Multiverse concepts in string theory
I recently happened upon a physics related blog and discovered that there has been an active debate within the physics community for the past few weeks, months and years about string theory and its relation to multiverse concepts. The blog is Not Even Wrong, http://www.math.columbia.edu/~woit/blog/, by Peter Woit, a mathematician and string theory skeptic at Columbia. His blog takes its name from his opinion about string theory: that it should not even be considered a part of physics, since it has failed for decades to make any testable predictions. Recently things have heated up with the publication of a new book by Leonard Susskind, the so-called father of string theory: The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. Depending on who you read, the book either finally exposes the intellectual bankruptcy of string theory, or else displays its incredible mathematical beauty, elegance and power. The New York Times book section recently had a scathing review of the book, which has triggered much of the current discussion. At this time the most recent blog post is here: http://www.math.columbia.edu/~woit/wordpress/?p=348 and it links to others, but the topic is far from exhausted. The reason for such contrasting opinions seems to come down to multiverse models and anthropic reasoning. Apparently modern formulations of string theory do not describe a single universe. Rather, there are a nearly infinite (or actually infinite, by some interpretations) set of possible parameters, all of which could represent a universe. Our observable universe is just one example of those. Susskind basically adopts Tegmark's level 2 multiverse to propose that all of the other string theory universes exist within an overall framework of cosmological inflation. The fact that we see a particular set of parameters is explained on anthropic principles, that in the other universes either there are no people, or they observe different parameters. To say that this reasoning is unattractive to many physicists would be an understatement. Anthropic reasoning seems to be one of the great dividing issues within the physics community. Anti-anthropicists see it as basically a matter of giving up on physics. If all universes and all sets of physical laws exist, then there is no possibility of explaining them, no deep underlying principles, and no scientific theory involved. It is just a matter of cataloging observations and putting them into some semblance of order. Further, no powerful predictions are possible since there are a virtually infinite number of models which match any set of observations. To these physicists, any physical constant or property whose value is left up to the anthropic principle is one which has departed from the realm of physics. On the other side there is a small but growing number of workers who accept anthropic reasoning for at least some constants. I'm not sure exactly why they are OK with it, but I get the impression that it comes down to the fact that there seem to be many physical constants that are fine-tuned for life. The alternative is that God created the universe specially for mankind, which is problematic for many well known reasons. (Apparently Susskind goes to some lengths in his book to discuss this issue.) I think another reason is that there are so many physical constants, and they seem so arbitrary, that the chance that they will just pop out of some simple theory is becoming more and more remote, so that these physicists have pretty much given up on that. Then there is the fact that the level 2 multiverse (separate universes as bubbles within a background of cosmic inflation) is pretty much compatible with modern day physics, so there is a place for multiple universes. Apparently Hawking is something of an anthropicist, and there is some discussion of this on the blog at http://www.math.columbia.edu/~woit/wordpress/?p=347. I think that part of the reason why multiverse models are seen as nonphysical is because physicists do not recognize or accept the concepts we have explored of measure. When Susskind writes about the so-called landscape of string theory, each point representing a different model, he and everyone else assumes that each point is equally probable, has equal measure. Given such an assumption of uniform measure, it would indeed be difficult or impossible to make predictions. In many of our discussions of multiverse models, we have explicitly or implicitly included the notion of measure, that some universes would be more common or more prominent than others. This is often linked to extensions of Occam's Razor, so that universes with relatively simple models would have higher measure than those that are more complex. Kolmogorov complexity is one way of defining a measure with this desirable property. This provides, in principle, a quantitative way of evaluating the measure of different universe models. Applying the anthropic
RE: Multiverse concepts in string theory
Hal wrote: I also get the impression that Susskind's attempts to bring disreputable multiverse models into holy string theory is more likely to kill string theory than to rehabilitate multiverses. Perhaps I am getting a biased view by only reading this one blog, which opposes string theory, but it seems that more and more people are saying that the emperor has no clothes. If string theory needs a multiverse then it is even less likely to ever be able to make physical predictions, and its prospects are even worse than had been thought. A lot of people seem to be piling on and saying that it is time for physics to explore alternative ideas. The hostile NY Times book review is just one example. Two words: Continental drift. Ok, continental drift is observable, whereas multiverses aren't, but it is worth remembering the ridicule heaped (up until not so very long ago) on those who dared to suggest what is now known as plate techtonics. Jonathan Colvin
Re: Multiverse concepts in string theory
