What did Hawking discover?

Updating Einstein’s Universe and Magic Universe

What did Stephen Hawking discover?

Stephen Hawking. Photo: Channel 4
















  • Is it fair?
  • Does it need updating?

Exploding black holes

Hawking’s theoretical discovery (1974) that we featured in “The Key to the Universe” remains, in my opinion, his chief claim to scientific fame. Here’s how his exploding black holes are explained in Einstein’s Universe.

Even when pictured at rest in its realm of distorted space and time, the massive body has to work continuously through time to maintain the curvature of space; in doing so it loses a little energy, or mass. Energy oozes out of strongly warped space like water from a squeezed sponge.

Stephen Hawking of Cambridge discovered the process as a remarkable extension of Einstein’s theory. Close to a black hole, space is warm and the consequences may be dramatic. Space seethes – that is the reason for the effect. If you take a census of what is present in a volume of high-grade ’empty’ space far away from any galaxy, and then discount all the expected things like a few atoms and plenty of particles of light passing through in all directions, something else remains. You cannot detect it in any ordinary way, but one of the strongest theories of modern physics insists that it is there – a surreptitious hint of everything that energy is capable of creating. The existence of ghostly particles predicted by the quantum theory has been confirmed by small effects on the ‘tuning’ of atoms.

Hawking realised that the intense curvature of space just at the edge of a black hole could convert some of these ghostly particles into durable particles of matter and light. It was in 1974 that he announced that black holes were warm and capable of exploding. The process can run away, draining the rest-energy of the black hole and abruptly abolishing it with a great outpouring of gamma-rays and sub-atomic particles. That would occur in our era only in very small black holes, which might have been created in the Big Bang at the origin of the universe.

Will black-hole explosions be seen? Astronomers might spot the burst of gamma-rays; alternatively, charged particles coming from the explosions will presumably swerve in the magnetism of the Galaxy and so broadcast detectable light and radio waves. Among the experts opinions vary widely about whether the small exploding black holes actually exist; much depends on what you think was happening during the Big Bang. Observations so far set a rough upper limit to their possible occurrence; if black-hole explosions occurred just once a year within a distance of 150 light-years, they should have been seen already.

  • Is it fair? I believe so.
  • Does it need updating? Yes.
  1. Exploding small black holes have still not been observed, despite the huge growth in telescope power.
  2. Hawking’s theory had a problem, concerning an apparent loss of information. There was no relationship between matter going into a black hole and matter coming out. This conflicted with tenets of Quantum Mechanics about the survival of information, and Hawking eventually conceded that it couldn’t be right. Since then there have efforts to fix the problem using theories of quantum gravity.

Neither point detracts from the fact that Hawking instigated a very important line of research. On the other hand, the efforts to reconcile Einstein’s General Relativity with Quantum Mechanics still seem to flounder in a sea of unverifiable conjecture. As I wrote for the 2005 edition of Einstein’s Universe, in “Afterword 2005: The Melting Pot”:

Top of the agenda of theoretical physics in I979 was the wish to reconcile General Relativity with the quantum theory. These great pillars of twentieth-century science were not just poles apart in their philosophical and mathematical styles. They flatly contradicted each other in the extreme conditions of a black hole or, more importantly, in the Big Bang. Einstein’s gravity should compel great concentrations of mass-energy to shrink to a geometric point – a singularity of zero size. The jiggling of matter that is the hallmark of the quantum theory ought to prevent anything ever getting quite so small.

The big aim, then as now, was to force General Relativity and the quantum theory to co-exist in a single theory uniting gravity, electromagnetism and the sub-atomic forces. Stephen Hawking had a clever notion, described in Chapter 8, that small black holes, the product of gravity, could explode and release sub-atomic particles in accordance with quantum theory. Nothing much has come of the idea, so far. More durably, Roger Penrose’s re-description of spacetime, using particle-like entities called twistors, might still in principle provide a framework for both gravity and the quantum theory.

  • Is it fair? I believe so, although I’d forgotten how dismissively it reads in respect of Hawking’s idea. The words reflect my general opinion about the floundering theories. In presenting him with the Royal Society’s Copley Medal in 2006, the president Lord (Martin) Rees said “Stephen Hawking has contributed as much as anyone since Einstein to our understanding of gravity.” That’s correct. The trouble is that the overall progress in understanding gravity has been very slow, and Einstein’s own theory has brushed off many challenges.
  • Does it need updating? About Hawking, no. About Penrose, yes but not here.

Turning now to Magic Universe, I start with another leading physicist who, like Peter Higgs, was less than gruntled with our Stephen.

A tussle about cosmic inflation

In a story in Magic Universe called “Big Bang: the inflationary Universe’s sleight-of-hand” there’s a sub-heading “The magician of Moscow”.

