This picture cannot be exact, because as soon as a localised wave packet has spread enough, it will be decohered into new more localised packets, so that intuitively one will get some kind of ‘fanning out’ of trajectories. In fact, such deviations from Newtonian behaviour are due both to the tendency of the individual components to spread and to the localising effect of the interaction with the environment, which further enhances the collective spreading of the components (because a narrowing in