A brief history of black holes, according to Stephen Hawking
The world-famous physicist discusses the content of his Reith Lectures, which air on Tuesday at 9am on Radio 4 FM
My talk is on black holes. It is sometimes said that fact is stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamt up by science-fiction writers, but they are firmly matters of science fact.
To understand them, we need to start with gravity. Although it’s by far the weakest of the known forces of nature, it has two crucial advantages over other forces. First, it acts over a long range. The Earth is held in orbit by the Sun, 93 million miles away, and the Sun is held in orbit around the centre of the galaxy, about ten thousand light years away. The second advantage is that gravity is always attractive, unlike electric forces, which can be attractive or repulsive. These two features mean that for a sufficiently large star the gravitational attraction between particles can dominate over all other forces, and lead to gravitational collapse.
Despite these facts, the scientific community was slow to realise that massive stars could collapse in on themselves, under their own gravity, and how the object left behind would behave. Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity, because matter could not be compressed beyond a certain point. Many scientists shared Einstein’s gut feeling. The principal exception was the American scientist John Wheeler, who in many ways is the hero of the black hole story. In his work in the 1950s and 60s, he emphasised that many stars would eventually collapse, and highlighted the problems that that posed for theoretical physics. He also foresaw many of the properties of the objects which collapsed stars become, that is, black holes.
During most of the life of a normal star, over many billions of years, it will support itself against its own gravity by thermal pressure caused by nuclear processes, which convert hydrogen into helium. Eventually, however, the star will exhaust its nuclear fuel. The star will contract. In some cases, it may be able to support itself as a white dwarf star. However, in 1930 Subrahmanyan Chandrasekhar showed that the maximum mass of a white dwarf star is about 1.4 times that of the Sun. A similar maximum mass was calculated by Soviet physicist Lev Landau, for a star made entirely of neutrons.
Then the Second World War intervened. Interest in the subject revived with the discovery of distant objects called quasars. The first quasar to be discovered (in 1963) was 3C 273. Many other quasars were soon discovered. They were bright, despite being at great distances. Nuclear processes could not account for their energy output, because they release only a fraction of their rest mass as pure energy. The only alternative was gravitational energy, released by gravitational collapse.
Gravitational collapses of stars were rediscovered. It was clear that a uniform spherical star would contract to a point of infinite density, a singularity. But what would happen if the star isn’t uniform and spherical? Could this cause different parts of the star to miss each other, and avoid a singularity. In a remarkable paper in 1965, Roger Penrose showed there would still be a singularity, using only the fact that gravity is attractive.
When Wheeler introduced the term black hole in 1967, it replaced the earlier name, frozen star. Wheeler’s coinage emphasised that the remnants of collapsed stars are of interest in their own right, independently of how they were formed. The new name caught on quickly. It suggested something dark and mysterious. But the French, being French, saw a more risky meaning. For years they resisted the name trou noir, claiming it was obscene. But that was a bit like trying to stand against le week-end and other Franglais. In the end, they had to give in. Who can resist a name that is such a winner?
This is an edited version of the lecture Professor Hawking delivered to the Royal Institution
The Reith Lectures begin on Tuesday at 9:00am on Radio 4 FM