The paradoxical solution to these problems [about the uniformity of the Universe] was to make the early expansion even faster, in a process called inflation. It achieved uniformity by enlarging the miniature Universe so rapidly that the creation of matter and radiation could not keep up. Nothing much happened until an average density of energy had been established throughout the micro-cosmos.

Inflation also had an economic effect, by vastly increasing the available energy — the cash flow of creation. To buy a well-stocked cosmos with a primordial speck seemed to require a gross violation of the law of conservation of energy, which says you can’t get something for nothing. Sleight-of-hand of a high order was therefore needed. Who better to figure out how the trick could have been done than an astrophysicist who was also an amateur magician? He was Andrei Linde of the Lebedev Institute in Moscow.

[later in the story] Linde announced his simpler and surer route to inflation at an international meeting of physicists in Moscow in the summer of 1981. Viscosity in the infant Universe, analogous to that experienced by a ball rolling in syrup, would make inflation possible. After his talk many physicists from the USA and Europe crowded around Linde, asking questions. Some volunteered to smuggle his manuscript out of the country to speed up its publication, because the censors would sit on it for months.

But next day Linde had a disagreeable task. He did the interpreting into Russian while Stephen Hawking, over from Cambridge, said in a lecture that Linde’s version of inflation was useless. This was in front of all the sages of Soviet physics — a formidable lot — and Linde was just an up-and-coming 33-year-old.

I was translating for Stephen and explaining to everyone the problems with my scenario and why it does not work,’ he recalled. ‘I do not remember ever being in any other situation like that. What shall I do, what shall do? When the talk was over I said that I translated but I disagreed, and explained why.’

Nevertheless it was at a workshop organized by Hawking in Cambridge in 1982 that a refined version of Linde’s scenario was generally adopted as the ‘new inflation theory’.

  • Is it fair? Yes. Hawking may have been wrong, although whether Linde was right is still an open question.
  • Does it need updating? Not in this connection, because there’s more about Linde and inflation in Magic Universe.

Imaginary time

Later in the same “Big Bang” story in Magic Universe:

How was the Big Bang provoked? Some experts wanted to set it off by colliding other, pre-existing universes. Stephen Hawking magicked the problems away by invoking imaginary time.

  • Is it fair? Yes. Magicked isn’t necessarily rude.
  • Does it need updating? The only question is whether I should explain imaginary time. It was the key new idea that Hawking wanted to popularise in A Brief History of Time, but the book left many readers boggled. While I’ll not add anything to Magic Universe, in fairness I’ll quote here from a 1996 public lecture by Hawking, called “The Beginning of Time”.

Hawking: It seems that quantum theory, on the other hand, can predict how the universe will begin. Quantum theory introduces a new idea, that of imaginary time. Imaginary time may sound like science fiction, and it has been brought into “Doctor Who”. But nevertheless, it is a genuine scientific concept. One can picture it in the following way. One can think of ordinary, real, time as a horizontal line. On the left, one has the past, and on the right, the future. But there’s another kind of time in the vertical direction. This is called imaginary time, because it is not the kind of time we normally experience. But in a sense, it is just as real, as what we call real time.

The three directions in space, and the one direction of imaginary time, make up what is called a Euclidean space-time. I don’t think anyone can picture a four dimensional curve space. But it is not too difficult to visualise a two dimensional surface, like a saddle, or the surface of a football.


If space and imaginary time are indeed like the surface of the Earth, there wouldn’t be any singularities in the imaginary time direction, at which the laws of physics would break down. And there wouldn’t be any boundaries, to the imaginary time space-time, just as there aren’t any boundaries to the surface of the Earth. This absence of boundaries means that the laws of physics would determine the state of the universe uniquely, in imaginary time. But if one knows the state of the universe in imaginary time, one can calculate the state of the universe in real time. One would still expect some sort of Big Bang singularity in real time. So real time would still have a beginning. But one wouldn’t have to appeal to something outside the universe, to determine how the universe began. Instead, the way the universe started out at the Big Bang would be determined by the state of the universe in imaginary time. Thus, the universe would be a completely self-contained system. It would not be determined by anything outside the physical universe, that we observe. [end of quote from Hawking’s lecture]

Black holes

The story about “Black holes: the awesome engines of quasars and active galaxies” in Magic Universe mentions Hawking twice:

Roger Penrose in Oxford, Stephen Hawking in Cambridge, Yakov Zel’dovich in Moscow and Edwin Salpeter at Cornell were among those who developed the theory of such stellar black holes. It helped to explain some of the cosmic sources of intense X-rays in our own Galaxy then being discovered by satellites. They have masses a few times greater than the Sun’s, and nowadays they are called microquasars. The black hole idea was thus available, ready made, for explaining the quasars and active galaxies with far more massive pits of gravity.

