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The delayed choice experiment

  1. Mar 30, 2012 #1
    This comes from Greene's Fabric of the Cosmos:

    Wheeler imagines a cosmic version of the delayed choice experiment in which the light source is not a laboratory laser but instead, a powerful quasar in deep space. The beam splitter is not a laboratory variety, either, but is an intervening galaxy whose gravitational pull can act like a lens that focuses passing photons and directs them toward earth. if enough photons from the quasar are collected, they should fill out an interference pattern on a photographic plate. But if we were to put another photon detector right near the end of one route, it would provide which-path information for the photons, thereby destroying the interference pattern. the photons could have been traveling for many billions of years.

    I'm confused about the words in bold. I don't see how a gravitational pull can act like a lens that focuses photons.
     
  2. jcsd
  3. Mar 30, 2012 #2

    Demystifier

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    I think this question is for the Special and General Relativity subforum.
     
  4. Mar 30, 2012 #3

    tom.stoer

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    One of the first successes of Einstein's theory of General Relativity was the correct prediction of light deflection in gravitational fields.


    Black_hole_lensing_web.gif

    simulated gravitational lensing (black hole going past a background galaxy); source: Wikipedia

    Further reading:

    http://en.wikipedia.org/wiki/General_relativity#Light_deflection_and_gravitational_time_delay
    http://en.wikipedia.org/wiki/General_relativity#Gravitational_lensing
    http://en.wikipedia.org/wiki/Gravitational_lens
     
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