[extropy-chat] Role of MWI and Time Travel

[Lee Corbin]

Serafino writes

> Lee wrote:
> > As for overpopulation [MWI, but backwards in time], 
> > I would simply suppose that the version of me 
> > who saw the photon go up would smoothly merge 
> > into the version of me who saw it go straight; 
> > if we branch into separate versions going forward 
> > in time, wouldn't it be very natural to merge 
> > into a single version going backwards?
> 
> Imagine a source of photons, a beam of photons going to 
> a beam-splitter, and two detectors (one for each path).

Okay.

> According to the 'collapse' interpretation when a 
> detector flashes it means that one photon arrived there 
> (while the other detector remains silent). 

Yes, in the ugly and bizarre Copenhagen interpretation,
the universe somehow manages to forget all about one
possibility, entirely!

> According to MWI both detectors flash, but each one 
> in its own 'world'.
> Now, what if we time-reverse this process? 
> According to the 'collapse' interpretation each photon
> goes from the detector (that flashed), to the beam-splitter,
> and then to the source. So only 50% of photons arrive
> at the source (because of the beam-splitter).

I thought we were talking about a single photon. Now, if
we are talking about single photons, then the photon would
just gently time-reverse its course, and go through the
beam splitter backwards, and hence to the photon source.
Even in the ugly and amnesiac Copenhagen version, I don't
see a real problem---do you?

> According to MWI each photon goes from the detector 
> that flashed, to the beam-splitter, and then to the source.
> But since we have here 2 worlds (merging) ... 2 x 50% = 100% 
> are the photons that arrive at the source. [Unless there is
> some big mistake].

Yes. Seems quite tidy.  Now I admit that I don't really think
very well about time-reversed situations. All I do is play
a mental recording of the forward motion backwards. So I can
not really think of any problem that would appear in any
interpretation.

Lee