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Beware Quantum Interpretations

Nature has recently published a special edition celebrating the 50th year of Hugh Everett’s Many World’s Interpretation of quantum mechanics (thanks to Slashdot for bringing this set of articles to my attention).

I think one of the things people forget about when discussing “quantum interpretations” is that they are just that: interpretations. The physics is self-contained within the mathematics and procedures of quantum theory. Like any good science, its strength is in its ability to predict experimental outcomes. The implications of the math may be murky, but the power of the theory is independent of interpretation (as long as you know how to set up problems!). Any valid interpretation of quantum mechanics is subjective, inherently experimentally indistinguishable from another self-consistent interpretation. If new predictions actually emerge from a specific interpretation then one interpretation is somehow experimentally distinguishable from another, and you aren’t really talking about interpretations anymore — you are talking about some structural, scientific part of the theory.

There are perhaps a dozen or more self-consistent interpretations of quantum mechanics on the market, most of them dating back over 50 to 80 years, all of them (apparently) experimentally indistinguishable. Some are downright silly, for example, the Copenhagen interpretation, while others tickle the imagination like Many Worlds. Fads will come and go, spontaneously favoring one over another. But despite what people like to believe, there is no experimental way (yet?!) to distinguish between Copenhagen and Many Worlds interpretations. Perhaps one charismatic theorists pushes for one for a few years or the community adopts it as a default favorite for a while, for no other reason than fashion. But the actual science that is done with quantum mechanics marches on independent of all that. The interpretations serve basically three roles: 1) psychological: it lets a physicist adopt something so they can sleep at night with a very odd, unintuitive mathematical framework; 2) insight: one interpretation may provide problem solving insight into new or old problems, which in turn might give one interpretation a creative advantage for inventing new ideas on a problem by problem basis; 3) new science: perhaps what you are dealing with isn’t an interpretation at all, but rather something fundamental (and thus experimentally distinguishable from other interpretations) and so exploring all its possibilities is natural (hedging your bet in case the theory can be augmented).

For all three reasons above, I personally enjoy thinking about the various quantum interpretations out there, so I certainly don’t want to discourage people who like to dabble. What I don’t like is when people (especially physicists like myself) start talking about how “the world really is” through these interpretations or speak of them as if they were accepted facts of reality or based on experimentally confirmed physics results, which is certainly not the case for MWI, for example. What we have is basically a powerful engine for calculating things, but the physical meaning of the engine can be viewed many ways and still function as the same engine. To ask what the engine is “really” doing becomes metaphysics because all you can apparently actually measure are the inputs and outputs.

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