What If There Is Only One Universe?

Article written: 4 Jun , 2009
Updated: 24 Dec , 2015
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When it comes to universes, perhaps one is enough after all.

Many theories in physics and cosmology require the existence of alternate, or parallel, universes.  But Dr. Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, explains the flaws of theories that suggest our universe is just one of many, and which also perpetuate the notion that time does not exist.  Smolin, author of the bestselling science book ‘The Trouble with Physics’ and a founding member of the Perimeter Institute, explains his views in the June issue of Physics World.

Smolin explains how theories describing a myriad of possible universes, or a “multiverse”, with many dimensions and particles and forces have become more popular in the last few years. However, through his work with the Brazilian philosopher Roberto Mangabeira Unger, Smolin believes that multiverse theories, which imply that time is not a fundamental concept, are “profoundly mistaken”.

Smolin says a timeless multiverse means our laws of physics can’t be determined from experiment.  And he explains the unclear connection between fundamental laws, which are unique and applicable universally, and effective laws, which hold based on what we can actually observe.

Smolin suggests new principles that rethink the notion of physical law to apply to a single universe.  These principles say there is only one universe; that all that is real is real in a moment, as part of a succession of moments; and that everything real in each moment is a process of change leading to future moments. As he explains, “If there is just one universe, there is no reason for a separation into laws and initial conditions, as we want a law to explain just one history of one universe.”

He hopes these principles will bring a fresh adventure in science.

If we accept there is only one universe and that time is a fundamental property of nature, then this opens up the possibility that the laws of physics evolve with time. As Smolin writes, “The notion of transcending our time-bound experiences in order to discover truths that hold timelessly is an unrealizable fantasy. When science succeeds, we do nothing of the sort; what we physicists really do is discover laws that hold in the universe we experience within time. This, I would claim, should be enough; anything beyond that is more a religious urge for transcendence than science.”

Source: Institute of Physics


42 Responses

  1. InvaderXan says

    I must say, the notion that the laws of Physics are actually time dependent is a fascinating idea…

  2. Lawrence B. Crowell says

    Here, here! I too think that there is only one cosmology which has what I call classical content. I do think that the grand quantum path integral for the universe contains amplitudes for alternate cosmologies (a’la string & M-theory), but I do think that the observable cosmology is uniquely einselected for classicality and the other amplitudes reduced to infinitesimal values.

    Smolin is doubtless taking this from a loop variable perspective, and this is a part of the string-loop wars. I tend to think string and loops are different or “dual” perspectives on these matters. My work with sphere packing and quantum codes is an attempt at some sort of unification of the two theories, or what might better be called “frameworks.”

    Lawrence B. Crowell

  3. Jon Hanford says

    I must concur with Lawrence B Crowell on this one. This does indeed appear to be an explanation through a loop variable perspective.

  4. Max Fagin says

    I’m not saying I agree or disagree with any multiverse theory of cosmology, but am I the only one whose humbug detector started going off while they were reading this?

  5. Lawrence B. Crowell says

    I don’t particularly sign on to the idea laws of physics vary with time. However, renormalization group equations do vary with energy, or the energy cut-off, and time is dual to energy. Yet most of any such time variation in field theoretic properties would have largely occurred in the first 10^{-30} seconds into the big bang.

    Smolin is a pretty smart guy out there. I think he takes too much of a dim view of string theory, which is so vast that we might want to call it a framework. Loop variable Q-gravity is more conservative than string theory, with some interesting possibilities with its causal network (spin-net) perspective on things. The biggest troubling element is the non-Lorentzian aspect of the Barbero-Immiziri parameter.

    Quantum gravity has three basic efforts underway. The first is of course string theory, and the second is Loop q-gravity. The third, which has lost popularity, is twistor theory of Penrose. I think the latter two might provide some constraints on string theory. This might avoid some of the ambiguities in string theory, and constrain the landscape problem.

    Lawrence B. Crowell

  6. Nexus says

    I am also wary of postulating a multitude of universes that we can’t ever observe, just to explain the one we can.

