OK, so I’m not an especially gung-ho physicist, which is why I heard this story via boingboing. This is interesting physics: all students of physics, when they first learn about quantum mechanics (QM) and modern physics, are told that the Copenhagen Interpretation of QM is correct and preferred; the party line, essentially. And that’s the last they hear of how to understand the theory: the emphasis in the education of a physicist now is on being able to manipulate the mathematical formalism to generate a solution to a problem. (I have to admit, that approach has appealed to my strongly pragmatic nature for a long time.) An interpretation is, as I understand it, a set of principles stated in English (which may, of course, be translated to a firmer symbolic logic notation) with which results of QM experiments may be predicted qualitatively. For example, Conservation of Energy is a principle which may form part of an interpretation.

The news bit is this:

[Harvard University Visiting Scientist Shahriar S.] Afshar has done a variation of the standard two-pin-hole “welcher-Weg” [lit. “which way”] optics experiment, in which he demonstrates that wave interference is present even when one is determining through which pinhole a photon passes. This result is in direct contradiction to Neils Bohr’s Principle of Complementarity, which would require in the quantum world that when one is measuring quantum properties, all wave interference phenomena must vanish. Afshar’s trick is to find the location of the minimum points of wave interference, place one or more wires at these minimum points, and observe how much light is intercepted when one is determining the pinhole through which the photons passed.

This experiment contradicts the Principle of Complementarity, which states that the wave behavior of an object can never be seen at the same time as its particle behavior. The experiment clearly shows both behaviors at the same time.

This is almost enough to get me excited about physics, again. But not quite. Anyway, there’s a good (read *lucid*) book on these philosophical issues of understanding QM: *The Structure and Interpretation of Quantum Mechanics* by R. I. G. Hughes. The links scattered about in the text above are taken from Cramer’s posting.

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