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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:
 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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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.
