As soon as Heisenberg returned to the German mainland, he told everyone what he had achieved. So long as one didn’t worry about tracking the final details inside an atom, he explained, remarkably accurate predictions about the light it would spray out could be made. Since the great Isaac Newton in the seventeenth century, science had been built on the assumption that, at least in principle, clarity could be found about every process we observe. Heisenberg seemed to be saying that didn’t have to be true.
Max Born accepted the new approach, pretty much, because Heisenberg’s results were so accurate. Einstein didn’t, but he was friendly with the whole Born family, and so had to tread carefully. He wrote to Born’s wife, with well-chosen ambiguity, “The Heisenberg-Born concepts leave us all breathless, and have made a deep impression.”
Einstein also was ambiguous because, though he objected to the way Heisenberg seemed willing to put causality to one side, he knew that physicists often missed out on important discoveries when they were too set in their ways. In 1895, for example, the German Wilhelm Röntgen had described the strange phenomenon of X-rays, and physicists who refused to accept the finding were soon proved wrong. But judgment has to be involved. In 1903 a distinguished French physicist described the equally strange new phenomenon of what he called N-rays, yet within two years they were shown to be just an experimental flaw, and physicists who had not resisted were proved wrong. Einstein wasn’t going to give a final public statement about Heisenberg’s work yet.
The Borns, for their part, suspected that Einstein was simply being polite. When Max Born probed, Einstein explained to him more of what he believed: “Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing.” To a closer friend, Einstein was even blunter: “Heisenberg has laid a big quantum egg. In Göttingen they believe in it. I don’t.”
Soon Born had to tell Heisenberg that Einstein wasn’t persuaded—and Heisenberg couldn’t bear that. His friends knew that although he tried hard to give the impression of being in control of himself, he was always on the edge of becoming frantic when he felt stressed. This was especially noticeable when he would start slamming out romantic pieces on the piano with a terrifying intensity. He loved being dominant, strong, triumphant. His insight into atoms should have been the achievement of a lifetime. Now the most respected thinker in the world was saying his insight was wrong.
Perhaps the answer, Heisenberg decided—much as George Lemaître would later do—would be to speak directly to Einstein and clear everything up in person.
SIXTEEN
Uncertainty of the Modern Age
HEISENBERG HAD NO IDEA how deeply Einstein opposed the theory he’d come up with that night on Heligoland.
To Einstein, probabilities were just a sign of gaps in our understanding. They were temporary fixes that, when science caught up, would be replaced by clearer understanding. After all, Uranus’s orbit had been a mystery until nineteenth-century astronomers had worked out how the unseen planet of Neptune was tugging on it. Infections had been a mystery until microscopes and other lab techniques became sophisticated enough to identify microbes.
Einstein believed that whatever was waiting in the outside world to be discovered couldn’t depend on the quirks of who the observer was or how he or she traveled. He’d had hints of that objective reality sitting contentedly with his pipe and a book at the cafés in Zurich, ignoring the busy student life around him, or sitting equally contentedly with a work pad and his pipe amidst the pandemonium of toddlers and guests in his and Marić’s first apartment in Bern. It arose even in the steady bemusement with which he’d viewed his great fame after 1919. Events seemed to rush past, to confuse us, to be chaotic: languages and cultures and children and words. But that was just appearances. Studied carefully enough, they were always very exact, very certain. That’s why he was proud, but not surprised, to have found the certainties of relativity.
Quantum mechanics, however, did not fit with that worldview.
There was good historical precedent for Einstein’s perspective. One of his greatest heroes was the Dutch Jewish philosopher Spinoza, and although he had lived three hundred years before, Einstein took solace in the fact that Spinoza, too, “was convinced of the causal dependence of all phenomena, at a time when the success accompanying the effort to achieve [that knowledge] was still quite modest.” Had Spinoza been able to live long enough, he would have seen our technological civilization find precisely the causal links that he had imagined to be there and use them to create our cities, our trains, and our aircraft.
