Short summary of The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics, Leonard Susskind, ISBN 978-0316016414.

Quantum systems only undergo unitary transformations, so they’re reversible, and no information can be lost in a quantum transformation. Stephen Hawking discovered that the presence of virtual photon pairs near a black hole’s horizon leads to its progressive evaporation, a process now known as Hawking radiation. Since photons are escaping, it’s possible to assign a (very low) temperature to the black hole, based on the black-body radiation created by these photons.

Information that falls into a black hole cannot be recovered, but the black hole ends up evaporating, leaving nothing behind. This is highly irritating to physicists, as it goes against the principle that all quantum transformations are unitary, and this reversible. During long debates between Susskind, Hawking and others, the following facts are established:

• There is a bound (the Bekenstein bound) on the amount of information inside a black hole.
• The information that enters the black hole seems to be lost, but it can (theoretically, over extremely long timeframes) be recovered using Hawking radiation.

The paradox is resolved by the fact that it’s impossible for an observer to see both things: if you enter the black hole, crossing the event horizon doesn’t feel like anything special. On the other hand, if you look at something fall into the black hole, it will eventually get radiated back. Since coming out of the black hole is impossible, it’s just all a matter of referential and no laws of physics get violated.

String theory sheds more light on this paradox: everything is made of strings that oscillate very fast, and are only localized when observed on long enough timescales (which are the only ones we can actually observe). For an outside observer, the strings slow down as they near the horizon, so they appear less and less localized, eventually covering the entire surface of the black hole.

Susskind also points out that we can try to study black holes through parallels with nucleons, and that cosmic horizons behave like inside-out black holes. The latter insight is the holographic principle: just as all the information poured into a black hole in some sense both remains at its surface and sinks inside, all the information contained in the visible universe might also be contained on its boundary.

[On the book itself: while good, the beginning is a little slow and the author’s self-esteem is a little too apparent throughout the book].