Light Can Be Wave and Particle
Also at the end of the summer of 1909, Einstein was invited to address the annual Naturforscher conference, the preeminent meeting of German-speaking scientists, which was held that year in Salzburg. Organizers had put both relativity and the quantum nature of light on the agenda, and they expected him to speak on the former. Instead, Einstein decided that he preferred to emphasize what he considered the more pressing issue: how to interpret quantum theory and reconcile it with the wave theory of light that Maxwell had so elegantly formulated.
After his “happiest thought” at the end of 1907 about how the equivalence of gravity and acceleration might lead to a generalization of relativity theory, Einstein had put that subject aside to focus instead on what he called “the radiation problem” (i.e., quantum theory). The more he thought about his “heuristic” notion that light was made up of quanta, or indivisible packets, the more he worried that he and Planck had wrought a revolution that would destroy the classical foundations of physics, especially Maxwell’s equations. “I have come to this pessimistic view mainly as a result of endless, vain efforts to interpret . . . Planck’s constant in an intuitive way,” he wrote a fellow physicist early in 1908. “I even seriously doubt that it will be possible to maintain the general validity of Maxwell’s equations.”44 (As it turned out, his love of Maxwell’s equations was well placed. They are among the few elements of theoretical physics to remain unchanged by both the relativity and quantum revolutions that Einstein helped launch.)
When Einstein, still not officially a professor, arrived at the Salzburg conference in September 1909, he finally met Max Planck and other giants that he had known only through letters. On the afternoon of the third day, he stepped in front of more than a hundred famed scientists and delivered a speech that Wolfgang Pauli, who was to become a pioneer of quantum mechanics, later pronounced “one of the landmarks in the development of theoretical physics.”
Einstein began by explaining how the wave theory of light was no longer complete. Light (or any radiation) could also be regarded, he said, as a beam of particles or packets of energy, which he said was akin to what Newton had posited. “Light has certain basic properties that can be understood more readily from the standpoint of the Newtonian emission theory than from the standpoint of the wave theory,” he declared. “I thus believe that the next phase of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and of the emission theories of light.”
Combining particle theory with wave theory, he warned, would bring “a profound change.” This was not a good thing, he feared. It could undermine the certainties and determinism inherent in classical physics.
For a moment, Einstein mused that perhaps such a fate could be avoided by accepting Planck’s more limited interpretation of quanta: that they were features only of how radiation was emitted and absorbed by a surface rather than a feature of the actual light wave as it propagated through space. “Would it not be possible,” he asked, “to retain at least the equations for the propagation of radiation and conceive only the processes of emission and absorption differently?” But after comparing the behavior of light to the behavior of gas molecules, as he had done in his 1905 light quanta paper, Einstein concluded that, alas, this was not possible.
As a result, Einstein said, light must be regarded as behaving like both an undulating wave and a stream of particles. “These two structural properties simultaneously displayed by radiation,” he declared at the end of his talk, “should not be considered as mutually incompatible.”45
It was the first well-conceived promulgation of the wave-particle duality of light, and it had implications as profound as Einstein’s earlier theoretical breakthroughs. “Is it possible to combine energy quanta and the wave principles of radiation?” he merrily wrote to a physicist friend. “Appearances are against it, but the Almighty—it seems—managed the trick.”46
A vibrant discussion followed Einstein’s speech, led by Planck himself. Still unwilling to embrace the physical reality underlying the mathematical constant that he had devised nine years earlier, or to accept the revolutionary ramifications envisioned by Einstein, Planck now played protector of the old order. He admitted that radiation involved discrete “quanta, which are to be conceived as atoms of action.” But he insisted that these quanta existed only as part of the process of radiation being emitted or absorbed. “The question is where to look for these quanta,” he said. “According to Mr. Einstein, it would be necessary to conceive that free radiation in a vacuum, and thus the light waves themselves consist of atomistic quanta, and hence force us to give up Maxwell’s equations. This seems to me a step that is not yet necessary.”47
Within two decades, Einstein would assume a similar role as protector of the old order. Indeed, he was already looking for ways out of the eerie dilemmas raised by quantum theory. “I am very hopeful that I will solve the radiation problem, and that I will do so without light quanta,” he wrote a young physicist he was working with.48
It was all too mystifying, at least for the time being. So as he moved up the professorial ranks in the German-speaking universities of Europe, he turned his attention back to the topic that was uniquely his own, relativity, and for a while became a refugee from the wonderland of the quanta. As he lamented to a friend, “The more successes the quantum theory enjoys, the sillier it looks.”49
CHAPTER EIGHT
THE WANDERING PROFESSOR
1909–1914
Zurich, 1909
As a self-assured 17-year-old, Einstein had enrolled at the Zurich Polytechnic and met Mileva Mari, the woman he would marry. Now, in October 1909, at age 30, he was returning to that city to take up his post as a junior professor at the nearby University of Zurich.
