# If photons have no (rest) mass, why would black holes attract light? [duplicate]

I was told that photons have no (rest) mass. However I thought that black holes are called "black" because no light can go escape the gravity force in their vicinity. I somehow think that, if light is just photons, then it should not be affected by gravity. Hence black holes could catch everything but light.

Do I miss something? Is light more than a bunch of photons? Or maybe black holes are not exactly what I think.

• The light thinks it's traveling in a straight line - it's space that's distorted. General Relativity. Jan 9 '15 at 15:58
• Keep in mind that photons do have mass, they just have no 'rest mass' but in relativity mass and energy are unified into a single framework which is refereed to as mass-energy. Jan 9 '15 at 17:03
• @Floris: "The light thinks it's traveling in a straight line - it's space that's distorted. General Relativity". Perhaps you should discuss this with John Rennie and you both should try to arrive at some unambiguous version of the theory. In his answer and comments here he claims curved space(time) is only a mathematical construct, just a trajectory and is not physical. Jan 9 '15 at 20:35

To make sense of why light is also affected by gravity one can argue that general relativity is an "extension" of special relativity, where mass and energy can be thought of being the same thing up to a fixed "exchange rate" given by the speed of light in the vacuum: $E=mc^2$. Photons don't have mass, but they do have energy according to $E = h\nu$, where $\nu$ is the frequency of the observed light. Indeed energy is one of the 10 components of the (symmetric) energy-stress tensor that figures in Einstein's fields equations for the geometry of space-time.
• Call me a pedant but the relation between photon energy and why it falls into a black hole is very remote and I think mentioning the $E=mc^2$ formula can be confusing. I am also quite sensitive about the popular spacetime geodesic vs. the spatial geodesic jumble. It should be stressed that the null or time-like geodesic is an idea of a "dynamical straight line" or "moving with a uniform velocity and direction".
• i think there is no mention of light falling into a black hole in my reply, and therefore $E=mc^2$ cannot possibly be related, as a consequence of what people read in here, to something I haven't mentioned. Feel free to post your answer as well if you feel that some points need clarifications. Jan 9 '15 at 18:11