The clearest, most accessible explanation yet, of the amazing world of quantum mechanics. How can matter behave both like a particle and a wave? Does a particle exist before we look at it or does the very act of looking bring it into reality? Are there hidden elements to reality missing from the orthodox view of quantum physics? And is there a place where the quantum world ends and our perceivable world begins? Many of science's greatest minds have grappled with these questions embodied by the simple yet elusive 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 theories. Nearly a century later, Albert Einstein showed that light comes in particles, and the experiment became key to a fierce debate with Niels Bohr over the nature of reality. Richard Feynman held that the double slit embodies the central mystery of the quantum world. Hypothesis after hypothesis, scientists have returned to this ingenious experiment to help them answer the deep questions about the fabric of our universe. With his extraordinary gift for making the complicated comprehensible, Anil Ananthaswamy travels around the world and through history, down to the smallest scales of physical reality we have yet fathomed for the answers. AUTHOR: Anil Ananthaswamy is an award-winning journalist who contributes regularly to New Scientist, as well as writing for Nature, National Geographic News, Discover, The Wall Street Journal and the Literary Review. He is the recipient of the Book of the Year award from Physics World (2010), the Nautilus Book Award (2015) and was longlisted for the Pen/E. O. Wilson Literary Science Writing Award (2016). He lives in London.