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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 19 2012: "If light photons have no mass, then how can gravity affect them? i.e., being pulled into a black hole."

    Gravity couples to energy, that energy can be "bottled up" in mass, but it doesn't have to be, it can also be "bottled up" in momentum ( E=sqrt[M^2c^4 + p^2c^2], where M is the mass, c is the speed of light and p is the momentum). Photons have energy and thus they react to gravity. All elementary particles have energy that allows them to heat or move something they hit, but not all of them have the additional form of energy that we call mass and is caused by the Higgs mechanism (you can easily distinguish the two kinds because particles without mass always travel at the speed of light while particles with mass can never reach the speed of light). Gravity does not distinguish between forms of energy so it acts on all particles, even the ones that don't have some of their energy bottled up in mass.

    Alternatively you can look at it from the viewpoint of general relativity where gravity is a warping of a patch of space-time and everything that exists within that patch of space-time will feel the effects of the warping.
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      Dec 19 2012: c - could not possibly be the measurement of what the speed of light is
    • Dec 20 2012: I get it. That's how light can push a solar sail . Maybe light is helping to push the galaxies apart from every direction. The ones not linked by gravity.

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