Chapter 10 – Edition 1

Some notes on Chapter 10: Wonderful, Wonderful Copenhagen

We emphasize to students that quantum mechanics needs interpretation because it challenges our (basically Newtonian) worldview. Interpretations are, of course, not testable scientific theories. They are ways of looking beyond the science for the “meaning” of the theory. While Copenhagen is the interpretation almost all physicists implicitly adopt in their work, in Chapter 14 we discuss nine other interpretations that currently contend.

Back when I studied quantum mechanics in graduate school it was generally implied that Copenhagen resolved all philosophical problems, that Copenhagen is the “right” interpretation. (Rudolph Peirels (check spelling?) objected to the name ‘Copenhagen interpretation’ because it implied other interpretations were possible.”) I can’t quite remember why we so blithly accepted the “collapse of the wavefunction”–everywhere, instantaneously.

The Copenhagen interpretation was developed in the early 20th Century, when the philosophical stance of positivism was in the air. (Positivism: “a philosophy which holds that the only authentic knowledge is that based on actual sense experience. Metaphysical speculation is thus avoided.”) Physicists certainly wanted to avoid “metaphysical speculation,” and in physics our “actual sense experience” are the output of our macroscopic experimental apparatus. Copenhagen went beyond positivism to claim that unobserved microscopic properties were not just irrelevant, but did not even exist until they were “observed” by the measuring apparatus. This is a stand that surely works, at least for all practical purposes. Copenhagen is a pragmatic stance, and physicists tend to be pragmatists. (A bumper sticker summary of “pragmatism”: If it works, it’s true.)

In our discussion of the Heisenberg microscope we rather glibly say that the wavelength of the light used must be smaller than the separation of the boxes (or the slit separation in the standard presentation) in order to tell which box the object came from.  It’s a rather tricky point we did not wish to belabor. (We had to make such decisions many times in writing   a book addressed to multiple audiences.)  A good way to give students a feel for this is to draw a series of arcs (wavefronts) emanating from each opening. When the wavelength is smaller than the separation, the wavefronts from the two openings essentially overlap and are thus indistinguishable.

The Heisenberg microscope argument is sometimes mistakenly offered as resolving the “wave-particle duality,” or even the paradox presented by quantum mechanics: it proves you can’t see wave and particle problems at the same time. That misses the point (as Heisenberg clearly emphasized). The problem is that you could have chosen to show that the object was in a single box (came from a single slit) OR you could have chosen to show that it was not in a single box (came from both slits). The essence of the reality problem posed by quantum mechanics depends crucially on the “free choice of the experimenters,” or our assumption of free will.

As we discuss (p. 109), Bohr was keenly aware of this and, in a version of Copenhagen dismissed any talk of experiments that might have been done but weren’t done. That’s essentially the “complementarity” argument. This is not just a denial of “free will” in the sense it is often popularly used, and in arguments by psychologists and philosophers. It’s saying that whenever the experiment demonstrating the object was wholly in a single box is chosen, those objects are indeed wholly in a single box. And when the alternate experiment is chosen, those objects are indeed not wholly in a single box. It’s postulating a totally deterministic world where our supposedly free choices are completely correlated with all the physical phenomena around. Philosophers call this a denial of “counterfactual definiteness.” John Bell calls this solution “more mind-boggling” than the problem it purports to solve.