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Opinions or facts about M theory

  1. Jan 11, 2013 #1
    Hi, so I just wanted some opinions on string theory, superstring theory, or M theory. Any of them would be fine probably, but M theory is the one I'm really curious about (since it's the latest and greatest, according to some of the books I've read). So the only opinions on M theory that I've heard are those of Michio Kaku and Brian Greene, but I wanted the opinion of people who are not as biased as them (whose lives' work is -besides popularization of science- string theory). I just don't see what's so special about it, and I'm not sure if it fits into the category of science. Because as Feynman said, science can be described with one sentence; "the test of all knowledge is experiment". As far as I can tell though, M theory hasn't been tested (forgive me if I'm wrong) and will be extremely difficult to test in the future. I've heard that its merits are that the mathematics are very beautiful and that it unifies all forces into one theory. It would be nice to have one theory, but (sorry to quote Feynman again) "we're exploring; we're trying to find out as much as we can about the world. People say to me: 'are you looking for the ultimate laws of physics?' No, I am not. I'm just looking to find out more about the world. And if it turns out there is a simple, ultimate law that explains everything, so be it, that would be very nice to discover. If it turns out it's like an onion with millions of layers and we're just sick and tired of looking at the layers, then that's the way it is. But whatever way it comes out, it's nature, it's there, and she's gonna come out the way she is. And therefore, when we go to investigate it, we shouldn't pre-decide what it is we're trying to do except to find out more about it!". Sorry for the long quote, I just love that guy and I think his wisdom in that quote really applies to what (I think) people are like with string theory (or M theory or whichever version you wish). I think that people are trying for some reason to decide that nature must be this way, nature must be symmetrical, and nature must have simple laws. Well, what if it's not like that?! I would really love the opinions of physicists, I'm just a teenager trying not to take everything that a few popular scientists say for granted. I don't know any of the math involved in the theory or anything, I just want to discuss its possible validity, its applicability to other problems in physics (so essentially the usefulness of it), and also my discomfort with the fact that people seem to be determined that what they think nature is should be right even if experiment does not say so yet.
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  3. Jan 12, 2013 #2
    M-theory is the high point so far of a very logical chain of investigations which also happens to resemble reality qualitatively but not, so far, quantitatively.

    The beginning was an alternative philosophy in the 1960s study of nuclear physics, which tried to account for all the hundreds of different short-lived nuclear particles by regarding them all as equally fundamental, rather than building them from quarks. String theory emerged from the attempt to make one of these "democratic" theories, and (as you would read in Greene and Kaku) supersymmetry and extra dimensions came as answers to internal consistency problems of string theory. Then in the mid-1980s people discovered how to make something that looks like the real world in string theory (heterotic strings with six of the dimensions as a small Calabi-Yau). Then in the 1990s the different flavors of string theory were found to be different limits of a membrane theory, M-theory.

    Meanwhile people kept writing down new ways to reduce string theory to four dimensions, so now we have all these models that are sort of in the right direction, but still not completely matching reality. Converging on reality is a slow hard process and we are still doing it, but if and when there is a calculable string model that matches all the measured numbers, it will be extremely predictive, because it will predict the next decimal place, and then the next... of all the fundamental quantities like particle masses. But first the models and the methods of calculation must be refined to the point that they do match reality.

    It is science, but it's unusually slow and hard even for theoretical physics, where it sometimes take decades to test a theory or to fix its mathematical problems. The very concept of a "string theory" took years to emerge, as people tried to figure out how to make it work. Then they had to start looking through all the possible string models. Meanwhile the mathematics proved to be deeper than anything seen in physics before, and string models very naturally produced all the ingredients of reality - gravity, the other sorts of forces, particles like electrons and quarks - just not yet in the precise arrangement that we see.

