Einstein: His Life and Universe by Walter Isaacson
Einstein refined his thought experiment so that the falling man was in an enclosed chamber, such as an elevator in free fall above the earth. In this falling chamber (at least until it crashed), the man would feel weightless. Any objects he emptied from his pocket and let loose would float alongside him.
Looking at it another way, Einstein imagined a man in an enclosed chamber floating in deep space “far removed from stars and other appreciable masses.” He would experience the same perceptions of weightlessness. “Gravitation naturally does not exist for this observer. He must fasten himself with strings to the floor, otherwise the slightest impact against the floor will cause him to rise slowly towards the ceiling.”
Then Einstein imagined that a rope was hooked onto the roof of the chamber and pulled up with a constant force. “The chamber together with the observer then begin to move ‘upwards’ with a uniformly accelerated motion.”The man inside will feel himself pressed to the floor. “He is then standing in the chest in exactly the same way as anyone stands in a room of a house on our earth.” If he pulls something from his pocket and lets go, it will fall to the floor “with an accelerated relative motion” that is the same no matter the weight of the object—just as Galileo discovered to be the case for gravity. “The man in the chamber will thus come to the conclusion that he and the chest are in a gravitational field. Of course he will be puzzled for a moment as to why the chest does not fall in this gravitational field. Just then, however, he discovers the hook in the middle of the lid of the chest and the rope which is attached to it, and he consequently comes to the conclusion that the chamber is suspended at rest in the gravitational field.”
“Ought we to smile at the man and say that he errs in his conclusion?” Einstein asked. Just as with special relativity, there was no right or wrong perception. “We must rather admit that his mode of grasping the situation violates neither reason nor known mechanical laws.”20
A related way that Einstein addressed this same issue was typical of his ingenuity: he examined a phenomenon that was so very well-known that scientists rarely puzzled about it. Every object has a “gravitational mass,” which determines its weight on the earth’s surface or, more generally, the tug between it and any other object. It also has an “inertial mass,” which determines how much force must be applied to it in order to make it accelerate. As Newton noted, the inertial mass of an object is always the same as its gravitational mass, even though they are defined differently. This was obviously more than a mere coincidence, but no one had fully explained why.
Uncomfortable with two explanations for what seemed to be one phenomenon, Einstein probed the equivalence of inertial mass and gravitational mass using his thought experiment. If we imagine that the enclosed elevator is being accelerated upward in a region of outer space where there is no gravity, then the downward force felt by the man inside (or the force that tugs downward on an object hanging from the ceiling by a string) is due to inertial mass. If we imagine that the enclosed elevator is at rest in a gravitational field, then the downward force felt by the man inside (or the force that tugs downward on an object hanging from the ceiling by a string) is due to gravitational mass. But inertial mass always equals gravitational mass. “From this correspondence,” said Einstein, “it follows that it is impossible to discover by experiment whether a given system of coordinates is accelerated, or whether . . . the observed effects are due to a gravitational field.”21
Einstein called this “the equivalence principle.”22 The local effects of gravity and of acceleration are equivalent. This became a foundation for his attempt to generalize his theory of relativity so that it was not restricted just to systems that moved with a uniform velocity. The basic insight that he would develop over the next eight years was that “the effects we ascribe to gravity and the effects we ascribe to acceleration are both produced by one and the same structure.”23
Einstein’s approach to general relativity again showed how his mind tended to work:
• He was disquieted when there were two seemingly unrelated theories for the same observable phenomenon. That had been the case with the moving coil or moving magnet producing the same observable electric current, which he resolved with the special theory of relativity. Now it was the case with the differing definitions of inertial mass and gravitational mass, which he began to resolve by building on the equivalence principle.
• He was likewise uncomfortable when a theory made distinctions that could not be observed in nature. That had been the case with observers in uniform motion: there was no way of determining who was at rest and who was in motion. Now it was also, apparently, the case for observers in accelerated motion: there was no way of telling who was accelerating and who was in a gravitational field.
• He was eager to generalize theories rather than settling for having them restricted to a special case. There should not, he felt, be one set of principles for the special case of constant-velocity motion and a different set for all other types of motion. His life was a constant quest for unifying theories.
In November 1907, working against the deadline imposed by the Yearbook of Radioactivity and Electronics, Einstein tacked on a fifth section to his article on relativity that sketched out his new ideas. “So far we have applied the principle of relativity ...only to nonaccelerated reference systems,” he began. “Is it conceivable that the principle of relativity applies to systems that are accelerated relative to each other?”
Imagine two environments, he said, one being accelerated and the other resting in a gravitational field.24 There is no physical experiment you can do that would tell these situations apart. “In the discussion that follows, we shall therefore assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.”
