I am trying to better understand the current scientific consensus (to the extent that such a thing exists) on the interpretation of quantum physics. I understand that this is still very much an active area of research but it seems to me that there is a general belief that decoherence is some sort of holy grail?
When I first was introduced to QT we were taught around the formalism of the Copenhagen interpretation (vague and unsatisfactory concepts of observations/measurements with a totally arbitrary line-in-the-sand between quantum and classical) and it was my understanding that the so called, "Measurement Problem" was still one of the big unsolved problems in physics. More recently however, I have gained the impression that a lot of people regard decoherence as the solution to this problem, but I have also seen specific claims that decoeherence does not attempt to resolve the measurement problem at all. Which is it? Is the measurement problem still a thing?
I have come across this paper which very explicitly claims that decoherence does not solve the measurement problem and never claimed to. So then, why are so many physicists behaving as though the problem has been solved?
A short outline of the paper:
It has lately become fashionable to claim that decoherence has solved the quantum measurement problem by eliminating the necessity for Von Neumann's wave function collapse postulate. For example, in a recent review in Studies in History and Philosophy of Modern Physics, Anderson (2001) states _The last chapter... deals with the quantum measurement problem....My main test, allowing me to bypass the extensive discussion, was a quick, unsuccessful search in the index for the word decoherence which describes the process that used to be called collapse of the wave function. The concept is now experimentally veried by beautiful atomic beam techniques quantifying the whole process." And again, in his response to the author's response (Anderson, 2001), Our diverence about `decoherence' is real.
In a somewhat similar vein, Tegmark and Wheeler (2001) state in a recent Scientic American article discussing the many-worlds" interpretation of quantum mechanics and decoherence, ...it is time to update the quantum textbooks: although these infallibly list explicit non-unitary collapse as a fundamental postulate in one of the early chapters, ...many physicists ... no longer take this seriously. The notion of collapse will undoubtedly retain great utility as a calculational recipe, but an added caveat clarifying that it is probably not a fundamental process violating Schreodinger's equation could save astute students many hours of frustrated confusion.
2 bonus questions!
It is my understanding that decoherence is a gradual process? Something that occurs quite rapidly but not instantaneously (correct?). So then, what does this have to say about the old notion of the discontinuous "qauntum leap"? Does the mainstream physics community still regard this "collapse" process as a discontinuous thing or is my understanding out of date?
This may not be worded correctly but here goes... When does decoherence occur? Take for example the double-slit experiment, if "collapse" occurs before the two waves interfere, then there will be no interference pattern. If "collapse" occurs after interference then there will be an interference pattern. This tell me that the process of "collapse" has a definite time of occurrence. So then, why does it happen when it does? What is preventing decoherence from occurring before the particle/wave passes through the slits? What is so different about a "measurement apparatus" compared to a molecule of nitrogen in the air or some other such thing? If I am to believe that it's simply a matter of having a large number of hidden degrees of freedom then where do you draw the line? Precisely how many free particles does an object need to have before we call it a "classical" object?!