Their homecoming restored, at least temporarily, some of the romance to their relationship. Mari was thrilled to be back in their original nesting ground, and by the end of their first month there she became pregnant again.
The apartment they rented was in a building where, they happily discovered, Friedrich Adler and his wife lived, and the couples became even closer friends. “They run a bohemian household,” Adler wrote his father approvingly. “The more I talk to Einstein, the more I realize that my favorable opinion of him was justified.”
The two men discussed physics and philosophy most evenings, often retreating to the attic of the three-story building so they would not be disturbed by children or spouses. Adler introduced Einstein to the work of Pierre Duhem, whose 1906 book La Théorie Physique Adler had just published in German. Duhem offered a more holistic approach than Mach did to the relationship between theories and experimental evidence, one that seemed to influence Einstein as he staked out his own philosophy of science.1
Adler particularly respected Einstein’s “most independent” mind. There was, he told his father, a nonconformist streak in Einstein that reflected an inner security but not an arrogance. “We find ourselves in agreement on questions that the majority of physicists would not even understand,” Adler boasted.2
Einstein tried to persuade Adler to focus on science rather than be enticed into politics. “Be a little patient,” he said. “You will certainly be my successor in Zurich one day.” (Einstein was already assuming that he would move on to a more prestigious university.) But Adler ignored the advice and decided to become an editor at the Social Democratic Party newspaper. Loyalty to a party, Einstein felt, meant surrendering some independence of thought. Such conformity confounded him. “How an intelligent man can subscribe to a party I find a complete mystery,” Einstein later lamented about Adler.3
Einstein was also reunited with his former classmate and note-taker Marcel Grossmann, who had helped him get his job at the patent office and was now a professor of math at their old Polytechnic. Einstein would often visit Grossmann after lunch for help with the complex geometry and calculus he needed to extend relativity into a more general field theory.
/> Einstein was even able to forge a friendship with the other distinguished math professor at the Polytechnic, Adolf Hurwitz, whose classes he had often skipped and who had spurned his plea for a job. Einstein became a regular at the Sunday music recitals at Hurwitz’s home. When Hurwitz told him during a walk one day that his daughter had been given a math homework problem she did not understand, Einstein showed up that afternoon to help her solve it.4
As Kleiner predicted, Einstein’s teaching talents improved. He was not a polished lecturer, but instead used informality to his advantage. “When he took his chair in shabby attire with trousers too short for him, we were skeptical,” recalled Hans Tanner, who attended most of Einstein’s Zurich lectures. Instead of prepared notes, Einstein used a card-sized strip of paper with scribbles. So the students got to watch him develop his thoughts as he spoke. “We obtained some insight into his working technique,” said Tanner. “We certainly appreciated this more than any stylistically perfect lecture.”
At each step of the way, Einstein would pause and ask the students if they were following him, and he even permitted interruptions. “This comradely contact between teacher and student was, at that time, a rare occurrence,” according to Adolf Fisch, another who attended the lectures. Sometimes he would take a break and let the students gather around him for casual conversation. “With an impulsiveness and naturalness he would take students by the arm to discuss things,” recalled Tanner.
During one lecture, Einstein found himself momentarily stumped about the steps needed to complete a calculation. “There must be some silly mathematical transformation that I can’t find for a moment,” he said. “Can one of you gentlemen see it?” Not surprisingly, none of them could. So Einstein continued: “Then leave a quarter of a page. We won’t lose any time.”Ten minutes later, Einstein interrupted himself in the middle of another point and exclaimed, “I’ve got it.” As Tanner later marveled, “During the complicated development of his theme he had still found time to reflect upon the nature of that particular mathematical transformation.”
At the end of many of his evening lectures, Einstein would ask, “Who’s coming to the Café Terasse?” There, with an informal cadre on a terrace overlooking the Limmat River, they would talk until closing time.
On one occasion, Einstein asked if anyone wanted to come back to his apartment. “This morning I received some work from Planck in which there must be a mistake,” he said. “We could read it together.” Tanner and another student took him up on the offer and followed him home. There they all pored over Planck’s paper. “See if you can spot the fault while I make some coffee,” he said.
After a while, Tanner replied, “You must be mistaken, Herr Professor, there is no error in it.”
“Yes, there is,” Einstein said, pointing to some discrepancies in the data, “for otherwise that and that would become that and that.” It was a vivid example of Einstein’s great strength: he could look at a complex mathematical equation, which for others was merely an abstraction, and picture the physical reality that lay behind it.
Tanner was astounded. “Let’s write to Professor Planck,” he suggested, “and tell him of the mistake.”
