introduced.
The main route from Wilmersdorf into Berlin was the Kaiser Allee, one of those wide, handsome boulevards that allow eager talkers to move along steadily while ignoring their dignified surroundings. Even so, Einstein would have had to be awake enough to spot their turn onto the narrower, undistinguished Gunzel Strasse.
Except for the word “photon,” Bohr’s recollection of ghost waves makes sense. At that time Einstein was pondering ghost waves or ghost fields that would steer the light quanta. The ghost-wave notion hinted at ways to link particles and waves. A particle might ride a wave like a master surfboarder. In physics, the closest thing to a union of wave and particle is known as a wave packet. Imagine a bull whip in the hands of a Berlin cabaret performer, a man dressed in tight, shiny clothes who is skilled enough to snap the cigarette from his lovely assistant’s mouth. Is that whip a thing of matter or is it energy? For the performer it is matter. He controls it, moving it in his hand like any other piece of solid earth. For the assistant, however, it is pure energy, exploding the cigarette from her lips with a poof. And if, God forbid, the performer slips and the whip kisses his assistant on the cheek, her face will explode in pain and her flesh will dissolve in an instant, leaving a hole where the sudden tide of energy rushed in. Meanwhile, the audience is aroused and confused. It does not know what it saw. The whip began as matter, but as it flew it disappeared into a blur and ended with energy’s crack. So it is left to the scientist to unravel the incident. Filming the whole thing in superslow motion to capture the snap in one thousand frames per second, and looking at the footage, the ob-
server sees something rare. Yes, the whip is solid all right, part of physical matter, but moving along the whip is a single bump of energy. It travels through the whip’s length, twisting and distorting the matter as it goes. That bump is a wave packet, matter and energy, united into a single thing. The bump is as close as a wave ever gets to being a unit of something, moving as a single form through the whip, sticking together, going one place and not another, knocking the cigarette from the assistant’s lip as surely as if it had been hit by a bullet. And yet it is clearly a wave moving through the whip, carrying energy as it flows. That wave packet is duality made briefly visible, displaying the properties of unit and wave together.
Einstein’s street, Haberland, was a single block with modern (for 1920) residences, apartments rather than townhouses. Einstein and Bohr turned into number 5, and passed the doorman. Bohr greeted Elsa with his Father Christmas smile, producing the Danish foods that he and his elfin helper had hauled across miles of greater Berlin. Einstein’s home was a large comfortable space, rather like the slice of Berlin that the two physicists had just crossed. Big and bourgeois, it provided a safe stage that Einstein could use and ignore at the same time. There was an ample dining room where Elsa, Bohr, and Einstein could sit and talk amidst wine and delicacies. Both men enjoyed pipes whenever they could and Einstein’s apartment was a free-smoking-zone. That cloud and the inevitable chatter about physics would have eventually chased Elsa from the scene.
Bohr was not converted to light quanta by Einstein’s ghost fields, and Einstein was not yet ready to convert him. The ghost-wave idea still had many problems. Especially, one could ask, what in nature serves as the cabaret performer’s whip and carries the wave packet? In pre-Einstein days people said that light waves moved through something called the ether, but Einstein had shown there was no ether. The ghost wave had as yet no physical meaning, and until he could take the step from grasping the idea’s utility to understanding how it worked, Einstein was unlikely to insist on his thoughts.
Bohr’s objectives were different. He wanted to understand light in concrete language and proven analogies. In particular, he did not want to contradict the correspondence principle. Powerful as his magic wand had proved, however, it had nothing to say about light quanta.
A radical like Einstein was perplexed by Bohr’s distaste for the anti-classical. Einstein was always ready to change ideas. He had broken with Newtonian concepts of absolute time and space when he proposed his theory of relativity. Then he modified major ideas in that first theory when he developed his second (general) theory of relativity. He knew he would have to revise those ideas still again if he ever succeeded at unifying relativity and electricity. So he was hardly likely to pause because some traditional analogy could not handle the new facts of the matter.
