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## If photons don't have mass, why do their paths "bend" in a gravitational field?

In general relativity, gravitation is a manifestation of the curvature of spacetime. The motion of all objects is affected by this curvature, regardless of whether they have mass or not. Light follows geodesic paths in spacetime, which are straight lines in flat spacetime, and curved paths in curved spacetime.

Note that by "mass" above I mean "invariant mass" as discussed in post #6 above, because it is the invariant mass that is zero for a photon. If you prefer to think in terms of "relativistic mass" (which is related to energy via $E = m_{rel} c^2$, note that all photons follow the same geodesics, regardless of their energy.

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jtbell