  7. Member

    My humbug detector on this concept is matched by an equal and opposite humbug detection on the concept of multiverses. So they really cancel each other out in my opinion.

  8. Astrofiend says

    # Fraser Cain Says:
    June 4th, 2009 at 8:51 pm

    “My humbug detector on this concept is matched by an equal and opposite humbug detection on the concept of multiverses. So they really cancel each other out in my opinion.”

    Agreed – I think that it is pushing the envelope pretty hard to even entertain the thought of having a firm opinion one way or the other on this. Either point of view could be feasible in reality; I don’t see Smolin’s arguments as entirely convincing, nor the arguments of the flipside view. It is great that we can begin to ask such questions in seriousness, but to expect to come close to answering them based on our current level of understanding and knowledge is a bridge too far.

    In reality, there either is or is not a bunch of universes out there in addition to our own. Unless somebody comes up with something very, very brilliant though, science is not currently in a position to address this question. Can science ever describe ‘ultimate’ reality? Maybe or maybe not – science and math have the wonderful habit of enabling us to describe observable reality. If extra universes are not observable, science can have no say about them one way or another, no matter how real they may be… It becomes an unanswerable question – unsatisfactory yes, but what the hell are we going to do about it?

  9. Pvt.Pantzov says

    i like his idea that physics laws can/may evolve with time.

  10. covariant says

    Since apparently no one here knows this, I feel obligated to point out that Smolin is considered a joke in the theoretical physics community. Literally no one takes what he says seriously. The guy is literally a crackpot who managed to get tenure somewhere (which, sadly, is by no means unheard of).

    All of his “ideas” are just strung together sciencey sounding words with no physical content. And this isn’t any different. I can’t even refute what he says because there’s simply no content to refute.

    Let me re-emphesize–he’s a joke. Literally. We actually laugh at him when he says these things. And then we die a little more inside.

  11. secondguess says

    the big bang theory states that eveything was compacted into a singularity, until the big bang. But what if there were other singularities similer to the one which started our universe, what then would we call our portion? I also wondered, if there were other big bangs, if they could be linked to the parallel universe theory

  12. secondguess says

    excuse the what if question, but i like to try and think about it alot

  13. Astrofiend says

    covariant Says:
    June 4th, 2009 at 10:22 pm

    “Since apparently no one here knows this, I feel obligated to point out that Smolin is considered a joke in the theoretical physics community.”

    Interesting. And what is your standing in the theoretical physics community?

  14. covariant says

    As you might’ve guessed by my use of the word “we”; theoretical physicist.

    I’m studying AdS/CFT, structure of gauge theories in general, and string theory in general. I looked into the stuff Smolin “studies” once–as an undergrad. And even then I was able to tell he was a crackpot.

    It’s a sad sign when guys like this get published (although comparing him with big-name or even medium-name theoretical physicists, he publishes little and has few citations). Part of that is due to journals matching referees to papers that have little experience in a subject, partly due to the odd incompetent referee, and partly due to journals trying to rush things off without adequate review.

    In the past 5 or 10 years, a few of the more general scientific journals have been slipping in quality for these reasons, and to try to appeal to a more “general” audience, or to try to generate controversy, since journals’ membership numbers are declining with more and more people using the arxiv (and with some scientists refusing to publish in some overly-priced journals or journals with other bad practices).

  15. Hon. Salacious B. Crumb says

    Hasn’t anyone heard of thought experiments?

    Ideas can be formulate to both create and destroy concept and confront postulates. So be it if it can advance the cause of the science behind its useful theories.

  16. Feenixx says

    I’m a little confused.
    Wasn’t it Smolin who originally proposed the (imo unsupportable) notion that Black Holes have different Universes “at the other end”? That our BH is somebody else’s Big Bang – provided this other Universe survives some kind of a process of Natural Selection?
    Didn’t he come up with Black Hole type entities which have no Event Horizon?
    … and different speeds of light for different wavelengths?

