Answer to Question #35640 in Astronomy | Astrophysics for ray

Question

If photon's have no mass why is light "bent" by gravity? Again, if photon's have no mass why can't they escape the pull of a black hole?

Answer 1

The first reason is, that in General Relativity, in contrast to Newton's gravity, the source of gravity is not only mass, but energy and pressure too, and, it will affect not only mass, but energy too. Photon possesses no mass, but have some energy, so gravity will change his trajectory.

The second reason is, that in General Relativity is actually not a force, but an geometry. The masses curve the spacetime, and we see it like gravity, changing the trajectory.

Photons can not escape black hole, as the have finite velocity. So, their velocity is less then orbital speed for black hole, and they can not leave it.