The way I would interpret this is to say that the photons always go
through both slits. Then, depending on the experimental setup, you can
entangle them with the universe such that coherence is lost, or you can
do it such that they remain coherent and interference can be observed.
This really happens at the point where they hit the detectors (or perform
some other irreversible operation), not before.
It's wrong to say that the photon went through one slit, even if you
put your eye up to it and *see* it go through. The photon went through
both slits. However, you entangled yourself (and hence the rest of the
universe with which you can interact) with the photon state such that
you were not able to see the consequences of the photon going through
both slits. Coherence is lost and the two possible paths can no longer
interfere. Effectivelly you have split yourself into two states which
can not interact, each of which sees the photon take a different path.
(I think it was B. Allsop who wrote:)
> >Surely any old physical "macro" event, any computer (quantum or not)
> >that records and represents the information, or even the information
> >traveling up the optic nerve or a rod or cone neuronn in the retina
> >firring... can do just as much to "collapse" a wave as phenomenal
> >conscious representations can.
I agree (mostly). There are many studies showing how this kind
of environmental interaction transforms the state from a quantum
superposition to a classical mixture. That is, interference between the
different branches is no longer possible. The exception is the quantum
computer, which is specially constructed in order to preserve the full
state information, and allow quantum interference effects to occur.
Hal