This chapter discusses further types of multiverse hypotheses: the branching worlds of Everettian quantum theory, the totality of possible worlds according to David Lewis’s modal realism, and the totality of mathematical structures, which – according to Max Tegmark – are all physically realized. I argue that suggested solutions to the probability problem in Everettian quantum theory are unconvincing. A recent proposal by Sebens and Carroll to derive the Born rule in the Everettian framework using considerations about self-locating belief turns out to suffer from a circularity problem because the central principle on which the derivation is based has no independent motivation besides, possibly, appeal to the Born rule itself. I further argue that it is not possible to coherently, in full seriousness, believe David Lewis's modal realism. Nor is it possible to coherently believe Max Tegmark's thesis that there is a multiverse of all mathematical structures in which they are physically realized. Tegmark's claim that our universe is itself a mathematical universe hypothesis is found to be incoherent.
The Copenhagen interpretation of quantum entanglement experiments is at best incomplete, since the intermediate state induced by collapse of the wave function apparently depends upon the inertial rest frame in which the experiment is observed. While the Many Worlds Interpretation of Everett, MWI, avoids the issue of wave function collapse, it, too, is a casualty of the special theory of relativity. This requires all events in the universe, past, present and future, to be unique, as in the block universe picture, which rules out Everett style branching. The benefits of MWI may be retained, however, by postulating a multiverse of discrete, parallel, block universes which are identical to each other up to certain points in the MWI trunk before they diverge according to the MWI branching. The quantum probability of an event then emerges from the number of parallel universes in which the event happens divided by the total number of universes. This means that the total number of such universes is finite. Such a picture is more easily envisaged by thinking of it as a purely mathematical structure, as in the Mathematical Universe Hypothesis proposed by Tegmark. However, while Tegmark wished to avoid contamination from Goedelian self referential knots, not only does such contamination appear to be inevitable, it brings an unexpected benefit. The mathematical hierarchy required by the enigmatic footnote 48a in the paper by Goedel leads to an explanation for a unitary evolution of deterministic quantum rules across the multiverse while accounting for quantum uncertainty within an individual universe. Other aspects of this structure, called here the Plexus, are discussed, including awareness of existence and other questions raised by the hypothesis.
The many-worlds interpretation (MWI) of quantum mechanics is currently experiencing a popular resurgence, propelled by such prominent and articulate physicists as Sean Carroll, David Deutsch, Max Tegmark, and Lev Vaidman. The consequences of MWI are mind-boggling: the spacetime universe of our experience is only one branch of an unimaginably fast-multiplying plethora of alternative universes held incommunicado. In this paper, we propose that the mass of Medusa's hair served up by MWI is due to a failure to embed the spacetime universe in the right space: spacetime is a slice of bread, not a splitting strand of pasta. By way of motivation, we first give a very simple presentation of Bell's inequality by comparing it to a ``quantum game show'', followed by a simple description of Aspect's 1985 experiment involving entangled photons which confirms the inequality. We interpret the paradoxical correlation between measurements as resulting from a process outside of spacetime that produces both the original entanglement and the measurements. This is followed by a brief presentation of MWI, and then by a pictorial comparison of the proposed process model and MWI. The final section lists a number of potential consequences of the model related to causality, determinism, free will, and consciousness, and points to further references that give a more in-depth and rigorous presentation of the process dimension model. The entire article is non-technical and requires no mathematical background other than high school mathematics and an understanding of basic concepts in probability. The physics involved in Aspect's experiment is also explained.