    Perhaps he’s putting material for a new bestselling book together, and this is the beginning of a viral marketing campaign. He might come up with something like Loop Quantum Gravity and sell it in book form as new and revolutionary.
    Fascinating and thought provoking ideas, but would they perhaps better be filed under “Philosophy”, rather than under “Physics”?

  17. damian says

    Well, your comments just blow the idea of walking with the Elves and dragons in Popular fantasy right out the window. 🙂

    Point is however, the multiverse (idiom) has been in human culture and storytelling for some time now.

    The reason the idea takes root is because its a familiar one.

    WTF: You say that Scientific integrity is slipping because Journals that make money from papers are seeking to increase their readership???

    Next stop; Fairies and elves. BTW< what was it with the Smurfs only having one girl??

    But im confusing alternate realities in fantasy with multiverse’s in string theory. Honest mistake,

    A joke in theoretical Physics? Like the fact that theorists cant seem to reconcile gravity into their math and end up postulating multiple dimensions and tiny strings we can never see, or dark matter compounded by dark energy and black holes that we cant see?

    Like i Mentioned, Dragons, Smurfs and Elves. One is a leap of imagination, the other is a leap of faith. One day, when we know better, I suppose we will have that laugh,
    I hope im still alive at that moment. I also wonder when we work it out whether it will foster that fundamental shift in human consciousness. This is what drives us towards these thought experiments no?

    Regards
    Damian K

  18. NilsO says

    Well, the Multiverse was invented in order to “explain” the curious fact that our observable universe is incredible fine-tuned to allow life to emerge and prosper. It would be highly improbable that this could happen in a one-time event, by chance.

    Therefore, of the 500 billion+ possible universes (of which all are presumed to exist, each possibly in infinite incarnations), at least one should be able to support life, and by default we inhabit this universe. Otherwise, we would not exist.

    The alternative explanation is probably even more unpalatable to scientists: That our Universe was designed by a Creator, where life was at least part og the great Plan.

    Thus, the scientists’ choice is between pest and cholera:
    * Either, an infinite Multiverse
    * Or, a single Universe designed by a Creator

  19. Vedic says

    I also can only agree with the single Universe consept.

    Bearing in mind that there are also only a finite number of Universes that *can* exist.

    There is one Universe of non infinite mass floating through time like a surfboard on the ocean.

    One might be able to jump off the board to be in ‘all times’ but one can only interact in the ever changing ‘here-and-now.’

    Good article many thanks

  20. Kipp White says

    Ok, I’m not a theoretical physicist, and I probably have an incomplete understanding of string theory, its many variants and others. But…. Do they actually require more than one universe to function? Or do they require more than 4 dimensions to function? Our observable universe seems to require 4 dimensions but does that mean that the other 7 (or how many others are required) don’t exist as part of this universe? I propose the possibility of one universe with as many dimensions as necessary to make all 4 forces coordinate and react as necessary. Why don’t we seem to experience these other dimensions? Maybe we do but we don’t recognize them as such. Maybe there’s some reason that we don’t. I guess the theoretical physicists well have to puzzle that out, if they decide they need to.

  21. Dark Gnat says

    I’ve never been a Multiverse fan. It might be a nice thought experiment, but to me it is no more provable or observable than God. (And in my opinion, multiverses are more far fetched than a Creator, but then again, maybe each Multiverse has it’s own Creator for all I know.)

    Unless multiverse theories can be tested, I tend to think of them as nothing more than mathematical experiments.

    I’ve never really thought of time as a form of energy, though. That’s an interesting concept. Without time, could *anything* happen?

  22. Anaconda says

    The frying pan calling the kettle black.

  23. covariant says

    Let me quickly address a few things mentioned:

    1. The “multiverse” idea from quantum mechanics has never been taken seriously by the physics community. It’s not functionally useful, and it’s not even really meaningful.

