Work by Robert Oppenheimer and others then led to the idea that such an object might be formed by the collapse of a massive star. Karl Schwarzschild then solved Einstein’s equations for the case of a black hole, which he envisioned as a spherical volume of warped space surrounding a concentrated mass and completely invisible to the outside world. The idea of black holes was rediscovered in 1916, after Einstein published his theory of gravity. This concept was so far ahead of its time that it made little impression. But he was correct in pointing out that any object must be invisible if its escape velocity exceeds the speed of light. So Michell’s proposal to find the mass of a star by measuring the speed of its light would not have worked. 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. Michell got the right answer, although he was wrong about one point. Light cannot escape from such a body, which would, therefore, be invisible to the outside world. 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. He knew the approximate speed of light, which Ole Roemer had found in the previous century. 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.
This “escape velocity“ depends only on the size and mass of the star. He knew that any projectile must move faster than a certain critical speed to escape from a star’s gravitational embrace. Michell asked himself how large this effect could be. 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. 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. Michell accepted Newton’s theory that light consists of small material particles. The idea came to him in 1783 while considering a hypothetical method to determine the mass of a star. This was the first evidence for the existence of physical associations of stars.īut perhaps Michell’s most far-sighted accomplishment was to imagine the existence of black holes. He argued that these were real systems of double or multiple stars bound together by their mutual gravity. By studying how the stars are distributed on the sky, he showed that many more stars appear as pairs or groups than could be accounted for by random alignments. Michell was also the first to apply the new mathematics of statistics to astronomy.
Using the measured value of the constant, Cavendish was able for the first time to calculate the mass and the average density of the Earth. The measurement yeilded a fundamental physical quantity called the gravitational constant, which calibrates the absolute strength of the force of gravity everywhere in the universe.
The black masses guide full#
Cavendish, who actually carried out the experiment after Michell’s death, gave him full credit for the idea. Michell conceived the experiment and built the apparatus to measure the force of gravity between two objects of known mass. This work earned him election in 1760 to the Royal Society, an organization of leading scientists. Michell suggested that earthquakes spread out as waves through the solid Earth and are related to the offsets in geological strata now called faults. After the catastrophic Lisbon earthquake of 1755, he wrote a book that helped establish seismology as a science. In 1750, Michell showed that the magnetic force exerted by each pole of a magnet decreases with the square of the distance. The range of his scientific achievements is impressive. Michell had numerous scientific visitors at Leeds, including Benjamin Franklin, the chemist Joseph Priestley (who discovered oxygen), and the physicist Henry Cavendish (who discovered hydrogen).
No portrait of Michell exists, but he was described as “a little short man, of black complexion, and fat.” He became rector of Thornhill, near Leeds, where he did most of his important work. Michell was born in 1724 and studied at Cambridge University, where he later taught Hebrew, Greek, mathematics, and geology. Although he was one of the most brilliant and original scientists of his time, Michell remains virtually unknown today, in part because he did little to develop and promote his own path-breaking ideas. This astonishing idea was first announced in 1783 by John Michell, an English country parson.