If photons do not have mass, why does light get attracted such that it cannot escape a black hole?

Question

Does gravitational force of black hole attract the light? Or is it something else?

Answer 1

According to general relativity(GR), gravity can be described as the curvature of spacetime. In GR there is thus no gravitational force. There only appears to be a force.

For example, a bullet shot in outer space will travel along a straight line. When a bullet is shot nearby a planet, due the local curvature of spacetime, the bullet will seem to be attracted by the gravitational force of the planet. However, according to GR, the bullet actually follows a straight line through a curved part of spacetime.

Because in this picture gravity is a purely geometrical effect it applies to all objects. It is for this reason that the observed gravitational acceleration is the same for all objects, independent of their mass (if you ignore air resistance, feathers fall just as fast as rocks) This even holds if the mass is zero, as for light.

In the special case of a black hole, the space is so strongly curved that close to the black holes there are no straight lines which point away from the black hole. It is therefore impossible for anything, including light, to leave the black hole.