Black Holes II: The Heat of Nothingness

Stephen Hawking’s major scientific contribution concerns the nature of black holes. He has demonstrated that they are hot.

I am not referring to the matter that becomes red hot by spinning and amassing together as it falls towards the black hole, making it visible in the heavens. No: Hawking has shown that even a calm black hole into which nothing is falling is still hot. Black holes are naturally hot.

No one has observed this heat yet. It is too weak to be picked up by any telescope, and in the black holes that we see it is usually overlayered by the tempestuous heat of the matter that is continually falling into it. Hawking’s prediction is theoretical at present: it lacks experimental confirmation. But his calculation has been repeated in many different ways, and the result is always the same. The result is judged by the scientific community to be persuasive. A black hole therefore is in all likelihood not so black after all. It is a moderate source of heat. If it was isolated in the middle of a starless sky, it would not seem black but resemble instead a small sphere emitting a pallid light.

This outcome surprised everyone. To be hot means to emit heat. But we thought we had understood that a black hole is a place from which nothing can escape – so how can heat emanate from it?

The key to Hawking’s calculation is that it involves quantum mechanics. Whereas the prediction that a black hole permits only an entry but never an exit is a prediction solely dependent on Einstein’s theory, general relativity – and this is an incomplete theory that overlooks quantum phenomena. Hawking’s calculation improves our understanding of a phenomenon that Einstein’s theory describes only up to a certain point and shows that something – a dim heat – escapes from black holes.

The heat of black holes involves both general relativity, namely the theory that describes the black hole itself, and quantum theory. Currently there is not yet consensus on a complete theory combining general relativity and quantum mechanics, and the heat of black holes is an indication of how to look for this combination. It is a theoretical benchmark for all attempts to solve the problem of combining the two great physics theories of the twentieth century. Black holes are not just amazing real objects in the heavens. They are also a laboratory for theoretically testing our ideas about space, time and quanta.

A cup of tea is hot because its molecules are in a very agitated state. The heat is the rapid movement of the molecules. But the surface of a black hole is not a concrete surface made of matter, like the surface of a ball or the surface of a cup of tea. It is merely a place of no return, where the force of gravity becomes incredibly strong. It is not a material surface composed of molecules. So, to what can we attribute the turmoil on the surface of a black hole, generating heat, if there is nothing there?

One possible answer is that it is elementary quanta of space that generate this heat. The heat of black holes foreseen by Hawking’s calculation could be the clue that reveals the existence of these ‘molecules of space’. The immensely powerful force of gravity acts on the surface of the black hole like a giant amplifier, revealing the infinitesimal trembling of the elementary grains of space. The heat of black holes is not the heat of some object or other: it is the heat of empty space itself, magnified by gravity. It is the elementary heat of nothingness.

Something puzzling follows from this reasoning: when we seek to combine the theory of gravity with quantum mechanics, it seems that it is not possible to do so without talking about heat. But why is this? Heat can be interpreted as lost information: to say that an object is hot is to say that its molecules are moving around a lot, but randomly, in a way that we cannot precisely reconstruct on the basis of the macroscopic behaviour of the object. If I burn a letter in a fireplace, a super-skilful investigator can in principle trace the text of the letter in the ashes or in the light emitted by the fire; but whatever falls into a black hole is lost for ever to those who are outside of it: if I throw a letter into a black hole, I will never know what was written on it. Black holes destroy all information. Where does it go?

Like a Gordian knot that symbolically closes access to Asia, a black hole is a mysterious object where all the marvels we have recently discovered about the world can be found linked together: time that slows, almost to the point of standing still; elementary quanta of space; lost information. A place in the universe where everything can enter and nothing ever leave, for all eternity … It causes a sense of unease. It challenges our theoretical understanding of the world. But are we really certain that anything that falls into a black hole can never escape? Never say never …