What’s interesting about black holes is the interaction between experiment and observations on the one hand, and the mathematical models we invent to explain them, on the other.

Einstein’s 1916 General Theory of Relativity immediately solved a century-old problem with the orbit of Mercury and predicted something new, the bending of light round the Sun, which was confirmed by observations during the 1919 eclipse of the Sun. But from these very first solutions of Einstein’s equations it seemed that something funny happened at a particular distance from a point-mass M, at distance 2GM/c². At first this seemed an academic point, because this distance is only three kilometres for the mass of the Sun, compared with its radius of one million kilometers.

Then in 1939 Oppenheimer and Sneider showed that, for a star above a certain mass, which we now know to be about 20 times the mass of the Sun, the end point of its evolution, after it has exhausted its nuclear fuel, is collapse to a point, a singularity, and that this would be hidden from view inside the ‘event horizon’. As their 1939 abstract says: