From Peter Rastall
Marcus Chown’s account of quantum theory is unnecessarily paradoxical (17 March, p 36). He asserts that a quantum object can be in several places at once. Observing it may make it appear at one of these places and disappear at the others. A mysterious and acausal action-at-a-distance seems to be required, which many people find worrisome.
One does not have to assume that the object is in several places at once. It is less confusing to assert that it has several possible positions, but not that it actually occupies any of them. Only when the object is observed (or, more generally, when it decoheres through its interaction with other objects) does it come to occupy one of these positions. The observation is a local event at the place where the object appears. Since the object is not actually in the other places, there is no need to tell it to disappear from them. No mysterious, acausal signals are required.
Confusions such as this arise if one regards the wave function as some kind of picture of the quantum object. Bohr pointed out long ago that this is incorrect. Perhaps one might also quote Heisenberg: the atom is not an object, it is a tendency.
Vancouver, Canada
