Event Horizon of A Black Hole
One of the most well-known examples of an event horizon derives from general relativity's description of a black hole, a celestial object so massive that no nearby matter or radiation can escape its gravitational field. Often, this is described as the boundary within which the black hole's escape velocity is greater than the speed of light. However, a more accurate description is that within this horizon, all lightlike paths (paths that light could take) and hence all paths in the forward light cones of particles within the horizon, are warped so as to fall farther into the hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time, and can actually be thought of as equivalent to doing so, depending on the spacetime coordinate system used.
The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body that fits inside this radius (although a rotating black hole operates slightly differently). The Schwarzschild radius of an object is proportional to its mass. Theoretically, any amount of matter will become a black hole if compressed into a space that fits within its corresponding Schwarzschild radius. For the mass of the Sun this radius is approximately 3 kilometers and for the Earth it is about 9 millimeters. In practice, however, neither the Earth nor the Sun has the necessary mass and therefore the necessary gravitational force, to overcome electron and neutron degeneracy pressure. The minimal mass required for a star to be able to collapse beyond these pressures is the Tolman-Oppenheimer-Volkoff limit, which is approximately three solar masses.
Black hole event horizons are especially noteworthy for three reasons. First, there are many examples near enough to study. Second, black holes tend to pull in matter from their environment, which provides examples where matter about to pass through an event horizon is expected to be observable. Third, the description of black holes given by general relativity is known to be an approximation and it is expected that quantum gravity effects become significant in the vicinity of the event horizon. This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Read more about this topic: Event Horizon
Famous quotes containing the words event, horizon, black and/or hole:
“We can glut ourselves with how-to-raise children information . . . strive to become more mature and aware but none of this will spare us from the . . . inevitability that some of the time we are going to fail our children. Because there is a big gap between knowing and doing. Because mature, aware people are imperfect too. Or because some current event in our life may so absorb or depress us that when our children need us we cannot come through.”
—Judith Viorst (20th century)
“Elvis’ disappearing body is like a flashing event horizon at the edge of the black hole that is America today.”
—Arthur Kroker (b. 1945)
“Today masses of black women in the U.S. refuse to acknowledge that they have much to gain by feminist struggle. They fear feminism. They have stood in place so long that they are afraid to move. They fear change. They fear losing what little they have.”
—bell hooks (b. c. 1955)
“Steam was till the other day the devil which we dreaded. Every pot made by any human potter or brazier had a hole in its cover, to let off the enemy, lest he should lift pot and roof and carry the house away.”
—Ralph Waldo Emerson (1803–1882)