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Gravity s effect on light

  1. Feb 4, 2012 #1
    gravity"s effect on light

    If light is bent by gravity as it passes a massive object like the sun isn't the speed of light retarded as it leaves the same massive object? If not how can gravity have the effect of bending light but not slow it down as it leaves?

  2. jcsd
  3. Feb 4, 2012 #2


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    Re: gravity"s effect on light

    There is an effect called "Gravitational Redshift" where the light leaving a massive body is red-shifted by the gravitational field of the emitting body. This does NOT change the velocity of the light, it just "stretches out" the wavelength.

    Actually, light from a distant source passing near a massive object follows the "geodesic", a curvature of space-time which deflects the path, as you describe above.

    But there is an additional effect: while travelling toward that mass it must feel a slight "blue-shift" and after passing that mass it feels the "red-shift". These are really small effects for an average-sized object, and it seems they would cancel each other. (not sure about this)
  4. Feb 4, 2012 #3


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    Re: gravity"s effect on light

    Light passing through a pane of glass slows down, but, resumes its normal speed once it exits the glass.
  5. Feb 4, 2012 #4
    Re: gravity"s effect on light

    The shortest distance between two points is a straight line. Any curved or circular path taken between two points will increase the distance from one point to the other. That's why in boxing a jab is quicker than a hook. One travels in a straight line while the other in a circular trajectory.

    The same with light. When light approaches a massive object it is suddenly forced to travel along a curved or warped space. Therefore it will take longer to reach us. Looking at gravity as a space-warper which forces moving matter to confrom to that space warpage as Einstein did instead of an attractor removes the seemingly paradoxical conunundrum.
  6. Feb 4, 2012 #5
    Re: gravity"s effect on light

    For a distant observer light slows down near the massive object. It's called Shapiro delay:


    As a reduction of the speed of light is equivalent to an increase of the refractive index gravitational lensing works similar to classical refraction.
  7. Feb 4, 2012 #6
    Re: gravity"s effect on light

    There are two possibilities. In a geometric theory as general relativity (GR) the speed of gravity is c, which is a constant, and lights bends because is moving in a curved spacetime (Light bending in curved spacetime)

    https://p.twimg.com/Ak1D4kuCIAEiIVN.gif:large [Broken]

    In non-geometric theories of gravity as field theory of gravity (FTG) the speed of gravity is not c but varies with gravitational field and lights bends because is subject to a gravitational force (Light bending in flat spacetime)

    https://p.twimg.com/Ak1EkH_CIAAJcuz.gif:large [Broken]
    Last edited by a moderator: May 5, 2017
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