Two months later, on September 15, 1930, the Nazis astonished themselves as well as the world by winning the second largest number of seats in the Reichstag. Hitler had found that power in the modern world comes only secondarily from the barrel of a gun. Foremost it comes from lies. The authority of Nazi lies rested on personal appeal and loyalty. Their explanations cast blame. They replaced understanding with excitement. And there was plenty of excitement in early October, when the Reichstag opened with its new membership. Demonstrations took place across Berlin. Count Kessler saw them in the part of town where Einstein caught his train to Potsdam. Kessler pronounced the demonstrators mostly “adolescent riff-raff” and watched them shout, “Germany awake! Death to Judah.” Hot-blooded ignorance was on the march.
A week after this terrifying scene, Einstein was back in Brussels for the opening of another Solvay Conference. This time, of course, there had been no controversy about inviting the Germans. Einstein came as a matter of routine. With Lorentz dead, the gathering had a new chairman, Paul Langevin, but the idea was the same: bring the world’s greatest physicists into a room and start talking. The official topic that year was “Magnetism.” Bohr came, looking (according to the photographs) much older than he had 10 years earlier when he first met Einstein. It was now six years since Fritz Haber had spoken
about how these “two … stand in such deep opposition.” The description still held. These two men still seemed to grasp the stakes far more deeply than their colleagues: Can science claim an objective knowledge of how nature governs itself or boast only of a practical understanding of what experiments will show? Bohr might have thought the matter settled, but Einstein had been saving something for him. The two men sat together; a photograph shows them sunk into easy chairs smoking pipes and talking.
Einstein proposed one of his elaborate contraptions for getting around Heisenberg’s principle, in this case the part about not being able to know both time and energy with great certainty. You could, Einstein proposed, create a hollowed ball and heat it up so that the interior would fill up with radiation. This was exactly the sort of experiment that Planck had considered 30 years before when he introduced the quantum h whose tortuous path had led them to their quarrel. In those early days Planck’s “blackbodies” had included a small hole through which radiation escaped for study. Einstein proposed that this hole be covered by a shuttered gate that moved so quickly it could release its radiation one photon at a time.
Bohr listened alertly. Give a point to Einstein there for having discovered photons in the first place.
Now attach a clock to the shutter so that we can tell the exact time it opens and a photon escapes. This system allows for a very precise knowledge of the time. That takes care of the time side of the uncertainty principle, but what about measuring the energy?
Here Einstein resorted to another of his famous discoveries, E = mc2. The photon that escapes is pure energy, as the long agony over the photon’s hυ had finally established. Any attempt to measure the photon’s energy directly would be subject to a variety of limits that would seem to validate Heisenberg. Einstein proposed an indirect measure. Weigh the blackbody before opening the shutter and then again after closing the shutter. This weighing, which can be extremely accurate, will show a drop in mass. Just as the sun loses mass every instant that it radiates energy into the universe, Einstein’s blackbody will lose mass as it radiates its little photon.
Sure, sure, the drop will be tiny, way too small for any real scale to
detect, but we are imagining an experiment here. The body loses mass and there is no principled reason that a scale could not measure it. Then, once we know the loss in mass, we can use the E = mc2 equation to compute how much energy that mass represents. We will have measured the time and energy of the photon with much greater accuracy than Heisenberg’s principle allows.
Bohr was thunderstruck. The room of physicists was stunned. Einstein had done it, gotten around Heisenberg, Bohr, and all those pragmatists who thought they had chased reality’s claims from the heart of physics. There was, after all, more in heaven and earth than in their equations.
Léon Rosenfeld, who later became Bohr’s longtime collaborator, was at Solvay 1930 and reported that, “During the whole of the evening [Bohr] was extremely unhappy, going from one to the other, and trying to persuade them that it couldn’t be true, that it would be the end of physics if Einstein were right, but he couldn’t produce any refutation.”
Bohr might have thought this experiment destroyed physics, but Einstein felt he had saved it by showing there was still more work to do. When he began his life as a physicist, the leaders were confident that Newton’s mechanics, Maxwell’s electromagnetism, and the new laws of thermodynamics summed up the whole of nature. By picking away at loose threads and examining areas that others had told him were pointless to question, Einstein had shown that more could be done. Through it all, Einstein had maintained a faith in God’s honesty. In his first theory of relativity he had made that faith a principle: The laws of nature are the same everywhere in the universe. This kind of principle can never be proved, but Einstein built on it, confident that “the Lord is … not malicious.” With quantum theory he again refused to believe that reality was permanently hidden.
The next morning, however, Bohr reported that he had more to say. It is true, Bohr agreed, that we can imagine a device that is so sensitive it can weigh the lost photon, but the measuring device will still have to be a scale of some kind. There are many different kinds of scales—doctors’ scales, grocers’ balances, and so on. Any system will do, but let us imagine the simplest one, a one-sided balance that points to
a chart. The balance moves up and down, and as it does, a pointer moves with it up and down across a scale. Because this balance moves to different positions for different weights it will move—very slightly, true, but move—when the blackbody loses its photon.
