A black hole is a celestial body so dense that light cannot escape its gravitational pull. It is formed when a star's gravity causes it to collapse into itself. Usually, pressure caused by heat from burning up hydrogen and other fuels balances out the star's gravity. However, when the star runs out of fuel, it is overcome by gravity and proceeds to collapse. A smaller star, with a mass less than one and a half times that of the sun, becomes a white dwarf. A star with a mass more than one and a half times that of the sun becomes a neutron star or, if it is massive enough, it collapses to a small point of great density-a black hole.

Because of its large mass, the black hole has a gravitational pull from which even light cannot escape. Light rays passing near the black hole are bent, and if they come close enough they are sucked in or trapped in orbit around the black hole. The trapped light rays define the event horizon of the black hole. The event horizon is the boundary past which nothing can escape from the black hole. Approaching the event horizon, time seems to slow down. A gravitational redshift occurs, as light turns red because it loses energy trying to escape from the black hole. Inside the event horizon, predictability and the laws of science break down in the presence of a singularity, or infinite curvature of spacetime. Because of its infinite density, the singularity causes all time and space to be the same. Meanwhile, the tidal forces, where the gravitational pull of the singularity is stronger closer to it than further away, tear matter apart as it nears the singularity.

The mass, and therefore the size, of a black hole increases whenever more matter is sucked in. A black hole also holds entropy, or disorder, and whenever the amount of entropy increases, so does the size of the event horizon. However, in order to contain entropy, a black hole must also have a temperature, which scientists could not accept for a long time. To have a temperature something has to radiate particles, and nothing can escape from a black hole. It turns out, however, that black holes do emit radiation, though not from inside the event horizon. Little particles are continually popping out of the vacuum of space, and one that popped up right outside the event horizon could escape and seem to be radiating from the black hole. If the black hole lost enough particles, its mass could become too small to keep up its immense gravity, and the black hole would explode. After the explosion, scientists theorize that there might be a tiny particle left called a boltzmon, which would contain all the information ever sucked in by the black hole. It would have an infinite number of states and would be very unstable, likely disappearing soon after it was created.

Of course, everything we supposedly know about what goes on inside black holes is only theory, since no one can actually go inside a black hole and return to tell the tale. Even if there were a way out, such as through a wormhole, the tidal forces of the black hole would tear the person apart before he or she got very far. Besides which, wormholes are very unstable; they can close at any time. A wormhole would have to have been formed at the beginning of the universe and have survived since then.