> >I think that MWI + decoherence = Copenhagen Interpretation.
> Not at all. It is a widespread misuse of decoherence.
Perhaps, but there is also a good (imo) use of decoherence,
i.e. in papers by Wheeler, Tegmark, etc.
> Actually decoherence justifies completely the *appearance*
> of wave collapse in the memory of observers, and this without
> any ontological collapse.
But standard quantum mechanics, in itself, is observer-free. It
predicts, for a certain observable, the distribution of the
value that would be found (performing measurements).
(Yes I know the interpretation due to von Neuman, Wigner,
etc. which is not observer-free)
> It is really simple: take a cat in a superposition state
> alive and dead:
> (a + d)
> I don't write the 1/sqrt(2) for reason of readibility.
> Now suppose a cosmic ray r hits the cat so that would the cat
> be alive the ray would bounce in the up direction, and if the
> cat is dead the cosmic reay end in some down direction.
> That ray acts as an elementary observer!
> QM says (by linearity of interaction and evolution) that, after
> the hit we have a superposition like:
> (a r-up + d r-down) (**)
> Now, we could still observe interferences showing the existence
> of a superposition of the dead and alive state of the cat, IF
> we were able to track the cosmic ray and quantumly erase the
> up/down distinction.
Yes. Or if (a r-up + d r-down) is an isolated system.
> But IF we were not able to track that ray,
> then QM forbids to factorize the r-state and we are confronting
> a mixture of cat being either alive or dead.
> You can verify it
> by adding the human observer state when he looks to state (**).
> This explained why isolation is needed to keep the interferences.
> It is not because decoherence collapse the state, but because
> decoherence entangles the state with the environment (here the
> cosmic ray), and the environment cannot practically be
Yes. Well, the environment can be disentangled, in a laboratory.
> The MWI view of decoherence is, like Charles said, the fission
> or the differentiation of the universes.
Yes, I know. But do these universes interfere? Or not? Why not?
And if you reverse the time arrow, what happens?
> Laroche in France made an experience for "observing" decoherence
> of a "schroedinger cat" by sending "schroedinger kittens" on it...
> From the MWI (or pure SWE) view this is an observation of the
> local "splitting" of the "universes".
Schroedinger's cat experiments show that if system is strongly
coupled to the environment, it remains localized in one state.
For intermediate coupling, the system hops back and forth
between the states. Finally, at low coupling, the system
follows coherent oscillations between the states, with a damping
rate vanishing as the coupling to the environment goes to zero.
And this mixing of the states can lead to the coherence gap
separating the energies of the superposition states.
Schmid: Phys. Rev. Lett. 51, 1506-1509 (1983).
Chakravarty, Leggett: J. Phys. Rev. Lett. 52, 5-8 (1984)
Friedman, Patel, Chen, Tolpygo, Lukens: Nature 406, 43-46 (2000).
Sometimes you can "see" the superposition of (all possible) states
(cat dead, cat alive) in our usual universe.
I do not understand why that local splitting of "universes"
is so essential.