[ExI] Passive Observations

[Lee Corbin]

Ian writes

> And read the description of Wheeler's delay-choice
> experiment, starting toward the end of page 1 and
> going on to page 2 here:
> 
> http://discovermagazine.com/2002/jun/featuniverse
> 
> This paragraph speaks directly to your example:
> 
> "By the time the astronomers decide which measurement
> to make - whether to pin down the photon to one
> definite route or to have it follow both paths
> simultaneously - the photon [ from a distant quasar ]
> could have already journeyed for billions of years,
> long before life appeared on Earth. The measurements
> made now, says Wheeler, determine the photon's past.

That's because this vintage of Wheeler doesn't believe
in MWI.  

> In one case the astronomers create a past in which a
> photon took both possible routes from the quasar to
> Earth. Alternatively, they retroactively force the
> photon onto one straight trail toward their detector,
> even though the photon began its jaunt long before any
> detectors existed."

One of the nice things about MWI is that the delayed choice
paradox doesn't arise. Two photons of course left that faraway
quasar so very long ago, one in one universe, one in another.
The question is whether they'll be allowed to recombine when
they approach the Milky Way. If at the last minute we decide
to allow them to interfere, then, duh, there were two photons
emitted.  If we decide not to allow interference, then there
still were two photons, but now one of them crashes into
our interference blocker (e.g. a rock) in one universe, and
the other passes via the other route right past the apparatus
in the other universe.

Lee