In 1982 an experiment that may have profound implications for scientific thought was carried out in Orsay, France, by the physicist Alain Aspect. 'Anybody who is not shocked by quantum theory has not understood it' proclaimed Niels Bohr. Ghost is the clearest writing I have seen on this difficult subject. Guiding the reader through the thoughts of Bohr, Einstein, Schrödinger, and Heisenberg, it sharpens the mind, and prepares one for an understanding of Aspect's ground-breaking work. Ghost is based on interviews with leading quantum physicists. It begins with the need for quantum behaviour to explain black-body radiation and the photoelectric effect. This is contrasted with electromagnetic theory, supported by Young's interference patterns. However, only Bohr and de Broglie could determine how electrons orbit the nucleus - as standing waves - thus explaining atomic structures and spectra. When Schrödinger and Heisenberg described the behaviour of subatomic particles, Quantum Mechanics was born. So why fuss over a theory that has given us transistors and lasers? Photons, should they pass one-at-a-time through Young's slits, know whether there's one slit or two, and build an interference pattern (or not) accordingly. So does the electron somehow manage to pass through both slits simultaneously? Try measuring this, and we're led to Heisenberg's Uncertainty Principle. Einstein never accepted the intrinsic nature of uncertainty, believing in a hidden but measurable mechanism operating at the subatomic scale, below light speed. An experiment was needed that could display Bohr's ghostly, co-operation-at-a-distance (i.e. faster-than-light 'signals') between photons. Fifty years later, Aspect's experiment conclusively illustrated this. Today's physicists concur that the philosophical implications are staggering, and this book admirably opens the subject to a wider audience.
