Mark P. Silverman: Atomic Physics

Can you imagine groups of physicists holding regular conferences on the interpretation of Newtonian mechanics with lively debates over whether the three laws of motion are complete, whether the objects to which they apply actually have such properties as velocity and acceleration, or whether such objects even exist if "Newton's friends" are not thinking about them? No? Well, neither can I. However, had my question referred to quantum mechanics, rather than Newtonian mechanics, the reality, whether one can imagine it or not, is that the interpretation of the theory has been discussed and debated for nearly 80 years in conferences, articles, books, radio broadcasts, and, more recently, even in theatrical performances. If you are interested in knowing in precise, but readable, detail what the ruckus is all about, Ghirardi's book will guide you through a labyrinthine maze of conflicting viewpoints regarding the peculiarities of quantum phenomena.

To the detriment of communicating a meaningful understanding of epistemological issues raised by quantum mechanics, the history of the subject is replete with eminently memorable, but generally nonsensical, aphorisms and images that are all too frequently quoted out of context or without regard for subsequent progress. We have Bohr's comment that anyone who can think about quantum physics without getting giddy doesn't understand the first thing about it. We have the image of Einstein standing by his office window, explaining to a visitor that the insane asylum across the way houses those madmen who have not thought about quantum mechanics. We have Feynman telling his students that no one understands quantum mechanics. We have John Bell's remark that nobody knows what quantum mechanics says about any particular situation. And in one of the most egregiously inappropriate images of all, which pervasively decorates book covers and conference posters, we have Schroedinger's cat in a linear superposition of dead and alive macroscopic states. (Schroedinger, at least, had the sense to refer to his example as "ridiculous". ) With leading physicists communicating sentiments like these to one another, or with other physicists persistently quoting them (a sin of which I am also guilty), it is no wonder that popular exegeses of quantum mechanics contain much of what Gell-mann called "quantum flapdoodle."

God's Cards is not flapdoodle. The author has written a meticulous and objective account of the principal theoretical features of quantum mechanics that clash with what we ordinarily take for granted as "reasonable" in trying to make sense of the physical world. He explains, in depth that I have not encountered in quantum physics textbooks (if the subject is encountered there at all) or in other books for popular reading, specifically how each quantum "mystery" challenges our classically-schooled common sense to provide an explanation. And he carefully examines the principal explanations that have been proposed, pointing out how they meet the challenge and what additional difficulties they engender.

Among the epistemological issues the author deals with, the most provocative in my opinion are (a) the completeness of quantum mechanics (as expressed in the subtleties of the Einstein-Podolsky-Rosen argument), (b) the nonlocality of quantum physics (as represented by entangled wave functions or state vectors of spatially-separated, coherently-produced particles), (c) the linear superposition of macroscopically distinguishable quantum states (as characteristic of the Schroedinger cat paradox), and (d) the so-called "collapse" or "reduction" of the wave function, whereby the measurement of a quantum system by a classically-described apparatus terminates the continuous evolution of the wave function and projects the system into one of its allowable states.

Among the various ways philosophically-oriented physicists have tried to make sense of the phenomena underlying these conceptual issues, which constitute what is generically referred to as "the quantum measurement problem", the author examines schemes that (a) posit the existence of precise, but unknowable, variables ("hidden variables") accounting for the uncertainties in quantum systems, (b) propose the existence of an infinity of universes evolving each instant, corresponding to the infinity of potential quantum outcomes ("many-worlds" theory), (c) attribute to environmental or intrinsically nonlinear interactions the "decoherence" which prevents observation of paradoxical macroscopic superpositions of quantum states ("dynamical reduction" theories), and (d)) rely upon human consciousness (e.g. "Wigner's friend") for the completion of a quantum measurement and reduction of the state description into a single observable outcome.

The author writes in a conversational style, patiently delineating these various schemes in a kind of gentle and nonjudgmental manner of a teacher who does not want to offend a student, no matter how outlandish the student's answer. Only at the end of the book does the author venture his own opinions as to the likely validity of the various unorthodox interpretations of quantum mechanics. (I use the word "unorthodox" guardedly, for there are various opinions as to what the "orthodox" interpretation is, and I do not believe the author's version would correspond entirely to my own.) In brief, he does not like the "many-worlds" interpretations because they appear redundant and sterile. He regards the emergence of consciousness in the physical world as deserving the utmost attention, but unlikely to be the essential reason for the reduction of the wave function. He places great importance on schemes in which the motion of quantum particles are guided by an underlying wave (pilot-wave theory), but notes that such approaches have so far failed to be generalized relativistically. For the same reason, he is skeptical of dynamical reduction theories. He mentions, but seems reluctant to judge, more recent schemes that look towards gravity or constellations of microscopic black holes as essential elements in the ultimate solution to the quantum measurement problem.

So what are we left with? "Hope", as far as I can tell. Basically, the author's gentle conclusion is to "watch with lively interest" the next generations of scientists who will undoubtedly find more "amazing mysteries" waiting for them. Of the latter possibility, I have no doubt, but, as to the general direction of quantum exegesis, I believe the author's competent exposition leads to a different conclusion.

Perhaps the quantum formalism and its valid (as opposed to fanciful) applications tell us simply and directly that the theory is to be interpreted as a calculus of probabilities describing the outcomes of observations on ensembles of similarly prepared quantum systems; that we do not see linear superpositions of cats because that is a misuse of the formalism, or superpositions of actually appropriate incoherent macroscopic physical systems because the quantum interference of their "HERE" and "THERE" states becomes negligibly small as some property (e.g. the mass) becomes macroscopically large. Perhaps, as the architects of quantum theory have remarked, the theory itself defines the kinds of meaningful questions we can ask of it, andÑproviding quantum theory gives a valid description of nature, as seems to be the case so farÑwhen we ask the "wrong" questions, we end up with paradoxical answers. Thus, we may avoid paradoxes, not by invoking farfetched interpretations, but by asking physically appropriate questions. If quantum theory gives a valid description of the physical world, then it is describing a world that has been operating without human consciousness for more than 10 billion years, and clearly such considerations as consciousness are entirely irrelevant to the outcome of physical processes.

Quantum phenomena can indeed seem strange, but quantum theory accounts for these phenomena adequately by predicting all that can be observed of them, not predicting what cannot be observed of them, and doing all this in conformity with the other main pillar of physics, special relativity¹. Is it possible, then, that the problem with the "quantum measurement problem" is that there is no problem?

These contrarian thoughts notwithstanding, I urge anyone with a passion for delving deeply into epistemological questions of quantum theory, which some physicists still debate hotly, to read this thorough and competent account by an author who has puzzled over these questions himself for many years.

1. Some physicists would take issue with this last assertion. See, for example, John Bell, Speakable and Unspeakable in Quantum Mechanics 2nd Ed, (Cambridge U.P., 2004).

Mark P. Silverman has investigated quantum phenomena by means of rf and microwave spectroscopy of atomic beams, electron interferometry, coherent laser spectroscopy, and nuclear spectroscopy. He discusses aspects of his quantum researches in his books, More Than One Mystery: Explorations of Quantum Interference (Springer 1995), Probing the Atom (Princeton U.P., 2000), and A Universe of Atoms, An Atom in the Universe (Springer, 2002).