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:
“It is known that Whistler when asked how long it took him to paint one of his “nocturnes” answered: “All of my life.” With the same rigor he could have said that all of the centuries that preceded the moment when he painted were necessary. From that correct application of the law of causality it follows that the slightest event presupposes the inconceivable universe and, conversely, that the universe needs even the slightest of events.”
—Jorge Luis Borges (1899–1986)
“The horizon is more than a convention of landscape painting, less than truth.”
—Mason Cooley (b. 1927)
“As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it—whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash.”
—Harper Lee (b. 1926)
“The more supple vagabond, too, is sure to appear on the least rumor of such a gathering, and the next day to disappear, and go into his hole like the seventeen-year locust, in an ever-shabby coat, though finer than the farmer’s best, yet never dressed.... He especially is the creature of the occasion. He empties both his pockets and his character into the stream, and swims in such a day. He dearly loves the social slush. There is no reserve of soberness in him.”
—Henry David Thoreau (1817–1862)