On Thu, Jan 28, 2021 at 10:46 AM 'Brent Meeker' via Everything List < [email protected]> wrote:
> On 1/27/2021 3:06 PM, Bruce Kellett wrote: > > 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. > > > As I recall, the experiment showed that you could erase the welcher weg > information *after *the Young's slits photons were already recorded, but > there was no test of not registering them at all...which is understandably > hard to arrange. That's why I had to postulate an experiment in outer > space. I suppose you could interpret the experiment as saying welcher weg > photons flying off into space will be registered somewhere sometime and > certainly won't have their information erased and so the experiment shows > that the interference pattern would be wiped out. But it's not actually > true that the idler photons will necessarily be registered somewhere > sometime. We see photons from the CMB. So photons can just get > redshifted so far that they no longer carry the necessary information. > I am not so sure that they didn't test the possibility of not registering the welcher weg photons at all. Calibration runs in which these photons are not detected always lead to the loss of interference. Photons interact only weakly with each other and with the atmosphere (and open space). That is why photons are useful for these experiments -- they don't lose coherence all that easily. So sending them out the window aimed at the sky is effectively to not ever detect them. The reddening due to the expansion of the universe does not destroy the information -- radio waves can still be polarized/detected or whatever. 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/CAFxXSLRcBztuD2Wa9FM8mZnL7Ro8PXFJJcP24xzzV5VvZeZ7Ag%40mail.gmail.com.