Using various mathematical calculations that can be made about an accelerated system, Einstein proceeded to show that, if his notions were correct, clocks would run more slowly in a more intense gravitational field. He also came up with many predictions that could be tested, including that light should be bent by gravity and that the wavelength of light emitted from a source with a large mass, such as the sun, should increase slightly in what has become known as the gravitational redshift. “On the basis of some ruminating, which, though daring, does have something going for it, I have arrived at the view that the gravitational difference might be the cause of the shift to the red end of the spectrum,” he explained to a colleague. “A bending of light rays by gravity also follows from these arguments.”25
It would take Einstein another eight years, until November 1915, to work out the fundamentals of this theory and find the math to express it. Then it would take another four years before the most vivid of his predictions, the extent to which gravity would bend light, was verified by dramatic observations. But at least Einstein now had a vision, one that started him on the road toward one of the most elegant and impressive achievements in the history of physics: the general theory of relativity.
Winning a Professorship
By the beginning of 1908, even as such academic stars as Max Planck and Wilhelm Wien were writing to ask for his insights, Einstein had tempered his aspirations to be a university professor. Instead, he had begun, believe it or not, to seek work as a high school teacher. “This craving,” he told Marcel Grossmann, who had helped him get the patent-office job, “comes only from my ardent wish to be able to continue my private scientific work under easier conditions.”
He was even eager to go back to the Technical School in Winter-hur, where he had briefly been a substitute teacher. “How does one go about this?” he asked Grossmann. “Could I possibly call on somebody and talk him into the great worth of my admirable person as a teacher and a citizen? Wouldn’t I make a bad impression on him (no Swiss-German dialect, my Semitic appearance, etc.)?” He had written papers that were transforming physics, but he did not know if that would help. “Would there be any point in my stressing my scientific papers on th
He also responded to an advertisement for a “teacher of mathematics and descriptive geometry” at a high school in Zurich, noting in his application “that I would be ready to teach physics as well.” He ended up deciding to enclose all of the papers he had written thus far, including the special theory of relativity. There were twenty-one applicants. Einstein did not even make the list of three finalists.27
So Einstein finally overcame his pride and decided to write a thesis in order to become a privatdozent at Bern. As he explained to the patron there who had supported him, “The conversation I had with you in the city library, as well as the advice of several friends, has induced me to change my decision for the second time and to try my luck with a habilitation at the University of Bern after all.”28
The paper he submitted, an extension of his revolutionary work on light quanta, was promptly accepted, and at the end of February 1908, he was made a privatdozent. He had finally scaled the walls, or at least the outer wall, of academe. But his post neither paid enough nor was important enough for him to give up his job at the patent office. His lectures at the University of Bern thus became simply one more thing for him to do.
His topic for the summer of 1908 was the theory of heat, held on Tuesday and Saturday at 7 a.m., and he initially attracted only three attendees: Michele Besso and two other colleagues who worked at the postal building. In the winter session he switched to the theory of radiation, and his three coworkers were joined by an actual student named Max Stern. By the summer of 1909, Stern was the only attendee, and Einstein canceled his lecturing. He had, in the meantime, begun to adopt his professorial look: both his hair and clothing became a victim of nature’s tendency toward randomness.29
Alfred Kleiner, the University of Zurich physics professor who helped Einstein get his doctorate, had encouraged him to pursue the privatdozent position.30 He also had waged a long effort, which succeeded in 1908, to convince the Zurich authorities to increase the university’s stature by creating a new position in theoretical physics. It was not a full professorship; instead, it was an associate professorship under Kleiner.
It was the obvious post for Einstein, but there was one obstacle. Kleiner had another candidate in mind: his assistant Friedrich Adler, a pale and passionate political activist who had become friends with Einstein when they were both at the Polytechnic. Adler, whose father was the leader of the Social Democratic Party in Austria, was more disposed to political philosophy than theoretical physics. So he went to see Kleiner one morning in June 1908, and the two of them concluded that Adler was not right for the job and Einstein was.
In a letter to his father, Adler recounted the conversation and said that Einstein “had no understanding how to relate to people” and had been “treated by the professors at the Polytechnic with outright contempt.” But Adler said he deserved the job because of his genius and was likely to get it. “They have a bad conscience over how they treated him earlier. The scandal is being felt not only here but in Germany that such a man would have to sit in the patent office.”31
Adler made sure that the Zurich authorities, and for that matter everyone else, knew that he was officially stepping aside for his friend. “If it is possible to get a man like Einstein for our university, it would be absurd to appoint me,” he wrote. That resolved the political issue for the councilor in charge of education, who was a partisan Social Democrat. “Ernst would have liked Adler, since he was a fellow party member,” Einstein explained to Michele Besso. “But Adler’s statements about himself and me made it impossible.”32
So, at the end of June 1908, Kleiner traveled from Zurich to Bern to audit one of Einstein’s privatdozent lectures and, as Einstein put it, “size up the beast.” Alas, it was not a great show. “I really did not lecture divinely,” Einstein lamented to a friend, “partly because I was not well prepared, partly because being investigated got on my nerves a bit.” Kleiner sat listening with a wrinkled brow, and after the lecture he informed Einstein that his teaching style was not good enough to qualify him for the professorship. Einstein calmly claimed that he considered the job “quite unnecessary.”33
Kleiner went back to Zurich and reported that Einstein “holds monologues” and was “a long way from being a teacher.” That seemed to end his chances. As Adler informed his powerful father, “The situation has therefore changed, and the Einstein business is closed.” Einstein pretended to be sanguine. “The business with the professorship fell through, but that’s all right with me,” he wrote a friend. “There are enough teachers even without me.”34
In fact Einstein was upset, and he became even more so when he heard that Kleiner’s criticism of his teaching skills was being widely circulated, even in Germany. So he wrote to Kleiner, angrily reproaching him “for spreading unfavorable rumors about me.” He was already finding it difficult to get a proper academic job, and Kleiner’s assessment would make it impossible.
