Quote:

Black holes almost certainly exist, and one of their basic properties is that they trap light. However, it is also true that nothing exceeds the speed of light. In fact, the theoretical prediction of black holes is due to the General Theory of Relativity, which is built on the principle that the speed of light in a vacuum is constant. The analogy of a cannonball falling back to Earth with the trapping of light in a black hole is only a crude and suggestive one that is not correct at a fundamental level (for one thing, the cannonball has mass, but light does not; it turns out that this difference is critical, because massless particles MUST travel at light velocity, but massive particles CANNOT travel at light velocity).

To understand fully why a black hole can trap light but the light still always travels at constant velocity requires an understanding of the General Theory of Relativity, but the essential point is that the black hole curves spacetime back on itself, so that all paths in the interior of the black hole lead back to the singularity at the center, no matter which direction you go (an analogy in two dimensions is that no matter which direction you go on the surface of the Earth in a "straight line" (what mathematicians call a "geodesic" or a "great circle"), you never escape the Earth but instead return to the same point. Imagine extending that analogy to the 4 dimensions of spacetime and you have a rough explanation for why light travels at light speed, but cannot escape the interior of a black hole.

---