A brief history of Stephen Hawking: A legacy of paradox

A brief history of Stephen Hawking:
A legacy of paradox
Stuart Clark, New Scientist, 14 March 2018

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Stephen William Hawking. 1942 – 2018. – Cosmologist, space traveller and hero.

‘I think most physicists would agree that Hawking’s greatest contribution is the prediction that black holes emit radiation,’ says Sean Carroll, a theoretical physicist at the California Institute of Technology. ‘While we still don’t have experimental confirmation that Hawking’s prediction is true, nearly every expert believes he was right.’

Experiments to test Hawking’s prediction are so difficult because the more massive a black hole is, the lower its temperature. For a large black hole – the kind astronomers can study with a telescope – the temperature of the radiation is too insignificant to measure. As Hawking himself often noted, it was for this reason that he was never awarded a Nobel Prize. Still, the prediction was enough to secure him a prime place in the annals of science, and the quantum particles that stream from the black hole’s edge would forever be known as Hawking radiation.

Some have suggested that they should more appropriately be called Bekenstein-Hawking radiation, but Bekenstein himself rejects this. ‘The entropy of a black hole is called Bekenstein-Hawking entropy, which I think is fine. I wrote it down first, Hawking found the numerical value of the constant, so together we found the formula as it is today. The radiation was really Hawking’s work. I had no idea how a black hole could radiate. Hawking brought that out very clearly. So that should be called Hawking radiation.’

The Bekenstein-Hawking entropy equation is the one Hawking asked to have engraved on his tombstone. It represents the ultimate mash-up of physical disciplines because it contains Newton’s constant, which clearly relates to gravity; Planck’s constant, which betrays quantum mechanics at play; the speed of light, the talisman of Einstein’s relativity; and the Boltzmann constant, the herald of thermodynamics.

The presence of these diverse constants hinted at a theory of everything, in which all physics is unified. Furthermore, it strongly corroborated Hawking’s original hunch that understanding black holes would be key in unlocking that deeper theory.

Hawking’s breakthrough may have solved the entropy problem, but it raised an even more difficult problem in its wake. If black holes can radiate, they will eventually evaporate and disappear. So what happens to all the information that fell in? Does it vanish too? If so, it will violate a central tenet of quantum mechanics. On the other hand, if it escapes from the black hole, it will violate Einstein’s theory of relativity. With the discovery of black hole radiation, Hawking had pit the ultimate laws of physics against one another. The black hole information loss paradox had been born.

Hawking staked his position in another ground-breaking and even more contentious paper entitled Breakdown of predictability in gravitational collapse, published in Physical Review D in 1976. He argued that when a black hole radiates away its mass, it does take all of its information with it – despite the fact that quantum mechanics expressly forbids information loss. Soon other physicists would pick sides, for or against this idea, in a debate that continues to this day. Indeed, many feel that information loss is the most pressing obstacle in understanding quantum gravity.

‘Hawking’s 1976 argument that black holes lose information is a towering achievement, perhaps one of the most consequential discoveries on the theoretical side of physics since the subject was invented,’ says Raphael Bousso of the University of California, Berkeley.

Read the full article in the New Scientist.

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