[later] Nuclear reactions are not the only way of extracting the rest energy of E=mc2 from matter. Black holes do it much more efficiently and are indifferent to the composition of the infalling material. If you are so far-sighted that your anxieties extend to the fate of intelligent beings when all the stars of the Universe have burned out, Roger Penrose at Oxford has the answer.

He described how energy might be extracted from a stellar black hole. Send your garbage trucks towards it and empty them at just the right moment, and the trucks will boomerang back to you with enormous energy. Catch the trucks in a system that generates electricity from their energy of motion, refill them with garbage and repeat the cycle. Technically tricky, no doubt, but there are hundreds of billions of years left for ironing out the wrinkles in Penrose’s scheme.

Incorrigible pessimists will tell you that even black holes are mortal. As Stephen Hawking at Cambridge first pointed out, they slowly radiate energy in the form of particles created in their strong gravity, and in theory they could eventually evaporate away. In practice, regular garbage deliveries can postpone that outcome indefinitely. Whether intelligent beings will enjoy themselves, living in a darkened Universe by electric light gravitationally generated from E=mc2, who can tell?

This reference is to the exploding black holes discussed earlier, and needs no further comment.

Over the top

There remains one quote from Hawking, in Magic Universe’s “Microwave background: looking for the pattern on the cosmic wallpaper”.

Progress resumed in 1992, with the release of results from NASA’s Cosmic Background Explorer satellite, or COBE, pronounced Koh-bee. Launched in 1989, this purpose-built spacecraft scanned the whole sky. Analysis of the results revealed, by statistical tests, the existence of features in the microwave background.

What startles me most is that we really can make a coherent story out of the Big Bang,’ remarked John Mather of NASA Goddard, who masterminded COBE. Others’ comments were less measured. ‘It’s like seeing the face of God,’ said George Smoot of the Lawrence Berkeley Lab, who presented the results to the American Physical Society in the form of a sky map. Stephen Hawking at Cambridge called it ‘the scientific discovery of the century, if not of all time.’

These remarks were over the top, because the COBE results were really rather crude. It was a magnificent achievement to confirm that temperature differences are present in the microwave background, as expected. But the widely publicized map did not really show which places in the sky were warm or cool, or how big the hotspots were. Derived from the statistics, it was abstract post-expressionism, like a Jackson Pollock painting.

  • Is it fair? Well it’s what Hawking said, according to Gordon Fraser of CERN.
  • Does it need updating? There’s much to come about the far better results from later spacecraft, NASA’s WMAP and ESA’s Planck, which had not been reported when I completed Magic Universe. But I’ll not withdraw my gripe about Hawking’s hyperbole concerning COBE.

It’s for the reader to judge whether the fact that those are the only mentions of Hawking and his scientific work, in 183-page book on relativity plus a 700-page Guide to Modern Science, is a reflection of his contributions, or of my stupidity.


Genius of Britain”, Channel 4 series, presented by Stephen Hawking et al., 30 May to 3 June 2010. (By the way, Paul Dirac gets his 4 minutes, but Hawking’s mentor, Roger Penrose, is mentioned very briefly.)

The Key to the Universe” produced by Alec Nisbett and written by Nigel Calder, BBC-TV and co-producers. (I suppose we should be pleased by the echo when Lucy & Stephen Hawking published a book for children in 2007 and called it George’s Secret Key to the Universe.)

Peter Higgs quoted in The Scotsman (Edinburgh) 2 September 2002.

N. Calder, Einstein’s Universe updated edition, Penguin 2005.

N. Calder, Magic Universe, Oxford UP, 2003.

S.W. Hawking, “The Beginning of Time” lecture 1996 see: http://www.hawking.org.uk/index.php/lectures/publiclectures/62

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2 Responses to What did Hawking discover?

  1. Anton Garrett says:

    I too feel some irritation with the Hawking bandwagon but I think that it is the media, not Hawking, who are (ir)responsible – Higgs was a little unfair. The media love the disparity between Hawking’s mental and physical faculties, and they have made him much better known than if he had done the same work but not been wheelchair-bound. Hawking suffers from a horrible disease, and some compassion is in order.

    As for his science, don’t forget Hawking’s earlier, ‘classical’ work with Roger Penrose on the singularity theorems in General Relativity. This is glorious work, which showed the inevitability of the Big Bang without having to solve Einstein’s field equations. That some of the assumptions involved ar now questioned scarcely detracts from the achievement.

    • Arijigoku says:

      Yes that point is worth emphasizing: when Hawking was asked about this on telly he said he wasn’t a genius nor comparable with Einstein nor Newton. He said he hoped he had the respect of his peers and i think he does.

      If we played the silly game of “Who’s the Cleverest Living Scientist”, i think it would probably be Ed Witten who’d win the most votes among physicists. Yet most people have probably never heard of him.

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