There was some validity to Kleiner’s criticism. Einstein was never an inspired teacher, and his lectures tended to be regarded as disorganized until his celebrity ensured that every stumble he made was transformed into a charming anecdote. Nevertheless, Kleiner relented. He said that he would be pleased to help him get the Zurich job if he could only show “some teaching ability.”
Einstein replied by suggesting that he come to Zurich to give a full-fledged (and presumably well-prepared) lecture to the physics society there, which he did in February 1909. “I was lucky,” Einstein reported soon after. “Contrary to my habit, I lectured well on that occasion.”35 When he went to call on Kleiner afterward, the professor intimated that a job offer would soon follow.
A few days after Einstein returned to Bern, Kleiner provided his official recommendation to the University of Zurich faculty. “Einstein ranks among the most important theoretical physicists and has been recognized as such since his work on the relativity principle,” he wrote. As for Einstein’s teaching skills, he said as politely as possible that they were ripe for improvement: “Dr. Einstein will prove his worth also as a teacher, because he is too intelligent and too conscientious not to be open to advice when necessary.”36
One issue was Einstein’s Jewishness. Some faculty members considered this a potential problem, but they were assured by Kleiner that Einstein did not exhibit the “unpleasant peculiarities” supposedly associated with Jews. Their conclusion is a revealing look at both the anti-Semitism of the time and the attempts to rise above it:
The expressions of our colleague Kleiner, based on several years of personal contact, were all the more valuable for the committee as well as for the faculty as a whole since Herr Dr. Einstein is an Israelite and since precisely to the Israelites among scholars are inscribed (in numerous cases not entirely without cause) all kinds of unpleasant peculiarities of character, such as intrusiveness, impudence, and a shopkeeper’s mentality in the perception of their academic position. It should be said, however, that also among the Israelites there exist men who do not exhibit a trace of these disagreeable qualities and that it is not proper, therefore, to disqualify a man only because he happens to be a Jew. Indeed, one occasionally finds people also among non-Jewish scholars who in regard to a commercial perception and utilization of their academic profession develop qualities that are usually considered as specifically Jewish. Therefore, neither the committee nor the faculty as a whole considered it compatible with its dignity to adopt anti-Semitism as a matter of policy.37
The secret faculty vote in late March 1909 was ten in favor and one abstention. Einstein was offered his first professorship, four years after he had revolutionized physics. Unfortunately, his proposed salary was less than what he was making at the patent office, so he declined. Finally, the Zurich authorities raised their offer, and Einstein accepted. “So, now I too am an official member of the guild of whores,” he exulted to a colleague.38
One person who saw a newspaper notice about Einstein’s appointment was a Basel housewife named Anna Meyer-Schmid.
In response to a congratulatory postcard from her, Einstein replied with a polite and mildly suggestive letter. “I probably cherish the memory of the lovely weeks that I was allowed to spend near you in the Paradies more than you do,” he wrote. “So now I’ve become such a big schoolmaster that my name is even mentioned in the newspapers. But I have remained a simple fellow.” He noted that he had married his college friend Mari, but he gave her his office address. “If you ever happen to be in Zurich and have time, look me up there; it would give me great pleasure.”40
Whether or not Einstein intended his response to hover uncertainly between innocence and suggestiveness, Anna’s eyes apparently snapped it into the latter position. She wrote a letter back, which Mari intercepted. Her jealousy aroused, Mari then wrote a letter to Anna’s husband claiming (wishfully more than truthfully) that Einstein was outraged by Anna’s “inappropriate letter” and brazen attempt to rekindle a relationship.
Einstein ended up having to calm matters with an apology to the husband. “I am very sorry if I have caused you distress by my careless behavior,” he wrote. “I answered the congratulatory card your wife sent me on the occasion of my appointment too heartily and thereby re-awakened the old affection we had for each other. But this was not done with impure intentions. The behavior of your wife, for whom I have the greatest respect, was totally honorable. It was wrong of my wife—and excusable only on account of extreme jealousy—to behave—without my knowledge—the way she did.”