Bohr’s preference for working with established concepts matched the way most physicists worked. Scientists typically are not so ready to trade in old ideas for the latest model. Despite relativity, many physicists, including Einstein’s beloved Lorentz, still believed in an ether that carried light through the universe the way air carries sound from mouth to ear. Most scientists were on Bohr’s side in this light quanta dispute.
This quarrel over light had a long history. Newton’s light experiments sent a beam through complex arrangements of prisms to show that light moved in straight lines. When a prism appeared to bend a light ray, Newton said the light behaved just like a tennis ball striking an oblique racket. Newton’s great English rival had been Robert Hooke. They quarreled constantly; however, Newton outlived his rival and had the pleasure of personally ordering the removal of the late Mr. Hooke’s portrait from the Royal Society wall. From the start, Hooke had disputed Newton’s particle theory by insisting that light was a wave that could be split in two. Bohr and Einstein, running on like rivers in springtime, were continuing a very old dispute.
For Einstein, a man like Bohr, who was devoted to explaining the world in visualizable terms was bound to be attractive; yet the two men’s compasses pointed toward different poles. Bohr was a pacer, an active man who radiated energy, circling like a predator looking for an opportunity to dash in and catch something live. Einstein was more of a still point, a lamp that shed light on whatever came near him. And they disagreed about physics’ heart. Bohr had come to Berlin to give a lecture. In it he publicly refused to quarrel with Einstein, saying simply, “I shall not here discuss the familiar difficulties to which the ‘hypothesis of light quanta’ leads … I shall not consider the problem of
the nature of radiation,” but in his visit with the Einsteins the nature of radiation is exactly what he discussed.
Bohr came at physics with very different tools from his host’s. Einstein sat taking pleasure in the delicacies that his colleague had brought. He wanted very badly to take one more step and understand radiation. Bohr, the pacer, did not look for meaningful steps and had made his discoveries without them. Einstein, even when surrounded by Bohr’s savory gifts, was apt to forget food when he became engrossed in talk. He once ate a pot of caviar without noticing that he was tasting something unusual. Still, the image of Einstein sitting in an overstuffed chair, using picturesque language, while Bohr paced about stringing together clumsy phrases probably gets close to what happened that day.
Strangely, Albert Einstein, the man living in the capital of physics, the conqueror of Isaac Newton, the world’s most acclaimed scientist, was an outsider, while Niels Bohr, the lone physicist from remotest Copenhagen, was the insider. Bohr had studied at Manchester with Rutherford, the greatest experimentalist of that day, and corresponded regularly with him after returning to Denmark. Rutherford had good reason to appreciate Bohr as the man who had firmly established the theory of the empty atom. Rutherford’s atomic theory had not been welcomed at first because philosophers had for thousands of years insisted that atoms, if they existed at all, were solid and indivisible, like a tiny piece of sand. A coal lump was a chunk of smaller, solid bits of coal. Rutherford had proposed, on the basis of his experiments, that atoms were mostly empty. Coal, he said, consists chiefly of empty space lightly sprinkled with electrons orbiting atomic nuclei. Spacious atoms made a radical concept and many physicists were uncertain about them, but Bohr was a loyal Rutherfordian and used the empty atom in his own quantum model. So Bohr’s success became Rutherford’s success, too, and won Bohr a prominent ally.
Einstein’s work did not bring him such automatic constituents. He did not build on the theories of others. The one great exception had been his attention to Planck’s quantum, yet even there he alarmed his potential ally by stressing the idea’s revolutionary side. Planck was no radical and he spent many years looking to see if there was some way he could derevolutionize his quantum. Meanwhile, Germany’s
leading experimentalist and the man whose work provided the original evidence for light quanta was Philipp Lenard, the Nobel Prize winner for 1905. Unfortunately, he detested Einstein’s approach to physics, so there was no Einstein-Lenard bloc the way there was a united Bohr-Rutherford front.
