Methodological Solipsism and the Multiverse

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It is well known that quantum physics deals with phenomena characterized by uncertainty. A typical expression of this uncertainty is the uncertainty principle, where, the more precisely we know the position of a particle, the more imprecisely we know its momentum (and vice versa).Opinions are divided as to whether this epistemological uncertainty is also an ontological uncertainty. Of some relevance here is the following observation by Quentin Smith: Heisenberg originally interpreted [the uncertainty] epistemically, but Bohr convinced him in private communications to accept a verificationist metaphysics, with its attendant ontological interpretation of the uncertainty relations (Smith 2002, 137) There are many competing interpretations of the quantum mechanical mathematical formalism. Of these interpretations, particularly well-known are the so-called Copenhagen interpretation of Bohr (where it is asserted that the position-view and the momentum-view of reality are complementary, and cannot be considered in the same context), and the many-universes interpretation of Hugh Everett. Roughly speaking, the latter asserts that whenever a measurement is made of a particle, the universe splits into copies, each one of which corresponds to the different possible outcomes of the measurement. Many scientists are happy to work with the many-universes interpretation-provided that their working with the theory has no ontological commitment. That is, they are not prepared to believe that the many universes actually exist. David Deutsch, who is one of the leading researchers in the field of quantum computation, is an exception. Indeed, he is the main advocate of the reality of the many universes; and he provides a number of important reasons why he is such an advocate. In the present essay, I intend to look at the arguments for and the arguments against the existence of the many universes, clarifying along the way what the meaning of exist in this regard should be considered to be. A first conclusion will be that while Deutsch's arguments are highly persuasive, there is one telling objection to the existence of the many universes-or multiverse, as the collection of them is becoming known-that must be addressed. My procedure then will be to show that the objection can in fact be met, using the so-called methodological solipsism approach introduced by Hans Driesch and employed subsequently by such theorists as Fodor and Putnam. The Many-Universes Interpretation of Quantum Physics It is not my intention in this essay to become bogged down in the minutiae of quantum physics, where the complexities of the mathematics are exacerbated by the lack of any single way of interpreting it. However, in order to provide a background for my subsequent arguments, I shall discuss briefly a typical and familiar quantum-mechanical scenario: the so-called experiment. My account is based on that of Penrose (1985). The results that may be expected in a performance of the two-slit experiment depend on the fact that sub-microscopic particles, such as electrons or photons, behave sometimes like particles and sometimes like waves. The experiment is simple to describe: typically, a beam of light is directed through a pair of closely-separated and parallel narrow slits to a screen located behind it. Penrose describes the scenario as follows (it is worth quoting him at length): The most evident manifestation of the light as particles . . occurs at the screen. The light arrives there in discrete localized units of energy.. .. Never is the energy of just half a photon... received. Light reception is an all-or-nothing phenomenon in photon units. Only whole numbers of photons are ever seen. However, a wavelike behaviour seems to arise as the photons pass through the slits. Suppose, first, that only one slit is open (the other being blocked off). After passing through, the light will spread out-by. …