Van Fraasen B. Quantum mechanics: an empiricist view

Oxford: Clarendon Press, 2000. — 558 p.Quantum theory grew up, from Planck to Heisenberg and Schroedinger, in response to a welter of new experimental phenomena: measurements of the heat radiation spectrum, the photoelectric effect, specific heats of solids, radioactive decay, the hydrogen spectrum, and confusingly much more. Yet this theory, emerging from the mire and blood of empirical research, radically affected the scientific world-picture. If it did describe a world ‘behind the phenomena’, that world was so esoteric as to be literally unimaginable. The very language it used was broken: an analogical extension of the classical language that it discredits, and redeemed at best by the mathematics that it tries to gloss.
Interpretation of quantum theory became genuinely feasible only after von Neumann's theoretical unification in 1932. Von Neumann himself, in that work, attempted to codify what he took to be the common understanding.
Astonishingly, the attempt led him to assert that in measurement something happens which violates Schroedinger's equation, the theory's cornerstone. As he saw very clearly, interpretation enters a circle when its main principle is Born's Rule for measurement outcome probabilities, while at the same time measurements are processes in the domain of the theory itself. Behold the enchanted forest: every road leads into it, and none leads out—or does the hero's sword cleave the wood by magic?
An empiricist bias will be evident throughout this book, but my own interpretation of quantum mechanics does not begin until Chapter 9. The first three chapters provide philosophical background; though they overlap my Laws and Symmetry, I have tried to make them interesting in their own right. The next four chapters mainly outline the achievements of foundational research, though with an eye to the philosophical issues to come. The negative part is to show that the phenomena themselves, and not theoretical motives, can suffice to eliminate Common Cause models of the observable world. The positive part is the conclusion that there are adequate descriptions of measurement—in the sense required for Born's Rule—internal to quantum theory. To make the book relatively selfcontained, Chapters 6 and 7 introduce all the quantum mechanics needed for the philosophical discussions to come.
From a purely philosophical point of view, the most important clarification reached since 1925 concerns the criteria of adequacy for interpretations of quantum mechanics. It appears at present that more than just one tenable interpretation, already in process of development, can meet those criteria.
I regret that I may have done little justice to the promising interpretations now underway which differ from my own, although I have tried to point to them as often as I could. I regard every interpretation as increasing our understanding, and believe that an awareness of what rival interpretations may be tenable is crucial to clarity. But that attitude already needs defence, for it involves views on what science is, and what philosophy can hope for.
I have also tried to take the philosophical debates somewhat further, into the fascinating cluster of problems that concern quantum-statistical mechanics and identical particles. At every point, but here especially, I was acutely aware of rapid progress in foundational research and of the kaleidoscopically changing philosophical debates. It is true that interpretation focuses on a single theory at one more or less definite historical stage—and yet, what we try to interpret is not static. Every time we understand a little more, we change what we are trying to understand. It is not surprising that scientists often become impatient with philosophy: what is ever achieved if every generation has to face the same questions again, with a new understanding of what is being asked, unable to rest on past answers? But philosophy does not create our predicament. It is only a myth that modern science had arrived at a clear and well-integrated worldpicture, or that contemporary science has already effectively given us a new one. At best, we are in process of replacing what never has existed by something that never will. It is only in this unendliche Aufgabe, this reaching for what we cannot finally have or hold, that understanding consists