    This is distinct from the other “multiverse” idea that there are different regions of the universe with different physical laws that look like effectively different universes. This is a more reasonable thing to think, because we see “condensation” effects like this happen in condensed matter systems whose physical properties can exhibit the same effect.

    2. Smolin definitely did not come up with the idea black holes could lead to other places. That idea was probably developed before he was even born. It’s also not an idea that’s taken seriously, since any “connection” like that would be unstable (unstable in the sense of “can’t exist” not “hey that’s kind of hard to do”).

    3. His theory does, however predict different speeds of light depending on wavelength. But it does this in a way that’s totally insane, because it breaks Lorentz invariance (essentially) infinitely much. Which is bad, because it’s known to hold to something like 1 part in 10^30. So his theory basically slaughters relativity, and is thus contradicted by every experiment done in the past 100 years.

    4. Journals’ quality is decreasing because they are being less rigorous, and accepting more crap, in an effort to make money. Last time I checked, that was bad science.

    5. You don’t understand string theory, I swear to god. You’re not qualified to criticize it anymore than you are qualified to criticize theories or RNA transcription, or heavy element abundance production in supernovas, or any other very technical topic.

    6. Quantum mechanics’ birth was in roughly 1900 with the discovery of things like the photoelectric effect. It was not a “finished” theory until the 1980s, when the last bits of the standard model of particle physics were finished (which, btw, is known to be incomplete in several ways, and is not really “finished”). What we think of as string theory was really started in the mid 1970s to describe QCD. So, before you go criticizing us for not having much to show (which is wrong in a lot of ways anyway) give us another 50 or so years.

    6. Dark matter and dark energy have nothing to do with each other, and are totally independent of string theory.

    7. We don’t see the other dimensions because they are “small.” I.e., the same reason you don’t notice that your table has ~1mm size bumps on it.

    8. Time is not “a form of energy,” that’s just crazy. It’s just a parameter.

  24. Duncan Ivry says

    Down to earth with simple questions:
    (a) Is there any observational (I don’t expect: experimental) evidence for multiple universes?
    (b) Has someone proposed any physics experiment to decide wether there are many universes?
    (c) Has someone proposed any physics experiment to decide wether there exists an entity which (or who?) has created the universe?
    The answers are, as far as I know: (a) no, (b) no, (c) no.
    Somebody may correct me.

  25. Jon Hanford says

    I’d like to thank the several individuals above who are involved professionally with this topic to take the time to post (and thanks to the several intelligent posts by other individuals above , too!). This is the main reason I like this site, seeing actual commentary by people ‘on the inside’ of some of these topics. I certainly learned something. As far as Smolin, the Wiki entry does touch on some of the controversy his ideas have caused. Of particular note, there is a link at the Wiki page to a 35 minute NPR interview/debate between Lee Smolin and (string supporter) Brian Green that is most interesting. @ covariant, how long have you been in the field? I’ve been closely involved with professional astronomy since 1978, so I was just curious when you came into your field. Thanks again for your input.

  26. brainw0rm says

    I read a book over 5 years ago called “The Fabric of Reality”, where David Deutsch (a quantum computing theorist) postulated that the confirmed observance of “shadow photons” is a convincing argument for the existence of a multiverse.

    I am not a physicist at all, but the book was written for smart laypersons and I did find myself convinced. Here is one of Deutsch’s papers: http://xxx.lanl.gov/ftp/quant-ph/papers/0104/0104033.pdf

    I am not attempting to promote his theory, just noting that the reasoning in the book seems pretty sound. The arguments did not dismiss time as a fundamental concept.

    -Skye Manning

  27. brainw0rm says

    (edit: ….postulated that the observation of “shadow photons”…)

  28. covariant says

    “The answers are, as far as I know: (a) no, (b) no, (c) no.”

    This is correct. People have certainly thought about how to detect these kinds of things, but no one’s thought of a sensible way to even talk about something like this.