There was an even deeper reason that Einstein was so enamored of the concept of causality. He didn’t believe in the tenets of revealed religion—he didn’t believe there was a divine force behind Moses’s tablets on Mount Sinai; he didn’t believe in the resurrection of any rabbi, however wise, from Galilee—but that is far from meaning he wasn’t religious. He thought that being an atheist was presumptuous, and he was awed at the intelligence manifested in natural laws. “This feeling is the guiding principle of [a scientist’s] life and work,” he wrote, “in so far as he succeeds in keeping himself from the shackles of selfish desire.”
So the very core of Einstein’s intellectual and spiritual life depended on the premise that all underlying reality was clear, exact, understandable. He was not going to believe that the universe was fundamentally unknowable.
In our earlier metaphor of actors changing clothes at a theater, Heisenberg would have been convinced that what had happened backstage was inherently a blur. From Einstein’s perspective, that was wrong. Obviously, each individual actor had to be changing his or her costume. It might be hard for us to see, peering into the poorly lit changing areas, but the fact that they all came out with different costumes proved that this had happened. The way electrons moved inside an atom, Einstein felt, was just the same.
Since Heisenberg knew little of Einstein’s deeper feelings, he still felt the great man could be persuaded. In early 1926, Heisenberg was invited to give a lecture in Berlin, which he knew Einstein would attend. Afterward, they fell into discussion, and Einstein invited him home. After exchanging pleasantries—Einstein asking about Heisenberg’s favorite teacher, Arnold Sommerfeld, whom he knew well—Heisenberg mentioned what was bothering him.
In the 1916 work on light hitting atoms, Heisenberg pointed out, Einstein hadn’t tried to describe what was going on inside individual atoms. He’d merely described what went in and then what came out. Heisenberg explained that this was exactly what he’d been trying to do in his great nighttime breakthrough on the island of Heligoland. Even so, Heisenberg remembered later, “to my astonishment, Einstein was not at all satisfied with the argument.”
“Perhaps I did use such philosophy earlier,” Einstein answered him, “. . . but it is nonsense all the same.” The question of what was observable in relativity was very different from what was observable in the micro-world, he explained. What had taken place in 1916 was just preliminary—a calculation that would account for what was observed. He still believed that underneath it all, electrons really did exist, and moved in some clear ways. He’d limited himself to input/output descriptions simply because with the technology he had, there was no way to get more details. In the future, that would clearly improve.
Einstein was blunter with people he knew well. He always had been. During a stay at the lodgings of his successor at the German University in Prague, Philipp Frank, who had become a close friend, Einstein once politely corrected Mrs. Frank’s inadequate attempt to fry liver in water, noting that fat or butter had a higher boiling point, and so would transmit heat more effectively. Ever after, the family called frying their meat in oil an example of “Einstein’s theory.” In one of their conversations, Philipp Frank made the same point that Heisenberg had: hadn’t it been Einstein himself who had popularized the approach of just looking at external details? Einstein answered, sardonically, “A good joke should not be repeated too often.”
To Michele Besso, Einstein was even more dismissive of Heisenberg’s theories. Heisenberg’s elaborate rules for transforming lists of what went into an atom into lists of what was observed coming out was, Einstein claimed, “a veritable witches’ multiplication table . . . Exceedingly clever, and [yet] because of its great complexity safe against refutation as being incorrect.”
Word of the august physicist’s objections began to spread. Perhaps Einstein was right. Heisenberg, after all, was proposing a total shift in what everyone had believed. What if his Heligoland listing of inputs and outputs really was just a temporary gimmick—a computational shortcut—to be used until a better description came up?
IN THE PERIOD during which Heisenberg first confronted Einstein, the tables showed signs of turning in the elder scientist’s favor. In January 1926, a graceful Austrian researcher, Erwin Schrödinger, had published a conventional, classical-style equation that, to many, no longer appeared to require relegating the movements inside an atom to the realms of unseeable mysteries. If his equation was correct, it seemed that it would return quantum mechanics to the strictly causal realm of physics that Newton and Einstein inhabited. Were that to be so, Schrödinger would be undermining Heisenberg’s insistence that only a fundamentally new view—one that didn’t even try to describe the inside of the atom in crisp, mechanical terms—could be accurate.
