Centuries before black holes became accepted science, a fat little man in 1780s Yorkshire imagined stars so massive that even light could not escape. He predicted black holes. This is the incredible story of John Michell and his "dark stars."
The Fat Little Country Reverend Who Invented Seismology
John Michell is one of the great unsung geniuses of 18th century science. Born in 1724, he studied at Cambridge University and later went on to teach there, instructing students in everything from Hebrew and Greek to math and geology. He was elected to the Royal Society in 1761, but in 1767 he left for northern England to take up the the post of rector of St. Michael's Church in the city of Leeds, where he spent the rest of his days.
Nobody quite knows what Michell looked like, as no portrait of him was ever painted. Even the most detailed contemporary description of him sounds like the writer couldn't be bothered to find out anything more about him:
John Michell, BD is a little short Man, of a black Complexion, and fat; but having no Acquaintance with him, can say little of him. I think he had the care of St. Botolph's Church Cambridge, while he continued Fellow of Queen's College, where he was esteemed a very ingenious Man, and an excellent Philosopher. He has published some things in that way, on the Magnet and Electricity.'
Despite his obscurity nowadays - and back then, judging by that rather half-assed description, Michell wasn't completely unappreciated. Some titans of science made the trip up to Leeds to visit him, including the great chemist Joseph Priestley, hydrogen discoverer Henry Cavendish, and even Benjamin Franklin.
His speculations on earthquakes proved remarkably prescient, writing that earthquakes act like waves of energy
beneath the Earth and that they are caused by discontinuities between different strata, essentially predicting the existence of fault lines. All this work earned him the later title of the father of seismology, and he developed a method to very accurately measure the mass of Earth.
Into The Black Hole Of History
So if Michell did all that and had so many legendary friends, why does he remain so little-known? Michell is a case study in the value of good self-promotion, or the lack thereof - he never really made an effort to fully develop his revolutionary ideas into full-fledged theories, and he didn't much care whether people associated his ideas with him or not.
Consider the torsion balance that could measure gravitational forces, allowing Earth's mass to be calculated. Yes, he came up with the idea and created the first such device, but he didn't really publicize his work very well. The balance only became famous when it was reinvented by the French scientist Charles-Augustin de Coulomb, and the torsion balance only became really useful when Michell's friend Henry Cavendish ran a famous experiment shortly after Michell's death to measure the mass of the Earth.
Cavendish was always very careful to give full credit for Michell, but you still had to actually read Cavendish's papers to discoverer Michell's role. For most, just knowing Cavendish had determined the Earth's mass was enough, and the finer details of the experiment - including the identity of the dead Yorkshire clergyman who had made it all possible - weren't really important. This is how a great scientist, reasonably famous and well-respected in his day, can quickly pass into total obscurity.
The Gravity Of Light
But Michell's most impressive scientific feat was coming up with the idea of black holes 150 years before any other scientists would start seriously considering the idea, and 200 years before they finally became a part of mainstream science. By his time, the speed of light was generally considered to be a finite quality and had already been calculated with some accuracy, but it hadn't gained the special place in physics that Albert Einstein and relativity later gave to it with E=mc^2.
Isaac Newton had imagined light as being composed of tiny particles called corpuscles, which differed from our modern understanding of the photon in one crucial respect. Corpuscles were thought to have mass, which meant that gravity could slow down the speed of light.
Michell came up with an ingenious method to work out the masses of stars, figuring that careful calculation of the speed of light coming from a given star would reveal its gravitational influence and, in turn, its mass. Now, while clever, that part of his theory was totally wrong. But in coming up with that idea, Michell imagined an extreme case where a star's gravity was so strong that even light could not escape. And that's how, in 1783, Michell came up with what would eventually become black holes.
Dark Stars And Black Holes
The basics of Michell's theory sound shockingly familiar. He wondered what would happen if a star's gravity was so immense that its escape velocity was greater than the speed of light. At the time, it wasn't yet known that there is nothing faster than the speed of light, but this detail didn't matter for Michell. The crucial point was that, because light could not escape the star, the star would be invisible to all outside observers.
In his 1783 letter to his friend Henry Cavendish, later published by the Royal Society, Michell explained his ideas:
If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity. This assumes that light is influenced by gravity in the same way as massive objects.
Yes, the details were wrong, but his basic idea was correct. Even so, a few specifics of Michell's work really do sound like they are ripped from the pages of a twentieth century astronomy textbook. He suggested the best way to detect these invisible stars would be to look for other star systems that showed the gravitational effects of two stars, and yet only one star was visible. This, he figured, would mean the other star was a dark star, and figuring out what percentage of such binary systems contained dark stars would help determine how common such stars were throughout the universe.
Michell was righter than he could have known. Currently, there are over a dozen stellar black hole candidates that we've discovered in the Milky Way. Every single one of them is part of a so-called X-ray compact binary system, in which a visible companion star has its matter slowly sucked away by its black hole partner. This has remained some of our best evidence for the existence of stellar black holes, and it's more or less the same idea that John Michell put forward 227 years ago.
But What About The Dark Stars?
Dark stars did differ from modern black holes in a rather fundamental way. Whereas black holes have the mass of the Sun concentrated to nearly a single point, dark stars are essentially normal stars, just many times larger. Michell had figured that a star with the same density of our own Sun but 500 times more massive would be big enough to be a dark star. While some of his underlying reasoning was incorrect, his basic idea about a star whose escape velocity was greater than the speed of light was correct.
So could there really be dark stars out there, stars of normal stellar density that are so unfathomably large that light cannot escape them? Unfortunately, this is a no on two counts. For a start, stars just aren't nearly as big as the 500 solar mass behemoths that Michell imagined. Until recently, astronomers believed the upper limit was about 150 solar masses, but the recent discovery of R136a1 changes that picture a little. It's anywhere between 250 and 320 times the mass of the Sun, making it twice as big as any previously known star.