On Thu, Jan 28, 2021 at 9:51 AM 'Brent Meeker' via Everything List < [email protected]> wrote:
> On 1/27/2021 2:28 PM, Bruce Kellett wrote: > > 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. > > > That's where I think there is still a gap in the theory. We know that in > the C60 double slit experiment the interference is wiped out because > sufficiently short wavelength IR photons from the buckyballs record their > position in the environment, presumably when they are absorbed in the > laboratory walls. But what would happen if they weren't registered any > where. What if the experiment were in outer space and the IR photons just > went off into infinity in a spherically symmetric wavefunction that never > "collapsed"? > I thought that was answered in one of Zeilinger's delayed choice experiments. The idler photons that carry the 'welcher weg' information do not have to be measured or intercepted. As long as they exist anywhere in the universe, the interference is destroyed. You have to actually 'quantum erase' the 'welcher weg' information they carry in order to restore the interference. Running the idler photons into the wall is not quantum erasure..... Bruce 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/CAFxXSLQAfwYCtRR8oZ%3Dd3AVhTpcDLP6Am3G73xoK%2BsujKA-_Pw%40mail.gmail.com.
