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Q&A: Black Holes

Q:
The way that I understand it, the reason a black hole looks "black" is because its gravity is so great, that light cannot escape it. In order for light to become "trapped", does its speed have to change? Is it possible for light to travel slower than the speed of light?

A:
Black Hole Illustration
According to Einstein's theory of relativity, light always travels at the speed of light in a vacuum (it can travel more slowly in a solid, liquid or gas, which is why a lens will bend or focus light, but that's another story). The reason that it bends around a black hole or a star or a galaxy is because space itself is warped by the gravitational fields of these objects. A light ray will follow this warp, like a ball rolling across a trampoline with a person standing in the middle, and appear to be bent. Inside the event horizon of a black hole this warping is so great that light is not slowed down but is doomed to follow paths that lead downward into the depths of the black hole.

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