John Michell

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John Michell (1724-1793) was Rector of Thornhill, England, and a friend of Henry Cavendish. Nowadays he is chiefly remembered for having been the first person to publish a prediction of gravitational shifts, and the critical r=2M radius of a compact gravitational object that we'd now call a black hole. Michell's pre-GR object, which could still radiate indirectly, is now referred to as a dark star.


Michell was one of the first to explore the theory of earthquakes, and suggested that they might originate as an event at a specific place (such as under a volcano), with the shockwave then being transmitted through the Earth to affect a wider region. A violent event occurring in the Earth's crust out at sea might shake cities inland, with the 1760 Lisbon quake being a likely example ... Lisbon's quake didn't necessarily mean that there was anything badly wrong actually underneath Lisbon, and didn't mean that the city (or other cities) were in danger of being suddenly swallowed up by the Earth – a reassuring idea to people who lived in there, as it meant that their cities were not in danger of plunging vertically into a Hellmouth!. If people could record the precise times that they experienced a 'quake (said Michell), we could calculate the location where the disturbance actually originated. Michell's ideas were put into practice ~125 years later, by John Milne (1850-1913), and are the basis of modern seismology.

Momentum of light:

One of Michell's more unsuccessful experiments was to try to demonstrate the existence of light-pressure. To do this, he used a reflector to focus sunlight onto one side of a compass needle, hoping to produce a deflection. The needle got hot, briefly went in the wrong direction (which Michell attributed to outgassing), and then melted.

Finding the mass of the Earth:

Michell developed an extremely sensitive torsion balance, with the aim of measuring the amount of gravity associated with a known amount of mass (the Gravitational Constant). If one knew the ratio between gravitational and inertial mass, and one knew the strength of the gravitational fields of planets (by the orbits of objects circling them), then one could calculate their masses.

Michell died before he could carry out the test, but Henry Cavendish inherited Michell's unfinished apparatus, continued to develop it, and used it to perform the Cavendish Experiment to calculate a value for the Earth's mass.

Gravitational shifts and the r=2M horizon radius

In 1783, Michell wrote a letter to Cavendish (published in the 1784 volume of the prestigious Royal Society's journal) discussing how we might measure the masses of distant stars. Michell's idea was that since light ought to be weakened when it climbs out of a star's strong gravitational field, we should be able (in theory) to see a shift in the characteristics of this starlight, that might tell us the star's surface gravity. Michell reasoned that when weakened light from a strong-gravity star was passed through a prism, it ought to be deflected differently due to its reduced energy, and this might result in the star's image being deflected differently by the prism, with the offset between the refracted images of different stars telling us the difference in surface gravity between them. In practice, a shift in the frequency of a star's visible light simply moves other frequencies into the visible range (making Michell's suggestion impractical), but the basic principle was sound, and modern spectroscopy instead uses identifiable "notches" in a star's spectrum as references for spectral shifts. While some sources tell us that Einstein was the first person to come up with the idea of gravitational shifts (in 1911), Michell was already going beyond this and exploring ways that future astronomers might be able to make practical use of the effect, back in the Eighteenth Century.

Michell also calculated that if a star was big enough or dense enough, its surface escape velocity could equal or exceed the speed of light, and we wouldn't be able to see the thing directly. The critical radius at which this happens can be written in simplified form as "r=2M", where M is the mass of the star, and various natural constants (such as c and G) are deliberately left out. Michell's distance is what we would now refer to as the Schwarzchild radius associated with the event horizon of a black hole under general relativity, and we now refer to the sort of object described by Michell as a "dark star", to distinguish it from GR1916's "black holes".

Although we couldn't see these dark stars directly, if a binary star system included one visible star and one dark star, we'd be able to identify the "dark" object by the motion of its visible companion (another idea often wrongly thought to have originated in the Twentieth-Century). Michell suggested a catalogue of binary stars in order to find what proportion were "dark", in the hope of producing some sort of statistical baseline for estimating the amount of other "dark", gravitationally-cloaked material there might be in the universe.

The history to Michell's "dark star" piece is discussed in Thorne (1994), and possible reasons for its obscurity until the mid-late C20th are discussed in Baird (2007).

See also

Background references

  • Isaac Newton, Opticks (1704-)
  • John Michell, On the means of discovering the distance, magnitude etc. of the fixed stars ... Philosophical Transactions of the Royal Society (1784)
  • Albert Einstein, On the Influence of Gravitation on the Propagation of Light (1911)
  • Isaac Asimov, Asimov's Biographical Encyclopedia of Science and Technology (revised 1972 edition) – lists most of Michell's key achievements, but doesn't mention his "dark star" paper
  • Clyde R. Hardin, The scientific work of the Reverend John Michell Annals of Science 22 27-47
  • Russell McCormmach, John Michell and Henry Cavendish: Weighing the stars, British Journal for the History of Science 4 126-155
  • Simon Schaffer, John Michell and black holes Journal for the History of Astronomy 10 42-43 (1979)
  • Gary Gibbons, The man who invented black holes [his work emerges out of the dark after two centuries] New Scientist 28 June pp.1101 (1979)
  • Hawking and Israel (eds), Three hundred years of gravitation (1987) Werner Israel, "Dark stars: The evolution of an idea", pp. 199-276
  • J. Eisenstaedt, De L'influence de la gravitation sur la propagation de la lumière en théorie Newtonienne. L'archéologie des trous noirs Archive for the History of Exact Sciences 42 315-386 (1991)
  • Kip S. Thorne, Black Holes and time warps (1994) especially Chapter 3