But perhaps Michell’s most far-sighted accomplishment was to imagine the existence of black holes. The idea came to him in 1783 while considering a hypothetical method to determine the mass of a star. Michell accepted Newton’s theory that light consists of small material particles. He reasoned that such particles, emerging from the surface of a star, would have their speed reduced by the star’s gravitational pull, just like projectiles fired upward from the Earth. By measuring the reduction in the speed of the light from a given star, he thought it might be possible to calculate the star’s mass.
Michell asked himself how large this effect could be. He knew that any projectile must move faster than a certain critical speed to escape from a star’s gravitational embrace. This “escape velocity“ depends only on the size and mass of the star. What would happen if a star’s gravity were so strong that its escape velocity exceeded the speed of light? Michell realized that the light would have to fall back to the surface. He knew the approximate speed of light, which Ole Roemer had found in the previous century. So it was easy for Michell to calculate that the escape velocity would exceed the speed of light on a star more than 500 times the size of the Sun, assuming the same average density. Light cannot escape from such a body, which would, therefore, be invisible to the outside world. Today we would call it a black hole.
Michell got the right answer, although he was wrong about one point. We now know, from Einstein’s relativity theory of 1905, that light moves through space at a constant speed, regardless of the local strength of gravity. So Michell’s proposal to find the mass of a star by measuring the speed of its light would not have worked. But he was correct in pointing out that any object must be invisible if its escape velocity exceeds the speed of light. This concept was so far ahead of its time that it made little impression.
— Excerpt from Cosmic Horizons: Astronomy at the Cutting Edge, edited by Steven Soter and Neil deGrasse Tyson, a publication of the New Press. Published at the American Museum of Natural History website, linked below. © 2000 American Museum of Natural History.
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