|MadSci Network: Physics|
as far as I know, you are not quite right. "decoherence" and "collapse of
wave function" mean the same thing. you are right that wave function
collapses when it is observed following Copenhagen interpretation. von
Neumann constructed a somewhat different theory of the measuring process,
especially of the collapse of the wave function:
before the measurement, let the system S (to be measured) and the measuring device A be in a well defined state. the measurement entangles the state of the system and the device, such that you can read off the device the state of the system. but coupling the system to the measuring device does not result in a collapse of the wave function. the observer has to read off the result from the measuring device, that means there is another measurement process involved, where the system (to be measured) consists of the system S and the device A. that's a never ending story called infinite regress. von Neumann said that the collapse of the wave function happens when the human observer perceives the result of the measurement.
the collapse of the wave function in this case is more related to the observer's knowledge about a system than in Copenhagen interpretation.
however, the spooky action at a distance, as you call it, has to be there, if we want to deal with a theory describing submicroscopic phenomena properly. this has been shown experimentally, by showing that the so called Bell's inequality is not satisfied. for further comments on Bell's inequality and non-locality in quantum physics see e.g. the MadSci questions Does Prof Aspect's work mean that information travels faster than light?, What are the implications of Quantum Mechanics being non-local? or search our archives for Bell's inequality.
I hope I could help you,
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