Pollock said:
Surely the principle of superposition is all in the mind.When a particle or system can have several equally probable values we don't KNOW which until we measure it.But that doesn't mean that it didn't have a definite value before the measurement;the electron must always have a spin either+/_ 1/2.Superposition is just an admission of our ignorance about a system.
One could equally ask of an unobserved particle: "Surely the principle of position is all in the mind".
While the concept of superposition, as much as position, is something that does take place in the mind, that's only because the mind is as good a place as any to entertain such concepts. But in and of itself the concepts would have only mathematical (and perhaps philosophical) interest if they weren't actually allied to the
practice of physics. There is a relationship between the concepts and that which can be physically observed (be it human or machine observation). And how one might exploit such concepts technologically.
The concept of super-position may very well express ignorance, but the concept of position is not any different. In any location where we do not intervene (ie. make a measurement) we can only
imagine what might be there: be it a particle having a position, or a particle having super-position.
If we adopt position as our preferred concept it is not any more reasonable than if we had adopted super-position as our preferred concept. For what would be the reason for using the concept of position? Both concepts take place in the mind because there is no immediately visible interaction occurring at the nominated location in space. One could even argue there's no physics taking place at all.
However once you factor in time (duration, intervals etc.) the concept of 'no immediately visible interaction' is seen to be too tight a restriction on what might actually be physically visible at a particular location in time and space. Could measurements made elsewhere constitute a measurement of what is happening at a given point? If nothing else events occurring at other points in space and time can provide a concept on what is at least
potentially visible at a particular location in space and time.
The concept of super-position (wave functions etc) emerges in this context. And they are particularly apt concepts precisely because we're not performing a localised interaction with the location in question. We are asking instead, what would happen
if we made a measurement at that location BUT, at the same time requiring that measurement not occur there - ie. so it can take place elsewhere instead. And not only that but the concept should also be consistent with measurements that are actually made at the location in question. And the concept of super-position (etc.) does a pretty decent job of satisfying that rather difficult problem. At least a whole lot better than just the concept of position.
As to whether super-position exists, the answer is obviously yes. How can we be talking about it if it doesn't exist? It is a very well defined concept. If you'd asked that same question a 100 odd years ago the answer might be no. It hadn't been conceived in it's modern form. Interestingly the roots of the concept are not entirely modern. Indeed one can trace the roots back to some very old ideas in Ancient Greece. The Stoics were particularly interested in some of the root problems, and how such might be logically resolved. Strangely enough. Not because they were studying the rather strange world of quantum mechanics, but because there are macro-scale problems that have a related difficulty (to do with the probability of naval warfare and whether the Gods know the outcome and if they do is it pre-ordained, etc). The Stoics elaborated propositional logic to deal with such problems (imposed on them by stressed out naval commanders).
C
