Of all the remarkable advances in scientific understanding that have been achieved over many centuries, it is the theory of quantum mechanics that is the most enigmatic. Introduced in the first quarter of the twentieth century, it grew to a beautiful mathematical structure that became the most revolutionary, broad-ranging, and successful of modern theories: yet despite the multitude of insights that quantum mechanics has given us, it remains the most baffling of all successful theories. Quantum mechanics has such mathematical precision and range of predictive power that it provides explanations for the stability of atoms and solid materials; for phase changes such as melting, freezing, and boiling; for the colors of heated materials, including a detailed description of the puzzling phenomenon of spectral lines; for low temperature superconductivity and superfluidity; and for the behavior of lasers, transistors, and television screens, not to mention the whole of chemistry and much of biology and genetics. How can it be that such a successful theory can still remain an enigma, even to the greatest of physical scientists?
It may be recalled that, for a time, that other great revolutionary physical theory, Einstein’s general relativity, introduced in about 1915, was supposed to have been understood by but a handful of people; yet today that theory is not considered to be exceptionally hard to grasp in its entirety, and it provides us with a mathematically consistent and philosophically satisfying picture of a universe. We may ask the question why has the same not happened with quantum mechanics—for indeed it has not: even one of the greatest quantum physicists of modern times, Richard Feynman, was led to write a few years ago in his book QED* (a book on quantum electrodynamics for the lay person) “I don’t understand it, nobody does.” Yet the answers that different physicists might give to this question would vary greatly, depending upon the views they might hold regarding the quantum theory.
It is not that the theory is particularly difficult to comprehend as a mathematical structure. Rather, the theory exhibits a baffling mixture of extreme physical precision with utter absurdity, with regard to the way it relates to the macroscopic world that we actually perceive. The irony of the situation was once captured very succinctly by Robert Wald, a colleague from the University of Chicago, in a dinnertable discussion about attitudes to the validity of the theory: “If you really believe in quantum mechanics then you can’t take it seriously.” This viewpoint expresses essentially that of those who support the “conventional wisdom” referred to as the “Copenhagen interpretation.” They indeed claim that the very precise formalism of the theory is not to be taken seriously as a picture of actual “reality.” They often assert, accordingly, that the whole question of quantum reality is a nonquestion. One should not think of the theory as providing us with a picture of actuality, they argue, but merely as giving us a calculational procedure that accurately provides the correct mathematical probabilities for the different possible outcomes of experiments. This, they say, is all that we should ask of a theory and not ask questions about “reality.” We do not need an understanding of the “actual” nature of the world; it is amply sufficient for our theory to make accurate “predictions”—something that quantum mechanics is indeed supremely good at.
Of course there is a macroscopic world that we can directly perceive, and that must be assigned some “reality,” but the submicroscopic world of quantum phenomena need not be. This was the position of Niels Bohr, originator in 1913 of the quantum model of the atom and of the “old” quantum theory that preceded the more refined theory of Heisenberg and Schrödinger, which came about in 1925–1926. Bohr also thought deeply about the newer quantum ideas and became the “high priest” of the Copenhagen viewpoint. Among other things, he was insistent that one should assign no “reality” to the quantum description of a state of a system between the “measurements” made by scientists, the results of these measurements themselves being the only things one should attach reality to.
Such a defeatist view was not shared by all the originators of quantum theory. In particular, Albert Einstein—who had himself been responsible for the first ideas leading ultimately to the quantum realization that particles were waves and waves were particles—found such a physical standpoint abhorrent (as did Schrödinger). To Einstein, the fundamental prerequisite of a scientific view of the world was a belief in a reality “out there” whose nature, no matter how strange, must be independent of how we might choose to examine it (even though by examining it we might indeed change that reality). This “objective reality” must hold at the quantum level of things—the submicroscopic world of molecules, atoms, and nuclear particles—just as it does at the macroscopic level of everyday experience. The aim of fundamental science, according to Einstein, is to find out what the world is actually like, and to discover the laws according to which that world behaves at all levels, including the level of quantum phenomena. If quantum theory does not provide a realistic view of events at the quantum level, no matter how unfamiliar, then that theory must be incomplete (or perhaps even just plain wrong). Thus Einstein was led to reject the quantum theory as not providing the appropriate description at submicroscopic scales.
