Regarding this article: A black hole is in a geometrical sense an end to the universe. If we picture the universe using Euclidean geometry, we can imagine going straight out forever. As we approach a black hole, the huge mass changes the geometry so that we also go on forever reaching the surface. We cannot therefore speak about the “inside” of a black hole. The final state of an observer falling down is the singularity at the center of the black hole. A singularity is a division by zero. This means the solution is not valid.

Sanford Aranoff
Monroe Township, N.J.

Someone falling into a black hole will indeed seem to take infinitely long to cross the event horizon, at least as seen from far away. But the falling person will have a different notion of time and will actually experience crossing the horizon and entering the black hole. What happens inside, only the falling observer will know, as no signals escape. The nature of the “singularity” inside a black hole—the region where the space-time curvature becomes infinitely sharp—nevertheless inspires active theoretical research .—Davide Castelvecchi