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Problem for String Theory?

  1. Sep 16, 2004 #1
    I've been trying to search for this on the Internet, but apparently there are not many links. I was wondering what the paper by Kachru about the 10^100 string vacua suggested really? Does it mean that there are 10^100 different formulations of string theory (formulations just like the original ones such as E8 X E8 or Type IIA)? If this is true, has there been any proposed mechanism to reduce this incredibly large number or rule it out? Just wondering what it meant, because if there are really such a fantasic number of different formulations then wouldn't it seriously pose problems for the theory?
  2. jcsd
  3. Sep 16, 2004 #2


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    a good place to start is with the Edge magazine interview with Leonard Susskind and the discussion Susskind had with Smolin, Steinhardt and others about the "Landscape"

    An often-used word for the great number of different vacua is the "String Theory Landscape"
    You can find more technical articles by searching at arxiv just with that as a keyword----looking for occurrences of "Landscape" in the title or abstract.

    the "Anthropic Principle" has been proposed by Leonard Susskind and others as a way out of the Landscape---to narrow things down to just the possibilities one wants to consider (e.g. with a reasonable size positive cosmological constant as astronomers observe, and so on)

    So one can also find articles, discussion, internet posts, if one searches for "Anthropic Principle"

    Another name that has been used for the 10100 different versions is the "Discretuum". This is by analogy with the familiar word "Continuum". It is like a continuum except it is not a continuous range of variation but all discrete. Tom Banks (another eminent figure like Susskind)
    has written about the discretuum.

    Mike Douglas, another prominent String theoretician, has devoted much of his research in the past year and a half to actually counting the vacua and analysing the Statistics of the String Theory Landscape.

    Depending on how one counts it one can get different estimates, like
    10150 instead of 10100, and Douglas has made a specialty of rigorously counting. So he has papers about that. For example
    "Statistics of String Vacua"

    searching arxiv for author = Douglas, or Susskind, or Banks will get authoritative (senior String people) articles.

    But the most accessible discussion is still I think the Edge interview and the following conversation, especially where they talk about the Anthropic Principle

    You ask a very good question when you say:"... has there been any proposed mechanism to reduce this incredibly large number or rule it out?"

    And the answer is, yes, the mechanism proposed is the Anthropic Principle.
    Last edited: Sep 16, 2004
  4. Sep 16, 2004 #3
    Could this be another way of saying that some principle of information and complexity may govern which solution of the "landscape" is chosen by reality? Or is it our particular state of complexity that is assumed to govern the choice? Thanks
  5. Sep 16, 2004 #4
    I think the consensus is that Reality doesn't "choose" which vacua will exist- but that all of the possible vacua already exist in a multiverse set-up-and that ours is just the one we find ourselves in- the landscape is all real like the landscape of the earth is all real- you can live in the jungle- but that doesn't mean that deserts and oceans don't exist- they are just elsewhere- [which is why the Anthropic principle is irrelevent: all possible worlds exist]
  6. Sep 16, 2004 #5
    Could M-Theory describe this landscape or is no longer a useful tool? In these other universes is matter still composed of strings? I think the title of 'elegant' does not really apply anymore to the theory.

    BTW...thanks marcus for the article, it has been very helpful.
  7. Sep 16, 2004 #6


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    this begs the question of whether string theorizing leads to something empirically connected with reality. if the (as yet untestable) edifice of string theory and its landscape are mathematical fantasy, then "reality" will be unable to connect with them enough to "choose".

    a scientific theory bets its life on the outcomes of future experiments---if it is not falsifiable in this sense then it is not meaningful and doesn't constitute science. I believe it has been shown that a certain version of the Anthropic Principle is not falsifiable----any "theory" which would depend on an appeal to such a form of anthropism would consequently not be considered by conventional standards to be part of science.
  8. Sep 16, 2004 #7


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    happily enough, there is an empirically testable form of the "landscape" idea which, however, has nothing to do with string theory
    this is the CNS (cosmological natural selection) model proposed by Smolin in 1995
    it makes certain predictions about, for instance, masses of neutron stars, which allow it to be refuted by future observation (if the observations turn out contrary to prediction). So it is a falsifiable model, offering a possible explanation for the values of key physical constants, and is legitimate science.

    this model has a huge (perhaps infinite, I dont know if that) variety of alternative universes which really exist and which have slightly different fundamental constants (like alpha, Lambda etc) from our own

    but the model does not invoke the Anthropic Principle and it does manage to be explanatory, predictive and testable by available means.
    Smolin introduced the term "landscape" in the sense of "evolutionary landscape" into Cosmology around 1995 with this theory.
    The Landscape idea was taken into String discourse in 2003 after the Kachru paper about the multitude of vacua.

    Scientific Alternatives to the Anthropic Principle

    ---from the abstract---
    ( Contribution to "Universe or Multiverse", ed. by Bernard Carr et. al., to be published by Cambridge University Press.)

