# If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

Is it because it has super massive gravity and energy? Or is it some sort of an anti-light component in it because it used to be a star?

• Why do you say "If nothing travels at the speed of light, except light"? Any massless particle and it's associate field travels at c. It is also the speed of gravity. I think the question is based on an incorrect premise. Commented Sep 25, 2013 at 0:41
• Exact question Answered here phys.vt.edu/~jhs/faq/blackholes.html#q4 Commented Nov 8, 2016 at 19:35

## 3 Answers

Gravity is a force, and it need not have a "speed"1.

A gravitational body sets up a gravitational field around itself. Note that by the time the light wave approaches the body, this field has already been set up. The gravitational force does not need to "reach out" and "catch up to the light" — it's already there.

In fact, it's not even really a force (though we can treat it as one to an approximation). Gravity bends the fabric of spacetime around it, messing with the meaning of a "straight line". From different reference frames, different lines appear straight. To the light wave, the inspiraling path seems "straight" and thus it follows that path. To an external observer, the light wave is not going straight.

1. It does, but that's the speed at which changes to the field propagate, not the speed at which it "catches up" with other things. Changes in the gravitational field propagate at lightspeed.

What happens is that the gravity produced by a black hole is simply so great that it warps the spacetime fabric in such a way that any light which crosses the event horizon is funneled in by the fabric itself - it's a crazy notion, and one hard to get your head around. This diagram might help:

• So the light is being pulled in by gravity even though gravity can't travel as fast as light? Commented Sep 25, 2013 at 0:10
• @Timtech Gravity is a strange thing - it warps the spacetime fabric, 'funneling' the light into itself. It is changing the very path of the light.
– user19
Commented Sep 25, 2013 at 0:11
• No - it's changing the fabric along which the light travels, just like a ball rolling around on a blanket. It might start out going straight, but if there is a depression in the blanket and the ball passes toward it, the ball will change course. A black hole creates a 'mega-depression'.
– user19
Commented Sep 25, 2013 at 0:15
• @Timtech Adding to what Undo said, and an alternative explanation, "light travels in a straight line" There is no fault with this statement. Gravity simply twists the space in such a manner that straight lines lead to inside the black hole. Remember, straight lines(even if parallel) may converge for non-Euclidean geometry. I hope I made myself clear. Commented Sep 25, 2013 at 0:23
• @Timtech why do you say gravity can't travel as fast as light? Commented Sep 25, 2013 at 0:43

Well, I have no astronomical background, so in layman's terms,

If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

I believe OP is confused over how a gravity wave can capture light if both can travel at most at the speed C.

It does not work quite that way. Say you move a massive BH from position X to position Y and an observer is at 1 LightYear away from Y. Then, the observer will not feel the gravitation effect of BH until 1 year because that's how much it will take a gravity waves to reach to observer. That is, its changes in gravitational field "Delta" propagate at the speed of light.

As others have mentioned, gravity bends/shapes the spacetime fabric, so after one year, at observer, spacetime is bend according to the gravity of BH at Y. Thus, any light that passes near to the observer will bend as it follows the spacetime. Plus, the curvature of space BH create in their vicinity is so severe that space is wrapped completely around itself and hence light can not escape.

Refer to this for simple explanation.

• Please edit this so it is an answer to the question or post it as a comment. Commented Sep 7, 2016 at 10:16