    “…a 35 minute NPR interview/debate between Lee Smolin and (string supporter) Brian Green that is most interesting”

    If this interview is the one I’m thinking of, I think listening to it is very telling of exactly how little physics Smolin actually knows. All of his answers were very evasive and confused, and, honestly, Brian Greene was way nicer than I would’ve been to him! His “descriptions” of the other “alternative” theories was telling, too, since basically all the theories he mentioned break just about all of the laws of physics.

    Loop quantum gravity really is a terrible theory, though… In addition to breaking many of the laws of physics, it doesn’t even include any particles! You have to add them all in ad hock and by hand. It’s whole strategy is to treat gravity–the weakest of all forces, by basically ignoring all the other forces (which is equivalent to not having particles in the theory). But you can’t just throw out large effects like that! It’s like calculating 10+1 = 1 by saying “well, I’d like to ignore the 10 because I don’t know what to do with it.” Crazy!

    And there are even more problems than that, too. I could go on for days!

    Strings, on the other hand, can actually solve real problems! AdS/CFT can (and are!) be used to do normal field theory calculations and this is of great interest to many people doing condensed matter physics who’d like to be able to solve hard problems easily like this.

    We can also use AdS/CFT to study models that look a lot like QCD (stuff with quarks). Though we don’t have an exact dual model to QCD yet. But it still gives us a great deal of insight into the behaviors that QCD can have that we couldn’t get other ways. We can even do real-time calculations in these strongly coupled QCD like theories that we don’t even begin to know how to do in real QCD!

    In fact, hydrodynamic calculations done in AdS/CFT are actually used to help us understand data from RHIC, I believe.

    Note that none of this has to do with “gravity” or anything. Strings tell us all about particles, in addition to gravity. That’s why they’re so exciting!

    And to answer your earlier question, I haven’t been doing string theory for long. Just a few years! I’m still fairly new and certainly have a lot to learn.

  29. Jon Hanford says

    The Lee Smolin/Brian Greene NPR debate is from 2006 and can be found here: http://www.npr.org/templates/story/story.php?storyId=5670911 . I mentioned it specifically to bring up the point of the weaknesses in Smolin’s theory and recent work in general. I appreciate covariant’s views on the current outlook for string theory in the future, and wish him well in future research.

  30. Lawrence B. Crowell says

    Fraser Cain’s statement, “My humbug detector on this concept is matched by an equal and opposite humbug detection on the concept of multiverses. So they really cancel each other out in my opinion.,” is interesting. The early universe was likely a superposition of states over different configuration variables for cosmologies. The universe evolved by adjusting its gauge theoretic content, or einselecting one particular configuration variable. This might be dual to the physics where the inflaton/Higgsian particle lands on a particular point on the landscape.

    The AdS/CFT presents a nice way in which particle physics and spacetime are joined in a conformal renormalization group flow. The running parameter of a CFT is the conformal group itself. The two dimensional case was worked by Zamolodchikov. Here the conformal group is PSL(2,C), which defines arcs on the Poincare 1/2 plane or disk. A field on this toy space is assumed to be defined by the Ladder operator of the Viraxoro algebra

    [L_m, L_m] = c_{m,n}L_{M+n}

    For a field W(z), holomorphic with W(z-bar) = 0, transforms under the Virasoro ladder operators . The central charge, determined by a kernel on the Witt algebra, decreases in a monotomic (log) manner by the renormalization group flow. This is the Zamolodchikov C-theorem. This should extend to RG flows on PSL(2,Q) Q = quaternion so the AdS seamlessly matches with particle physics.

    I am not quite as jaundiced against loop quantum gravity (LQG), though I am more of a string/M thinker. I do think the basic idea of Astekar is interesting and does match with general relativity better in some ways than string/M theory. It seems to me that LQG might be some sort of target map from a D3-brane with constraints not described in string/M-theory.

