John Michell and the Effect of Gravity on Light

Lensing by a black hole

Lensing by a black hole

On December 25, 1724, English natural philosopher and geologist John Michell was (probably) born. He is best known as both a theorist and an experimenter, who was the first to propose the effects of gravity on light, later resulting in the physics of general relativity and black holes.

John Michell was born in Nottinghamshire, however, his exact birth place and date remain unknown. He earned his Master’s degree drom Queen’s College of Cambridge University around 1752 and probably in the early 1760s, he earned a BD. Michell was elected fellow of the Royal Society in the same period and held the Woodwardian Chair of Geology. He was appointed rector of St. Michael’s Church of Thornhill, near Leeds in 1767 and held this post for the rest of his life. [1,2]

Michell began researching on artificial magnets already in the 1750s and he managed to demonstrate that the magnetic force exerted by each pole of a magnet decreases with the square of the distance. Michell also designed an experimentational device that was probably used later on by Cavendish to measure the force of gravity between the masses in the laboratory to receive the first accurate value for the gravitational constant ‘G’. In the field of Astronomy, Michell demonstrated how stars were distributed in the sky and he argued that there were numerous pairs or groups of stars and the first evidence was provided for binary stars and clusters.

Around 1783, Michell wrote a paper, intending to discover a method to determine the mass of a star. In it, Michell referred to Newton’s theory of light and reasoned that when light particles were emitted by a star, it’s gravitational pull reduced their speed and thus, an observable shirft in the starlight was the result. Initially, Michell thought that he could measure how much the speed of light was reduced by passing it through a prism. He could conceivably compare the refracted images of different stars to determine the difference in their surface gravity, and from that, calculate their respective masses. Back then, Ole Rømer’s measurements of the speed of light were known and Michell also knew of the concept of ‘escape velocity’. Michell asked himself, what would happen if stars were so incredibly massive and their gravity so strong that the escape velocity was equal to the speed of light. The conclusion was that these stars would probably be invisible to the astronomers and that there could be numerous stars like these that emitted no light and stay indetectable to scientists. On this day, most scientists believe that most galaxies have black holes in their center. [1]

As revolutionary as Michell’s thoughts were, he was wrong in his initial proposal to measure a star’s mass by the speed of light, because from Einstein’s relativity theory (1905) it is known that light moves through space at constant speed regardless of the local gravity strength. The scientific research around black holes was probably rediscovered after Einstein published his theory of gravity. Karl Schwarzschild is known to have contributed significantly to the topic and solved Einstein’s equation for the case of a black hole. He envisioned it as a sperical volume of warped space surrounding a concentrated mass and invisible to scientists on Earth. It was then suggested in the work of Robert Oppenheimer and his colleagues that an object like this may be formed by the collapse of a massive star. The physicist John Wheeler coined the term ‘Black Hole’ in 1968. [3]

At yovisto, you may be interested in a short video lecture describing Black Holes by Tom Jerrett.

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