Bohr also attracted allies by generating important work for them to do. He had leapt onto physics’ center stage in 1913 when he published a series of three papers that described the hydrogen atom in quantum terms. His basic idea was beautifully simple. In the hydrogen atom one electron orbits the atom’s nucleus, just as the moon orbits the earth. From time to time—and this is unlike anything our moon does—the electron changes its orbit. When it does so it absorbs or emits energy. How much energy? A quantum’s worth. Other physicists could sink their teeth into Bohr’s idea. They could apply the theory to more complicated atoms, or look at what happens in magnetic and electric fields. All this work showed that Bohr’s theory was powerful, yet incomplete. More work had to be done and physicists in Cambridge, Munich, and Göttingen rose to the challenge.
Einstein’s work was different. He was like Moses come down from the mountain with his laws written on tablets. You could accept them, as happened with relativity, and apply them as needed, or you could refuse them and continue to worship Baal or some other idol. In neither case did schools pop up to sort through any loose ends. Either there were no loose ends, or they were so loose that only an adventurer with Einstein’s abilities and obsessions dared confront them.
Bohr’s more pragmatic approach did not appeal to Einstein. When first asked what he thought of Bohr’s atom, Einstein had replied with a polite dismissal, “Important, if right.” That was the comment he sometimes made when asked about a notion that made no physical sense and had no evidence to support it. Bohr’s theory required the electron to change its orbit instantly. That is to say, the electron is following one path and suddenly is someplace else following a different path. This leap from one place to another without going through the space between—the so-called quantum jump—made no more physical sense than the Virgin’s assumption into heaven. Einstein’s calling it “important if” was typically sly.
Einstein abandoned his scorn for Bohr’s atom, however, when he
learned that the Bohr model could explain a seeming contradiction in Bohr’s own theory. The spectral lines of some hydrogen atoms did not match the predicted pattern. Bohr showed that his equation worked perfectly if you supposed the atom was really a helium atom with a helium nucleus but only one electron. Einstein was stunned by this bonus success and changed his attitude right away, “This is an enormous achievement. The theory of Bohr must then be right.”
Scientists often link two seemingly unrelated facts as Bohr had linked atomic structure with its spectral lines, but scientists usually make the link by finding an unexpected common ground. Newton had found a law that linked the way apples fall to earth with the way the moon orbits the earth. But Bohr made his link without adding such an explanatory idea. His equation used the quantum hυ but the quantum had no meaning in the equation. It explained nothing. It just was. Yet it worked and worked with astounding precision. Even years later, Einstein expressed his wonder at Bohr’s success, writing, “That [quantum radiation’s] insecure and contradictory foundation was sufficient to enable a man of Bohr’s unique instinct and tact to discover the major laws of the spectral lines and of the electron-shells of the atoms together with their significance for chemistry appeared to me like a miracle—and appears to me as a miracle even today. This is the highest form of musicality in the sphere of thought.” No wonder then that Paul Ehrenfest had reacted to Bohr’s success by saying, “If this is the goal, I must give up doing physics.”
Einstein and Bohr’s first meeting had something of the mutual caution you expect whenever a prophet and a poet come together. Each knew the other was doing something related to and yet profoundly unlike his own work. It produced an Alphonse and Gaston exaggeration to their politeness.—After you, Herr Bohr.—No, after you, Herr Professor Einstein. And their mutual thank you letters have the tone of the obsequious headwaiter who has received an overlarge tip.—Einstein: “Not often in life has a person, by his mere presence, given me such joy as you did.”—Bohr: “To me it was one of the greatest pleasures ever to meet you and talk with you.”
Einstein flashed his cards a bit later in his thank you note. “I have learned much from you, especially about your attitude toward scien-
tific matters.” Attitude, not facts or theory. Bohr’s poetic attitude, his radical pragmatism, his search for analogies that worked without deepening physical meaning was as far from Einstein’s realistic attitude as a scientific approach could be. During the coming years their well-mannered dispute would strip scientific imagination naked, showing plainly what it strives for, what authority it recognizes, and what part is a matter of personal taste.