    It is explained in detail why the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science. Cases which have been claimed as successful predictions from the AP are shown to be not that. Either they are uncontroversial applications of selection principles in one universe (as in Dicke's argument), or the predictions made do not actually logically depend on any assumption about life or intelligence, but instead depend only on arguments from observed facts (as in the case of arguments by Hoyle and Weinberg). The Principle of Mediocrity is also examined and shown to be unreliable, as arguments for factually true conclusions can easily be modified to lead to false conclusions by reasonable changes in the specification of the ensemble in which we are assumed to be typical.
    We show however that it is still possible to make falsifiable predictions from theories of multiverses, if the ensemble predicted has certain properties specified here. An example of such a falsifiable multiverse theory is cosmological natural selection. It is reviewed here and it is argued that the theory remains unfalsified. But it is very vulnerable to falsification by current observations, which shows that it is a scientific theory.
    The consequences for recent discussions of the AP in the context of string theory are discussed.
    ---end quote---
    Last edited: Sep 16, 2004
  9. Sep 16, 2004 #8
    one thing about the Smolin natural selection through black hole reproduction thingy that I haven't seen fully explained: [I haven't read "THe Life of the COsmos" which probably answers this] when we are talking about new universes being born from black holes- what is the mechanism conjectured? is it some kind of pinching of the spacetime manifold that creates a new bubble universe that extends into large extra dimensions of some kind? or does a new universe [or bundle of universes] "virtually" emerge as a result of the natural quantum computation of the event horizon which chaotically computes wavefunctions to the limit of Bekenstein's bound and thus generates [or "projects"?] an ensemble of quantum universes [I hope that isn't stupid :shy: ]- or something else?
    Last edited: Sep 16, 2004
  10. Sep 16, 2004 #9


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    this is a great question
    in L. Modesto's paper which gets rid of the BH singularity the spacetime just continues, I want to get back to this, have to go out now
    there appears to be no dimensional problem! the new universe (starting as bubble and expanding) does not interfere with ours! have to think a lot more about this
  11. Sep 16, 2004 #10
    I think it is important that we can distinquish this difference Marcus, although you might poofoo this, it takees us directly to the question onhow LQG and STrings sees itself.

    I often like to compare this to the ideas of Platonic figuing and Pythagorean physics( that ole music thing we like to talk about in amplitude consideration? :smile:

    These traits are distinctive as much as background independance(BI) versus back ground dependance(BD)

    The vast perceptional attributes given geometrical model here has to have been able to arise from some place and is limited, if the discription of this modelling cannot expalin where it arose from?

    So in having you point back to Susskind and the EDge talk you have revitalize the points about how each sector is manifesting itself in term of abstractions in relation to Topos and String Topology respectively. They are each assign to the ideas of this modelling and based on the subject of dicretium and continuity.

    Do me proud and say okay? :grumpy:
  12. Sep 16, 2004 #11


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    we need to get a link to Leonardo Modesto's paper and put it together with what is described in Susskind's "Scientific Alternatives..." and the usual LQG picture of the BigBang bounce

    one thing Susskind says in "Scientific Alternatives" is that directions down the hole are forward in time and that whatever emerges from the BH ex-singularity is in the future. The baby universe is in the future direction from ours so it cannot have a causal effect on us---it cannot make anything happen "back" on us.

    this is a good thing about black holes, it occurs to me

    geometrically there is no "bumping" or crowding problem---the continuum just continues onwards at the bottom of the hole. (should we not say "bottom" but core or maybe even the "navel" of the hole?) anyway whatever you call that spot, it is where the bounce/continuation happens.
    we only get worried about this when we start imagining our universe embedded in some absolute space----if ours has a little bud and it starts to swell won't that hurt us? will there be enough room for us and it?
    but there isnt any room in which cosmology happens, there is only this one 4D (possibly topologically complicated) continuum.

    forgive the imprecision. this is not as clear as it should be and I think there is an idea of "quantum continuum" a sort of relational matrix that should take the place of the familiar differentiable manifold continuum we got from Riemann around 1850 and is such an enduring mental fixture.

    very tentative on my part. but the question is crucial and requires whatever response I can muster :smile:
  13. Sep 16, 2004 #12
    Actually, when thinking about it, doesn't it easily solve the question of whether our Universe seems so finely-tuned? That is actually a good part of this landscape. Also, a little doubt, if all these universes sprang out of eternal inflation, does it mean that new universes are being created constantly?
  14. Sep 16, 2004 #13


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    The reason the Smolin evolutionary landscape explains the finetuning is that it relates it to reproductive success, by budding. The evolutionary model is that features of the universe will prevail if they favor long cosmic lifespan (no early collapse) and production of lots of stars eventually turning into black holes.
    The CNS (cosmic natural selection) scenario is NOT "eternal inflation" which as usually understood has no evolutionary progress---values for the physics constants are random in each "bubble"

    however in CNS new universes are constantly emerging---with their own periods of rapid inflation---from the black holes of previous universes, so it has some of the same visual character as the "eternal inflation" scenario.

    the main logical difference is that CNS makes predictions about what will be found when one measures the masses of neutron stars----it can be proven wrong---and therefore it has explanatory power: if it is right then (as you say) CNS can explain the apparently selected values of certain parameters like the fine structure constant and other main constants of the standard models of physics and cosmology.

