4 The Solvay Battles
The issue around which Einstein chose to launch his attack involved one of the cornerstones of quantum theory: Werner Heisenberg’s famous uncertainty principle.
The inability to give us a picture of a real physical situation (aka hidden variables) is built into the very structure of quantum mechanics. Perhaps the strongest evidence for this is provided by the uncertainty principle. This principle, formulated in 1927, deals with a strange limitation of the language of quantum theory: it cannot speak of certain pairs of properties. One example is the specification of the position and velocitya of an electron. Using the language of quantum mechanics we can write down a description of an electron being at some definite place—but that description will specify that the electron can have any velocity at all. Conversely, we can elect to write down a quantum-mechanical statement of the fact that the electron is moving at a definite velocity—but that description will specify that it can be at any location at all.
Are we just being stupid? Maybe with a little more work we could cook up a quantum-mechanical description of an electron with a perfectly definite position and velocity. Unfortunately, no matter how hard we try we find ourselves unable to find such a specification—and indeed, it can be shown that the mathematical structure of quantum theory is such as to prohibit such descriptions.b My Predictor does not even talk our language.
But perhaps we have been misunderstanding the nature of the electron. Perhaps the electron is just not the sort of thing that has a definite location. Could it be a little fuzzy, or have a fluctuating shape, so that it is impossible to specify with perfect accuracy its position? Maybe an electron is less like a tiny, point-like particle and more like a region of bad weather. Things might be pretty stormy across the Northeast—but a more precise specification of the bad weather’s location is simply impossible.
But this will not do, for quantum theory is perfectly capable of describing an electron located at a precisely specified place. It is only pairs of properties that cannot be simultaneously described. Such pairs are termed “complementary.” Furthermore, it is only some pairs that are complementary: the theory is perfectly capable of simultaneously specifying the position and mass of an electron, for instance.
Is this a fatal limitation of quantum mechanics? Or is it some kind of insight into the very nature of the microworld? Is the uncertainty principle a problem or a discovery? Is it an expression of the fact that quantum mechanics is incomplete, a mere half-theory that must be supplanted by a fuller theory? Or is it a profound insight into metaphysics and the nature of reality?
Einstein took the first view. Another of the creators of quantum mechanics, Niels Bohr, took the second. Einstein wanted a more talkative Predictor; Bohr thought that this desire was naive. They battled it out for years.
Two memorable interchanges took place at historic “Solvay” conferences in the years just following the creation of quantum theory. Founded by the Belgian industrialist Ernest Solvay in 1911, these meetings take place in Brussels and have been devoted to fundamental issues in physics and chemistry. Meeting at irregular intervals, they continue to this day.
At the Solvay Conference in 1927—the very year in which Heisenberg enunciated his uncertainty principle—Einstein came up with a “thought experiment” that he felt revealed a way to circumvent it. Such thought experiments do not need to be performed: they are mental exercises designed to bring out certain elements of a situation, just as a novelist might place characters in a particular state of affairs to watch what they do. Einstein invented a series of steps designed to reveal to the experimenter two complementary quantities that, according to Heisenberg’s principle, could not be so revealed.
Figure 4.1
Niels Bohr. Also one of the creators of quantum mechanics, Bohr argued that Einstein’s search for a more complete theory—one that would describe microscopic reality—was misguided. Indeed, Bohr argued, the refusal of quantum theory to do so was not a problem but a discovery—a profound philosophical insight. Photo courtesy of the American Institute of Physics Emilio Segre Visual Archives.
A participant has given a first-hand description of what happened. Each day
Einstein came down to breakfast and expressed his misgivings about the new quantum theory, every time [he] had invented some beautiful [thought] experiment from which one saw that [the theory] did not work. … Pauli and Heisenberg, who were there, did not pay much attention, “Ah well, it will be all right, it will be all right.” Bohr, on the other hand, reflected on it with care and in the evening, at dinner, we were all together and he cleared up the matter in detail.1
Three years later, at the next conference, Einstein arrived armed with a second thought experiment.
It was quite a shock for Bohr—he did not see the solution at once. During the whole evening he was extremely unhappy, going from one to the other, 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. I shall never forget the sight of the two antagonists leaving [the meeting], Einstein a tall majestic figure, walking quietly, with a somewhat ironic smile, and Bohr trotting near him, very excited.2
This time it took all night for Bohr to discover the error in Einstein’s reasoning.
