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Re: [Phys-l] Reality versus models of realty



It's far too easy to end up in very silly places when discussing what one might mean by the nature of "reality". I have no intention of wading into this beyond the following comment.

The observation is produced by the observer.


This is not an accurate characterization of the current state of understanding of quantum theory. What's called "measurement" in textbooks is a physical process in which interference between states (usually, but not necessarily, macroscopically distinct ones) is destroyed, usually to a fantastic degree of accuracy -- but (almost) never perfectly. (As an aside, it's this latter point that most interests me personally, especially as concerns the topic of this thread.) The complete state resulting from a physical process of this kind is a superposition of distinct (i.e. orthogonal -- or very nearly, anyway) states. This is usually called "decoherence", and it is a ubiquitous occurrence in nature. In each of these (approximately) distinct "branches" of that superposition, subsystems (such as physicists and their lab equipment) have the experience of a single, more or less definite quasiclassical world. As far as this goes, there is no need whatever for observers, consciousness, or any of that ridiculous mumbo-jumb conjured up in a desperate effort to make sense of it all.

What's less clear is what to make of the picture of reality this leaves you with, or whether it needs to be added to. Roughly speaking, there are two broad camps here. In one camp, this is what you get. None of these branches is any better or worse than any other, just different, and they're all equally "real".

For many, that's just way too much. (And the better you understand it, it really is a quite fantastic picture of "reality"!) They seek to add SOMETHING to the theory which "picks" a branch and makes THAT one the "real" one. This turns out to be quite hard to do in a way that really makes sense, even by fiat, thought that is what Bohr tried to do, and what you find in most textbooks.

Either way, the resulting picture of the world is interesting and fantastic. And with just a little luck, what's really going on will even turn out to be experimentally accessible. Mechanisms which "pick" a branch more or less necessarily require a CHANGE to quantum mechanics of some kind, and that should eventually be measurable. Hope is much greater now that these are not simply metaphysical questions.


David Craig


<http://web.lemoyne.edu/~craigda/>