How is it possible that a force that affects mass (gravity) can affect massless particles like photons?
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1$\begingroup$ Because, in GR, gravity is not treated as a force but as a warp in space-time; space-time itself changes its shape in the presence of mass and the light follows paths accordingly. $\endgroup$– NeutronStarCommented Jun 12, 2014 at 22:00
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$\begingroup$ physics.stackexchange.com/questions/34352/… physics.stackexchange.com/questions/18900/… physics.stackexchange.com/questions/54701/…? physics.stackexchange.com/questions/62411/…? $\endgroup$– dmckee --- ex-moderator kittenCommented Jun 12, 2014 at 22:57
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1$\begingroup$ In part, because referring to things like photons as "massless" particles is something of a misnomer. Calling them "rest-massless" would be more fitting, but that never caught on, probably because it's a lot more awkward to say. Photons do actually have mass, just not rest mass. They are carrier particles, and carry energy. Their mass is only that which is accounted for by this carried energy. A particle with literally no mass at all, of any kind, would either not exist, or else would not be meaningfully different from not existing. $\endgroup$– Matthew NajmonCommented Jan 24, 2015 at 14:01
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In special relativity, Einstein developed a relation between energy and mass based on both the particles momentum and it's rest mass. We often times say that gravity acts on mass, but the effect of gravitational lensing indicates that gravity acts on energy rather than mass (or from a different perspective, on both rest mass and momentum). Thus, since a photon has energy and momentum, it can fall under the effects of gravity. This is why we think of general relativity as a curve in spacetime: because even if a particle has no rest mass, it can still bend due to momentum changes.
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4$\begingroup$ I don't think it's quite right to say that "gravity acts on energy". Mass-energy curves (acts on) spacetime and photon world lines are light-like geodesics in the curved spacetime - gravity is curved spacetime. $\endgroup$ Commented Jun 12, 2014 at 23:01