For centuries, people have used the most advanced technology of their era as a metaphor for the universe. Ancient philosophers compared nature to living organisms. During the Industrial Revolution, the universe became a giant clockwork machine governed by precise laws. Today, in an age dominated by computers, a different question has emerged: What if the universe itself is performing a computation? At first glance, this idea sounds strange. Computers sit on desks and run software. The universe contains stars, galaxies, atoms, and people. Yet many scientists and philosophers have noticed that the laws of nature often resemble the rules of an information-processing system. Given the state of the universe at one moment, physical laws determine what happens next, much like a computer updates its state according to a program. This raises a fascinating possibility. Perhaps the universe is not merely described by computation. Perhaps it is computational at its deepest level. From Phys...