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If light photons have no mass, then how can gravity affect them? i.e., being pulled into a black hole.

Perhaps light, from its source, follows the curvature of space-time as created by gravitational objects such as stars and black holes.

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    Dec 20 2012: Gravity doesn't directly affect light. Strong gravity curves space-time so from our perspective the light travels a curved path, but if we were in the same location as the light it would appear that the path was straight, If you paddle a canoe past a whirlpool your path is curved by the whirlpool even though you are paddling in a straight line. The whirlpool has no direct effect on your canoe but it curves the water through which you are moving
    • Dec 20 2012: If you look at it from GR, gravity doesn't directly affect anything, but make no mistake: photons are nothing special to the force of gravity, since photons have energy and that's all that gravity cares about. To further illustrate this point a photon will actually exert a gravitational pull on other matter, including other photons.
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        Dec 20 2012: I was trying to keep it simple but from a standard model-quantum mechanics stand point photons have no mass or they wouldn't be able to travel at "c". I've never seen any experimental results that indicate two photons interacting due to gravity.
        • Dec 20 2012: "I've never seen any experimental results that indicate two photons interacting due to gravity."

          Well, they do: it's used in models of the very early universe.

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