About observability

lugita15

OK, then referring to that paper can you try and answer my questions?

Demystifier

Not everything, only the unitary time evolution of the quantum state (wave function).

It cannot be predicted, because collapse into a one definite state is a nonunitary event, which Schrodinger equation does not describe.

No. Or if you disagree, try to find a statement in the paper you mentioned which confirms your view.

A naive original version of Copenhagen interpretation can be disproven by quantum Zeno effect experiments, but it is still possible to introduce a refined version of Copenhagen interpretation which is compatible with quantum Zeno effect experiments.
See also
http://xxx.lanl.gov/abs/1009.4072

martinbn

Demystifier, I don't follow. He says clearly "in the absence of collapse", then you keep saying "no, because, collapse....".

Also, why is quantum Zeno a disproof of Copenhagen?

lugita15

Yes, but isn't the whole point that we have the appearance of collapse into a pointer state without actually having real collapse, so that everything stays unitary?
I have to find time to reread the Zurek paper, but if I recall it just discusses how the interaction Hamiltonian selects a preferred pointer basis, so I don't think I'll find any confirmation there.
That's really remarkable if true. I guess I'll have to look into that as well.

Demystifier

Please quote the whole sentences, not part of sentences out of context!

Copenhagen, or at least a naive version of Copenhagen, claims that there is a strict border between classical and quantum. In this sense, it claims that measurement cannot be described by quantum mechanics, and that there can be no partial collapse.

Quantum Zeno (or more generally, decoherence) demonstrates that there is no strict border between classical and quantum. More precisely, the density matrix evolves continuously during a finite (but typically very short) time from quantum to classical.
In this sense, measurement can at least partially be described by quantum mechanics. In particular, by certain physical definition of measurement apparatus, something closely related to "partial collapse" (that is, partial diagonalization of the density matrix) is possible.

Demystifier

That is the point of many-world interpretation of QM (which to a large extent rests on decoherence) but that is not the point of decoherence per se.

I agree, but then why did you say something which was not said in the paper?

lugita15

So if many worlds has an explanation of which pointer state the wavefunction appears to collapse into, doesn't that mean that unitary time evolution is sufficient to tell you what the state is?

Demystifier

But I didn't say that many worlds explains to WHICH pointer state the wavefunction appears to collapse. It definitely does not. At best, many worlds explains why the wave function appears to collapse in only one pointer state, but it definitely does not tell WHICH one.

lugita15

I'm a bit confused. In the absence of real collapse, the entire future history of the system is determined by unitary Schrodinger evolution, so if you know the initial quantum state and the Hamiltonian then the future is completely predictable, correct? So then how is it that these two pieces of information are insufficient to tell you which pointer state the wave function will appear to collapse into?

Demystifier

That's because in the many-world interpretation you need to distinguish two levels of reality:
(i) the multi-world, and
(ii) the single world we actually see
By knowing Hamiltonian, initial quantum state, and the Schrodinger equation, you can predict everything about the multi-world. In particular, you can predict that this multi-world will consist of MANY ordinary worlds. But it does not tell how to single out one particular single ordinary world out of this big set containing many ordinary worlds.

lugita15

Are you saying that in different worlds decoherence will lead to the wave function appearing to collapse into different pointer states, so that ignorance about what world you are going to branch into translates into uncertainty about what pointer state you will see the wave function appear to collapse into?

Demystifier

Yes, that's more-or-less what the many-world interpretation claims.

See also this very readable, good, short recent paper on it:
http://lanl.arxiv.org/abs/1110.0549 [to appear as a Brief Review in Modern Physics Letters]