Le 13-févr.-06, à 09:44, Hal Finney wrote (in part): In many of our discussions of multiverse models, we have explicitly or implicitly included the notion of measure, that some universes would be more common or more prominent than others. This is often linked to extensions of Occam's Razor, so that universes with relatively simple models would have higher measure than those that are more complex. The necessity of simplicity could perhaps be a consequence of comp, but this remains to be shown. But even if that is the case, I don't see how simplicity would make the model having a higher measure, unless you attach consciousness to particular individuals in particular universe, but this can be done only by identifying the first person and some arbitrary particular third person description. And the UDA shows this cannot be done (with comp). Physics is a science, and that means it needs to work with theories that can be tested and disproven. We are a long way from being able to come up with any experiment that a working physicist in his lab could run to see whether multiverse models are correct. (And no, quantum suicide doesn't count!) The day physicists will understand the logician's sense of model and theories, things will be pretty much clear. If we agree that physicists obey the same laws as the particles they describe, then, even just the two slits experiment entails mutiverses, and confirms QM which is literally a mutiverse theory (even with the collapse, which is just an invention for cutting on the typical quantum contagion of the superpositions). Also, note that the 0-universe, 1-universe, infinity-of-universes are all on the same par. Nobody has ever tested the existence of a primitive physical universe nor of the existence of Aristotelian Prime Matter, and other common sense idea on which those physicalist ideas derive. Note also that the theory of Matter given by the loebian (introspective) machine is 100% testable. I also get the impression that Susskind's attempts to bring disreputable multiverse models into holy string theory is more likely to kill string theory than to rehabilitate multiverses. String theory relies entirely on QM and so inherits all its interpretation problems. Except that in String Theory, like Quantum Cosmology, the wave collapse is still more unintelligible. Witten makes the points in a conference some years ago. According to him String Theory is still very fuzzy on the whole wave aspect of strings, above its traditional role as computation tool. Perhaps I am getting a biased view by only reading this one blog, which opposes string theory, but it seems that more and more people are saying that the emperor has no clothes. If string theory needs a multiverse then it is even less likely to ever be able to make physical predictions, and its prospects are even worse than had been thought. A lot of people seem to be piling on and saying that it is time for physics to explore alternative ideas. The hostile NY Times book review is just one example. To be sure I disagree that string theory is not testable, and I think the multiverse idea is testable and already indirectly tested and that it is the most certain consequence of QM. Now, as a theory of everything, string theories, like QM and actually the whole physics enterprise, are flawed at the start, because those approach relies (consciously or not) on a hardly clear or coherent theology inherited mainly from Aristotle, and which tends to put the mind-body problem under the rug. Bruno http://iridia.ulb.ac.be/~marchal/
Re: belief, faith, truth
Le 30-janv.-06, à 22:07, Benjamin Udell wrote, in part, sometimes ago (30 January): Most people, however, do have some sort of views, which are or have been significant in their lives, about what are traditionally called metaphysical questions -- God, freedom, immortality, psycho-physical relationships, etc. Many have one or another kind of metaphysical faith. It seems increasingly clear to me that Bruno is doing a machine metaphysics, or a computer metaphysics, or a metaphysics of, by, and for computers or machines Yes. I am interested in what machines (and other entities) can prove about themselves. And also about what is true about themselves, but that those machines/entitities cannot prove, but can deliver as true in a way or another. The propositional parts of those discourse has been captured by the modal logical systems G and G* respectively (Solovay 1976). (I can't remember why Bruno opts for machines instead of computers.). I use computer for universal machine. Ordinateur in french. All loebian machines I talk about are universal machine. All universal machine believing in classical tautologies and in the laws of addition and multiplication, and in some induction formulas is lobian. It's a shame that the word metaphysics is ruled out by (if I remember correctly, it was in a post a while back) reaction of intellectuals in Belgium. In Belgium, in France and in other countries, I'm afraid, among most scientists, I mean. I rule out also metaphysics because I don't know what it means. Historically it concerns the books which were on the sides of the books on physics in the texts by Aristotle (but is this a legend?). In metaphysics, meta has not the same sense that meta in computer sciences and mathematical logics. Create confusions. Moreover, machine metaphysics is kind of catchy in its alliterative way. Sure. Look: digital machine metaphysics is a branch of metamathematics! Metaphysics is not religion but instead a philosophical study of questions which are among the important ones in religion. Philosophy, however, can be applied in living, so the distinction is not a barrier impenetrable in practice (or, therefore, in theory either) I don't even really believe in any precise frontiers between all those things. It is useful only for the curriculum vitae and for searching job and getting social profile, but any fundamental questioning is up to eventually move frontiers or suppress some. Bruno http://iridia.ulb.ac.be/~marchal/
Re: belief, faith, truth
