Greater Minds: Stephen Hawking on black holes

  • Stephen Hawking gave this year’s Reith Lectures on the subject of black holes.
  • Hawking combined quantum mechanics and relativity to show black holes emit ‘Hawking radiation’.
  • Hawking radiation shows that unpredictability is inherent to the structure of the universe.
  • Hawking has been a world-renowned physicist for 50 years since being diagnosed with motor neurone disease and given two years to live.


Stephen Hawking outside Gonville & Caius College, Cambridge, in 2015 (Lwp Kommunikáció [CC / Flickr])

I’ve been writing the Greater Minds series for more than a year and a half now, so it’s something of an oversight that I’m only just getting to the mind often lauded as the greatest of them all: Stephen Hawking. As he gave this year’s Reith Lectures, a broadcasting event I always look forward to, now is as good a time as any to correct the oversight.

The two half-hour lectures are:

Do black holes have no hair? (podcast and transcript)


Black holes ain’t as black as they are painted (podcast and transcript).

The great demystifier

Hawking is best known as a science communicator. His synthesised voice is famous from his lectures and documentaries in which he demystifies the most mystifying areas of fundamental physics, and from his occasional appearances in The Simpsons. To fully comprehend what an achievement that is, we have to remember that he writes his lectures at around a word per minute by twitching a cheek muscle.


Composite image from Chandra X-Ray Observatory and Hubble Space Telescope of Galaxy NGC 1275 with a black hole at the centre (NASA’s Marshall Space Flight Center [CC / Flickr])

In the Reith lectures, he returns to the subject of his most ground-breaking research: black holes. If Hawking will be remembered for one discovery, it is for showing that black holes do not guzzle all matter and energy in their vicinity as had been previously thought. While his second lecture describes the process by which it happens, he is modest enough not to give the phenomenon its proper name of ‘Hawking radiation’. That was only mentioned in the questions and answers by comedian and presenter Dara O’Briain, who also pointed out that Hawking’s discovery was ‘the first theoretical prediction that required both quantum mechanics and relativity’.

Which points to the problem with writing about Hawking and his work: I can’t claim more than the haziest understanding of it. Hawking himself is clear and easy to follow, and represents the limit of my understanding of black holes and quantum physics. The BBC have made it even easier by providing an animated description of black holes as he describes them:

Hawking radiation slays Laplace’s demon

The idea of a star collapsing into a point of infinite density that curls space time around it boggles my mind. I suspect it takes the years of study and thought that Hawking has put into the subject to fully comprehend it. Not content with mere boggling, Hawking’s second lecture considers some of the wider implications of that. Perhaps it should be called metaboggling.


Portrait of Pierre-Simon Laplace by Sophie Feytaud, 1841 (Luestling [Wikimedia Commons])

He invokes Laplace’s Demon, the intellect conceptualised by the natural philosopher Pierre-Simon Laplace. In 1814, before Einstein had introduced the concept of quantum uncertainty, Laplace suggested that if an intellect could know every force and item in existence at a given moment of time, it would be possible to predict all forces and items at any point in the future. The omnipotent intellect has become known as Laplace’s demon.

Laplace’s demon is a thought experiment rather than a natural experiment, as the idea of measuring every force and particle in the universe presents some minor practical difficulties. In the two centuries since Laplace conceptualised his demon, we’ve learned more about both the vastness of the universe and the minutiae of sub-atomic particles. Gathering the information Laplace describes appears enormously more difficult than it would have done to Laplace, and he never thought it was possible.

Engines of unpredictablility

The point of Laplace’s demon is not that anyone should try to become it, but to suggest that the future can be predicted from the present if the present is sufficiently well known. It’s a principal that Hawking calls scientific determinism.

Hawking radiation is a silver bullet aimed at the conceptual heart of Laplace’s demon, because it has no relationship with whatever form it previously took.


Stephen Hawking in 2008 (NASA HQ PHOTO [CC / Flickr])

Anything that falls into a black hole is ripped apart at the sub-atomic level and reduced to mere mass. No trace of whatever it was before that remains. So far, so meh for Laplace’s demon. It’s quite capable of predicting that the carbon molecule now forming part of one of your eyelashes will join the mass of a particular black hole in a couple of billion years.

Yet the Hawking radiation emitted by that black hole is stripped of any and all information available to Laplace’s demon right now. And if Laplace’s demon cannot see what will emerge from that black hole, even if it knows everything there is to know about the universe as it is right now, the principal of scientific determinism collapses and Laplace’s demon is spagehettified in a black hole of its own logic.

Black holes may be the devourers of all matter and energy down to light itself, but they are also engines of unpredictability.

Humour and stubbornness

Hawking’s emphasis is very much on the physics that he has made his life, and he only alludes to his remarkable life in answer to direct questions. He is as well known for his disability as for his scientific achievements. He was diagnosed with motor neurone disease when he was 21, when he went from being a young scientist with his whole life ahead of him to being told his life would consist of two years of deterioration until he died.


Mosaic portrait of Stephen Hawking (Charis Tsevis [CC / Flickr])

There is no right time to receive news like that, but there have been worse ones. Hawking’s deterioration has been matched by advances in medical technology that not only kept him alive but enabled him to use his monumental intellect. More than fifty years after his diagnosis, his condition has reached the point where he only has control of a few of his facial muscles, yet he is still able to communicate so eloquently that 20,000 people applied for the 400 places in his Reith Lectures.

On being asked how he kept going during the most difficult times in his life, he credited his sense of humour and said it was important not to become angry. His daughter had a different take: ‘I think he’s enormously stubborn’.

He concluded the second lecture with an observation on Hawking radiation, which may have been an inspiration to Hawking himself at times:

If you feel you are in a black hole, don’t give up. There’s a way out.

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Posted in Greater minds, Wednesday Pontification

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