Supermassive black holes, which are millions of times the sun's mass, form over hundreds of millions of years by both feeding on material around them and by merging with other black holes. In order to escape, one would have to travel faster than the speed of light, and since nothing can travel faster than the speed of light, that black-hole meal is doomed. Once anything crosses the event horizon, it can never, ever leave. Surrounding that singularity is the event horizon, the invisible spherical boundary that marks the entrance to the black hole. Since no known force can stop the collapse, once material forms a black hole it keeps on squeezing down until it becomes a singularity - a point of infinite density. However, black holes can only form under the most extreme conditions. But in the late 1930s, it became clear that nature could indeed allow black holes to exist when Indian physicist Subrahmanyan Chandrasekhar found that above a certain density, no force can overwhelm gravity. Early physicists assumed that this situation would never be found in nature. If you compressed the mass of an object into a space smaller than that radius, its gravitational pull would overwhelm every known force and nothing could escape. It was later realized why this radius was so special. But that solution contained a peculiar feature: the theory behaved strangely at a specific radius, known today as the Schwarzschild radius. He was trying to find the solution to the gravitational pull of a single, solitary, symmetric ball of matter - such as the sun at the center of our solar system. Physicist Karl Schwarzschild accidentally discovered black holes in 1916, when he was figuring out a particular solution to Einstein's general theory of relativity.
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