On Thu, Jan 28, 2021 at 3:08 AM Bruno Marchal <[email protected]> wrote:
> > Also, worlds interfere statistically, by do not interact at all. A term in > a superposition cannot interact with any other terms, but we can make them > interfering, like with the two slits. > Your grasp of the relevant physics is rather tenuous, I'm afraid Bruno. The idea of "worlds interfering statistically without interacting" is just a nonsense. There can only be interference if there is an interaction. And there certainly is an interaction between the photons on the two possible paths in the two slit experiment. The two paths arrive at the screen with different amplitudes and phases -- if the signs are the same, they add. But if the signs are different they cancel -- partially or completely depending on the relative amplitudes. The trouble is that David Deutsch has really screwed up the understanding of "worlds" for a lot of people. He has talked as though each path in the two slit case is a separate "world", and then has to resort to magic to reproduce the interference. The Everett concept of a "world" is a "relative state", in which an "observer" sees a definite result. This idea was made more precise with the introduction of the idea of decoherence, and generalized entanglement with the environment. If "worlds" are defined as the result of decoherent histories, then Deutsch's confusion should not arise. A "world" is the result of (FAPP irreversible) decoherence. There is no decoherence at the slits in the two slit experiment, so no separate "worlds" are formed. If you induce decoherence by measuring at the slits, then the interference pattern disappears -- you have certainly created a separate "world" for each path, but these can no longer interfere. That is part of the definition of the "worlds" that are created by irreversible decoherence. So the concept of "world" is, indeed, well-defined in physics. It might not be defined in logic or metaphysics, but this is of no concern to the working physicist -- we know perfectly well what we mean by "a world". And we can readily tell when someone is talking nonsense by claiming that "worlds interfere statistically without interacting". The superposition of the paths in the two slit case extends right to the screen: that is what produces the interference -- superposition means that the two components are added together with their intrinsic phases intact. If you destroy the superposition at any point, such as by interacting with the paths at the slits, there is no more interference -- you have produced separate "worlds" that can no longer interact so there is no interference. As Scott Aaronson is fond of saying: quantum computers work by interference, so the computations must all occur in one "world". As Scott recently posted: "BREAKING: President Biden signs executive order banning people from saying "Quantum computers solve problems by just trying all possible solutions in parallel"." Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLQfi1Ximm14BrP659fyrt5O3GH%3DLXH6ntt6GMkUaRbB-g%40mail.gmail.com.