    Lawrence B. Crowell

  31. jimmNJ says

    So where are these other Universes? All around us? Behind the CMB? Over the rainbow? I’m sorry, I can’t accept any of these options. It’s like the medieval theologians debating how many Universes could fit on the head of a pin Check that — Angels. You can define as many dimensions as you like but the more sophisticated the math gets, the more it’s like “Alice’s Adventures in Wonderland”. You may call me derogatory names to assert your superiority but — come on, let’s get real.

  32. Torbjorn Larsson OM says

    Some odds and ends:

    @ secondguess:

    “call our portion”

    It is still the observable universe, placed within a larger bubble or pocket universe of ours. If the multiverse ever is tested good, people may want to change. But for now it is premature.

    I’m not sure what you mean by “the parallel universe theory”.

    @ HSBC:

    “thought experiments”

    But this is not a thought experiment which gives testable predictions. It is untestable metaphysics, not science.

    @ NilsO:

    “the Multiverse was invented in order to “explain” the curious fact that our observable universe is incredible fine-tuned”

    No, to my understanding it wasn’t, as covariance points out – it is, to my understanding (not a theoretical physicist) a natural ground state of such theories as string theory or chaotic inflation.

    That is, it takes specific constraints to confine them to one universe only. For example, finding that all parameters are forced to the values we find for some fundamental reason or other. [Using bayesian reasoning, what are the a priori odds for that? Slim to none. ;-)]

    I have no idea why you wave the religious argument of “fine-tuning” around.

    As it so happens, it is falsified.

    For example, Victor Stenger has modeled universes by varying the 4 parameters that specify the features of any standard cosmology universe: electron and proton mass, strength of EM and strong interactions. Varying them 10 orders of magnitude around present values he finds that half of the stars will still have lifetime over 1 Gy, enough for life.

    Thus fine-tuning is an invalid ‘prediction’. ‘Prediction’, since there never was any scientific basis for making one AFAIU. No wonder, it is woo-woo teleology, a method found leading nowhere and so tested invalid long since. Science use causality instead.

    @ Duncan:

    “an entity which (or who?) has created the universe?”

    Again with the religion.

    As it happens, there is a paper from -03 that shows that universes are zero energy entities (as expected from virtually flat curvature in the now standard cosmology). This is no mean feat, as general relativity has no single energy condition, but apparently it is doable.

    Now, as the paper points out, this means that if a universe spontaneously tunnel from an underlying eternal universe, which it can if it is zero energy, there can’t be a third entity acting in such a “creation” process. Which is bad news for those who believes in supernatural agents. But also answers your question: no such entity is required.

    Another means to see this is in chaotic inflation, where Linde points out that even if you must eventually run into a singularity following a worldline back (a derivation even I understand, it’s that simple :-o), the upper bound for those worldlines can be pushed unboundedly back in those cosmologies.

    So even before we knew that universes are zero energy and so formally eternal, physically with unboundedly old worldlines, the simple claim was to evade initial conditions by predicting that the multiverse always existed and is in steady state condition.

    As to propose tests, people have proposed the weak anthropic principle to arrive at a testable prediction for multiverses. (That is, all parameters that aren’t constrained by other things will be found lying close to the maximum of a likelihood distribution.)

    Unfortunately it is difficult to get to those distributions, as Smolin alludes to. AFAIU the latest attempt is Linde’s, derived from steady state conditions of multiverses, to evade the “youngness problem” of statistics one of Smolin’s references criticize on an earlier attempt.

    Oh, and one of those critics, Boussou, has his own attempt, the “causal entropic principle” or something such, but it has been criticized for treating entropy from gravitation differently AFAIU, a technical question beyond me.

    Lacking this there is one or two tests of the WAP anyway, in such cases where you can mod out the distribution by another. (The cosmological constant is one such parameter tested.) So far, it’s a go IIRC. Don’t hold your breath though.

    But as covariant says above, give them 80 years or so, who knows? “Don’t know” is a valid state of science, and definitely doesn’t mean “no”.

  33. Anaconda says

    Like I stated above: “The frying pan calling the kettle black.”

    Why?

    Because both sides have let mathematical constructs become unhinged from observation & measurement.