Einstein’s objections to the indeterminism in quantum theory are well known, and he wrote to Max Born, in 1926: “The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice.” Yet this indeterminism did not appear to have been Einstein’s main objection to the theory. It was the apparent subjectivity of “physical reality,” inherent in the Copenhagen interpretation, that he could not abide. Einstein made many attempts to show that quantum theory was actually inconsistent, and up to and around 1930 he was involved in many arguments with Niels Bohr.
Although the general consensus was that Bohr won these arguments, Einstein never gave up. In 1935 he produced, with Boris Podolsky and Nathan Rosen, a paper pointing out a disturbing implication of quantum mechanics (now called the EPR effect after the three men): certain experimental situations could be set up in which the very choice of the particular way in which a particle is to be observed in one region of space would seemingly instantaneously affect the state of a second particle that could be very distant from the first. In fact the effect would be totally independent of the distance between the two particles.
This was very disturbing for Einstein, and he referred to the effect as “spooky action at a distance.” Not only did this “action” not get smaller as the distance increases, as would a normal, self-respecting force, but the instantaneous nature of this action seemed incompatible with the picture of the world that Einstein himself had provided in his special theory of relativity. (In fact there is no implied observational conflict between this “spooky action” and Einstein’s special theory, but there is a serious conceptual conflict.)
Einstein’s followers, most notably David Bohm in 1952 (and also Prince Louis Victor de Broglie, the founder of the idea of a universal wave/particle duality), later developed a theory of “hidden variables” according to which the uncertainties in quantum behavior could arise because of the random motions of some underlying substructure or “parts” that cannot be directly observed but provide the needed “reality” for subquantum level objects. To be consistent with the effect described above, however, it would be necessary for any “realistic” view of quantum mechanics, such as the de Broglie-Bohm theory, to be essentially nonlocal. Thus the “spooky” instantaneous actions would have to be built in as part of “reality” according to the theory. This would be very much in conflict with the spirit of Einstein’s relativity theory.
For a while, physicists did not worry themselves much with such matters, these being regarded more as questions of philosophy than of serious physics. Things began to change, however, when in 1964 the Northern Irish physicist John Bell published a paper pointing out a mathematical inequality that showed that the “spooky action” of quantum theory could actually be put to experimental test. A version of the EPR effect put forward by Bohm was subsequently used as the basis of a number of experiments, most notably one performed by Alain Aspect in Paris in the early 1980s which convincingly confirmed that this “spooky” nonlocal quantum effect is, in a queer sense, really true, and is now part of observed physical reality.
Though there are probably more different attitudes to quantum theory than there are quantum physicists, the particular polarity of views expressed by Einstein and Bohr encompasses an important part of this spectrum of opinion. Jeremy Bernstein, in his new book Quantum Profiles, provides us with an unusual insight into this controversy by addressing the physical issues indirectly: through the opinions and attitude of some of the major personalities involved. There are just three chapters (two of which have epilogues), one on Bell, one on John Wheeler (an influential disciple of Bohr’s), and one on Einstein’s lifelong friend and colleague Michele Besso. The chapters on Bell and Wheeler are mainly based on personal interviews, while the one on Besso is based on his very extensive correspondence with Einstein.
