Some of the physicist’s greatest hits – from singularities in gravitational collapse to a quantum theory of gravity

Physicists working on Einstein’s theory of gravity noticed that it allowed for singularities – points where spacetime appeared to be infinitely curved. But it was unclear whether singularities were real or not. Roger Penrose at Birkbeck College in London proved that singularities would indeed form in black holes. Later, Penrose and Hawking applied the same idea to the whole universe and showed that Einstein’s theory predicted a singularity in our distant past. It was the big bang.

Black holes have their own set of laws that mirror the more familiar laws of thermodynamics. Hawking came up with the second law, which states that the total surface area of a black hole will never get smaller, at least so far as classical (as opposed to quantum) physics is concerned. Also known as the Hawking area theorem, it created a puzzle for physicists. The law implied that black holes were hot, a contradiction of classical physics that said black holes could not radiate heat. In separate work, Hawking worked on the “no hair” theorem of black holes, which states that black holes can be characterised by three numbers – their mass, angular momentum and charge. The hair in question is other information that vanishes when it falls into the black hole.

Nothing can escape a black hole, or so physicists once thought. Hawking drew on quantum theory to show that black holes should emit heat and eventually vanish. The process is slow for normal black holes. It would take longer than the age of the universe for a black hole with the same mass as our sun to evaporate. But smaller black holes evaporate faster, and near the end of their lives release heat at a spectacular rate. In the last tenth of a second, a black hole could explode with the energy of a million one megaton hydrogen bombs.

A popular theory in cosmology holds that the fledgling universe went through a period of rapid inflation soon after the big bang. Hawking was one of the first to show how quantum fluctuations – minuscule variations in the distribution of matter – during inflation might give rise to the spread of galaxies in the universe. What started as a tiny difference grew into the cosmic structure we see, as gravity made matter clump together. Recent maps of the heavens that pick up the faint afterglow of the big bang reveal the kinds of variations Hawking worked with.

Hawking has spent much of his time trying to develop a quantum theory of gravity. He started out applying his idea of Euclidean quantum gravity to black holes, but in 1983 teamed up with Jim Hartle at Chicago University. Together they proposed a “wave function of the universe” that, in theory, could be used to calculate the properties of the universe we see around us.