Topic of the Moment – 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.

 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 1067 years.

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