In 1974, British physicist Stephen Hawking used quantum field theory in curved spacetime to show that in theory, the force of gravity at the event horizon was strong enough to cause thermal radiation to be emitted and energy to "leak" into the wider universe from a tiny distance around and outside the event horizon. Our current understandings of quantum physics can be used to investigate what may happen in the region around the event horizon. A classical black hole is pure empty spacetime, and the simplest (nonrotating and uncharged) is characterized just by its mass and event horizon. (Although nothing can travel through space faster than light, space itself can infall at any speed.) Once matter is inside the event horizon, all of the matter inside falls inexorably into a gravitational singularity, a place of infinite curvature and zero size, leaving behind a warped spacetime devoid of any matter. Credit: Andrew Hamilton, JILAĪlternatively, using a set of infalling coordinates in general relativity, one can conceptualize the event horizon as the region beyond which space is infalling faster than the speed of light. At the event horizon, the infalling speed equals the speed of light. Outside/inside the horizon (red), the infalling speed is less/greater than the speed of light. Picture of space infalling into a Schwarzschild black hole at the Newtonian escape speed. The essence of a black hole is its event horizon, a theoretical demarcation between events and their causal relationships. The region beyond which not even light can escape is the event horizon an observer outside it cannot observe, become aware of, or be affected by events within the event horizon. Nothing can travel that fast, so nothing within a distance, proportional to the mass of the black hole, can escape beyond that distance. They were first predicted by Einstein's 1915 theory of general relativity, before astrophysical evidence began to mount half a century later.Ī black hole can form when enough matter and/or energy is compressed into a volume small enough that the escape velocity is greater than the speed of light. Black holes are astrophysical objects of interest primarily because of their compact size and immense gravitational attraction.
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