I feel that it is a matter of tremendous good fortune that Bernstein was able to interview John Bell when he did—in January 1989—for Bell is no longer with us, having unexpectedly died of a cerebral hemorrhage in October 1990, at the age of sixty-two. This is a tragic loss. In my own opinion, Bell’s contributions to our understanding of quantum mechanics have been unique and fundamental. He opened the way to a striking class of new experiments which, in the words of the physicist and philosopher Abner Shimony, provide, perhaps for the first time, experiments that can actually test one’s philosophical viewpoint concerning the nature of reality. In my opinion, there is an additional tragedy in Bell’s loss. He could not but be respected by all who thought deeply about quantum mechanics whatever their persuasions. Even the “Copenhagen hard-liners” had to respect his views. In the later years of his life, Bell gave strong support and encouragement to the ideas of some of those who, in recent years, have dared to challenge conventional quantum theory with model rival theories of their own. Although all of these new theories have their weakness, Bell has made this activity respectable. I believe that this is vitally important. For, before too long, we shall indeed need a new theory. I profoundly hope that the new-found respectability for such searchings will not die with Bell. In his account, Bernstein gives us a warm and friendly insight into the personality of this subtly humorous, gentle, and admirable man, in addition to letting us in on Bell’s profound and clear views on the puzzles of quantum mechanics, and on Bell’s insightful commentaries concerning Bohr’s and Einstein’s views.
The chapter on John Wheeler is also illuminating. Wheeler is a fascinating man: a paradoxical union within one personality of the revolutionary and the profoundly conservative—a perfect gentleman with a wild delight in explosives (and a leading figure in the development of the hydrogen bomb). I liked Wheeler’s summary of John Bell’s approach to quantum theory: that Bell would rather be clear and wrong than foggy and right. There is certainly something of the Einstein/ Bell type of view in the first of these alternatives, as opposed to the Bohr/ Wheeler type of view in the second. Bell always made it clear that his sympathies lay with Einstein in his controversy with Bohr, and that Einstein’s philosophical standpoint was far superior to that of Bohr—even though it was Bell’s own insights that finally led to a refutation of Einstein’s “local realism.” It just seemed to Bell that it was a “pity” that Bohr has (in this important respect, at least) turned out to be right!
There is indeed a profound fogginess in Bohr’s (and indeed Wheeler’s) expression of views. For myself, I do not find this obscurity helpful, and in this respect my sympathies are definitely with Bell and Einstein. Yet there is a deep point, also, in Wheeler’s stressing the mathematical elegance of the formalism of quantum theory. While it is at odds with good philosophy, quantum theory enjoys a profound harmony with mathematics.
My personal belief is that the resolution of these puzzles can never be achieved within the theory as it stands. We must search for a new theory with a mathematical elegance even greater than that of present-day quantum mechanics—to which quantum theory can be only an excellent approximation—and which will also make profound philosophical sense. I am sure that Bell would agree with this. I am not so sure about Wheeler, who seems more concerned with seeing “why” quantum mechanics is “utterly right” than in exploring how it might actually turn out eventually to be wrong.
In the chapter on Besso, we learn more about Einstein himself, his generous but dedicated personality and his powerful views about quantum mechanics. There is something here of historical interest both in relation to Einstein’s scientific persona and the strains of his family life. It is a useful supplement to the existing biographies of Einstein.
I found Bernstein’s book fascinating to read (perhaps because I have known the remarkable scientists featured in two of the three chapters). Perhaps I learned less that was new about their views on quantum mechanics than I had hoped I might but I did gain some new insights. However, there are several curious errors or misprints throughout the book that caught my attention, such as the conflation “Max Bohr,” and there are some others that are also genuinely confusing. Also, in his description of Galileo’s Dialogues, Bernstein asserts that it is “Sagredo” who puts forward Galileo’s own views, whereas I had always thought this was normally “Salviati.” Bernstein’s book is valuable in that it emphasizes the fact that quantum theory is science and not mysticism, and that one should be extremely wary of the frequent associations that are made between these two very different ways of thinking about the world. The book provides a valuable addition to an important debate about our understanding of the universe at its most fundamental level.
March 28, 1991