STEPHEN HAWKING (b.1942)

1971 – UK

1974 Elected fellow of The Royal Society. 1977 Appointed Professor of Gravitational Physics at Cambridge. 1979 Appointed Lucasian Professor of Mathematics.

‘During the first moments of the big bang that marked the birth of the universe some areas were forced by the turbulence to contract, rather than expand. This could have crushed matter into black holes that ranged in size from a few micrometres to a metre (their masses ranging from a few milligrams to the mass of a large planet). This multitude of black holes may still exist’

Hawking sought to extend the existing General Theory of Relativity, uniting it with quantum theory by suggesting that mini black holes weighing up to billions of tons and having the size of a proton would thus be susceptible to gravitational laws and would also need to obey quantum laws. Implications of Hawking’s theory are a universe with no beginning or end, nor any boundaries to it at all. A mini black hole has not yet been detected nor is there any circumstantial evidence for their existence.

In 1974 Hawking proposed a mechanism by which black holes transform their mass into radiation and particles that leave the hole, with the result that they slowly evaporate. Hawking had noticed the similarity between the event horizon of a black hole and the second law of thermodynamics. The law says that an isolated system will always tend to gain entropy and become more chaotic, and if left to itself will never become more ordered. The surface of a black hole can only stay the same size or swell – it can never shrink or become more ordered. Heisenberg’s Uncertainty Principle formulated in 1927 shows that it is not possible to be certain of both an object’s momentum and its position at the same time, as the way of determining the question will distort either one or the other. On an everyday scale, this distortion is so small that it does not matter, but on the level of sub-atomic particles it is crucial and leads to quantum effects in which particles apparently ignore the rules of classical physics and jump and pop up seemingly at random. One astonishing aspect of Heisenberg’s Uncertainty Principle is what it says about empty space. It says there is no such thing as empty space, because empty space would be a precise state and there is no such thing as a precise state. To create what is probably empty space, pairs of ‘virtual’ particles must oscillate either side of the zero that is empty space. These pairs are positive particles and negative antiparticles. When they come together, they annihilate each other, but they are constantly moving in and out of space.

artists' conception of a black hole

Hawking realised that this interchange of virtual particles was happening all along a black hole’s event horizon, which is an edge in space. Negative particles would be drawn into the black hole and positive particles pushed out. The negative particles are what stop the size of the black hole from ever decreasing. The expelled positive particles emerge as heat, miniscule – a few millionths of a degree above absolute zero – but in theory measurable. So black holes are not black, but emit heat radiation, which came to be known as Hawking radiation. The amount of Hawking radiation escaping from an evaporating black hole is inversely proportional to the square of its mass. Extra rotational energy is thought to be extracted from a black hole when its spin is slowed by the twisting of magnetic field lines close to its event horizon.

Hawking went further to suggest that just as just as a star losing radiation gradually dwindles, so a black hole would eventually evaporate into pure radiation.

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