In Hawking’s Forward to the 1996 edition of A Brief History of Time, he admits that he and his publishers had no idea the book would sell so many copies. In this edition, he updates the science, adds a chapter on wormholes, and discusses how the “dualities” of conflicting theories of physics suggest a unified theory that may need to be expressed in different ways for different situations. Hawking points out that improvements to scientists’ understanding of fluctuations in the cosmic microwave background—electromagnetic radiation from the beginning of the universe—suggest the possibility that the cosmos has no beginning or end.

The chapter opens with an anecdote of a woman at the lecture of a famous astronomer. She defiantly tells the astronomer that the world is a flat disc carried on the back of a giant turtle and the universe is “turtles all the way down” (1), referencing an ancient myth. Hawking posits how and why a person would now know that this idea is rather silly.

Ancient Greek philosopher Aristotle provided two clear indications that the Earth is round: 1) because its shadow on the full moon during an eclipse is round and 2) because the North Star gets lower in the sky as a person moves south.