John Michell, Gravity and Black Holes

Newton shows that the gravitational attraction of an object is proportional to its mass.

If light is affected by gravity, a massive enough object will have such a strong gravitational field that no light will be able to escape it.

Einstein explains gravity as a distortion of space-time, meaning that massless light is affected by gravity.

The velocity of light will appear to diminish.

In a 1783 letter to Henry Cavendish at the Royal Society, British polymath John Michell set out his thoughts on the effect of gravity. The letter was rediscovered in the 1970s and found to contain a remarkable description of black holes. Newton’s law of gravity states that an object’s gravitational pull increases with its mass. Michell considered what might happen to light if it is affected by gravity. He wrote: “If the semidiameter of a sphere of the same density with the sun were to exceed the sun in the proportion of 500 to 1, a body falling from an infinite height toward it would have acquired at its surface a greater velocity than that of light, & consequently, supposing light to be attracted by the same force… all light emitted from such a body would be made to return towards it.” In 1796, French mathematician Pierre-Simon Laplace came up with a similar idea in his Exposition du Système du Monde.

Source: The Science Book: Big Ideas Simply Explained (2014) by DK Publishing.