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The Structure of the Multiverse


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The idea that quantum theory is a true description of physical reality led Everett (1957) and many subsequent investigators (e.g. DeWitt and Graham 1973, Deutsch 1985, 1997) to explain quantum-mechanical phenomena in terms of the simultaneous existence of parallel universes or histories. Similarly I and others have explained the power of quantum computation in terms of ‘quantum parallelism’ (many classical computations occurring in parallel).


However, if reality – which in this context is called the multiverse – is indeed literally quantum-mechanical, then it must have a great deal more structure than merely a collection of entities each resembling the universe of classical physics. For one thing, elements of such a collection would indeed be ‘parallel’: they would have no effect on each other, and would therefore not exhibit quantum interference. For another, a ‘universe’ is a global construct – say, the whole of space and its contents at a given time – but since quantum interactions are local, it must in the first instance be local physical systems, such as qubits, measuring instruments and observers, that are split into multiple copies, and this multiplicity must propagate across the multiverse at subluminal speeds. And for another, the Hilbert space structure of quantum states provides an infinity of ways of slicing up the multiverse into ‘universes’, each way corresponding to a choice of basis. This is reminiscent of the infinity of ways in which one can slice (‘foliate’) a spacetime into spacelike hypersurfaces in the general theory of relativity. Given such a foliation, the theory partitions physical quantities into those ‘within’ each of the hypersurfaces and those that relate hypersurfaces to each other. In this paper I shall sketch a somewhat analogous theory for a model of the multiverse.


The Structure of the Multiverse by David Deutsch.




Originally posted on Time Travel Portal, rescued from Archive.org.

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