Einstein had by then become slightly more tactful, especially with those he placed on a pedestal, such as Planck and Lorentz. “We won’t tell him he made a mistake,” he said. “The result is correct, but the proof is faulty. We’ll simply write and tell him how the real proof should run. The main thing is the content, not the mathematics.”5
Despite his work on his machine to measure electrical charges, Einstein had become a confirmed theorist rather than experimental physicist. When he was asked during his second year as a professor to supervise laboratory work, he was dismayed. He hardly dared, he told Tanner, “pick up a piece of apparatus for fear it might blow up.” To another eminent professor he confided, “My fears regarding the laboratory were rather well founded.”6
As he was finishing his first academic year at Zurich, in July 1910, Mari gave birth, again with difficulty, to their second son, named Eduard and called Tete. She was ill for weeks afterward. Her doctor, contending that she was overworked, suggested that Einstein find a way to make more money and pay for a maid. Mari was annoyed and protective. “Isn’t it clear to anyone that my husband works himself half dead?” she said. Instead, her mother came down from Novi Sad to help.7
Throughout his life, Einstein would sometimes appear aloof toward his two sons, especially Eduard, who suffered from increasingly severe mental illness as he grew older. But when they were young, he tended to be a good father. “When my mother was busy around the house, father would put aside his work and watch over us for hours, bouncing us on his knee,” Hans Albert later recalled. “I remember he would tell us stories—and he often played the violin in an effort to keep us quiet.”
One of his strengths as a thinker, if not as a parent, was that he had the ability, and the inclination, to tune out all distractions, a category that to him sometimes included his children and family. “Even the loudest baby-crying didn’t seem to disturb Father,” Hans Albert said. “He could go on with his work completely impervious to noise.”
One day his student Tanner came for a visit and found Einstein in his study poring over a pile of papers. He was writing with his right hand and holding Eduard with his left. Hans Albert was playing with toy bricks and trying to get his attention. “Wait a minute, I’ve nearly finished,” Einstein said, as he handed Eduard to Tanner and kept scribbling his equations. “It gave me,” said Tanner, “a glimpse into his immense powers of concentration.”8
Prague, 1911
Einstein had been in Zurich less than six months when he received, in March 1910, a solicitation to consider a more prestigious job: a full professorship at the German part of the University of Prague. Both the university and the academic position were a step up; however, moving from the familiar and friendly Zurich to the less congenial Prague would be disruptive for his family. For Einstein, the professional considerations outweighed the personal ones.
He was again going through difficult periods at home. “The bad mood that you noticed in me had nothing to do with you,” he wrote to his mother, who was now living in Berlin. “To dwell on the things that depress or anger us does not help in overcoming them. One must knock them down alone.”
His scientific work, on the other hand, was giving him great pleasure, and he expressed excitement about his possible new opportunity. “It is most probable that I will be offered the position of full professor at a large university with a significantly better salary than I now have.”9
When word of Einstein’s possible move spread in Zurich, fifteen of his students, led by Hans Tanner, signed a petition urging officials there “to do your utmost to keep this outstanding researcher and teacher at our university.” They stressed the importance of having a professor in “this newly created discipline” of theoretical physics, and they extolled him personally in effusive terms. “Professor Einstein has an amazing talent for presenting the most difficult problems of theoretical physics so clearly and so comprehensibly that it is a great delight for us to follow his lectures, and he is so good at establishing a perfect rapport with his audience.”10
The Zurich authorities were so eager to keep him that they raised his salary from its current 4,500 francs, which was the same as he made as a patent examiner, to 5,500 francs. Those attempting to lure him to Prague, on the other hand, were having a more difficult time.
The faculty department at Prague had settled on Einstein as its first choice and forwarded the recommendation to the education ministry in Vienna. (Prague was then part of the Austro-Hungarian Empire, and such an appointment had to be approved by Emperor Franz Joseph and his ministers.) The report was accompanied by the highest possible recommendation from the best possible authority, Max Planck. Einstein’s theory of relativity “probably exceeds in audacity everything that has been achieved so far in speculative science,” Planck proclaimed. “This principle
has brought about a revolution in our physical picture of the world that can be compared only to that produced by Copernicus.” In a comment that might later have seemed prescient to Einstein, Planck added, “Non-Euclidean geometry is child’s play by comparison.”11
Planck’s imprimatur should have been enough. But it wasn’t. The ministry decided that it preferred the second-place candidate, Gustav Jaumann, who had two advantages: he was Austrian, and he was not Jewish. “I did not get the call to Prague,” Einstein lamented to a friend in August. “I was proposed by the faculty, but because of my Semitic origin the ministry did not approve.”
Jaumann, however, soon discovered that he was the faculty’s second choice, and he erupted. “If Einstein has been proposed as the first choice because of the belief that he has greater achievements to his credit,” he declared, “then I will have nothing to do with a university that chases after modernity and does not appreciate merit.” So by October 1910, Einstein could confidently declare that his own appointment was “almost certain.”
There was one final hurdle, also dealing with religion. Being a Jew was a disadvantage; being a nonbeliever who claimed no religion was a disqualifier. The empire required that all of its servants, including professors, be a member of some religion. On his official forms, Einstein had written that he had none. “Einstein is as unpractical as a child in cases like this,” Friedrich Adler’s wife noted.
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