Long dreamed of yet unseen, invisible stars intrigued scientists and the public as well
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 unable to imagine, though, that a star could shrink enough to exceed the density needed to make it invisible. That leap of imagination came from J. Robert Oppenheimer and Hartland Snyder in 1939 (the same year Einstein wrote a paper denying that black holes could exist). Oppenheimer and Snyder calculated that a sufficiently massive star would collapse under its own gravity. “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists,” they wrote.
Oppenheimer soon took charge of the Manhattan Project to build the atomic bomb, and nobody paid much attention to his collapsed stars until the 1960s. They were discussed at a symposium in Dallas in December 1963 and a few weeks later at a meeting in Cleveland. Someone even uttered the phrase “black hole” to refer to them. (The term’s first astrophysical use in print appeared in Science News Letter’s coverage of the Cleveland meeting, in the issue of January 18, 1964.)
But the name “black hole” didn’t catch on until John Archibald Wheeler used it in a talk in 1967. Scientific study of black holes then began in earnest. Stephen Hawking studied them, showing that they could actually emit a weak form of radiation since named for him. Astronomers sought them, compiling impressive evidence that they really exist, based on the motion of stars and other matter in their vicinity. (Michell, in fact, had suggested just this approach for detecting an invisible star’s presence.) In 2016 gravitational waves provided an unmistakable sign of two black holes colliding.
So almost nobody now doubts that they exist. But while Michell, Oppenheimer, Wheeler, Hawking and many others imagined what a black hole must be like, none of them had ever seen one.
Science’s history records other cases of imagining phenomena that defy the imagination before proof of their discovery. Paul Dirac imagined antimatter before anybody found it in nature. Alexander Friedmann imagined the expansion of the universe before astronomical observations confirmed it. Ancient Greek philosophers imagined atoms 2,500 years before microscopy became sufficiently sophisticated to take their picture. All these successful imaginations had been considered by some an affront to common sense or conventional logic. Their confirmations, as with the new black hole image, reinforce the lesson that apparent absurdity is not a sound argument against existence.
Perhaps the fact that human imagination conceived of black holes, despite the truth of their absurdity, is one of the reasons they captivate the imagination of almost everybody whose brain, as Carl Sagan would say, is not made of wood.
Black holes have consumed the mental energy of the world’s great physicists, astronomers and mathematicians, revealed secrets about objects in space and about spacetime itself, fascinated the public at large, inspired science fiction books and films, and illustrated the fantastic magnitude of nature’s creative (and destructive) power. The black hole has become the poster child for outrageously incomprehensible astronomical phenomena.
And now there is an image available to put on the poster.
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