# Why are massless photons affected by gravity?

Mentor
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 the following FAQ:

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 (as far as we know) follow the same geodesics, regardless of their energy. This has been verified, for example, by comparing the deflection of visible light as it passes close to the sun, with the deflection of radio waves from distant sources.

The following forum members have contributed to this FAQ:
jtbell

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Manpriyam