Dark Stars

1783: John Michell

In November 1783, the Fellows of London's Royal Society heard a remarkable idea: countless numbers of the galaxy's stars might be dark.

The idea was conceived by Rev. John Michell, rector of the village of Thornhill, who had established himself as one of Britain's leading scientists (known as the time as natural philosophers). After a deadly earthquake in Lisbon, for example, he proposed that quakes travel as waves through the solid Earth, establishing the modern science of seismology. He also conducted pioneering work in magnetism, and conceived both the theory and the devices that would later help scientists measure Earth's mass.

Michell combined contemporary knowledge about gravity and the nature of light to devise a way to measure the masses of stars.

Sir Isaac Newton had described light as "corpuscles" (particles), which were subject to the pull of gravity in the same way as particles of matter. And Ole Roemer had measured the speed of light at about 183,000 miles (295,000 km) per hour, which is just two percent away from today's measurements.

Michell surmised that a star's gravity would slow the particles of light it emitted into space. He hoped to use that effect to measure a star's mass.

As part of his work, Michell calculated that a star could be dense enough to prevent light from escaping into space. The light particles would climb away from the star's surface, just as a rifle bullet or cannonball climbs away from Earth's surface. But just as Earth's gravity pulls those objects back to the ground, a dense, heavy star would pull light particles back to its surface, making the star impossible to see from afar. In other words, it would be a "dark star."

Michell's calculations showed that the surface gravity of a star as massive as the Sun but just four miles (6 km) in diameter would be strong enough to prevent light from escaping, which is the same as modern calculations of black holes.

Michell's ideas varied from modern concepts of a black hole in several key ways, though. For one, Michell envisioned that a dark star would behave like a normal star, doing the same "starry" things as the Sun and all the other stars. He also thought that light could actually leave the surface of such a star and travel a little way into space before falling back, which would make the star visible from an orbiting planet; it would be cut off only from the more-distant universe.

Despite the differences, however, Michell was on the right track. His insights and calculations gave the Royal Society — and the rest of the world — a first glimpse at exotic dark stars.

Resources

The Eighteenth-Century “Dark Star”

John Michell

Black Holes and Time Warps: Einstein’s Outrageous Legacy, by Kip Thorne, 1994

Timeline