Hi again,
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Originally Posted by Dave2042
Finally, I thought I'd clarify the issue of correspondence between QM and classical. This is in fact a directional issue. QM taken to a large-scale limit confirms and agrees with classical physics (or we'd have trouble explaining why civil engineering works) - no conflict. Not the same the other way, though. Classical applied to small-scale phenomena (eg atoms) fails spectacularly, and while semi-classical approaches can sometimes be useful, they clearly fail to capture something essential at that scale, and not for want of trying.
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Just wanted to emphasise Dave's point here because I think it is important. Another simplistic way you could say it is that classical physics is a special case of quantum physics (e.g., a quantum system minus entangelment/coherence = classical).
And as Steve points out (and I sort of brushed over) how you deal with the "observer" and her "measurements" in a particular situation largely determines which approach (quantum or classical) will be more useful.
Craig, I think your quest
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Originally Posted by CraigS
I am deliberately attempting to find ways of avoiding distinguishing QM over classical at the fundamental levels, as I think it should be perfectly feasible and legitimate to do so.
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is doomed to failure (no offence intended!). Quantum and classical essentially describe how the world works but they cover different domains - they are only useful and "correct" in their particular domain. Classical physics will never be able to properly describe a quantum system, and it would be very difficult to describe a classical system using quantum physics (not becuase it's wrong, but because the mathematics would become intractable).
This may not be a good analogy (becasue they are both classical models) if you look at Newton's laws of motion (e.g., F=m.a) and special relativity, we know that Newton's laws of motion are "wrong" becuase they don't take into account relativistic effects, but in 99.9999% of cases you'd get by with Newton's laws. We also know that special relativity is "wrong" becasue it only holds in inertial reference frames (hence the need for general relativity), but again, how often are you going to need GR when calculating the trajectory of a cricket ball? OK, if you are going to play cricket on horizon of a black hole you would, but then you'd have more important things to worry about than hitting the ball
I guess the point I'm trying to make is that mathematical theories always have a domain of applicability - as you cross the domains one theory may be able to morph into another (GR -> SR -> Newton), or maybe not. You just have to know which one to use in a particular situation. For me whether "reality" corresponds to the mathematics becomes a bit of a philosophical question (and therefore there may be no right or wrong answer), but one thing we do know is that quantum physics is an excellent theory for describing quantum systems, and has features that can never be derived from a classical model.
John.