Both times Bohr had succeeded in refuting Einstein’s arguments. But Einstein remained unconvinced nevertheless. He wrote to Schrödinger: “The soothing philosophy—or religion?—of [complementarity due to] Heisenberg–Bohr is so cleverly concocted that for the present it offers the believers a soft resting pillow from which they are not easily chased away. Let us therefore let them rest. … This religion does damned little for me.”3
Over the years the two battled it out. But their scientific disagreements never became personal. Indeed, Einstein held great affection for Bohr. Writing to a friend: “Bohr was here, and I am as much in love with him as you are. He is like an extremely sensitive child who moves around the world in a sort of trance.”4
It cannot be said that the great debates between Einstein and Bohr ever reached a definite resolution. Rather they just seemed to peter out. Some people continued paying attention to the question, but by and large the mainstream did not.
Maybe it was a matter of simple exhaustion. Einstein and Bohr had wrestled over the matter without reaching a conclusion: why go over the same ground yet again?
Figure 4.2
Bohr and Einstein … in the midst of a furious battle? Although they disagreed profoundly, their disagreements were never personal. In fact, they had deep affection and respect for one another. Photo courtesy of the American Institute of Physics Emilio Segre Visual Archives.
Or maybe it was like the famous paradox of Zeno: in order to go from here to there, first you need to cover half the distance, then half the remaining distance, and so on. This seems to prove that motion is impossible. But do I care? Not at all: I can’t think of a satisfactory rebuttal, but that doesn’t stop me from walking across the room. Similarly, maybe you don’t have to know what quantum theory means in order to use it.
At any rate, thinking about such issues was just “not done” in those days. To many people it seemed a bit unprofessional, maybe even juvenile. Grownups did not waste time doing such things. Somehow, admitting to an interest in the nature of reality felt like admitting to a fascination with ESP or reincarnation.
On the one hand, the problem seemed to have no practical consequences. Whichever side you took made not the slightest difference to the conduct of your research. Accustomed to playing the hardball of theories that made specific, testable predictions, of conducting experiments that yielded detailed, verifiable results, stewing over such matters had been striking many people as just a bit fluffy.
It is also a matter of the technology available. In the great debates over the creation of quantum theory, the thought experiments of Einstein and others were just that: thought experiments. They could not actually be conducted. It was technically impossible. And in science, experiment and observation are paramount. Pure thought is all very well, but it gets you only so far.
The astonishing thing about the battle over the adequacy of quantum theory is that the battle simply seemed to vanish for many years. In my own experience, I can testify that when I was a student studying quantum mechanics not a single professor so much as mentioned the enigmatic nature of the theory, the mysteries surrounding its interpretation, or the great debates that had so animated Bohr and Einstein. The same is true of every textbook I ever read. Indeed, it was not until 1985 that a single graduate-level textbook so much as mentioned Bell’s Theorem—more than two decades after he had discovered it. Undergraduate texts took even longer to get around to the subject.5 And if our professors and our textbooks did not refer to the subject … why then, we students did not either. The subject was out of bounds.
The Science sections of bookstores nowadays are crammed with popular expositions of the mysteries of quantum mechanics. This has not always been so. For more than two generations following the close of World War II virtually no such books appeared discussing these issues.6 At the other end of the publishing spectrum, the Physical Review, one of the premier scientific journals in the world, had for years an explicit policy of refusing to publish any paper on such matters that did not explicitly make new quantitative experimental predictions.7 Most of the great debates among the theory’s founders would have remained forever unpublished under such a guideline.
So for decades the questions were relegated to the margins.
Einstein was trying to show that quantum mechanics was incomplete—a mere half-theory—and that it should be supplanted by a more complete one that would describe in detail the hidden reality of the microworld. So far he had failed. But he had not given up. And the next step he took led directly to Bell’s wonderful discovery into the nature of quantum reality.
a. Technically the theory speaks of momentum, but momentum is just the electron’s velocity multiplied by its mass so the distinction doesn’t matter.
b. Years later a variant theory was proposed by David Bohm that evaded this problem. (It is briefly discussed in chapter 14.) This theory does not, however, succeed in providing the sort of mechanistic description of the microworld that Einstein was looking for.
Notes