Mentor

## 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.

The following forum members have contributed to this FAQ:
jtbell
 PhysOrg.com science news on PhysOrg.com >> A robot that runs like a cat (w/ Video)>> Global cooling as significant as global warming, research shows>> 'Chase and run' cell movement mechanism explains process of metastasis

 Similar Threads for: If photons don't have mass, why do their paths "bend" in a gravitational field? Thread Forum Replies Special & General Relativity 16 Classical Physics 2 Special & General Relativity 51 Special & General Relativity 7