A Brief History of Time

Stephen Hawking, the Cambridge physicist, tries to answer in plain language some of the biggest questions in physics: where the universe came from, what time is, and whether there is a limit to what we can know. Over twelve chapters he moves from the ancient Greeks to Einstein's picture of space and time, the expanding universe and the big bang, the uncertainty principle, particles and forces, black holes (including his own finding that they glow), the direction of time, time travel, and the hunt for one theory that explains everything.

This summary follows the 1996 updated and expanded edition, which added the chapter on wormholes and time travel; the 1988 original had eleven chapters and an introduction by Carl Sagan. Later printings, including the 2017 one, reprint the 1996 text and add an appendix in which Hawking updates a few topics. Where we mention it, we say so.

Notes: physics has moved since the text was written. In that 2017 appendix Hawking himself says that in 1998 two teams of astronomers found the expansion of the universe is speeding up, a cause now called dark energy; this makes the book's talk of a possible recollapse, and its remark that the cosmological constant is generally believed to be zero, out of date. He also says that in 2004 he concluded he had been wrong to hold, as the book does, that information falling into a black hole is lost. Gravitational waves, which the book says had not yet been observed directly, were first detected directly in an experiment announced in 2016. The book's estimate for the age of the universe (ten to twenty thousand million years) has since been narrowed to about fourteen billion. Inflation is now widely accepted but not proven; the no boundary proposal and string theory remain unconfirmed. What the book says about God and creation is Hawking's own view, reported as his, and this summary takes no position on it.