A Brief History of Time Summary & key ideas explained

A famous scientist once gave a public talk on astronomy. He explained how the Earth circles the Sun, and how the Sun circles the center of a vast swirl of stars called our galaxy. When he finished, an old woman at the back stood up and told him he had it all wrong. The world, she said, is a flat plate resting on the back of a giant tortoise.

The scientist smiled and asked what the tortoise was standing on.

She gave him a pitying look. "It's turtles all the way down!"

Stephen Hawking opens his book with this story, and then asks an uncomfortable question: why are we so sure we know better? Where did the universe come from? Did it have a beginning? Will it end? Why does time move only forward?

Hawking spent his life on these questions as a physicist at Cambridge. In 1988 he published his answers for everyone, not just specialists. Someone warned him that each equation he included would halve his sales, so he kept exactly one: E = mc². The book went on to stay on the London Sunday Times bestseller list for 237 weeks.

In the next few minutes, you'll travel from ancient Greece to the edge of a black hole and back to the first instant of time. And it starts with a surprisingly humble idea about what science can and can't prove.