What Richard Feynman described as “the central mystery” of quantum physics is encapsulated in the experiment with two holes, also known as the double-slit experiment. In this experiment, a particle going through one of a pair of holes seems to be aware of what is going on at the other hole, and changes its behaviour according to whether that hole is open or closed. This is closely linked to the puzzle of entanglement, where one particle instantly reacts to what is happening to another particle, even when they are widely separated. And in a final example of the mind-boggling nature of the quantum world, these effects seem to operate across time as well as space: What is going to happen in the future affects the behaviour of a particle now.InThe Quantum Mystery, John Gribbin, the best-selling author ofIn Search of Schrödinger’s Cat, describes the history of the double-slit experiment, the wave-particle duality of the quantum world, and the latest experiments which show these bizarre effects at work before our very eyes.