Hi, Bruno, You're tending -- too selectively, arbitrarily -- to try to go by what was meant by words many hundreds and even some thousands of years ago. Original or early meanings can be very illuminating, but a lot has happened since then, and there is some degree of _stare decisis_ in these matters. Words like property (originally, a non-essential differentia, i.e., an idiosyncrasy) and physics (which meant pertaining to growth, especially to plants as growths, as in _phyton_, plant) could not withstand the standard which you apply. And it's best if you seem to prioritize your theory much, much higher than you prioritize the correcting of wrongs which you believe to have been done in theologically related politics over a thousand years ago. And you can't simultaneously do that and justify the use of the word theology as the correction of an ancient wrong and restoration of the original, legitimate meaning. People just don't care _that_ much about pedigree or ancient politic! s. And if you don't really care that much either, but are basically just seeking a justification, it's good to pick one which will bear up under the weight which you place on it, so that you don't give the appearance of over-prioritizing such ancient occurrences. And if you do actually care that much, then you should consider whether you care about your machine theory even more. It's good to give your intended audience the sense that you share understand their concerns and are familiar with the same intellectual world as they are. Your best arguments on terminology all seem founded in the present, e.g., the vagueness of the word metaphysics, plus its causing opprobrium among scientists. However, if its causing opprobrium among scientists is a sufficient objection, then the opprobrium which the word theology will cause among scientists is a sufficient objection too. The opprobrium would likely be even greater, and the objection, therefore, that much stronger. Add to that, the opprobrium which would be caused by your use of the word theology among religious people generally in proportion as your theory were to gain fame or notoriety. What will they say? Worst-case scenario: They'll say and believe that you're founding a religion of worship of some Big Machine in the Sky. Imagine having myriad academic people and highly religious people united, as strange bedfellows, against you, and declaring against that which they call the nightmarish bastard offspring of a shotgun wedding between religion and science. Even Romeo's Juliet's circumstances were less forbidding. Of course lots of people have startling and evocative theories, deriving physics from various abstract considerations. The odds of your particular theory's becoming famous seem small to a comparatively ignorant outsider like me. Yet, if you have confidence in the persuasiveness of yourself your theory, then you should think very carefully before actually naming your theory as a new and scientifically based competitor in the religious field, and subjecting your theory, your intentions, and yourself to wild caricatures which people will take as gospel and spread as gospel and which will form the basis of their dispositions to act in regard to you if the occasion ever arises or is made to arise. Now, metaphysics and theology both seem like bad ideas for names, given the intellectual climate. Nevertheless, between the two, I think metaphysics is preferable, for the reasons that I've stated here elsewhere. As to the meaning of metaphysics, the biggest problem is the number of people, for whom it is synonomous with supernatural issues, in languages other than English (I'm told that such is the primary meaning of the Spanish metafisica.). Not much that one can do about that, but at least most such people are far from your intended audience. There might indeed be confusion over the use of the meta- prefix. But I suspect that most people take the word metaphysics as a whole, it's a familiar word. Certainly, for what it's worth, in English many will take it as a whole, because that's the kind of language that English is, words are heard in English differently than they are, for instance, in German, whose speakers like a feel for the elements of a compound. ! In English we often just traffic the whole word unanalyzed across speech. It is quite possible in English to go through life without any awareness, for instance, that the words cross and crusade are cognate. The word metaphysics will be taken in various senses developed within philosophy itelf. Its primary sense is not book after the physics book, obviously, whatever its ancient origin, about which hardly anybody worries in construing it in current philosophical use. (Do you think Bruno just means a book after a physics book? Nobody will ask such a question.) About the best that one can do is to try to come up with a meaning which stands up in terms of modern
Re: Multiverse concepts in string theory
Hal wrote: I also get the impression that Susskind's attempts to bring "disreputable" multiverse models into "holy" string theory is more likely to kill string theory than to rehabilitate multiverses. Perhaps I am getting a biased view by only reading this one blog, which opposes string theory, but it seems that more and more people are saying that the emperor has no clothes. If string theory needs a multiverse then it is even less likely to ever be able to make physical predictions, and its prospects are even worse than had been thought. A lot of people seem to be piling on and saying that it is time for physics to explore alternative ideas. The hostile NY Times book review is just one example. String theory isn't going to be killed until there's a replacement available, and any replacement is likely to face the same issue of describing a large collection of universes of which only a small subset can support life.So Iwouldn't beconcerned about more effort being devoted to looking for alternatives to string theory. In the mean time, the multiverse meme continues to spread. Take the review of Susskind's bookin American Scientist (http://www.americanscientist.org/template/BookReviewTypeDetail/assetid/49558)for example: In the end, however, good though this book is, I was left feeling that the argument was not carried to its logical conclusion. Despite his justified scorn for intelligent design, Susskind retains a hint of this worldview in his own attitude. It was Galileo who said that the book of Nature is written in mathematics, and almost all physicists subscribe to this view. When we contemplate the power and simplicity of constructions like general relativity, there is a temptation to carry intelligent design to an extreme in which God wrote the equations, from which all else follows. Frequently this perspective is quite explicit, as with Einstein (recall Bohr's admonition, "Stop telling God what to do!"). The landscape picture derails this thinking to some extent, but Susskind just transfers the quasi-religious awe to string theory, whose mathematical results he repeatedly describes as "miraculous." But if life on Earth is a random accident in a universe where only chance yielded laws of physics suitable for life, why stop there? Perhaps string theory itself is nothing special and only part of a wider spectrum of possible prescriptions for reality. If the search for a unique and inevitable explanation of Nature has proved illusory at every step, is it really plausible that suddenly string theory can make everything right at the last? Reading Susskind's book should make you doubt that possibility, in which case we may have reached the end of the search for underlying simplicity that has driven physics since the beginning. A comment made by Steven Weinberg in his 1977 book The First Three Minutes sums things up well: "The more the universe seems comprehensible, the more it also seems pointless." Pointless to look for meaning in our existence in the universe, and also (according to Susskind) pointless to look for meaning in physics. To a physicist, this is a pretty depressing conclusion, but there is some consolation: The beauty we perceive in the laws of physics perhaps tells us as much about the human aesthetic response as it does about any fundamental design of the universe. In short, physics is a human creative art on the same level as painting and music, and that is reason enough to be proud of what the subject has achieved.
Re: Multiverse concepts in string theory
Dear Wei and Friends, I have been following this thread with some interest (Hal initial post was wonderful, BTW!)and echo the comments of Wei here, but I would offer a note of caution: we must be very careful that the elevation of string theory (SUSY) to almost dogmatic "Sacred Cow" status does not bode well for many of us, particularly those that have found that its most fundamental assumption, the existence of a supersymmerty relation between bosons and fermions, has never even come close to matching experimental observation. Maybe, just maybe, SUSY is a good theory or maybe it is just a very elegant bit of pure mathematics. Remember, just because a mathematical theory can be shown to be self-consistent and elegant, there is no requirement that that theory have anything to do with the physical world we experience. I find that the choices presented by Weinberg and the Intelligent Design advocates are not the only possibilities. Consider that we still do not have a consistent and faithful model of observers within our physics and thus can not even start to coherently consider what the notion of "comprehensibility" means in the context of physics. ;-) Onward! Stephen - Original Message - From: Wei Dai To: everything-list@eskimo.com Sent: Monday, February 13, 2006 7:06 PM Subject: Re: Multiverse concepts in string theory Hal wrote: I also get the impression that Susskind's attempts to bring "disreputable" multiverse models into "holy" string theory is more likely to kill string theory than to rehabilitate multiverses. Perhaps I am getting a biased view by only reading this one blog, which opposes string theory, but it seems that more and more people are saying that the emperor has no clothes. If string theory needs a multiverse then it is even less likely to ever be able to make physical predictions, and its prospects are even worse than had been thought. A lot of people seem to be piling on and saying that it is time for physics to explore alternative ideas. The hostile NY Times book review is just one example. String theory isn't going to be killed until there's a replacement available, and any replacement is likely to face the same issue of describing a large collection of universes of which only a small subset can support life.So Iwouldn't beconcerned about more effort being devoted to looking for alternatives to string theory. In the mean time, the multiverse meme continues to spread. Take the review of Susskind's bookin American Scientist (http://www.americanscientist.org/template/BookReviewTypeDetail/assetid/49558)for example: In the end, however, good though this book is, I was left feeling that the argument was not carried to its logical conclusion. Despite his justified scorn for intelligent design, Susskind retains a hint of this worldview in his own attitude. It was Galileo who said that the book of Nature is written in mathematics, and almost all physicists subscribe to this view. When we contemplate the power and simplicity of constructions like general relativity, there is a temptation to carry intelligent design to an extreme in which God wrote the equations, from which all else follows. Frequently this perspective is quite explicit, as with Einstein (recall Bohr's admonition, "Stop telling God what to do!"). The landscape picture derails this thinking to some extent, but Susskind just transfers the quasi-religious awe to string theory, whose mathematical results he repeatedly describes as "miraculous." But if life on Earth is a random accident in a universe where only chance yielded laws of physics suitable for life, why stop there? Perhaps string theory itself is nothing special and only part of a wider spectrum of