John Michell and the “dark star”


John Michell (1724-1793

John Michell (25. Dezember 1724 – 29. April 1793) was an Anglican clergyman whose scientific work spanned a wide range of subjects from astronomy to geology, optics, and gravitation.

Michell conceived, sometime before 1783, the experiment now known as the Cavendish experiment. It was the first to measure the force of gravity between masses in the laboratory and produced the first accurate values for the mass of the Earth and the gravitational constant. He wrote a lucid exposition of the nature of magnetic induction. His most important geological essay was entitled “Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes” (Philosophical Transactions, li. 1760), which showed a remarkable knowledge of geological strata. He was thus one of the founders of seismology.


Simulation: A galaxy is passing behind a Black Hole. John Michell did expect the deflection of the light and the darkness of the “Star”

More recently, Michell’s main “claim to fame” is considered to be his letter to Cavendish, written in 1783 and published in 1784, on the effect of gravity on light. This paper was only generally “rediscovered”in the 1970s and is now recognised as anticipating several astronomical ideas that had been considered to be 20th century innovations. Michell is now credited with being the first to study the case of a heavenly object massive enough to prevent light from escaping (the concept of escape velocity was well known at the time). Such an object, which he called a “dark star” (the predecessor of the modern idea of a black hole under general relativity) would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. Michell also derived the radius for such an object based on its mass, which corresponds roughly to what is called the Schwarzschild Radius in general relativity. Michell also suggested using a prism to measure the gravitational weakening of starlight due to the surface gravity of the source (”gravitational shift”). Michell acknowledged that some of these ideas were not technically practical at the time, but wrote that he hoped they would be useful to future generations. By the time that Michell’s paper was “resurrected” nearly two centuries later, these ideas had been reinvented by others.



Source: Dave Armstrong, Science and Christianity: Close Partners or Mortal Enemies? (2013)

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