Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site sdcsvax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!cornell!uw-beaver!tektronix!hplabs!sdcrdcf!sdcsvax!davidson From: davidson@sdcsvax.UUCP (Greg Davidson) Newsgroups: net.physics Subject: Re: QM and Multiple Worlds Message-ID: <1049@sdcsvax.UUCP> Date: Mon, 19-Aug-85 17:52:20 EDT Article-I.D.: sdcsvax.1049 Posted: Mon Aug 19 17:52:20 1985 Date-Received: Sun, 25-Aug-85 04:42:54 EDT References: <486@talcott.UUCP> Organization: EECS Dept. U.C. San Diego Lines: 112 The replies to both of my previous articles have missed the point, which may be more subtle than I thought. Let me try again with more care and detail, because I think that the point is an important one. I invite others to help me do so. I apologize for the length of this article. I will first present the point in detail, then answer the criticisms made by those who replied to my earlier articles. You may wish to skip the latter, but don't if you're going to post a followup to this one. Finally, I'll give some references. First suppose that every elementary event occurs in all possible ways by forking off a whole universe for each possible outcome of the event. For ``observers'' along any world-line, it is irrelevant (because unobservable) whether some outcomes are more probable, or whether all are equally likely. It is also irrelevant whether outcomes are constrained by some ``laws of physics'' or whether all conceivable outcomes generate universes. Note that the latter is a simpler theory, hence preferable by Occam's razor. Now in such a system, the notion of observer is rather bizarre (hence my earlier quotes). Particle configurations identical to what we think of as observers pop into and out of existence in all possible ways at all possible points, complete with false memories (brain states not corresponding to any past events along their world-line) false sensations (sensory neuron states not corresponding to any external events, if any, etc. Even when observers are part of what we think of as a normal universe, with true memories and true sensations, this is entirely an accidental occurrance. Only some successor universes will preserve this kind of special relationship. There is no reason to attribute any significance to those world-lines which do preserve what we think of as normality. So to summarize, MWTs do violence to the notion of observers, and thus to the practice of physics. To belive in a MWT is to believe ourselves to be illusions, and is therefore absurd. (Please note that I'm talking here about our physical continuity being illusory, not about our psychological selves being so, the latter being quite another issue.) A MWT unconstrained by any laws of physics is preferable, because simpler, to one incorporating laws of physics, hence MWTs directly do violence to the notion of physics. Now let me answer some of the replies I've received. Doug Gwyn wrote: > [1] Not all the alternate worlds are equiprobable! [2] There is no > observable difference between the alternate-worlds QM and the > Copenhagen QM. [1] Maybe, but irrelevant to my point. Also, Occam's razor would suggest abandoning any theory of structure in the generation of successor worlds. [2] Agreed. Yet note that in an unconstrained MWT, there will be observers who witness events consistent with any physical theory you want to posit. Thomas M. Breuel wrote: > [1] The 'multiple worlds interpretation' of QM is not a physical theory: > you cannot design an experiment to disprove it, since it postulates that > there is not interaction between its different worlds. [1] Many physical theories give the same results. Ptolemaic epicycles give the same results as Keplerian ellipses in explaining observations of the planets (and were embarassingly superior to Copernican circles). Does that mean that Ptolomy's model of the universe as consisting of crystalline spheres is as good a model of the Solar System as the heliocentric model? (Disregarding later information from telescopes, spacecraft, etc.). Choosing among equivalent models is part of physics. As I understand it, MWT was offered because of philosophical problems with CI, not because CI fails to explain events. Unfortunately, both MWT and CI have terrible philosophical problems. Does anyone know of a third model without such problems? > [2a] It makes no sense to speak of 'observing differences' among worlds, > or [2b] to generalise notions of normality to a fictitious ensemble of > such separate worlds. [3a] WE are the dwelles of THIS world, and [3b] by the > very defition of the word 'world' (in this context), [3c] the best thing > that natural science can do is to describe statistically the physical > laws that govern this world. [2] I do not speak of physically observing such differences. The examining of any MWT is the act of mentally considering such differences. Sorry to have led you astray here. [3a] The implication of an unconstrained MWT is that ``WE'', along with ``THIS world'' are illusions (except for unobservable instantaneous time slices. [3b] The definition of the word `world' merely reflects conventional, pre-QM ideas. The compound world-line is more modern, but is a bit peculiar, since the word `world' is generalized to meaning a particle or system of particles considered collectively. I don't see that either implies [3c]. [3c] Even this is not possible. The best thing that science can do is to construct theories, check them against observations and less ponderable criteria (such as elegance), and investigate their consequences. For some very interesting reading on these issues, I recommend some fictional stories designed to present relevant thought experiments. The earliest I know of are Jorge Luis Borges' stories ``The Garden of Forking Paths'' which explores the nature of MWTs, and ``The Library of Babel'', which explains how all possible writings (read `universes') can be generated by a finite library of books (read `world states'). Both of these can be found in his collection ``Labyrinths'', New Directions Pub. Corp., 1964. A more recent, and more devastating exploration of MWTs is Larry Niven's story ``All the Myriad Ways'', in his collection by the same name [Ballantine Books, 1971], and probably in other collections as well. A scholarly and systematic examination of the notion of an observer, with a good coverage of the above mentioned ideas of false memories and false sensations is covered in ``The Story of a Brain'' in the collection ``The Mind's Eye'' edited by the philosopher Daniel Dennett and the physicist and computer scientist Douglas Hofstadter. I cannot recommend this latter collection too strongly (but can't give a full citing - my three copies are all lent out - but its still in print). As usual, I welcome comments, but if you don't see my point or follow my logic, you may wish to reply to me directly. I'll post any direct replies which I find interesting. _Greg Davidson Virtual Infinity Systems, San Diego