The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics

  • Leonard Susskind
Little Brown: 2008. 416 pp. $27.99 9780316016407 | ISBN: 978-0-3160-1640-7

The idea of a black hole in space produced by the collapse of a massive star dates back to the 1930s, but it was only in the 1960s that astrophysicists began to understand the details. Evidence now suggests that black holes do exist, and are a key element of the great cosmic story. Yet some of their predicted properties remain puzzling and threaten cherished physical laws.

One long-running conflict concerns the fate of the material that implodes to form a black hole. In his new book The Black Hole War, theoretical physicist Leonard Susskind of Stanford University, California, describes how he sparred with Stephen Hawking of the University of Cambridge, UK, about this thorny issue.

Black holes earn their name because their gravity is so strong they trap even light, appearing black from the outside. According to Einstein's general theory of relativity, the ball of matter that implodes to create the hole continues to shrink, meeting an uncertain fate at the centre and leaving behind it a region of empty space. Because physicists believe nothing can go faster than light, no information or material should escape the hole. Practically all the information about the collapsed star would be lost from the outside Universe, making it impossible to tell whether the star was made of matter, antimatter or green cheese; once inside the hole, its external physical properties would be the same.

A twist arose in 1975 when Hawking argued that black holes are not perfectly black. By applying quantum mechanics to the formation process of a black hole, he predicted that it must radiate heat. Because heat is a form of energy, this radiance would have to be paid for by gradually reducing the hole's mass, because energy and mass are equivalent in general relativity. In time, the hole would evaporate away completely, leaving behind just heat energy, mostly in the form of photons, with a few other particles such as neutrinos and electrons.

On an immense timescale, vastly longer than the current age of the Universe, the black hole would convert the entire star into heat. Hawking concluded that the heat energy would look the same whatever the star was made of originally. Many physicists did not like this conclusion. A principle of quantum mechanics is that information is never created or destroyed in a quantum process. For example, if you throw an encyclopaedia into a furnace, it might seem that the information is irretrievably lost, turned into incoherent heat. According to quantum mechanics, however, the information is lost only for practical purposes. The infrared photons emitted by the embers still contain the original information, but in a hopelessly scrambled form that renders it inaccessible to us. Leading particle physicists, led mainly by Susskind and Gerard 't Hooft of the University of Utrecht in the Netherlands, declared that the same would be true of black holes — the information about the original star would be enfolded in the emitted heat. It might be jumbled up, but it should all be there.

Hawking begged to differ. I recall him going through the argument during a conversation in 1978. He reasoned that crucial bits of information about subatomic particles cannot be carried by photons, so other types of particles such as electrons or quarks would also be needed. But there are not enough of these other particles in the radiation to embody all the information, because most of the heat energy from a black hole is in the form of photons. The conundrum became known as the information paradox. It may seem like theoretical nit-picking, but the paradox goes to the very heart of physical theory and its description of reality.

The momentous conclusion that a black hole swallows and permanently obliterates physical information didn't bother Hawking, whose background was in gravitational theory and space-time geometry rather than particle and quantum physics. With theorist Roger Penrose of the University of Oxford, UK, he proved that space and time could have boundaries or edges, called singularities, at which information might enter or leave the Universe. The general theory of relativity predicts that such a singularity lurks at the centre of a black hole, where the gravitational field and space-time warp become infinite. As a consequence, the imploding star's information might disappear from space and time through the hole's singularity. Hawking was sufficiently confident to place a bet with theoretical physicist John Preskill at the California Institute of Technology in Pasadena.

An uncertainty in Hawking's argument was the fate of the singularity. If the black hole evaporated completely then the singularity must presumably disappear too, but the details depend on an incomplete field of physics known as quantum gravity. In recent years, work in string theory — to which Susskind has made seminal contributions — has placed quantum gravity on a more secure foundation. Armed with such arguments, Susskind and others gathered support for the position that information is conserved, against Hawking's claim. In 2004, Hawking called a press conference and announced to the world that he had changed his mind. Black holes did not, after all, irreversibly annihilate information, he said. The bet with Preskill was duly settled in the form, appropriately enough, of an encyclopaedia.

The Black Hole War charts the ups and downs of this lengthy yet good-natured dispute. Susskind skilfully explains the subtleties of the physics that underlie the issue, and includes anecdotes to enliven the technical details. He has stuck to his guns for many years, but nevertheless resists the temptation to gloat over Hawking's eventual capitulation.

Is the matter laid to rest? I don't think so. Hawking justified his reversal by sketching out a calculation, but quantum gravity is still too unrefined for a rigorous proof. The weak point is that, in quantum gravity, the singularity can be replaced by a space-time region with a complicated and changing topology, allowing information to shift from one region of space-time into another disconnected one, perhaps from our Universe to a newly born 'baby universe'. Susskind dismisses this possibility, but the matter is far from resolved. It may be that if we consider the entire 'meta-verse' of all spatial regions, information is never lost. But if we restrict attention to a single universe, or connected region of space, then information can in fact leak out. Deciding the matter is a task for a future generation of theoretical physicists.