Erwin Schrödinger, about two decades after his 1926 breakthrough
Heisenberg tried to fight back. But whenever he attempted to best Schrödinger in a debate, somehow he got flummoxed. Schrödinger was more than a decade older than Heisenberg and had a Viennese superiority and calm that drove Heisenberg to distraction. (He also had a private life that the Boy Scout–like Heisenberg could never grasp. Schrödinger had worked out his equation over Christmas 1925 at a luxurious Alpine resort—accompanied by one of the various mistresses his wife was happy for him to travel with—delicately placing a single pearl in each of his ears when he needed quiet.)
Heisenberg had a dilemma: if you’ve made a great discovery and then been disbelieved, what do you do next? In desperation, he turned back to his most central belief. He was being criticized for saying that it was a waste of effort to try to trace the clear paths that electrons follow within atoms. Well, that’s what he would face directly. He would go further than simply asserting one couldn’t measure the behavior of those electrons; he would prove it.
Besides being scorned by Einstein and shown up by Schrödinger, Heisenberg had one other humiliation from his past that motivated him and had prepared him particularly well for this new challenge. Back in his student days, under Sommerfeld in Munich, he had been called to take his Ph.D. oral exams—the final step before obtaining his doctorate—at the unheralded age of twenty-one. Since Sommerfeld was the physics department’s respected chairman and Heisenberg was his prize student, everyone assumed the oral exams would be a formality. But the faculty at Munich also included the elderly experimentalist Professor Willy Wien. Heisenberg had been registered for a course under Wien just before the exams, but he had skipped it almost entirely. He had never liked experimental work, he was excited about the forthcoming degree ceremonies, and anyway, he knew he was brighter than anyone else at the university. What could a harmless old experimentalist possibly do to hurt him?
Wien recognized he was no longer as respected as he had once been, and he’d also had a much harder life than Heisenberg—being raised on a rural estate that his parents had been forced to sell after a drought; repeatedly dropping out of school. He also believed that experimentation was the true basis of all advances in science. Sommerfeld, the theorist, had all the glory now, and Wien couldn’t attack him—he was too powerful. Sommerfeld’s student, however, would be different.
When Heisenberg entered the seminar room in the Theoretical Physics Institute at 5 p.m. for his oral exams, there Wien was, sitting beside a now slightly apprehensive Sommerfeld. Wien began the questioning mildly enough, asking Heisenberg how a certain new electronic laboratory device worked. Heisenberg didn’t know. Sommerfeld tried to switch the topic, raising theoretical questions where Heisenberg’s knowledge of mathematics would let him do well. Wien waited till they were done and then returned to his polite questions: Could Mr. Heisenberg perhaps now tell him how a radio circuit worked? Heisenberg tried to figure it out but then got lost, for these were details he’d never studied. Then Wien asked how an oscilloscope worked. Finally, Wien asked: Could Heisenberg even tell him how an ordinary microscope worked?
Heisenberg stumbled out of the seminar room two hours later, his face flushed, unwilling to speak to anyone. He told his father that his career in physics was over. Only the intervention of Sommerfeld—whose top grade for Heisenberg balanced Wien’s equivalent of a failing F—allowed Heisenberg to get his degree.
That had been in 1923. Now, these few years later, after meeting Einstein in 1926, if there was one thing Heisenberg had gone over and over again in his mind, it was how to calculate how much a microscope could magnify what it was aimed at, and how exactly that process worked. This was the understanding he would use to show that no one could ever follow the detailed paths an electron took within an atom. It was also a good way to refute Schrödinger: “The more I think about the physical portion of Schrödinger’s theory, the more repulsive I find it,” Heisenberg confided to his friend Wolfgang Pauli later in 1926. And to his mentor Bohr: “I got the idea of investigating the possibility of determining the position of a particle with the aid of a gamma-ray microscope.” Now he proceeded like no one had before.