    So string theory eminently deserves the level of attention it has received. But getting to the payoff is a multi-decade process with many twists and turns, like one of those proofs in mathematics that takes decades to unfold. We still don't know if the mainstream approach to getting reality from string is correct, or maybe some new direction is required - there are many "deviant" possibilities. It is also still logically possible that no form of string theory has anything to do with reality; but the odds of that are very low.
  4. Jan 12, 2013 #3
    Wow, thank you for your answer, you've changed my opinion quite a lot. I'll probably want to look into this more in the future (when I'm more advanced in my education)... Do you know what math is used in string theory? And what other prerequisites there are?
  5. Jan 12, 2013 #4
    All of it?

    String theory is really an outgrowth of quantum field theory (QFT), which is already the standard language of particle physics. If you can understand quantum field theory, then M-theory is not that big a leap, you just need to add geometric ideas of dynamical space (as originated in Einstein's theory of gravity) to the QFT framework.

    It's possible, even likely, that this familiar form of M-theory will eventually be proven equivalent to some completely new mathematical framework. If and when that happens, there will be a new foundation for physics, and a different math to learn (though it would have to reduce to the familiar math when necessary).

    But for now your main challenge is quantum field theory - and that is already going to give you plenty of conceptual problems. A sketch of how it works can be seen in a famous little book by Richard Feynman called "QED". If you grasp the picture in that book, then string theory (in its simplest form) is quite similar. But your main problems going forward will be making sense of quantum mechanics (about which so many sensational and contradictory things are said), and then making sense of quantum mechanics applied to fields (how particles come from fields, what a virtual particle is, etc).
  6. Jan 12, 2013 #5
    OK, that sounds like a long way ahead of where I am right now... And I love QED! I've read it and understood it (at least I think so). But I think I've got a couple of years to go at least until I arrive at string theory...
  7. Jan 15, 2013 #6

    So far it's theory...theoretical, as you posted, unproven...but has some fascinating insights....

    Kaku and Greene do a good general job of explaining string theory.

    Perhaps the most dynamic explanations and extensions of their string theory discussions can be found in Leonard Susskinds THE BLACK HOLE WAR, "My battle with Stephen Hawking to make the world safe for quantum mechanics." ...very little math

    An abbreviated summary can be found here:


    check it out and if you like it, consider Susskind's book....cheap, used, online.

    I found his views, insights and explanations VERY interesting....

    PS: Susskind was right it seems, Hawking not so much!
  8. Jan 15, 2013 #7


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    AFAICT string theory is a research program that, after 30 years, hasn't worked out. However, many people have invested their careers in it and don't want to admit that it hasn't worked out. The apparent nonexistence of supersymmetry at the electroweak scale doesn't disprove string theory, but it makes it much less attractive. There are other candidates for a theory of quantum gravity, such as loop quantum gravity. You can find some good generic discussion of the pros and cons of string theory in the WP article: http://en.wikipedia.org/wiki/String_theory
  9. Jan 15, 2013 #8
    I don't think that's at all a fair characterization of why many people are sticking around with string theory. A significant number of researchers seem to have begun looking at it as essentially a mathematical pursuit which has shown us some new ways of thinking about QFT (without making any assumptions either way about whether or not strings of any variety actually exist). To quote from Mikhail Shifman's address at last year's Frontiers Beyond the Standard Model conference:

    The whole speech is worth reading. Of course, I can't comment about every string theorist everywhere—and I'm sure there are still those convinced that it is phenomenologically true and pursue it solely for that reason—but my experience in talking to string theorists at my university about their motivations is very much in line with Shifman's analysis. You only need to look at how mathematical structures originating in string theory like holographic duality have been applied with some success to problems in condensed matter physics—where there's certainly no assumptions about the ontology of strings—to see its value as a research program. Maybe deep down most string theorists still hope it will lead to a ToE, but I think your cynical explanation for their continued involvement with the field is unfair and not in line with reality.
    Last edited: Jan 15, 2013
  10. Jan 15, 2013 #9


