Hi Eric,

You are asking important questions which have been discussed by this group but to which imo, no satisfactory answer was given.

First I'll widen the problem to place it in context. I can think of at least five ways that QM manifest itself:
1) Quantum Randomness
2) Spooky action at a distance
3) Superposition
4) Coherence/Decoherence
5) Interference

The first two phenomena, randomness,and spooky action at a distance can easily be expained by many-worlds and COMP.

1) Randomness:
A quantum die appears to be random because when it is thrown, all the faces come up, but, because of the many-worlds we can observe only one face.

2) Spooky action at a distance
When a quantum phenomena appears to invove spooky action at a distance, there is no action at all but an old magic trick. When a distant observer, makes his observation, the universe (or himself) has already split and his observation is just a confirmation of which branch his consciousness has followed. The rabbit is already in the hat before the beginning of the trick.

The last three phenomena cannot be explained so easily with many worlds +COMP because they involve the simultaneous and possibly interfering existence of several worlds. It is as if our consciousness occupies simultaneously a multiplicity of "classical?" worlds. In fact it appears that for consciousness to exists it MUST span several "classical?" worlds.

In other words, simply assuming the plenitude, filled with a multiplicity of observer moments, is not sufficient to explain superposition, coherence/decoherence and interference. As consciousness experiences its own propagation from one observer moment to another observer moment, it is constrained by interference. Why is this necessary? There is another layer besides many-worlds, and COMP. What in the nature of consciousness makes such a layer important?

George Levy

Eric Cavalcanti wrote:

I think this discussion might have already took place
here, but I would like to take you opinions on this.

How do we define (de)coherence? What makes interference
happen or be lost?

Take the a double-slit-like experiment. A particle can take
two paths, A and B. We can in principle detect which path
the particle went through.

Suppose we can make the detecting apparatus 'non-interfering'
enough so that the particle is not grossly deflected by the
detection, but can still reach the screen. We know that the
result of this thought-experiment is that interference does not

The first answer is that the paths have 'decohered'. But what
exactly does that mean? In a MWI perspective, I like the
explanation that the two universes A and B are different by a
large number of particles: the electrons in a wire, which carry
the amplified pulse of the detector, which then reach a
computer, and such and such. Something of the order of 10^23
particles have changed state.

Now suppose we use some kind of very slow detector. The
detection is made by, say, a very slow process such that not
many particles (suppose only one particle, even though I don't
know how to make that detector) change their state before the
interfering particle reaches the screen. After that, we can amplify
this information and know which path the particle went through.
Again, I believe interference would not be possible. But it is a
little harder to say why.

Before anyone says that *some* other particle has changed
state, and that should be enough for the decoherence, suppose
now an experiment with a charged particle, say, an electron.
We make it go through paths A and B by steering it with
magnetic fields. Certainly, it has interacted with *something*,
ot it could not be steered. It interacted with the photons of the
EM field, or in last analysis, with the electrons that are
generating it. We could use electric fields, so that the interaction
is more evident. On the other hand, a photon being reflected
by a mirror is also interacting with something, but that does
not prevent the interference from happening, as is well known.

Therefore interaction by itself does not cause decoherence.
And if it is just a large number of particles changing state that
does, then what is the threshold? Would an experiment with a
few-particle-delayed-detector as described above allow
interference? Or is it the 'information' that causes decoherence?
If that is the case, how does one define 'information'?


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