If Einstein really wanted to see an electron, Heisenberg reasoned, he’d have to shine a light wave or some other energy down on the atom to light up the electron in there. But electrons are small. If the blast of light was strong, it would overpower the electron, jarring it out of position. Yet if the blast of light was weak, it couldn’t be precisely aimed enough to see the tiny electron. It’s much the way that, however carefully you use a gauge to measure the air pressure in a car’s tire, you are inevitably letting a little bit of air out, so that the very act of taking the measurement makes your reading incorrect.
Heisenberg managed to prove that any super-microscope had to suffer from the same problems: it would be useless for observing the electron without influencing it. If you get a clean view of an electron’s position, you’ll knock it out of line with the light you’re using to see it, and so won’t be able to tell exactly what direction it had been traveling. (That’s because individual packets of light carry a distinct momentum “punch” as they travel: it’s very small, but enough to “push” a tiny electron.) But if you want to be so gentle that you don’t knock it away from where it’s traveling, then you won’t have enough clarity to see exactly where it began. You can choose to measure either where the electron is or how fast and powerfully it’s traveling, but you can’t measure both with full accuracy at once. You’re always going to be a little bit unsure—uncertain—about the complete mix.
This is the basis of the famous uncertainty principle, which Heisenberg published in February 1927. It was irrefutable. It ended centuries of belief that the universe followed an inherent perfect order. It revolutionized physics.
And Einstein would have nothing to do with it.
SEVENTEEN
Arguing with the Dane
THE DISAGREEMENT BETWEEN Einstein and most other quantum physicists first came to a head at the Brussels conference in October 1927. This was the same gathering where Lemaître buttonholed Einstein about the lambda. As if it weren’t enough for him to be fending off one set of uncomfortable ideas, now he had two—and one fight would, in time, reinforce his determination in the other.
If the meeting had taken place just a year earlier, Einstein would have enjoyed the support of many of his assembled colleagues. Up until that time, many of the attendees had shared Einstein’s initial responses to Heisenberg’s ideas. Before Heisenberg’s imagined work with a gamma-ray microscope yielded the uncertainty prin
ciple in early 1927, physicists were skeptical about his theories regarding the quantum universe. They, like Einstein, were impressed that his early computations had had such success in accounting for how electrons responded to blasts of light, but weren’t convinced that reality could be so unclear, so vaguely glued together, that at the most detailed level we really had to accept uncertainty forever.
When it emerged in February 1927, several months before the conference, however, the uncertainty principle robbed Einstein of many potential allies. The principle, most physicists agreed, did seem to show that views into the atom were inherently closed off. Heisenberg, they conceded, appeared to have been right—which meant that Einstein (whom many of his colleagues would have known to be disdainful of the younger scientist’s theories) had to be wrong.
Einstein had been invited to open the conference, since everyone wanted to see how he would deal with the new challenge from the quantum theorists, and how he would defend his traditional views about causality. He declined, however. He wasn’t in a position to tell all of Europe’s scientists what to think—not yet, and not in the magisterial way in which he’d been able to lay out the details of general relativity. His feelings were still a hunch, a suspicion, an almost visceral belief that “an inner voice” told him this could not be how the world worked.
So Einstein sat politely through the opening sessions and watched as Niels Bohr stood up to weigh in on the issue. Now middle-aged, Bohr had become the leader of the pro-Heisenberg faction. As he’d aged the odd appearance he’d had as a young man had become more attractive. His habit of speaking slowly and softly, with long pauses for thought, gave his words majesty.
Bohr began the conference by recapping the changes that Europe’s scientists—there were almost none of any significance at the time in the United States—had been experiencing. Since the decline of medieval scholasticism, Bohr recounted, there had been at least some efforts in the West to bring reason to bear on the material world. This was not a fettered reason, predetermined to come up with conclusions that matched what the church wanted to hear. Rather, this was a reason, an intellectual inquiry, that was convinced it could uncover every fact of nature, however laborious the process might be and however many centuries it might take. This program of inquiry took for granted the belief that what was out there, in the real world, truly existed and could—in whatever detail we wished—ultimately be understood.
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