A profound question arises as a result of this faint glow. When the black hole has gone, what has happened to everything that fell in? Because of the unique production mechanism of Hawking radiation, plucked as it is out of the vacuum in the vicinity of the event horizon, the radiation would seem to have nothing to do with whatever has fallen into the black hole during its lifetime. It is very difficult therefore to see how any information about anything that fell in, or indeed the star that collapsed to form the black hole in the first place, could be preserved, imprinted somehow, in the radiation. Indeed, Hawking’s original calculation appeared very clear on this point. The radiation, the remnants of the black hole, contains no information at all.
One of the pioneers of modern black hole research, Leonard Susskind, tells the story of a meeting in a small San Francisco attic room in 1983 at which Hawking first raised this question and answered it, incorrectly as it turns out. Susskind’s first-hand account of the tremendous intellectual struggle Hawking’s question generated is called The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics. Susskind has a way with titles. He once co-authored a paper called ‘Invasion of the Giant Gravitons from Anti-de Sitter Space’. He writes that ‘Stephen claimed that information is lost in black hole evaporation, and worse, he seemed to prove it. If that was true … the foundations of our subject were destroyed.’
Susskind was referring to one of the pillars of modern physics: determinism. If we know everything about a system, be it a simple box of gas or the Universe, we can predict how it will evolve into the future and how it looked in the past. This is an ‘in principle’ statement of course. It’s not possible in practice to know everything about the past and future, because we always have incomplete information about any real physical system. But in science, unlike modern-day politics, principles matter. If Hawking was right, black holes would render the Universe fundamentally unpredictable and the foundations of physics would crumble.
We now know that Stephen Hawking was wrong – information is not destroyed and physics is safe – as Hawking himself came to accept with delight, not regret, not least because the ongoing programme of research stimulated by his original claim continues to propel us towards a new understanding of space and time and the nature of physical reality.
In the last edition of A Brief History of Time, Hawking writes that he eventually changed his mind in 2004 and conceded a bet he’d made with John Preskill (whose work we’ll meet later). After a further argument about the merits of cricket and baseball, which he also lost, Hawking gave Preskill an encyclopaedia of baseball. At the time of writing, Hawking notes, nobody knew how the information gets out of the black hole – just that it does. What was clear, however, is that the information would be very hard to decode. ‘It’s like burning a book,’ he writes. ‘Information is not technically lost, if one keeps the ashes and the smoke – which makes me think again about the baseball encyclopaedia I gave John Preskill. I should perhaps have given him its burnt remains instead.’
Beyond the horizon
Imagine you find a watch lying on the ground. On close inspection you are compelled to marvel at its delicate sophistication and exquisite precision. The mechanism was surely designed; there must have been a creator. Transpose ‘watch’ for ‘Nature’ and this is the argument for God presented by clergyman William Paley in 1802. We now understand that the argument is seriously undermined by the overwhelming evidence in support of Darwin’s theory of evolution by natural selection. The watchmaker is Nature, and it is blind. ‘There is grandeur in this view of life,’ wrote Darwin, ‘with its several powers, having been originally breathed into a few forms or into one; and that, while this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.’
But what of the fixed law of gravity, a prerequisite for the existence of the planets on which the endless forms evolved? Or the laws of electricity and magnetism which glue the animals together? Or the menagerie of subatomic particles out of which we are made? Who or what laid down the laws; the framework within which everything cycles on?
The story of modern physics has been one of reductionism. We do not need a vast encyclopaedia to understand the inner workings of Nature. Rather, we can describe a near-limitless range of natural phenomena, from the interior of a proton to the creation of galaxies, with apparently unreasonable efficiency using the language of mathematics. In the words of theoretical physicist Eugene Wigner, ‘The miracle of the appropriateness of the language of mathematics for the formulation of the laws of physics is a wonderful gift which we neither understand nor deserve. We should be grateful for it.’11 The mathematics of the twentieth century described a Universe populated by a limited number of different types of fundamental particles interacting with each other in an arena known as spacetime according to a collection of rules that can be written down on the back of an envelope. If the Universe was designed, it seemed, the designer was a mathematician.
Today, the study of black holes appears to be edging us in a new direction, towards a language more often used by quantum computer scientists. The language of information. Space and time may be emergent entities that do not exist in the deepest description of Nature. Instead, they are synthesised out of entangled quantum bits of information in a way that resembles a cleverly constructed computer code. If the Universe is designed, it seems, the designer is a programmer.
But we must take care. Like Paley before us, we are in danger of over-reaching. The role of information science in describing black holes may be pointing us towards a novel description of Nature, but this does not imply we were programmed. Rather we might conclude that the language of computing is well suited to describing the algorithmic unfolding of the cosmos. Put in these terms, there is no greater or lesser mystery here than Wigner’s miracle of the appropriateness of the language of mathematics for the formulation of the laws of physics. Information processing – the churning of bits from input to output – is not a construction of computer science, it is a feature of our Universe. Rather than spacetime-as-a-quantum-computer-code pointing to a programmer, we might instead take the view that earth-bound computer scientists have discovered tricks that Nature has already exploited. Viewed in this way, black holes are cosmic Rosetta Stones, allowing us to translate our observations into a new language that affords us a glimpse of the profoundest reason and most radiant beauty.