Where will it move? To its new position, yes, but as the photon departs, its momentum will push the balance in the opposite direction. In short, as the balance moves to its new position, it will sway a bit, as balances always do when you change the weights involved. We cannot predict exactly how the balance will sway because we cannot predict the direction taken by the escaping photon, but we know it will rock gently like a suitcase tag swaying on a train.
Perhaps Einstein’s famous hair stood even more starkly on end as Bohr managed to slip an uncertainty into the system. Einstein had always been troubled by his inability to calculate exactly what direction an emitted photon will take.
Now, Bohr continued, it is while this balance sways in its unpredictable path that the clock attached to it will measure the time. But we know from general relativity that the time on a moving clock differs from the time of observers. Of course the equations of general relativity would let us translate that clock’s time to our time if we knew the clock’s speed and direction, but in this case we do not know how the clock is swaying, so we cannot calculate the time difference exactly. Thus, although we can know the energy to great precision, there is still an uncertainty about the time. Heisenberg’s principle holds.
Bohr might as well have slapped Einstein with a fish. The founder of relativity had forgotten that time is not absolute. Still, even in this defeat, Einstein remained a mensch and rose to the moment. He was much the better mathematician and helped Bohr work through the calculations to show that the uncertainty between time and energy would, after all, be as Heisenberg’s principle said. Quantum mechanics continued to describe experimental outcomes without revealing an underlying reality.
Einstein’s inability to find inherent contradictions in quantum mechanics appears finally to have cracked the optimism that had been fundamental to his scientific character. After that morning he was no
longer sure he had the imagination to find the secret behind the compass. He had told Count Kessler that doing profound physics was really very simple, you looked for the point where physics contradicted itself and found a way to resolve the contradiction. When he said that, he had not doubted his talent for solving the puzzles. Now the contradictions in quantum physics seemed to be right on the table—if you do one set of experiments you can show that light and electrons are waves; if you do another set of experiments, you show these very same things are particles—yet he could not reduce the paradox to one experiment that brought the contradictory measurements together.
He believed that quantum mechanics was both right and a dead-end, providing no pathway to the rest of physics. For the rest of his professional life he looked for a unified theory that would provide an entryway into a coherent, realistic quantum physics. He did not succeed, and few colleagues joined him in the search. For them, Bohr had won, scientific realism had fallen and with it the belief that the world is objectively comprehensible.
Many physicists said that what had become of Einstein was a great tragedy. Physics had to do without him. Einstein never saw himself as tragic, nor his defiance as futile. His state of mind was, as he once said about Max Planck, “akin to that of the religious worshipper or the lover; the daily effort comes from no deliberate intention or program but straight from the heart.” When those late Renaissance dreamers imagined that the natural world could be explained in natural terms, many had opposed their idea as blasphemy and presumption that was dangerously apt to lead one away from God and toward materialism. The critics had a point. The search for natural laws proved so successful that for many savants the search became an end in itself. They did not follow the Renaissance expectation that discovering order would bring them closer to God. Einstein’s talk of using physics to find the secret of the Old One sounded poetic and eccentric. With the triumph of quantum mechanics and Bohr’s complementarity, physicists put away all pretense of striding closer to knowing what was out there beyond ourselves.
For the professional physicist this change was of minor interest, and you sometimes read a professional who denies that the Einstein-
Bohr debates were of much importance. The difference between logical laws that work because they are efficient and natural laws that work because they get the meaning right is of no interest to their pragmatic labors. Quantum mathematics provided a way to test predictions and organize the complete results of any experiment. It was the amateurs who, without knowing clearly what had happened, reacted to the change in physics. Quantum physics had become permanently paradoxical and alien, a set of techniques that lay people could read about over and over and yet never seem to really understand. Like the most advanced music, painting, and literature of the period, physics took a turn that its old audience could neither follow nor grow into. The subject could not be dismissed; after all, physicists had discovered the secret of blowing up the world, but it seemed to say nothing meaningful about either their lives or their world.
Einstein refused to follow his colleagues down that pathway. A few weeks after Bohr’s refutation at Solvay, Einstein was with his friend Paul Ehrenfest in the Netherlands, participating in a panel discussion on quantum physics. As Einstein criticized quantum mechanics, some of the distinguished scientists grew quietly restless. A speaker on the floor then defended quantum theory as complete and consistent. Einstein was not happy. “I know this business is free of contradictions,” he granted but waved triumphant opinion aside as he had done a dozen years earlier in the Reichstag, “yet in my view it contains a certain unreasonableness.” The satisfied revolutionaries in the room could only stare. Their pathfinder was defying them all, determined still to find the enduring face behind all our changing facts.