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As often presented, QM asserts that the system collapses under measurement. But for a fundamental theory, measurement as an external thing doesn't make sense. T
by jostylr 11y ago
As often presented, QM asserts that the system collapses under measurement. But for a fundamental theory, measurement as an external thing doesn't make sense. Thus, when does collapse happen? This is that cat thing.
One can shrug and say, we know what we need for computations. But a fairly fundamental theory should be cleaner. Of course, there is a perfectly simple theory (Bohm's) involving Schrodinger's equation and a guiding equation for particles that has all of this quantum formalism derivable from its dynamics.
A paper by Bell that goes into this point more clearly than I can articulate: http://www.tau.ac.il/~vaidman/IQM/BellAM.pdf http://www.tau.ac.il/~vaidman/IQM/BellAM.pdf
- stared 11y agoDecoherence-based interpretations (Everett, Żurek, Fuchs...) do not have measurement as anything separate - it's an emergent phenomenon based on the (already present) rules of unitary evolution. So for me its a clear argument why some of the interpretations make more sense that the others.
- cygx 11y agoDecoherence alone doesn't solve the collapse problem.
- stared 11y agoCould you expand on that? Once you get decoherence, instead of 'measurement' or 'collapse' you get splitting into subspaces, which get practically separated. Sure, modeling decoherence is not always easy. (And, 'collapse' is often a nice simplification for practical purposes.)
- jostylr 11y agoYes, practical separation occurs. But you still have to explain why our experience is that of a single outcome when the only thing in the theory (the wave function) says nothing of the kind. Now, this can be rectified by using many worlds, for example. Particularly, integrating out the wave function to get a mass density on 3-space (non-relativistic version) does produce a valid theory which has multiple realities whose evolutions internally would appear to have single outcomes even when that is not actually true. Fuzzy tv reception with multiple overlapping channels is the mental model here. However, that is not just decoherence. One has to add something. And the question is, why choose a theory that denies the truth of our common experience in such a fundamental way when one does not need to do so? Pilot-wave theory is, subjectively, very simple and natural. It is not epicycles upon epicycles. In fact, quantum theory practically screams out for it with the existence of the probability current and the many insights that that current drives. There are at least 5 very distinct ways to derive pilot-wave theory from quantum notions. One last thing to ponder. Wave functions are defined on the configuration space of the particles. The specific space is crucial to defining the evolution operator. Fermions and bosons emerge naturally by considering the configuration space of unlabeled particles. But if you have no particles, why is a configuration of them useful at all?