What's up in this case

1. drag

1,341
Greetings !

Here's an example by Steven Weinberg I read sometime
ago (in SciAm I think):

Imagine a light beam coming from a quasar billions of
light years away. It encounters galaxy superclusters
on the way and is split so that gravitational lensing
is observed when we look at the quasar from the Earth.
Now, let's take the uncollapsed WFs of photons in the beam.
We now have the choice of detecting the photons in
a 1 or 2 slit (as in the double-slit experiment) configuration.
If we choose to use a single slit and aim at one of the images
we'll collapse the WF of the photon (let's say we have
just 2 images). If we use two slits we'll collapse it and
get the ussual difraction pattern.

If we try to interpret these results it appears abvious
that we've decided upon the path - if any, that the photon
took during all these billions of years.

The above example is of course intresting in its own right,
and was used by Weinberg for emphasis of some of the "conscious
mind" stuff in various interpretations of QM. However, I'll be
using it in this post in a somewhat different way,
and want to inquire about physics rather than its
interpretations (though separating the two is not
that easy at this point).

First let's suppose that our 2 WF splits are both trapped
for a while in an orbit around two different black holes,
before they reach us. Let's further assume that the
light beam is extremely intense. So much that such
an amount of light at a close orbit would be able to affect
the BHs.

Now let's play the "what if" game:
The BH's are affected by the light (the WFs of
some of the photons in the beam will have to collapse for that
to happen, I guess) and we detect the shifts in its
radiation due to these gravitational distortions
before the quasar's beam reaches us.

Now, let's say that some WFs did not collapse. I assume
we'll agree that these will not affect the BHs initially ?
And now, we run one of the above versions of the experiment.
What will happen in each case ?

Thanks !

Live long and prosper.

Last edited: Nov 21, 2003
2. drag

1,341
Well, anybody ?

Anyway, here's a simpler way to look at it that
I cooked up:
There exists a variation of the double slit experiment
when there's a mirror with equal chances to reflect the
photon in 2 different directions, eventually
the split WF converges after passing through a single slit -
sepearate for each part and far enough not to allow them any
difraction. Now you again have the choice of measuring
in different ways.

I'll add something to this experiment - at each separate path
of the split WF we'll position something that will be affected
by the passage of the photon. I'll be able to check that
something (in case the WF actually collapses in one of the paths)
before I detect the photon.

Now, I run the experiment and detect the photon at the end.
The same dellema as in the enitial case follows :

If I measure the difraction pattern then would it seem
like the photon never passes anywhere ?

Or if I measure one of the cases and collapse the WF "deciding"
upon a certain path - would I see my "past" changing - or
maybe I won't even know it happenned ?

Thanks !

Live long and prosper.

3. arcnets

513
drag,
if I understand you correctly, you ask if it's possible do detect which path the photon took, and still observe the interference pattern. AFAIK, this is not possible.
The standard answer is: If you make such a measurement, you alter the initial state so that the interference is destroyed.

I guess your idea is that gravity lenses/black holes may be a special kind of detector that does not obey this rule. I don't know anything about quantum gravity, so I really can't help here.

4. drag

1,341
Greetings !
Of course that's not what I'm asking.
I've not explained my Q sufficiently in the
second post.

Anyway, I figured a few things here as I considered
it some more, so, if I may, I'd like to ask a
different question:
Does a particle's WF, while it is uncollapsed,
cause external effects (like space-time curvature =
gravity, electric fiedls and so on) ?

Thanks !

Live long and prosper.