The essential mystery - How an electron can be in more than one place at a time

Abstract Quantum mechanics is an extraordinarily successful scientific theory. But it is also completely mad. Its discovery forced us to confront all the cosy notions about physical reality that had been gained from our naïve interpretation of classical physics. Although the theory quite obviously works , it appears to leave 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. This paper is based on a talk delivered at the St Cross Centre for the History and Philosophy of Physics “ The Nature of Quantum Reality ” Conference in 2017. It addresses quantum theory’s only mystery and uses this as a platform to explore the Bohr-Einstein debate and the process of scientific theorising about the nature of quantum reality.