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What's up in this case

  1. Nov 21, 2003 #1


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    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. jcsd
  3. Nov 22, 2003 #2


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    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.
  4. Nov 23, 2003 #3
    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.
  5. Nov 24, 2003 #4


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    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.
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