# The career of Stephen Hawking

The world celebrated Prof. Stephen Hawking’s 70th birthday on 8 January 2012. We take a look back over his life and work.

**A brief history of Hawking**

Hawking first shot into the public consciousness with the 1988 release of his popular-science book *A Brief History of Time*, which would remain on the *Sunday Times* bestseller list for an unprecedented four years.

Early in his career, Hawking and his Cambridge colleague Roger Penrose had developed new mathematical tools for use in Einstein’s general relativity.

Using these, they showed that singularities – such as those thought to be at the centre of black holes – are a common feature of the theory, and that, if the effects of quantum mechanics are ignored, the universe must also have begun at a point of infinite density and energy.

It is perhaps for his work on black holes that Hawking is best known.

First, he mathematically proved a theorem by the American physicist John Wheeler that any black hole can be completely described by just three properties – its mass, angular momentum and electric charge.

He then formulated a set of principles that would apply to black holes and be analogous to the usual laws of thermodynamics. Until then, the concept of black holes had presented a problem in that it appeared they allowed the second law of thermodynamics to be broken.

The second law of thermodynamics says that the amount of entropy in a system will always increase. As entropy is a measure of disorder, it would be possible to decrease the total entropy in the universe simply by throwing mass into a black hole.

Hawking’s discovery that the area of a black hole also never decreases led the Mexican-Israeli physicist Jacob Bekenstein to suggest that in some way the area *is* the entropy of the black hole. The problem with this was that if a black hole has an entropy then it must have a temperature, and must therefore radiate heat – so it would no longer be black.

Black holes are thought to do precisely this through what is now known as Hawking radiation, which arises due to quantum fluctuations at the black hole’s event horizon. ‘Empty’ space is actually filled with pairs of virtual particles popping into and out of existence, and whereas they would normally vanish effectively instantaneously, in this case, for each pair, one is trapped by the black hole and the other escapes.

If enough time passes, black holes will radiate away all their energy and completely evaporate. The lifetime of a black hole is proportional to the cube of its mass – for a solar-mass black hole it would be around 10^{67} years.

**Cosmology and space advocacy**

Hawking then turned his attention back to the universe as a whole.

Along with Jim Hartle of the University of California, Santa Barbara, he developed a cosmological model in which the universe has no boundary in spacetime – overturning his own earlier theory that it must have begun with a singularity.

He later proposed a “top-down” cosmological model in which, rather than the universe beginning with a specific set of initial conditions from which it involved into the state that we see today, the present in some sense “selects” the past in much the same way as the observing of a quantum system causes its wavefunction to collapse from superimposed states to one definite condition – such as a living cat or a dead one.

More recently, Hawking has become involved in “space advocacy”, arguing that colonising space is vital to the long-term survival of humanity.

Long-term survival is something that Hawking is very familiar with – having now reached 70 years of age despite a prognosis in his early 20s that, with motor neurone disease, he would only have a few years left to live. Physics has been much better off for his longevity.

An exhibition, Stephen Hawking: A 70th birthday celebration is running at the Science Museum until 9 April