tmb@talcott.UUCP (Thomas M. Breuel) (08/13/85)
In article <1031@sdcsvax.UUCP> , davidson@sdcsvax.UUCP (Greg Davidson) writes: >> From: gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) >>> The multiple worlds interpretation has some severe problems ... >> Not all the alternate worlds are equiprobable! There is no >> observable difference between the alternate-worlds QM and the >> Copenhagen QM. It happens that there IS a small chance that > The probability of the multiple worlds is irrelevant to the dwellers > therein. Each world is a complete spacetime continuum separate from > the others. In some moments, at some places, in some of them, there is > Maybe the latter worlds are less common, or maybe your thoughts fo > normality, including your notions of what is probable, are due to > the peculiar accidents of your world. Once you accept the multiple 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. It makes no sense to speak of 'observing differences' among worlds, or to generalise notions of normality to a fictitious ensemble of such separate worlds. WE are the dwelles of THIS world, and by the very defition of the word 'world' (in this context), the best thing that natural science can do is to describe statistically the physical laws that govern this world. Thomas.
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (08/15/85)
> 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. The multiple-worlds theory was proposed (by Everett and Wheeler, as I recall) as an alternative to conventional (Copenhagen) quantum theory. It was later shown to make exactly the same predictions. For this reason, it seems to have fallen into neglect. There IS an interaction between the multiple worlds in this theory; but it is just the forward branching process, which is of course inherently probabilistic. "The world" as normally used in conversation actually would encompass the knowable past of our particular slice and all possible futures, i.e. an infinite number of the other "multiple world" slices. It is unfortunate that "world" has these two different meanings when discussing this theory. It certainly is confusing.
davidson@sdcsvax.UUCP (Greg Davidson) (08/20/85)
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
brad@looking.UUCP (Brad Templeton) (08/24/85)
To me, in the multiple worlds interpretation, you should not talk about "an observer moving down his world line." An observer IS his world line, by definition. You didn't observe the photon as a particle, you *are* the world line that saw the photon as a particle. This translates as saying that the definition of a consciousness is an entity that is (or I suppose to make it clearer I could say traverses) a single world line in a universe of many. To me, this is also Zen. What this means is that you're a figment of my imagination, and I'm also a figment of yours, and it isn't inconsistent to say this. Now, being a concious human world-line is being a semi-continuous path though a large dimensional space of all possible configurations of space time. That's what makes us special. What QM and related topics might be showing is that the path is only semi-continuous, and doesn't have to be consistent at the lowest level, as long as it follows certain probabilistic rules. So this notion is not "absurd", just somewhat unusual and perhaps distasteful to our conceptions of consciousness. As a note, I am not saying I believe all this. Simply that is is a possible consistent theory and should not be dismissed out of hand as absurd (which I used to do.) -- Brad Templeton, Looking Glass Software Ltd. - Waterloo, Ontario 519/884-7473
peter@baylor.UUCP (Peter da Silva) (08/28/85)
> 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 How on earth do you figure that an unconstrained MWT is simpler or preferable. The fact that an unconstrained MWT produces a different observed universe than the one we see here is enough to reject that hypothesis. > 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. I see. What's happened is that you don't understand Occam's Razor, so you're applying it too broadly. Occams razor simply says that out of (n) hypotheses you should select the simplest and most testable hypothesis so that you know when you're wrong as soon as possible. It doesn't say the simples explanation is right, as you (and the creationists) seem to think. Since your view of how the EWG multiple worlds model works predicts a complete breakdown of causality we know it's wrong (or have you observed this breakdown of causality?). The next thing to do is look at the actual EWG theory. That I'll leave to someone better versed in it than I. > 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. Last I heard Borges wasn't a physicist. > 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. Niven never postulated impossible worlds, as you do. His MWT is vanilla EWG. His objection is that EWG makes volition meaningless. He's probably right, but so does determinism. -- Peter (Made in Australia) da Silva UUCP: ...!shell!neuro1!{hyd-ptd,baylor,datafac}!peter MCI: PDASILVA; CIS: 70216,1076
davidson@sdcsvax.UUCP (Greg Davidson) (09/10/85)
