I think this discussion might have already took place here, but I will post this to take your opinions on the topic.
How do we define (de)coherence? What makes interference happen or be lost? Taking the double-slit experiment in mind, with paths A and B, the first answer that comes to the mind is that the particle should not be detected while passing through the slits in a way that we can tell which path the particle went through. I like to understand this as if the universes A and B have changed enough, in the state of a macroscopic number of particles, in order for us to have access to that information. So we could postulate that different universes do not interfere. But is that detection merely the interaction of the particle with anything? I don't think so. For instance, suppose we use electrons in a double-path experiment. They take the two paths by being steered with magnetic fields, and then meet again to interfere in a screen. I believe this experiment would also produce interference, but correct me if I'm wrong. Well, the electrons surely interacted with photons of the magnetic field, or in last analysis with the electrons that are producing the magnetic field. Or we could use an arrangement of electric fields - before anyone argues that 'magnetic fields do no work' or something - in such a way that the interaction with some massive particle would be evident. On the other hand, a photon being deflected by a mirror is also interacting with something, but still that does not prevent interference from happening. If these experiments produce the interference pattern, then the mere interaction of the particle with anything does not by itself cause decoherence. What then does? Is it only when a large enough number of particles change their states? But what would be the threshold? Or is it the 'information'? But then how do we define 'information'? -Eric.
