Many great scientific minds have grappled with the 'double slit' experiment. Thomas Young devised it in the early 1800s to show that light behaves like a wave, and in doing so opposed Isaac Newton's view that light is made of particles. But then Albert Einstein showed that light comes in quanta, or particles. Thus, quantum mechanics was born. This led to a fierce debate between Einstein and Niels Bohr over the nature of reality-subatomic bits of matter and its interaction with light-as revealed by the double slit experiment. Richard Feynman held that it embodies the central mystery of the quantum world. Decade after decade, hypothesis after hypothesis, scientists have returned to this ingenious experiment to help them answer deeper and deeper questions about the fabric of the universe. How can a single particle behave both like a particle and a wave? Does a particle, or indeed reality, exist before we look at it, or does looking create reality, as the textbook 'Copenhagen interpretation' of quantum mechanics seems to suggest? How can particles influence each other faster than the speed of light? Is there a place where the quantum world ends and the familiar classical world of our daily lives begins, and if so, can we find it? And if there's no such place, then does the universe split into two each time a particle goes through the double slit? With his extraordinarily gifted eloquence, Anil Ananthaswamy travels around the world and through history, down to the smallest scales of physical reality we have fathomed. It is the most fantastic voyage you can take.