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How do black holes attract light?

  1. Apr 4, 2017 #1
    I recently studied about black holes and hiw they attract light too. But does light have mass? If it doesn't, I dont see any way how gravitational forces affect light. I have heard a bit about wave particle duality. But could you make it more clearer? I have no idea of relativity or particle physics or any other advanced stuff like that.
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  3. Apr 4, 2017 #2


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    Gravity from black holes (or any object with mass) curve space-time. Time slows down near the object. When the light follows a straight space-time line it curves toward the object.
  4. Apr 4, 2017 #3
    In general relativity, gravity is a curvature of space time.
    You can imagine that the ball is placed on the trampoline, you will see the curvature of its surface, if you place a small ball nearby, the ball will start to move to the center due to the curvature.

    Similar case for black hole, then the light is attracted because it follow the curvature created by black hole.
  5. Apr 5, 2017 #4
    Mass is not related to gravity. All energy creates gravity and all energy is affected by gravity. In fact, Einstiens equations for gravity have no mention of mass.

    You could even (theoretically) create a black hole out of nothing but light.
  6. Apr 5, 2017 #5


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    I wouldn't quite word it that way, but I get what you're trying to get across. Objects with mass certainly create gravity, but an amount of energy equal to mc2 will create the same amount of gravity that the mass m would.
  7. Apr 6, 2017 #6
    Depends on what you call "mass". Light has zero "invariant mass" but it has energy, which is "relativistic mass".


    Invariant mass (in units where c=1) is simply sqrt(E^2 - (px^2 + py^2 + pz^2)), or simply the norm of four-momentum. For particles moving at the speed of light, it's zero: E^2 = (px^2 + py^2 + pz^2) - they move by the same distance in space and in time coordinates.

    Term "mass" nowadays almost always implies "invariant mass", but this has the unfortunate side-effect that when people hear that "particle X has zero mass", they incorrectly assume that it is not affected by gravity.
  8. Apr 6, 2017 #7


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    Who are these people?
    Last edited by a moderator: Apr 6, 2017
  9. Apr 6, 2017 #8
    Does it follow a straight space-time line? Two straight lines intersect in one point only but in folded space-time two light beams can intersect in more than one point.
  10. Apr 6, 2017 #9


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    I am not expert enough on this to give an authoritative answer, but I know that the path of light is found by solving a geodesic equation in relativity. I'm afraid that my calling it a "straight line" might have been too informal.
    Right. I think that is why gravitational lensing happens, where light from a single source that would be blocked by a large gravitational object appears to be coming around both sides.
    Last edited: Apr 6, 2017
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