0
$\begingroup$

This question already has an answer here:

Please no hate for lack of knowledge:

I am somewhat fascinated with the subject of black holes. However, I do not understand a concept which is constantly attributed with black holes: that a black hole can attract light and therefore light cannot escape it. What I don't understand is, how? Gravity, as far as I understand, only attracts matter. However, light has no mass (right?), so how can a black hole attract light?

$\endgroup$

marked as duplicate by John Rennie, Qmechanic Jul 17 '15 at 6:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2
$\begingroup$

You misunderstand. All objects have some escape velocity, which is the velocity needed for anything (photons or matter) to escape from that object's gravitational field. And that's not the velocity it needs to maintain under some sort of constant thrust, but the initial velocity it needs to, shall we say, coast away from the object. For a black hole that escape velocity is greater than the speed of light.

You might be thinking, "But photons don't have mass." In fact, your original question says that. Perhaps you've heard of gravitational lensing in which a very massive object can bend the light from an object beyond it. Yes, gravity can bend light. While photons have no rest mass, they do acquire relativistic mass at the speed of light and therefore, respond to gravity.

$\endgroup$
0
$\begingroup$

Look at the paragraph "gravity and the photon" in this link:

In the relativistic framework, i.e. large velocities, any energy is also a relativistic mass:

For the photon this means the following equation:

photonrelmass

m is the relativistic mass of a photon with energy h*nu.

Gravity attracts relativistic mass, and the photon has one. Read the link further to acquire some intuition.

$\endgroup$
  • $\begingroup$ I am curious what is the reason for the negative vote? the links? $\endgroup$ – anna v Jul 17 '15 at 12:28
  • $\begingroup$ Probably because you used the concept of relativistic mass and tried to connect it with Newtonian gravity equation not using general relativity.I dont know exactly,I didnt vote down. $\endgroup$ – Paul Jul 17 '15 at 15:49
  • $\begingroup$ Relativistic mass is an unnecessary(and non mainstream) word which just mean energy,mass and energy curves spacetime and photons which have energy follow geodesics in spacetime. $\endgroup$ – Paul Jul 17 '15 at 16:01
  • $\begingroup$ @Paul did you notice the link? it is a solid standard university link. This is possibly the only relevant case where relativistic mass earns its keep. One can talk in the framework of gravity, and one can talk about general relativity geodesics. The question is in the framework of gravity, and in that framework the statement is correct, because energy acts as a mass in the case of the photon. Relativistic mass is useless in particle physics but earns its keep in gravitational situations. $\endgroup$ – anna v Jul 17 '15 at 18:55

Not the answer you're looking for? Browse other questions tagged or ask your own question.