A World of Many Worlds

Marisol de la Cadena, Mario Blaser

Milena Radović

The purpose of this paper is to give an overview of modal realism and many worlds interpretation of quantum mechanics and compare the concepts of many worlds in those two theories. Main source for modal realism is David Lewis' book On the Plurality of Worlds (1986). For many worlds interpretation I rely on Bryce DeWitt and Lev Vaidman. First part of the paper concerns modal realism, second is about many worlds interpretation and third is the comparison of those two. Many worlds in modal realism and many worlds interpretation have the same phenomenology, but are different ontologically. The worlds of modal realism are causally and spatiotemporally separated and have always been, while the worlds of many worlds interpretation are part of a branching universe. That means they were the same world at some point in the past, even though they are equally separated, as the worlds of modal realism are, after the branching.

Shan Gao

It is shown that the components of the wave function of a measuring device, each of which represents a definite measurement result, do not correspond to many worlds as assumed by the many-worlds interpretation, because all components of the superposed wave function can be observed in our world by protective measurement, and they all exist in one world. According to the many-worlds interpretation, each component of the wave function of a measuring device that represents a definite measurement result corresponds to each world among the many worlds (Barrett 2011). This means that in one world there is only one component of the superposed wave function and the other components do not exist, and thus these components that cor-respond to the other worlds cannot be observed in this world. As a result, in every world the whole superposed wave function of the measuring device cannot be measured. It is unsurprising that the existence of such many worlds may be consistent with the results of conventional impulse measurements1, as the many-worlds interpretation is just invented to explain the emergence of these results, e.g. the definite measurement result in each world always denotes the result of a conventional impulse measurement. However, this does not guarantee consis-tency for all types of measurements. It has been known that there exists an-other type of measurement, the protective measurement (Aharonov and Vaid-

Rob Clifton

In their article “On What It Takes To Be a World”, David Albert and Jeffrey Barrett raise “a rather urgent question about what the proponents of a many-worlds interpretation [of quantum mechanics] can possibly mean by the term ‘worlds’” (1995, 35). I argue that their considerations do not translate into an argument against the Many-Worlds conception of a world unless one requires that the dispositions that measurement devices display through the outcomes they record be explainable in terms of facts particular to the worlds in which those devices do their recording. Granting that their conception of a world takes away the possibility of such an explanation, a Many-Worlds proponent can claim that the Universal quantum state, which does not represent a fact about any world in particular, is enough to ground the dispositions of measurement devices.