How does a black hole absorb light knowing that it is massless?

Answer 1

It was formerly believed that only mass was affected by gravity. When Albert Einstein proposed his Theory of General Relativity, which suggested that gravity was actually a distortion in spacetime, scientists put it to the test during the solar eclipse on May 29, 1919, by measuring to see if the sun's gravity bent the light from distant stars as it passed by -- and found, indeed, that it did.

Now back to black holes: all objects with gravity have a certain escape velocity (the velocity at which an object must move at in order to escape the certain object's gravity's spacetime snare; in other words, the velocity at which you would have to throw an object off the earth in order for it to never fall down) that depends on the ratio of the mass of the object to the square of its radius. If this ratio is sufficiently great enough so that the Universal Law of Gravitation can be rearranged to give a value for escape velocity that is greater than c (the speed of light), then the object with escape velocity > c is a black hole.

When light passes by the event horizon (the radius from a black hole at which the greater-than-c escape velocity takes effect), the light will bend around and around the black hole instead of merely being deflected, as the star's rays were when they passed by the sun during the eclipse. Just as an object thrown off the earth with insufficient velocity falls back down to earth, light falls down into the black hole.

Answer 2

A black hole doesn't "absorb" light in the usual sense of absorption observed in materials. Instead, the immense gravitational pull of a black hole causes light (or any other form of electromagnetic radiation) to be captured by its strong gravitational field, preventing the light from escaping. So it's not that the black hole absorbs the light because of its mass, but rather because of its gravitational effects.