Under a Dark Star

July 07, 2009

Welcome to Black Hole Week at Twisted Physics: Part Deux. We think of black holes as a 20th century invention, dating back to 1916, when Einstein published his theory of general relativity and fellow physicist Karl Schwarzschild used those equations to envision spherical section of spacetime so badly warped around a concentrated mass that it is invisible to the outside world. But the true "father" of the black hole concept was a humble 18th century English rector named John Michell -- a man so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later.

Born in 1724, Michell attended Cambridge University and wound up teaching there for a time, before becoming rector of Thornhill, near the town of Leeds. He is described somewhat unflatteringly in contemporary accounts as "a little short man, of black complexion, and fat," who was nonetheless "esteemed a very ingenious Man, and an excellent Philosopher." For a small-town rector, he had some pretty impressive scientific connections: Benjamin Franklin, Joseph Priestley and Henry Cavendish all visited him at some point in his career.

Michell's research interests were all over the map. He started out looking into magnetism, then made a few waves after the big Lisbon earthquake of 1755, proposing that earthquakes propagate as waves through solid earth -- thereby helping establish the field of seismology.

He conceived and designed the experimental apparatus later used by Cavendish to measure the force of gravity between masses in the laboratory to get the first accurate value for the gravitational constant ("G"). And he was the first to apply statistical methods to astronomy, studying how stars were distributed in the night sky and arguing that there were far more "pairs" or groups or stars than would happen with random alignments. His analysis provided the first evidence for binary stars, and star clusters.

But it was a paper Michell wrote in 1783 that proved the most revolutionary. He didn't set out to "invent" black holes; he was just casting about for a handy method to figure out the mass of a star.

This was before scientists knew that light was both particle and wave, and Michell sided with the pro-particle Newton. And since light was made of particles, he figured that when they were emitted by a star, that star's gravitational pull would reduce their speed -- much like what happens when you toss an apple into the air. He thought he could measure how much the speed of light was reduced and from that, calculate the mass of a star.

It was a sensible enough scheme: Ole Roemer had measured the speed of light the century before, so Michell had a ballpark figure with which to work. He also understood the concept of "escape velocity" -- namely, any light particle must move faster than a certain critical speed in order to escape from a star's gravitational pull. And that critical speed would be determined by the mass and size of the star.

Here's where Michell found himself pondering an intriguing "what if?" scenario: what would happen if a star was so massive, and its gravity so strong, that the escape velocity was greater than the speed of light? Well, what happens when you throw an apple into the air without sufficient velocity to escape the Earth's gravity? It falls back down to Earth. Michell figured the same thing would happen to particles of light emitted by a super-massive star more than 500 times the mass of the sun: it would fall back to the surface, rendering that star invisible to astronomers.

Michell even thought it might be possible to indirectly detect such "dark stars" if they had a luminou "twin" circling them -- a binary star system -- making him doubly prescient. It's one of several different methods modern astronomers use to infer the existence of black holes.

Today we define black holes as volumes of space in which gravity is so strong, not even light can escape. It might be said that John Michell, that short, fat rector, was born under a dark star. He never achieved sufficient escape velocity for his ideas to break out of Thornhill. He died in quiet obscurity, and his notion of a "dark star" -- that Newtonian precursor to our modern notion of a black hole -- was forgotten until his writings re-surfaced in he 1970s. Consider it a form of conceptual Hawking radiation: eventually, his ideas found their way into the light.

Image: Title and excerpt from Michell's 1783 paper in which he first described the concept of a "dark star." Source: Philosophical Transactions of the Royal Society of London, Vol. 74, p.35, 1783.

Comments

[Gordon Pasha]

Neither Einstein nor Schwarzschild ever heard the words "black hole", at least in the context used in this article. Neither of them understood the Schwarzschild solution in modern terms. The understanding that the solution corresponds to a black hole was developed only in the 1960s.

Regards

Gordon

[Peter Lund]

Rømer, please.

(Or possibly Römer but Rømer is preferred.)

Otherwise, thank you :)