Quantum mechanics is an extraordinarily successful scientific theory. It is also completely mad. Although the theory quite obviously works, it leaves us chasing ghosts and phantoms; particles that are waves and waves that are particles; cats that are at once both alive and dead; and lots of seemingly spooky goings-on. But if we're prepared to be a little more specific about what we mean when we talk about 'reality' and a little more circumspect in the way we think a scientific theory might represent such a reality, then all the mystery goes away. This shows that the choice we face is actually a philosophical one.

Here, Jim Baggott provides a quick but comprehensive introduction to quantum mechanics for the general reader, and explains what makes this theory so very different from the rest. He also explores the processes involved in developing scientific theories and explains how these lead to different philosophical positions, essential if we are to understand the nature of the great debate between Niels Bohr and Albert Einstein. Moving forwards, Baggott then provides a comprehensive guide to attempts to determine what the theory actually means, from the Copenhagen interpretation to many worlds and the multiverse.

Richard Feynman once declared that 'nobody understands quantum mechanics'. This book will tell you why.

Jim Baggott is an award-winning science writer. He trained as a scientist at the University of Oxford before embarking on post-doctoral research studies at Oxford and at Stanford University in California. He gave up a tenured lectureship at the University of Reading after five years in order to gain experience in the commercial world. He worked for Shell International Petroleum for 11 years before leaving to establish his own business consultancy and training practice. Jim's many books include Quantum Space (OUP, 2018), Mass (OUP, 2017), Origins (OUP, 2015), Higgs (OUP, 2012), The Quantum Story (OUP, 2011), and A Beginner's Guide to Reality (Penguin, 2005).

"This is a superb book. Indeed it is the book I wish I had read when I was an undergraduate student in philosophy of science, keen to understand the philosophical implications of various interpretations of quantum mechanics. Jim Baggott has set for himself a very ambitious task: namely, to unpack the realist commitments at stake in the century-long debate on the completeness (or incompleteness) of quantum mechanics that began with Niels Bohr and Albert Einstein in the 1920s-1930s. It is rare to find this level of philosophical engagement with thorny foundational issues among physicists writing popular science books... This book is sheer joy to read." - Michela Massimi, Philosopher of Science and editor of Philosophy and the Sciences for Everyone

"Jim Baggott proves once again to be a master popularizer, this time investigating with wit, depth, very wide angle, and remarkable equilibrium, what is perhaps the most obscure and fascinating mystery of modern science: what does quantum theory tell us about the world?" - Carlo Rovelli, author of The Order of Time and Seven Brief Lessons on Physics

"Jim Baggott has written a highly readable, fair-minded and well-researched account of the ongoing debate about the nature of quantum reality. Amongst popular accounts of the subject, it is the most accessible and enlightening one I have come across." - Harvey R. Brown, Philosopher of Physics and author of Physical Relativity: Space-time structure from a dynamical perspective

"An engaging tour of the mysteries of quantum mechanics and the controversies of its interpretation, with the rare bonus of some substantial and well-grounded philosophy of science, synthesised from Baggott's wealth of knowledge and experience." - Jon Butterworth, author of A Map of the Invisible