    A prior commenter summed it up well: How many angels can fit on the head of a pin?

  34. Lawrence B. Crowell says

    The question is whether we can understand the observable universe with a minimum of consideration given to so called other universes. The 11 dimensional bulk of M-theory may contain other cosmologies. Each of these are some Feynman path integral that describes a process which takes a vacuum configuration and transforms it to another. In Tegmark’s language of “bird vs. frogs,” in the bulk a super observer (the bird) observes only this vacuum to vacuum transition. From the perspective of the frogs, real observers, things do not appear in such a perfect manner. So from the frog’s perspective in some cosmology there are thermal distriubtions and the like, which correspond to big bang and the like.

    Each path integral contains amplitudes for other cosmologies. These are configuration variables for other spacetimes and field theoretic data on them. The inflationary period is then in part some process where one particular amplitude is einselected, or its value approaches unity, while the others become tiny. The frogs are observers that emerge in the resulting cosmology. They are not able to access data about other cosmologies (due to horizons etc), and so deduce what they can from measurements and observations they make locally. In this setting the frogs might be able to understand their universe with minimal reference to other universes that exist with some sort of classical content to them. A universe or cosmology with classical content is one where the quantum amplitude for its configuration variable is unity or nearly so.

    So quantum gravity will predict metric fluctuations which are then maybe tiny overlaps between the einselected cosmology, that with clossical content, and other tiny amplitudes for different configuration variables in the cosmic Feynman path integral from vacua |0)-i to |o)_f. Ideally in this perpective the frog can understand their cosmology as some einselection process which uniquely selects or einselects their observable cosmology from the others. Maybe if we become skilled at quantum gravity experiments we might be able to detect metric fluctuations and determine how they pertain to different configuration variables.

    The entire bulk might have many of these processes where vacua |0)-i transition to |o)_f, just as our more ordinary world has many particles. Yet, for us “frogs,” we might be able to understand our local universe with minimal reference to them and ideas about multi-universe ensembles that have some “real” content.

    I hope this helps a bit. In the string vs. loops argument the problem comes down to how string theory predicts maybe too much, and is really a framework more than a theory, and LQG predicts too little. the minimalism of LQG has some attractive features, such as one does not have to work with 10^{500} different landscape outcomes and the like, but as Covariant points out getting some basic particle physics to work in that picture is like getting a camel through the eye of a proverbial needle.

    Lawrence B. Crowell

  35. Torbjorn Larsson OM says

    “Smolin is considered a joke in the theoretical physics community”

    I suspected as much, especially after the last (?) thread on Cosmic Variance blog, where he got quite a drumming from a good physicist whose name at the moment slips my mind. And thanks all for the interview reference, it will be a good treat!

    But it is really enough with the references given in the Physics World article. Smolin’s main reference is an essay placed in an essay collection of philosophers like Bergson and physicists like Poincaré from an obscure publisher (Clinamen Press Ltd). For all we know it can be a vanity edition that Smolin has initiated.

    Smolin’s claim here is based on connecting difficulties to extract probability distributions for observables in multiverses with metaphysics on time. This is wrong on so many levels.

    First, you can’t use metaphysics in physics. (To see what Smolin is about it is illuminating to find this quote in his essay: “However my view is that this question is not one that can be settled by philosophical argument alone. Were that possible the problem would already be resolved.” That is, he is prepared to consider philosophy as physically valid at the outset.)

    Second, as covariant points out, time is just a parameter, and it is neither decided by distributions nor such a problem for these theories. Smolin compares apples and oranges.

    The basic error however is when he presents these ideas of time as an often used “assumption”. Going back to the essay, the total argument is actually the sole idea of Julian Barbour. The Wikipedia claims that it is a “controversial view”, and I find no reason to believe otherwise. Barbour’s claim seems to me to boil down to that one can mod out time in the same way that one can mod out a voltage bias. That doesn’t mean that time doesn’t exist any more than potentials doesn’t. At best it is untestable metaphysics, it can never be “an assumption”, and it looks to me totally naive and besides the point of using time in science.

