• back

What is it like to fall into a black hole? We consider two astronauts. One approaches the black hole, and the other stays a safe distance away.

We assume that the astronaut approaching the black hole can send out signals in various directions, including back to the other astronaut. As the first astronaut approaches the black hole, the first thing the distant astronaut would notice is the redshift in the signals received. The magnitude of the redshift increases as the first astronaut becomes closer to the Schwarzschild radius.

Before the Schwarzschild radius is reached, another effect becomes noticeable. The paths of photons sent out by the first astronaut are not straight lines. They bend. The only direction in which the astronaut can aim a beam and not have it bend is straight up. If the beam is not aimed sufficiently close to the vertical, the bending will be so great that the light will not escape. Only light aimed into a cone about the vertical, called the exit cone, will escape. As the first astronaut moves closer to the Schwarzschild radius, the exit cone becomes smaller. At a distance equal to (3/2)Rs, photons aimed horizontally go into orbit around the black hole. The sphere of orbiting photons is called the photon sphere. If you were to look straight out, along the horizon, you would see the back of your head.

The second astronaut never actually sees the first astronaut reach the Schwarzschild radius. The gravitational time dilation is so great that, as Rs is approached, the second astronaut thinks that it takes the first astronaut an infinite amount of time to reach Rs. The time dilation makes the first astronaut appear to slow down as Rs is approached.

From the point of view of the first astronaut, there is no such respite. The Schwarzschild radius is reached very quickly. If the black hole is of sufficiently small mass, the tidal forces would tear the first astronaut apart. However, if the black hole is massive enough, the tidal forces might be survived and the astronaut crosses Rs. When this happens, we say that the astronaut has crossed the event horizon. If the black hole is massive enough, the astronaut might not notice anything unusual, except that escape is impossible!.

Once inside the black hole, the inevitable journey to the center continues. The gravitational time dilation is so great that time passes slowly. However, the headlong rush through space continues. Outside the black hole, it is time that rushes on while distance is covered slowly. It is as if crossing the event horizon has interchanged to roles of space and time.

The second astronaut can tell nothing about what is going on inside the black hole. In fact, the only properties of a black hole that can be deduced are its mass, radius, electric charge and angular momentum. (So far, we have assumed zero angular momentum. We will discuss rotating black holes below.) The external simplicity of black holes is summarized in a theorem that states that black holes have no hair.

So far we have been discussing non-rotating black holes. The structure of a rotating black hole is somewhat more complicated than that of a non-rotating black hole, and is depicted schematically.

The situation shown is for the case in which the angular momentum per unit mass, J/M, is less than GM/c. For the case shown, there are two infinite redshift surfaces instead of a single event horizon. Between the two surfaces, the roles of space and time are reversed, just as inside the event horizon in the non-rotating case. The region between the outer infinite redshift surface and the event horizon is called the ergosphere. The name results from the fact that there is a way to extract energy from the black hole by moving particles through the ergosphere in the correct trajectory.