Einstein: His Life and Universe by Walter Isaacson
By now the siren song of a unified theory had come to mesmerize Einstein. “Over it lingers the marble smile of nature,” he told Weyl.8On his steamer ride through Asia, he polished a new paper and, upon arriving in Egypt in February 1923, immediately mailed it to Planck in Berlin for publication. His goal, he declared, was “to understand the gravitational and electromagnetic field as one.”9
Once again, Einstein’s pronouncements made headlines around the world. “Einstein Describes His Newest Theory,” proclaimed the New York Times. And once again, the complexity of his approach was played up. As one of the subheads warned: “Unintelligible to Laymen.”
But Einstein told the newspaper it was not all that complicated. “I can tell you in one sentence what it is about,” the reporter quoted him as saying. “It concerns the relation between electricity and gravitation.” He also gave credit to Eddington, saying, “It is grounded on the theories of the English astronomer.”10
In his follow-up articles that year, Einstein made explicit that his goal was not merely unification but finding a way to overcome the uncertainties and probabilities in quantum theory. The title of one 1923 paper stated the quest clearly: “Does the Field Theory Offer Possibilities for the Solution of Quanta Problems?”11
The paper began by describing how electromagnetic and gravitational field theories provide causal determinations based on partial differential equations combined with initial conditions. In the realm of the quanta, it may not be possible to choose or apply the initial conditions freely. Can we nevertheless have a causal theory based on field equations?
“Quite certainly,” Einstein answered himself optimistically. What was needed, he said, was a method to “overdetermine” the field variables in the appropriate equations. That path of overdetermination became yet another proposed tool that he would employ, to no avail, in fixing what he persisted in calling the “problem” of quantum uncertainty.
Within two years, Einstein had concluded that these approaches were flawed. “My article published [in 1923],” he wrote, “does not reflect the true solution of this problem.” But for better or worse, he had come up with yet another method. “After searching ceaselessly in the past two years, I think I have now found the true solution.”
His new approach was to find the simplest formal expression he could of the law of gravitation in the absence of any electromagnetic field and then generalize it. Maxwell’s theory of electromagnetism, he thought, resulted in a first approximation.12
He now was relying more on math than on physics. The metric tensor that he had featured in his general relativity equations had ten independent quantities, but if it were made nonsymmetrical there would be sixteen of them, enough to accommodate electromagnetism.
But this approach led nowhere, just like the others. “The trouble with this idea, as Einstein became painfully aware, is that there really is nothing in it that ties the 6 components of the electric and magnetic fields to the 10 components of the ordinary metric tensor that describes gravitation,” says University of Texas physicist Steven Weinberg. “A Lorentz transformation or any other coordinate transformation will convert electric or magnetic fields into mixtures of electric and magnetic fields, but no transformation mixes them with the gravitational field.”13
Undaunted, Einstein went back to work, this time trying an approach he called “distant parallelism.” It permitted vectors in different parts of curved space to be related, and from that sprang new forms of tensors. Most wondrously (so he thought), he was able to come up with equations that did not require that pesky Planck constant representing quanta.14
“This looks old-fashioned, and my dear colleagues, and also you, will stick their tongues out because Planck’s constant is not in the equations,” he wrote Besso in January 1929. “But when they have reached the limit of their mania for the statistical fad, they will return full of repentance to the spacetime picture, and then these equations will form a starting point.”15
What a wonderful dream! A unified theory without that rambunctious quantum. Statistical approaches turning out to be a passing mania. A return to the field theories of relativity. Tongue-sticking colleagues repenting!
In the world of physics, where quantum mechanics was now accepted, Einstein and his fitful quest for a unified theory were beginning to be seen as quaint. But in the popular imagination, he was still a superstar. The frenzy that surrounded the publication of his January 1929 five-page paper, which was merely the latest in a string of theoretical stabs that missed the mark, was astonishing. Journalists from around the world crowded around his apartment building, and Einstein was barely able to escape them to go into hiding at his doctor’s villa on the Havel River outside of town. The New York Times had started the drumbeat weeks earlier with an article headlined “Einstein on Verge of Great Discovery: Resents Intrusion.”16
Einstein’s paper was not made public until January 30, 1929, but for the entire preceding month the newspapers printed a litany of leaks and speculation. A sampling of the headlines in the New York Times, for example, include these:
January 12: “Einstein Extends Relativity Theory / New Work Seeks to Unite Laws of Field of Gravitation and Electro-Magnetism / He Calls It His Greatest ‘Book’ / Took Berlin Scientist Ten Years to Prepare”
January 19: “Einstein Is Amazed at Stir Over Theory / Holds 100 Journalists at Bay for a Week / BERLIN—For the past week the entire press as represented here has concentrated efforts on procuring the five-page manuscript of Dr. Albert Einstein’s ‘New Field of Theory.’ Furthermore, hundreds of cables from all parts of the world, with prepaid answers and innumerable letters asking for a detailed description or a copy of the manuscript have arrived.”
