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