    In summary, as for the physics, I also find “no content”.

  36. Torbjorn Larsson OM says

    Looking back I wasn’t clear enough. Smolin references a personal essay as presenting ‘an often used assumption’. Moreover, an essay isn’t even a peer reviewed publication. Thus he is using a typical crackpot tactic of bloviating on empty references.

  37. Torbjorn Larsson OM says

    “Loop quantum gravity really is a terrible theory, though… In addition to breaking many of the laws of physics, it doesn’t even include any particles! […] But you can’t just throw out large effects like that!”

    Interesting observation (well, I knew of the “by hand”, but I never thought of it as “throwing out effects” :-/), convincing and easy to follow.

    Another convincing and less easy argument I’ve seen, though still achievable for outsiders, is that they start with statics and never (this was – 06ish, I think) gets to dynamics. They can’t derive a harmonic oscillator!

    And when consistently they try, fail, and still claims it is correct because at least it looks like an attempt and they get ‘corresponding results’, it sort of make them seem like mathematicians painting physics pictures with crayons instead of the real deal. Their idol Einstein was a physicist, for relativities sake!

    [Sort of as opposed to string theory, which AFAIU instead starts with physicists and dynamics of the string, but has problems with the ‘statics’ of specific solutions.]

  38. Torbjorn Larsson OM says

    “in such cases where you can mod out the distribution by another.” D’oh! Actually I think they (Weinberg?) constrained the distribution by some other means, which was why it wasn’t a problem.

  39. covariant says

    “Another convincing and less easy argument I’ve seen, though still achievable for outsiders, is that they start with statics and never (this was – 06ish, I think) gets to dynamics. They can’t derive a harmonic oscillator!”

    Well, it’s worse than that. They can’t reproduce ANY known physics, of any kind. They can’t show their theory reduces to GR, they can’t show it reduces to real QM, they can’t show anything!

    String theory is designed to have GR and QM built in! Although, getting the correct spectrum of particles out of it is a bit difficult ;), this is due to how constraining string theory is–you can’t just put things in by hand in this theory!

  40. Lawrence B. Crowell says

    Loop quantum gravity is really a system of constraint conditions. The Wheeler-DeWitt equation HY[g] = 0 in spinorial form leads to LQG. The whole thing is simply a system of constraints. Period! Dynamics? Bah, some LQG guys, such as Rovelli, abolish the notion! When they do try to construct a four dimensional system for “time,” a four-spinor, they get a physically dubious construction necessarily real.

    String theory OTOH hand is a dynamical system. Early on a problem which made some GR guys wince is that the curvature is defined as an expansion around string modes against a flat background. The GR guys threw up their hand on this. Yet string/M theory appears to work well with AdS, where this problem is less difficult with conformal symmetry on flat space. So the “bimetric” problem is at least ameliorated.

    However, if we want to Wick rotate the AdS problem to dS, sinse AdS is really a sort of “fiction,” and we want to look at anisotropies and local quantum gravit issues, then we need a system of constraint. Might it be that LQG is precisely the sort of system we might consider?

    Lawrence B. Crowell

  41. Drunk Vegan says

    NilsO Says:
    June 5th, 2009 at 6:04 am

    “Thus, the scientists’ choice is between pest and cholera:
    * Either, an infinite Multiverse
    * Or, a single Universe designed by a Creator”

    ————————

    Who says?

    A religious person would have to admit that it’s the height of arrogance to suggest that there’s anything that a (all-powerful, anyway) Creator couldn’t do.

    So why wouldn’t a Creator make an infinte number of universes – even, perhaps, and infinite number that were ALL favorable to life? When you start including pixie dust in science, there’s no end to the possibilities.

  42. Jon Hanford says

    Thanks, covariant and LBC for those last two posts, as it helped me sum up current thinking on LQG and string theory in a cogent manner.

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