From our frame of reference on Earth, objects slow to a stop as they approach black holes in space because time gets infinitely stretched by the strong gravity near the black hole. If astronauts accidentally falling into a black hole tried to signal back to Earth by flashing a light, what kind of “telescope” would we need to detect the signals?
ANSWER: As we know that the outside universe is completely isolated from the black holes by the event horizon, anything that goes into the black hole’s event horizon, will never be able to communicate with the outside universe. The event horizon is not a physical boundary of the black hole, but it is made up of the light rays which are bent by black hole’s gravity. Once the light rays enter into the horizon, there is no escape for them. The singularity of the black hole will be in their future. The escape velocity at the horizon is the speed of light, and after crossing the horizon the escape velocity becomes infinite. As we know nothing can move faster than light, anything that falls into the black hole, there is no escape. So as the person trying to send signals to the outside universe from the interior of the black hole, the signals will be returned to the black hole and never comes out. So as the signals won’t come out, no telescope can detect them.