In an earlier article, I presented problems with the multiple worlds theory (MWT) as an interpretation of quantum mechanical reality. Some of the problems are old, first presented by Bohr, Heisenberg, and other early QM theorists. Some were unconventional ideas suggested by me, with inspiration from a variety of sources. I asked for participation in exploring this often baffling topic. These problems are as much philosophical as physical, as they are constrained by the latter, but must be addressed using the tools of the former. It seems like a good game we can all play, if we're careful to stick to the rules and tread cautiously - this territory is very slippery. Unfortunately, the two postings to reply so far have attempted to dismiss the matter rather than address it. I'll use the most recent as an example. If you read it through, I think you'll see that it was not a responsible posting, although it partially addresses some of my points. The author's text is indented with tabs. My original text indented with >'s. From: peter@baylor.UUCP (Peter da Silva) Subject: Re: QM and Multiple Worlds Date: 28 Aug 85 11:34:34 GMT Organization: The Power Elite, Houston, TX > 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 How on earth do you figure that an unconstrained MWT is simpler or preferable. The fact that an unconstrained MWT produces a different observed universe than the one we see here is enough to reject that hypothesis. As follows. The conventional MWT, as I understand it, says that each quantum mechanical event splits the universe into a superposition of states, one for each possible outcome. As observers, being restricted to one universe/state, we can observe only one outcome. We infer the existence of superpositions by their effect on the probability of occurrance of events; yet, those superpositions ``collapse'' or ``branch away from us'' (depending on which interpretation you're using) as soon as they interact with our process of observation . Yet all outcomes possible by physical law actually occur in some universe/state. Because we cannot see beyond our own universe/state, we actually have no idea of the true probability of events. We could just happen to be on a very low probability worldline in the state space of branching universes. The universe we wind up in always has a probability value of 1, since we can only observe it after the fact. We cannot predict, since all predicted outcomes always occur. (If you're familiar with the Anthropic Principle in Cosmology, the argument here has a similar [unpleasant] flavor.) We need only extend this line of reasoning a little bit to see that the presence of physical laws constraining the generation of states is also unobservable. Given branching universes, if all possible outcomes of events were to occur, with no physical laws to constrain them, there would be observers along worldlines inferring all kinds of physical laws. Imagine us as such observers, with a deceptive history implying certain physical laws, now being presented with evidence of multiple worlds. What magnificent irony! > 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. I see. What's happened is that you don't understand Occam's Razor, so you're applying it too broadly. Occams razor simply says that out of (n) hypotheses you should select the simplest and most testable hypothesis so that you know when you're wrong as soon as possible. It doesn't say the simples explanation is right, as you (and the creationists) seem to think. Since your view of how the EWG multiple worlds model works predicts a complete breakdown of causality we know it's wrong (or have you observed this breakdown of causality?). The next thing to do is look at the actual EWG theory. That I'll leave to someone better versed in it than I. The principle you mention sounds like a nice principle for guiding experimentation, but bears no relationship to Ockham's razor. I just checked the Encyclopedia of Philosophy (page 8-307) and found that though I'd spelled his name wrong, I understood his principle quite well. They quote him as saying: ``What can be done with fewer [assumptions] is done in vain with more.'' They go on to say: `The principal use made by Ockham of the principle of parsimony was in the elimination of pseudo-explanatory entities....' Well, in a MWT system, physical laws become pseudo-explanatory entities. No observation can support their existence. I object to your insulting style of argument. You attack my argument by belittling my understanding and comparing me with creationists! Yet you neither check my argument carefully nor offer an alternative. The problem of how to interpret quantum mechanical reality is a real problem. Great physicists tried hard to reject QM because of it. Now that we have been forced to accept QM, we must grapple with it. > 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. Last I heard Borges wasn't a physicist. He was not. I recommend his stories for their clear presentation of the philosophical consequences of certain physical models; models which physicists have suggested, but which have neither been accepted nor rejected. You will not find the answers to these problems in existing writings by physicists, you will only find the questions there. > 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. Niven never postulated impossible worlds, as you do. His MWT is vanilla EWG. His objection is that EWG makes volition meaningless. He's probably right, but so does determinism. His story is particularly devastating because it sticks closely to the vanilla MWT, yet obtains bizarre results. I'm glad you agree that such models pose serious problems for volition. I would not agree that determinism does. For clear arguments supporting the compatibility of determinism and volition, I recommend Daniel Dennett's new book ``Elbow Room; the kind of free will worth having.'' Its published by MIT Press. Dennett is a professional philosopher who pays close attention to the work of physicists and often collaborates with the physicist and computer scientist Douglas Hofstadter. So how about it folks? Am I the only one disturbed by the philosophical implications of QM? Do you think that physics books have the answers? If you see a hole in my arguments (or Einstein's, for that matter, but check carefully if you think you see a hole in his!), please show me. If I've misunderstood the claims of QM, let me know that too. If you have another interpretation which avoids some of these problems, lets have it! _Greg Davidson Virtual Infinity Systems, San Diego