Author: Tom Siegfried / Source: Science News
Black holes capture everything they encounter. From subatomic particles to stars, solids, gases, liquids and even light, everything falls irretrievably in.
And even more assuredly, black holes capture the popular imagination.Thinking about space, as humans have since they first gazed at the points of light decorating the nighttime sky, triggers the mind to imagine things that cannot be experienced here on Earth. And black holes expand the imagination more dramatically than any other marvel that astronomy has to offer. A black hole is a cosmic vacuum cleaner, sucking stardust into the most literal of bottomless pits, a contortion of spacetime exerting irresistible gravitational attraction, a nothingness that can obliterate all somethings. It’s a hole in space, black because light cannot escape its attraction. It is therefore invisible. Hard to imagine.
Yet black holes have in fact long been imaginable, even if nobody knew they were real. In 1784, the English geologist and clergyman (and amateur astronomer) John Michell surmised that for a large and dense enough star, Newtonian gravity would be too strong for light to escape. He believed (as had Newton) that light is a stream of particles (as commonly accepted in those days). Michell calculated that the velocity of light particles would be insufficient to escape the gravity of a star as dense as the sun but 500 times its diameter. “Their light could not arrive at us,” he wrote.
A decade or so later the French mathematician Pierre-Simon Laplace also speculated that “invisible bodies” could exist in space. Laplace considered a star with the density of the Earth and 250 times wider than the sun. Its Newtonian gravitational pull would not allow light to leave its surface. “The largest bodies in the universe may thus be invisible by reason of their magnitude,” he declared. (You can read a thorough account of Michell’s and Laplace’s black hole papers in the Journal of Astronomical History and Heritage.)
Real black holes emerge not from Newtonian gravity, though, but rather from Einstein’s theory of gravity — general relativity. Einstein hid black holes (even from himself) in his equations. But the German astronomer Karl Schwarzschild pried the concept out of those equations during World War I, shortly before he died after falling ill at the Russian front. Schwarzschild was…
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