On Friday, September 20, 2002, at 10:03 AM, Wei Dai wrote:

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> On Thu, Sep 05, 2002 at 12:08:39PM +0200, Bruno Marchal wrote: >> This comes from the fact that MWI is explained most of the time >> in the context of non relativistic QM (which assumes time and space). >> But this problem disappear once you take into account the >> space time structure of relativistic QM, where roughly speaking >> moment of time are handled by "parallel" universes (see Deutsch FOR). > > I got Deutsch's book, but it doesn't mention relativistic QM at all. > Can > you elaborate on what the MWI of relativistic QM is, or point me to > another paper or book, or give me a page number in FOR that deals with > this? This topic dovetails (no pun intended) on several points I've made as well, so I'll add some comments. * Deutsch's "Fabric of Reality" is a slender book, with only the first few chapters really making his main point (about how the single- and double-slit experiments already "proved" the MWI interpretation a century ago, had we known what to look for, and that quantum computers make the point as well). I don't recall whether he says much about relativistic vs. nonrelativistic QM, but I'll take your word that he says nothing. His focus is on the quantum aspects, not cosmology or relativity or a unified theory, so this is not too surprising. * Much more is said in a book I have recommended a couple of times here: Lee Smolin's "Three Roads to Quantum Gravity." Also, his earlier book, "The Life of the Cosmos." * The idea is this: -- conventional ("classical") QM assumes Newtonian space and time, i.e., a universal coordinate system -- conventional ("classical") relativity (SR and GR) assumes a non-Newtonian, non-constant space and time, via Lorentz transforms on a Minkowski spacetime, but it has no quantization a la QM -- in other words, two very different spacetimes. This is sometimes characterized as the "very small" (quantum effects) vs. the "very large" (astrophysics), and experiments at most ranges don't produce contradictions, as gravity effects are miniscule at the usual quantum levels and quantum effects are miniscule at cosmological or astrophysical scales. However, understanding black holes will almost certainly require a unification of these two theories or outlooks. And of course a coherent, unified theory ought not to have two radically different views of spacetime. * Einstein attempted to merge the two, but failed. Beginning in the 1970s, with the work of Ashtekar, Witten, Rovelli, Crane, Susskind, Baez, and many others, progress was made toward unifying the models. The quantum gravity program, as pursued by the several different schools (strings and branes, spin foams, twistors, etc.), is to unify these two fundamentally different outlooks. As of now, this hasn't happened. * Personally, I think there is much of interest in the "discrete at Planck scales" relational approach. --Tim May