January 25 (page 1): “Einstein Reduces All Physics to One Law / The New Electro-Gravitational Theory Links All Phenomena, Says Berlin Interpreter / Only One Substance Also / Hypothesis Opens Visions of Persons Being Able to Float in Air, Says N.Y.U. Professor / BERLIN—Professor Albert Einstein’s newest work, ‘A New Field Theory,’ which will leave the press soon, reduces to one formula the basic laws of relativistic mechanics and of electricity, according to the person who has interpreted it into English.”
Einstein got into the act from his Havel River hideaway. Even before his little paper was published, he gave an interview about it to a British newspaper. “It has been my greatest ambition to resolve the duality of natural laws into unity,” he said. “The purpose of my work is to further this simplification, and particularly to reduce to one formula the explanation of the gravitational and electromagnetic fields. For this reason I call it a contribution to ‘a unified field theory’... Now, but only now, we know that the force that moves electrons in their ellipses about the nuclei of atoms is the same force that moves our earth in its annual course around the sun.”17 Of course, it turned out that he did not know that, nor do we know that even now.
He also gave an interview to Time, which put him on its cover, the first of five such appearances. The magazine reported that, while the world waited for his “abstruse coherent field theory” to be made public, Einstein was plodding around his country hideaway looking “haggard, nervous, irritable.” His sickly demeanor, the magazine explained, was due to stomach ailments and a constant parade of visitors. In addition, it noted, “Dr. Einstein, like so many other Jews and scholars, takes no physical exercise at all.”18
The Prussian Academy printed a thousand copies of Einstein’s paper, an unusually large number. When it was released on January 30, all were promptly sold, and the Academy went back to the printer for three thousand more. One set of pages was pasted in the window of a London department store, where crowds pushed forward to try to comprehend the complex mathematical treatise with its thirty-three arcane equations not tailored for window shoppers. Wesleyan University in Connecticut paid a significant sum for the handwritten manuscript to be deposited as a treasure in its library.
American newspapers were somewhat at a loss. The New York Herald Tribune d
The New York Times, for its part, raised the unified theory to a religious level by sending reporters that Sunday to churches around the city to report on the sermons about it. “Einstein Viewed as Near Mystic,” the headline declared. The Rev. Henry Howard was quoted as saying that Einstein’s unified theory supported St. Paul’s synthesis and the world’s “oneness.” A Christian Scientist said it provided scientific backing for Mary Baker Eddy’s theory of illusive matter. Others hailed it as “freedom advanced” and a “step to universal freedom.”20
Theologians and journalists may have been wowed, but physicists were not. Eddington, usually a fan, expressed doubts. Over the next year, Einstein kept refining his theory and insisting to friends that the equations were “beautiful.” But he admitted to his dear sister that his work had elicited “the lively mistrust and passionate rejection of my colleagues.”21
Among those who were dismayed was Wolfgang Pauli. Einstein’s new approaches “betrayed” his general theory of relativity, Pauli sharply told him, and relied on mathematical formalism that had no relation to physical realities. He accused Einstein of “having gone over to the pure mathematicians,” and he predicted that “within a year, if not before, you will have abandoned that whole distant parallelism, just as earlier you gave up the affine theory.”22
Pauli was right. Einstein gave up the theory within a year. But he did not give up the quest. Instead, he turned his attention to yet another revised approach that would make more headlines but not more headway in solving the great riddle he had set for himself. “Einstein Completes Unified Field Theory,” the New York Times reported on January 23, 1931, with little intimation that it was neither the first nor would it be the last time there would be such an announcement. And then again, on October 26 of that year: “Einstein Announces a New Field Theory.”
Finally, the following January, he admitted to Pauli, “So you were right after all, you rascal.”23
And so it went, for another two decades. None of Einstein’s offerings ever resulted in a successful unified field theory. Indeed, with the discoveries of new particles and forces, physics was becoming less unified. At best, Einstein’s effort was justified by the faint praise from the French mathematician Elie Joseph Cartan in 1931: “Even if his attempt does not succeed, it will have forced us to think about the great questions at the foundation of science.”24
The Great Solvay Debates, 1927 and 1930
The tenacious rearguard action that Einstein waged against the onslaught of quantum mechanics came to a climax at two memorable Solvay Conferences in Brussels. At both he played the provocateur, trying to poke holes in the prevailing new wisdom.
Present at the first, in October 1927, were the three grand masters who had helped launch the new era of physics but were now skeptical of the weird realm of quantum mechanics it had spawned: Hendrik Lorentz, 74, just a few months from death, the winner of the Nobel for his work on electromagnetic radiation; Max Planck, 69, winner of the Nobel for his theory of the quantum; and Albert Einstein, 48, winner of the Nobel for discovering the law of the photoelectric effect.
