Fifty years ago, Stephen Hawking published one of his most celebrated papers. In it he posed a puzzle about black holes that has still not been settled half a century later. If anything, the subtlety and complexity of Hawking’s puzzle have only grown as others have attempted to solve it. And so, accordingly, has the significance of the paper, which seems now to point towards deep connections in fundamental physics and perhaps to offer clues about what is needed for a proper theory of quantum gravity.
Black holes are closely associated with Hawking. They are formed from stars, somewhat more massive than our sun, that have exhausted their fuel and collapsed under their own gravity. According to Einstein’s theory of general relativity, which furnishes the modern view of gravity, the collapse is a runaway process that ends with all the star’s mass squeezed into a space with zero volume and infinite mass density: a singularity.
The gravitational field of such an object is so strong that once anything comes closer to the singularity than a particular threshold called the event horizon, it can never escape. Not even light can get away: hence, black hole.
In a classic 1974 paper, Hawking argued that black holes are not actually so black after all. Thinking about a black hole in terms of quantum mechanics, the theory that governs fundamental particles, he showed that there should be a steady stream of radiation emanating from just outside the event horizon.
This effect, called Hawking radiation, has never been observed – it’s too dim to see in the images of black holes that astronomers have now obtained. But Hawking’s argument is generally accepted. It implies that black holes don’t last for ever. Hawking radiation means that their mass will gradually decrease: the black hole evaporates, eventually vanishing entirely.
In his 1976 paper, Hawking showed that this scenario of evaporating black holes poses a problem. When an object falls into a black hole, all information it contains – for example, about its shape, composition, the positions of its atoms – is lost to any observers outside the event horizon. But if a black hole can vanish, that information seems to have disappeared entirely from the universe.
Quantum mechanics, however, forbids such loss of information. In principle, one can always reverse-engineer a process by collecting all the information about it. A burnt book, for example, could be reconstructed in theory by exhaustive tracking of every molecule it contained. But there seemed to be no prospect of that for a black hole. The only imprint it leaves on the universe after it has evaporated is the Hawking radiation it emitted during its lifetime, which seems to be just random and scrambled.
This problem became known as the black hole information paradox. Arguments about it have raged during the past 50 years, and not even Hawking could solve it (despite claiming at one point that he had). It seems something has to give: either quantum mechanics is wrong to insist on “information conservation”, or general relativity isn’t properly predicting the features of black holes.
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The argument has become more refined in the intervening years, so that there are actually three possible cracks in physics: in quantum mechanics, or in “locality”, the idea that an event “here” can’t instantly affect an event “there” (because general relativity insists they are separated in time and space), or in the thermodynamic arguments behind Hawking radiation.
Most proposed resolutions of the black hole information paradox let go of one of these assumptions. In one view, black holes don’t evaporate fully but leave tiny remnants called fuzzballs. Others are content to modify quantum theory.
But the most exciting option renounces locality. Typically this approach argues that the information is encoded in the Hawking radiation but in a way that’s almost impossible to decode. The implication here is that spacetime itself – the water we all swim in – is not fundamental but “emergent”, and can cease to be relevant in a case like this. If that’s true, it would be as seismic in physics as general relativity itself was in 1916.