possible prescriptions for reality. If the search for a unique and inevitable explanation of Nature has proved illusory at every step, is it really plausible that suddenly string theory can make everything right at the last? Reading Susskind's book should make you doubt that possibility, in which case we may have reached the end of the search for underlying simplicity that has driven physics since the beginning. A comment made by Steven Weinberg in his 1977 book The First Three Minutes sums things up well: "The more the universe seems comprehensible, the more it also seems pointless." Pointless to look for meaning in our existence in the universe, and also (according to Susskind) pointless to look for meaning in physics. To a physicist, this is a pretty depressing conclusion, but there is some consolation: The beauty we perceive in the laws of physics perhaps tells us as much about the human aesthetic response as it does about any fundamental design of the universe. In short, physics is a human creative art on the same level as painting and music, and that is reason
Re: belief, faith, truth
I doubt Marchal's ideas will be made widely known or popularized in the foreseeable future. The problem isn't with the name of his theory, or with any problem with Bruno per se beyond this: There doesn't seem to be an easily reducible way to summarize the theory in a manner that is digestible to anyone beyond the highly specialized in similar fields. I certainly understand the basics of some of his ideas, but when it gets into all his logical analysis I just have never found myself willing to devote myself to the time required to really get into the detail of where he is coming from. And I would consider myself highly interested in these topics and at least reasonably intelligent. Even something as mundane as the MWI (to this group at least) runs into a brickwall when presented to the layperson. You should see the conversations I have with my wife. Tell people everything is made of strings. Or space and time can be warped and curved. They may not understand the science and math behind it at all, but at least you are speaking their language. The world is not ready for his ideas. Even for the most part the world of scientists in my opinion. Danny Benjamin Udell wrote: Hi, Bruno, You're tending -- too selectively, arbitrarily -- to try to go by what was meant by words many hundreds and even some thousands of years ago. Original or early meanings can be very illuminating, but a lot has happened since then, and there is some degree of _stare decisis_ in these matters. Words like "property" (originally, a non-essential differentia, i.e., an idiosyncrasy) and "physics" (which meant "pertaining to growth, especially to plants as growths," as in _phyton_, "plant") could not withstand the standard which you apply. And it's best if you seem to prioritize your theory much, much higher than you prioritize the correcting of wrongs which you believe to have been done in theologically related politics over a thousand years ago. And you can't simultaneously do that and justify the use of the word "theology" as the correction of an ancient wrong and restoration of the original, legitimate meaning. People just don't care _that_ much about pedigree or ancient politic ! s. And if you don't really care that much either, but are basically just seeking a justification, it's good to pick one which will bear up under the weight which you place on it, so that you don't give the appearance of over-prioritizing such ancient occurrences. And if you do actually care that much, then you should consider whether you care about your machine theory even more. It's good to give your intended audience the sense that you share understand their concerns and are familiar with the same intellectual world as they are. Your best arguments on terminology all seem founded in the present, e.g., the vagueness of the word "metaphysics," plus its causing opprobrium among scientists. However, if its causing opprobrium among scientists is a sufficient objection, then the opprobrium which the word "theology" will cause among scientists is a sufficient objection too. The opprobrium would likely be even greater, and the objection, therefore, that much stronger. Add to that, the opprobrium which would be caused by your use of the word "theology" among religious people generally in proportion as your theory were to gain fame or notoriety. What will they say? Worst-case scenario: They'll say and believe that you're founding a religion of worship of some Big Machine in the Sky. Imagine having myriad academic people and highly religious people united, as strange bedfellows, against you, and declaring against that which they call the nightmarish bastard offspring of a shotgun wedding between religion and science. Even Romeo's Juliet's circumstances were less forbidding. Of course lots of people have startling and evocative theories, deriving physics from various abstract considerations. The odds of your particular theory's becoming famous seem small to a comparatively ignorant outsider like me. Yet, if you have confidence in the persuasiveness of yourself your theory, then you should think very carefully before actually naming your theory as a new and scientifically based competitor in the religious field, and subjecting your theory, your intentions, and yourself to wild caricatures which people will "take as gospel" and spread as gospel and which will form the basis of their dispositions to act in regard to you if the occasion ever arises or is made to arise. Now, "metaphysics" and "theology" both seem like bad ideas for names, given the intellectual climate. Nevertheless, between the two, I think metaphysics is preferable, for the reasons that I've stated here elsewhere. As to the meaning of "metaphysics," the biggest problem is the number of people, for whom it is synonomous with "supernatural issues," in languages other than English (I'm told that such is the primary meaning of the Spanish "metafisica."). Not much that one can do about
