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Speed of light in gravity

  1. Nov 3, 2009 #1
    “It is a well proven fact, that the speed of light is reduced in a gravitational field. As a consequence, a light beam, which passes a big object, is bent towards the object.”

    I read the above quote on the internet but I am not sure if it is a well proven fact that c is reduced in a gravitational field.

    What is the accepted wisdom in physics on this issue?
  2. jcsd
  3. Nov 3, 2009 #2

    George Jones

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    It depends on what one means by "the speed of light".

    All observers who measures the speed of a photon that whizzes by in their local (vacuum) neighbourhoods, near or far from large masses and even inside black holes, get the same result c.
  4. Nov 3, 2009 #3


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    Near a big mass the speed of light is reduced when measured by a distant clock. Measured locally it is still c due to gravitational time dialtion:

    The same happens in accelerated frames of reference;
  5. Nov 3, 2009 #4
    Is there an interpretation where the "speed of light" in a gravitational field can be said to vary.

    All observers measure the speed of light to be c locally but can the speed of light be inferred as having altered at some remote gravitational potential?
  6. Nov 3, 2009 #5


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    From http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames: [Broken]
    Last edited by a moderator: May 4, 2017
  7. Nov 3, 2009 #6


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    You can work in a weak-field approximation with a flat background, and treat gravitation as a perturbation. You then get g11/g00 (coordinate dx / coordinate dt) different from 1. Interpreting this as a different speed of light, you can calculate e.g. light deflection like you do in an optics problem. That's exactly what Einstein did in his original paper on GR.
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