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    LQG has not been shown to recover classical GR. String theory has. In AdS/CFT we have plausibly our only working example of fully defined quantum gravity - I say plausibly because although AdS/CFT has passed many tests, it has not been rigourously proved. String theory may or may not model our universe, but its preeminence among approaches to quantum gravity is well justified. Furthermore, I believe it has lessons for LQG. In AdS/CFT, classical GR is recovered in the large N limit, while a form of the conjecture is believed to hold even for small N. In the small N regime, the bulk geometry seems to be coarse and "quantum-like". A tensor network picture has been proposed whose formal elements are very similar to the spin networks of LQG, as pointed out by Brian Swingle, building on work from Guifre Vidal's group. Thus AdS/CFT has a regime in which it has many of the problems of LQG, and a regime in which those problems are solved, and a parameterization that moves one between those regimes. The recent paper by Bianchi and Myers indicates that LQG researchers are looking at the links between LQG and AdS/CFT.

    Singh, Pfeifer, Vidal Tensor network decompositions in the presence of a global symmetry
    Swingle Constructing holographic spacetimes using entanglement renormalization
    Bianchi, Myers On the Architecture of Spacetime Geometry
    Last edited: Jan 15, 2013
  11. Jan 15, 2013 #10


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    That's a funny interpretation, Atyy. The senior author is Bob Myers, whose main research has been in string. Now he is crossing over and working with a young LQG guy. Moreover the AdS/CFT angle is only one aspect of their paper, it gets a brief mention.

    I think Bob Myers is first rate! and I am glad to see him recognizing Bianchi's research on entanglement entropy as interesting (Bianchi already has several papers on this general topic that have gotten a lot of attention, at least by LQG standards).

    The fact is that LQG is increasingly attracting the attention of people outside the Loop community. The other paper, by Giuffre Vidal et al is also by outsiders showing this growing interest. In their abstract they say: "On the other hand, the resulting tensor network can be interpreted as a superposition of exponentially many spin networks. Spin networks are used extensively in loop quantum gravity, where they represent states of quantum geometry. Our work highlights their importance also in the context of tensor network algorithms, thus setting the stage for cross-fertilization between these two areas of research."

    I think it's nice to see the lines of demarcation breaking down, and it is on the part of people like Bob Myers who have never written a LQG paper in their lives. :biggrin:

    Another recent example was Brendt Müller http://arxiv.org/abs/1212.1930 , an outstanding non-LQG guy.

    This may be for various reasons. I suspect a large part is driven by the fact that LQC recovers classical FRW cosmology, and cosmology is almost sure to be the main testing ground for theories of quantum geometry+matter. But there could be other reasons.

    EDIT another example just came up today! Two NCG people Marcolli and van Suijlekom have constructed a "noncommutative" generalization of the LQG Hilbert space. This looks very interesting. Instead of an orthonormal basis of spin networks (as in usual LQG) they have an orthonormal basis of "gauge networks", a generalization of spin networks.

    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom
    (Submitted on 15 Jan 2013)
    We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. The configuration space of quiver representations (modulo equivalence) in the category of finite spectral triples is studied; gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences...

    The idea is instead of a network or assemblage of quanta or chunks of ordinary geometry space, they are dealing with quanta/chunks of noncommutative or spectral-triple-type space. This can include ordinary as a special case, and also the ALMOST ordinary type of geometry---socalled "almost commutative" construction. that was what Connes and co-workers used to realize the Standard Model of particle physics.

    So there seems considerable potential in what Marcolli van Suijlekom are doing. Reformulating LQG but where the vertices of the spin network are chunks of a different kind of space that might be able to give rise to matter.

    And it is yet another crossover by non-Loop people into LQG-land.
    Last edited: Jan 16, 2013
  12. Jan 16, 2013 #11


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    That should have been "who had never". :biggrin:
  13. Jan 16, 2013 #12

    Physics Monkey

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    Just a comment, Myers goes by Rob not Bob.
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