Title: Is the Multiverse Hypothesis capable of explaining the Fine Tuning of Nature Laws and Constants? The Case of Cellular AutomataAuthors: Francisco José Soler Gil, Manuel AlfonsecaThe objective of this paper is analysing to which extent the multiverse hypothesis provides a real explanation of the peculiarities of the laws and constants in our universe. First we argue in favour of the thesis that all multiverses except Tegmark's <<mathematical multiverse>> are too small to explain the fine tuning, so that they merely shift the problem up one level. But the <<mathematical multiverse>> is surely too large. To prove this assessment, we have performed a number of experiments with cellular automata of complex behaviour, which can be considered as universes in the mathematical multiverse. The analogy between what happens in some automata (in particular Conway's <<Game of Life>>) and the real world is very strong. But if the results of our experiments can be extrapolated to our universe, we should expect to inhabit -- in the context of the multiverse -- a world in which at least some of the laws and constants of nature should show a certain time dependence. Actually, the probability of our existence in a world such as ours would be mathematically equal to zero. In consequence, the results presented in this paper can be considered as an inkling that the hypothesis of the multiverse, whatever its type, does not offer an adequate explanation for the peculiarities of the physical laws in our world. Read more (558kb, PDF)
Title: The Multiverse Interpretation of Quantum MechanicsAuthors: Raphael Bousso, Leonard SusskindWe argue that the many-worlds of quantum mechanics and the many worlds of the multiverse are the same thing, and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics.Decoherence - the modern version of wave-function collapse - is subjective in that it depends on the choice of a set of unmonitored degrees of freedom, the "environment". In fact decoherence is absent in the complete description of any region larger than the future light-cone of a measurement event. However, if one restricts to the causal diamond - the largest region that can be causally probed - then the boundary of the diamond acts as a one-way membrane and thus provides a preferred choice of environment. We argue that the global multiverse is a representation of the many-worlds (all possible decoherent causal diamond histories) in a single geometry.We propose that it must be possible in principle to verify quantum-mechanical predictions exactly. This requires not only the existence of exact observables but two additional postulates: a single observer within the universe can access infinitely many identical experiments; and the outcome of each experiment must be completely definite. In causal diamonds with finite surface area, holographic entropy bounds imply that no exact observables exist, and both postulates fail: experiments cannot be repeated infinitely many times; and decoherence is not completely irreversible, so outcomes are not definite. We argue that our postulates can be satisfied in "hats" (supersymmetric multiverse regions with vanishing cosmological constant). We propose a complementarity principle that relates the approximate observables associated with finite causal diamonds to exact observables in the hat. Read more (475kb, PDF)
When cosmologist George Ellis turned 70 last year, his friends held a party to celebrate. There were speeches and drinks and canapés aplenty to honour the theorist from the University of Cape Town, South Africa, who is regarded as one of the world's leading experts on general relativity. But there the similarity to most parties ends.For a start, Ellis's celebration at the University of Oxford lasted for three days and the guest list was made up entirely of physicists, astronomers and philosophers of science. They had gathered to debate what Ellis considers the most dangerous idea in science: the suggestion that our universe is but a tiny part of an unimaginably large and diverse multiverse.