Of the remaining twenty-six attendees, more than half had won or would win Nobel Prizes as well. The boy wonders of the new quantum mechanics were all there, hoping to convert or conquer Einstein: Werner Heisenberg, 25; Paul Dirac, 25; Wolfgang Pauli, 27; Louis de Broglie, 35; and from America, Arthur Compton, 35. Also there was Erwin Schrödinger, 40, caught between the young Turks and the older skeptics. And, of course, there was the old Turk, Niels Bohr, 42, who had helped spawn quantum mechanics with his model of the atom and become the staunch defender of its counterintuitive ramifications.25
Lorentz had asked Einstein to present the conference’s report on the state of quantum mechanics. Einstein accepted, then balked. “After much back and forth, I have concluded that I am not competent to give such a report in a way that would match the current state of affairs,” he replied. “In part it is because I do not approve of the purely statistical method of thinking on which the new theories are based.” He then added rather plaintively, “I beg you not to be angry with me.”26
Instead, Niels Bohr gave the opening presentation. He was unsparing in his description of what quantum mechanics had wrought. Certainty and strict causality did not exist in the subatomic realm, he said. There were no deterministic laws, only probabilities and chance. It made no sense to speak of a “reality” that was independent of our observations and measurements. Depending on the type of experiment chosen, light could be waves or particles.
Einstein said little at the formal sessions. “I must apologize for not having penetrated quantum mechanics deeply enough,” he admitted at the very outset. But over dinners and late-night discussions, resuming again at breakfast, he would engage Bohr and his supporters in animated discourse that was leavened by affectionate banter about dice-playing deities. “One can’t make a theory out of a lot of ‘maybes,’ ” Pauli recalls Einstein arguing. “Deep down it is wrong, even if it is empirically and logically right.”27
“The discussions were soon focused to a duel between Einstein and Bohr about whether atomic theory in its present form could be considered to be the ultimate solution,” Heisenberg recalled.28 As Ehrenfest told his students afterward, “Oh, it was delightful.”29
Einstein kept lobbing up clever thought experiments, both in sessions and in the informal discussions, designed to prove that quantum mechanics did not give a complete description of reality. He tried to show how, through some imagined contraption, it would be possible, at least in concept, to measure all of the characteristics of a moving particle, with certainty.
For example, one of Einstein’s thought experiments involved a beam of electrons that is sent through a slit in a screen, and then the positions of the electrons are recorded as they hit a photographic plate. Various other elements, such as a shutter to open and close the slit instantaneously, were posited by Einstein in his ingenious efforts to show that position and momentum could in theory be known with precision.
“Einstein would bring along to breakfast a proposal of this kind,” Heisenberg recalled. He did not worry much about Einstein’s machinations, nor did Pauli. “It will be all right,” they kept saying, “it will be all right.” But Bohr would often get worked up into a muttering frenzy.
The group would usually make their way to the Congress hall together, working on ways to refute Einstein’s problem. “By dinner-time we could usually prove that his thought experiments did not contradict uncertainty relations,” Heisenberg recalled, and Einstein would concede defeat. “But next morning he would bring along to breakfast a new thought experiment, generally more complicated than the previous one.” By dinnertime that would be disproved as well.
Back and forth they went, each lob from Einstein volleyed back by Bohr, who was able to show how the uncertainty principle, in each instance, did indeed limit the amount of knowable information about a moving electron. “And so it went for several days,” said Heisenberg. “In the end, we—that is, Bohr, Pauli, and I—knew that we could now be sure of our ground.”30
“Einstein, I’m ashamed of you,” Ehrenfest scolded. He was upset that Einstein was displaying the same stubbornness toward quantum mechanics that conservative physicists had once shown toward relativity. “He now behaves toward Bohr exactly as the champions of absolute simultaneity had behaved toward him.”31
Einstein’s own remarks, given on the last day of the conference, show that the uncertainty principle was not the only aspect of quantum mechanics that concerned him. He was also bothered—and later would become even more so—by the way quan
Einstein may have lost the debates, but he was still the star of the event. De Broglie had been looking forward to meeting him for the first time, and he was not disappointed. “I was particularly struck by his mild and thoughtful expression, by his general kindness, by his simplicity and by his friendliness,” he recalled.
The two hit it off well, because de Broglie was trying, like Einstein, to see if there were ways that the causality and certainty of classical physics could be saved. He had been working on what he called “the theory of the double solution,” which he hoped would provide a classical basis for wave mechanics.
“The indeterminist school, whose adherents were mainly young and intransigent, met my theory with cold disapproval,” de Broglie recalled. Einstein, on the other hand, appreciated de Broglie’s efforts, and he rode the train with him to Paris on his way back to Berlin.
At the Gare du Nord they had a farewell talk on the platform. Einstein told de Broglie that all scientific theories, leaving aside their mathematical expressions, ought to lend themselves to so simple a description “that even a child could understand them.” And what could be less simple, Einstein continued, than the purely statistical interpretation of wave mechanics! “Carry on,” he told de Broglie as they parted at the station. “You are on the right track!”