    might be worth your while reading
    Scientific Alternatives to the Anthropic Principle

    (it is not a lot of formulas, it's based on reasoning rather than technicality, and I believe it is an important development going beyond stuff like
    "eternal inflation" and "bubbles" and the Anthropic Principle)
    Last edited: Sep 16, 2004
  15. Sep 16, 2004 #14
    .......rambling.........I always wonder what happened with a space that gets very small what kind of heat it could generate

    :cry:......Schwarzchild, where are you...... :cry:
    Last edited: Sep 16, 2004
  16. Sep 16, 2004 #15


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    it has been studied in the LQG context and found to lead to a bounce--a continuation (in which inflation is generic) "out the other end" so to speak

    here is Modesto's paper
    Disappearance of the Black Hole Singularity in Quantum Gravity

    one of many papers about the bounce in quantum cosmology
    Genericity of Big Bounce in Isotropic Loop Quantum Gravity
  17. Sep 16, 2004 #16
    okay......where was that you got the bubble?
  18. Sep 17, 2004 #17
    Question: it points out in the article that the new solutions found to string theory would be non-supersymmetric ones, does this then mean that if supersymmetry is discovered it would automatically eliminate the other solutions?
  19. Sep 17, 2004 #18
    I have read some useful information on the Internet and string/M-theory seems even more complicated and confusing now! These are the questions that I can't really understand:

    (a) string theory (as I understand it) had always predicted a very large ensemble of universes, i.e. the Multiverse, and all these different universes had different physical constants, different elementary particles... How come though, it seems to be that only recently the idea of landscape has been introduced since the Kachru paper suggesting that there are such a large number of solutions to the theory? Wasn't it always a prediction of string theory that there were so many universes? Why only now does it seem as such a horror to string theorists?

    (b) Would all these universes be born out of a single Big Bang followed by an epoch of inflation (leading to eternal inflation thus creating all these new universes) or is each universe created by its own Big Bang in different regions of space with its own inflationary period?

    (c) If a full non-perturbative description of M-theory is realized, would it mean that it would account for these other universes or would it only describe just a few tiny corners of the landscape? That is, would string theory still apply to other universes or would another theory have to do this?

    Just a few doubts!!!!
  20. Sep 17, 2004 #19


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    these seem like really good questions to be asking, Curious6.
    I'm personally not familiar enough with String Thought to reply effectively but will wait to see what the others say.

    I had not heard that it had always predicted a very large ensemble of universes and if you have an online source for that I would be glad to have it!

    I had been informed (not first hand!, but I thought reliably enough) that Witten and an important faction with him had not accepted the Anthropic Principle and the Multiverse theory that goes along with it. Their attitude was one of reluctance---let's not appeal to this unless we absolutely have to. I had no inkling that Multiverse was established String doctrine prior to January 2003. So I will be especially interested to hear about it, if it was.

    Just to clarify a point: the Inflation Scenarios of folks like Alan Guth and Andrei Linde are a different matter. Those scenarios exist independently of String. In that context a Multiverse was invented to explain how a scalar field with just the right properties to cause just the right inflation could have arisen, because they calculated that such a field was extremely unlikely. so they postulated an infinite chaotic universe of quantum fluctuation where even extremely unlikely events eventually occur.

    (this multiverse premise has been made unnecessary in LQG because in that context it has been shown that an early episode of inflation is inevitable or as some recent authors say "generic" to the LQG big bang
    Date/Hossain "Genericity of Inflation in LQG")

    one can still imagine an "eternal inflation" or "chaotic inflation" Multiverse, if one so desires, but the original logical need that caused people like Andrei Linde to postulate such a thing has now been remedied in a somewhat simpler fashion.

    but that is inflationary cosmology! With String I thought the hope had always been to say why the numbers in the standard model and cosmology are what they are, by constructing models which explain why they must be what they are and moreover make testable predictions.
    this is what one customarily has expected of physical theories from Newton on down. So I thought the hope was that String thinking would eventually explain, for instance, what causes the cosmological constant to be what it is observed to be. I had not heard, as I mentioned earlier, of Multiverse tendencies in String prior to the Kachru paper. Obviously there's a lot I just dont know! so maybe you, or some other knowledgeable PF person can provide a source.
  21. Sep 17, 2004 #20
    marcus, this is an article by BBC which states how scientists came up with parallel universes idea in the context of M-theory:


    Also, in the Edge article you posted, Smolin seems to suggest that a multiverse was a characteristic of M-theory theorists seemed to turn their back to and ignored.

    By the way, I mentioned eternal inflation because in the Edge article Susskind suggests there is a link between eternal inflation and this huge number of string vacua (which supposedly have arisen from this period of inflation). However, a problem becomes apparent here: if there are about 10^500 string vacua or even more (even though this is a very large number it is still finite) as string theory suggests it conflicts with the view that inflation is eternal, as in the scenario suggested by some physicisits (leading thus eventually to more than the string vacua predicted by the theory and continuously increasing).
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