would you say the Copenhagen Interpretation is still the Orthodox interpretation of QM?

]]>I’m not interpreting the squared amplitudes of a superposition as probabilities!

My whole point is that in decoherence theory, there are no probabilities at level of the system+measuring device+environment wavefunction.

Classical probabilities only emerge after we trace over the environment. After that, we obtain a density operator wich describes the system+measuring device.

The classical probabilities are the eigenvalues of the density operator.

]]>I think our main source of disagreement is you assume once you see a measurement outcome, it is obvious the probability distribution has collapsed. To me, all that’s obvious is you’re entangled with that particular outcome and you have no way of accessing the other, which nonetheless keeps existing as an unaccessible part of the system’s state, thus effectively splitting into two separate realities.

“This doesn’t mean there are infinitely many guitar strings in parallel universes”: precisely. That’s why “parallel universes” is a misnomer. There is only one wave function one can split into many components which cannot interact. ]]>

“QM became stronger — as if it needed any strengthening.”

The Copenhagen interpretation was not able to derive the preferred basis, it was chosen by an ad hoc rule. This was an important problem of the interpretation.

In decoherence theory, the preferred basis of the system+measuring device Hilbert-space is generated by the unitary dynamics of the system+measuring device+environment.

So QM needed strengthening and it is stronger now.

32. David

Sorry, but I completely disagree with you.

“Decoherence explains how a superposed state “splits” into two classical ones, but I don’t see it as endorsing CI. In fact, decoherence does not predict a collapse but a splitting, which is much more in line with MWI! For the collapse you still need the measurement postulate.”

No, decoherence theory gives you a density operator and the eigenvalues of this operator are classical probabilities.

And classical probabilities collapse by definition, when you learn the outcome of a measurement. This fact has nothing to do with quantum mechanics. If I throw a classical dice, then the probabilitiy distribution is (1/6, 1/6, 1/6, 1/6, 1/6, 1/6). But when I learn that the result is 3, then the probability distribution collapses to (0, 0, 1, 0, 0, 0). In my opinion, there is nothing more to explain here.

“These we call “parallel universes”, which is probably a misnomer since we’re only talking about different parts of one wave-function. That is, decoherence taken seriosuly is MWI.”

A superposition has nothing to do with parallel universes. I can represent the motion of a classical guitar string with a Fourier-series, but this doesn’t mean that there are infinitely many guitar strings in parallel universes, there is only one.

]]>Here’s the picture: there is only one wave function carrying all the information about the experimental device and the experimenter. When the measurement is performed, two “regions” of the function decohere, meaning they become unable to interact with each other, therefore seeming like two effective separate wave-functions. These we call “parallel universes”, which is probably a misnomer since we’re only talking about different parts of one wave-function. That is, decoherence taken seriosuly is MWI.

If you want to explain the classical outcome without retorting to MWI, you have to postulate the collapse of the original wave-function, that is, the fact that after decoherence the wave-function “chooses” one of its parts and discards the other. That seems to me as completely ad-hoc and unjustified, since the previous interpretation already gave the results we see! ]]>

What Scott said. QM became stronger — as if it needed any strengthening.

“Bohr did not need to explain how the wave function collapsed because it was never, to him, a physical wave to begin with.

Yeah, it just about our knowledge of world, not the world itself . . . ?

]]>NO, their work is based on decoherence theory. Decoherence is a physical phenomena, you can measure it in the lab, and it solves the “preferred basis problem” of the Copenhagen interpretation. So the Copenhagen interpretation actually became stronger.

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