“A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: ‘What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.’ The scientist gave a superior smile before replying, ‘What is the tortoise standing on?’ ‘You’re very clever, young man, very clever,’ said the old lady. ‘But it’s turtles all the way down!’”

This famous story has several versions, but all contain an elderly lady who’s sure the world rides on a giant tortoise. The point of the story is that many people believe in myths about Earth and the universe that, though dramatic and compelling, make little sense, especially when one thinks them through. The purpose of science is to look past humanity’s original assumptions about reality, then propose theories, and believe only those that the evidence supports.

“What do we know about the universe, and how do we know it? Where did the universe come from, and where is it going? Did the universe have a beginning, and if so, what happened before then? What is the nature of time? Will it ever come to an end? Can we go back in time?”

Hawking offers some of the fundamental questions about reality. While some, like time travel, seem to be the realm of science fiction, scientists take these questions seriously and have, over the past several decades, made great strides in answering them. The book provides a summary of those discoveries, and Hawking entices the reader’s curiosity by introducing these existential and provocative considerations early on.

“When asked: ‘What did God do before he created the universe?’ Augustine didn’t reply: ‘He was preparing Hell for people who asked such questions.’”

The point of Hawking’s quip is that it’s futile to speculate on what happened before the Big Bang, since “before” doesn’t exist outside the universe. The nature of anything beyond observable or deducible existence is thus forever closed off to humans. Instead, science starts with what it knows, goes back as far as it can, and halts at the moment when all knowledge about the universe disappears at the moment of its creation.