Re: MWI experiment proposal

From: Bruno Marchal <>
Date: Tue, 18 Jun 2002 12:02:04 +0200

At 8:22 +0200 18/06/2002, scerir wrote:
>I think that MWI + decoherence = Copenhagen Interpretation.

Not at all. It is a widespread misuse of decoherence.
Actually decoherence justifies completely the *appearance*
of wave collapse in the memory of observers, and this without
any ontological collapse.

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 elemntary 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. 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

The MWI view of decoherence is, like Charles said, the fission
or the differentiation of the universes.

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".

There is something fundamental here: if you accept the strict
linearity of description of a system, you are lead by the formalism
itself to the apparent non linearity of the description of the
relative subsystems.
This has been explained by Hugues Everett in 1957.
Also, the modern champion of the decoherence theory, Roland
Omnes, realised so clearly that decoherence makes "collapse"
useless that he considers you *need* to be irrational for
keeping QM and *unicity* of the universe (and collapse!).
Now he defends that form of irrationnality because he dislike
the many-thing idea, but this is aknowledged wishful thinking.

Received on Tue Jun 18 2002 - 03:00:03 PDT

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