David Lewis and Schro dinger’s Cat

Abstract In ‘How Many Lives Has Schro dinger’s Cat?’ David Lewis argues that the Everettian no-collapse interpretation of quantum mechanics is in a tangle when it comes to probabilities. This paper aims to show that the difficulties that Lewis raises are insubstantial. The Everettian metaphysics contains a coherent account of probability. Indeed it accounts for probability rather better than orthodox metaphysics does. On 27 June 2001, not four months before his untimely death, David Lewis delivered the third Jack Smart Lecture at the Australian National University. His title was ‘How Many Lives has Schro dinger’s Cat?’ > and he spoke on a topic which is absent from his published writings, the interpretation of quantum mechanics. More specifically, he discussed the ‘no-collapse’ interpretation of quantum mechanics pioneered by Hugh Everett III [1957]. Lewis allowed that this interpretation offers initial theoretical attractions, but also argued that it suffers from irremediable flaws. If you have heard of Everett because of his association with the ‘many-worlds’ interpretation of quantum mechanics popularised by Bryce Dewitt [1970; 1973], you might suppose that there is some affinity between the no-collapse interpretation and Lewis’s philosophical realism about possible worlds. But this would be a mistake. While Everett’s interpretation does add extra ‘branches’ to the reality recognized by common sense, these additions fall far short of Lewis’s multiplication of worlds. For a start, the extra ‘branches’ that Everett adds to reality all lie within the actual world that evolves from the actual initial conditions in line with the actual laws of physics-these branches by no means include all possibilities. Moreover, Everett’s branches are best conceived, not as sundering’s of the whole universe, but rather as entities that spread out causally at finite speeds, ‘like ripples on a pond’, as Lewis puts it. For example, in the Schro dinger’s Cat experiment, firstthe photon branches into a deflected and undeflected version when it passes through the half-silvered mirror; thenthe detector branches into a triggered and untriggered state when it interacts with the photon; thenthe poison bottle branches into a smashed bottle and an unsmashed bottle under the influence of the detector; and so on, culminating in the cat branching into a live and dead cat, and the human observer branching into a self who sees a live cat and a self who sees a dead cat. >