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Ontological status of extra dimensions

  1. Nov 16, 2008 #1
    M-theory, the latest version of string theory, posits 11 dimensions (ten space dimensions, one time dimension). Earlier versions posited ten or fewer dimensions.

    Brian Greene and other string theory proponents envision the first three "extra" space dimensions (beyond the three space dimensions we know well) as akin to a tiny sphere affixed to each locus of our normal three dimensions. These tiny spheres, with their additional three dimensions, are not as yet detected b/c they are so tiny. At our scale, and even at the scale of the most powerful microscopes we have today, the additional dimensions are undetectable because they are "folded" back on themselves, contrary to the "unfolded" dimensions we normally experience. Thus space seems smoothly three-dimensional for us. Even Greene admits imagination fails him in trying to describe what it really means to have additional dimensions beyond these six space dimensions.

    I believe that physical theories should ultimately attempt to describe physical reality: what is really real? Accordingly, what on earth does it mean to have ten space dimensions? It seems likely that such postulates are merely mathematical artifacts not indicative of physical reality.

    For example, envision an electron (or whatever particle you prefer) at location x, y, z. With seven additional space dimensions, it's full physical location would be described as x, y, z, a, b, c, d, e, f, g. And t for time. This means that for every particle we can pinpoint at a given locus x, y, z, there are actually seven possible "real" locations for that particle. In other words, every particle we locate in 3-space has seven versions. What forms the boundaries between the location of these particles? If there are indeed distinct dimensions beyond the three we know of, how do particles in these additional dimensions interact? I know Greene's imaginary spheres are just that - imaginary - but they are meant to suggest some physical reality of the extra dimensions. It seems to me that mathematical physics has gotten ahead of the need for empirical realism.

    This is one reason why I think people like Lee Smolin and John Moffat are probably correct in generally rejected the validity of string theory.
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  3. Nov 17, 2008 #2


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    Hello Tam,

    I don't disagree with the general tenor of your post. Welcome by the way, I see you are just on your 8th post.

    My feeling is that Stringy Thought (:smile:) doesn't need to be critiqued much anymore because the string bubble has started to collapse. There may be people who are sensitive bout this and react defensively, but for a lot of people it just isn't interesting any more.

    All we need to do is keep some objective information in view for the sake of people who are slow to catch on and still imagine studying it in grad school. But if they can't figure out for themselves what's going on that's basically their own lookout.

    I do like to watch the string indices, like popular booksales, research paper publication and citation rates, because they are objective reality checks. I don't want to risk fooling myself about the rate changes are occurring.

    But mainly for me the interesting question is not about string at all, it is where do we go from here?

    Wilczek's new book (Lightness of Being) is an important part of my perspective on the future. It's totally non-string of course. You might get something out of it. He's a Nobel particle theorist very much geared to what will show up at LHC.

    But in my view the exciting questions are not so much how the standard PARTICLE model will evolve but what new model of the CONTINUUM is emerging. A new model continuum will IMHO automatically change the picture of matter as soon as the standard particle model is reconstructed on the new version of spacetime. It probably won't be a (smooth) differential manifold with the same dimensionlity at all scales. Could be 4D at large scale with dimensionality declining towards 2D as you zoom in to micro scale. I.e. more like a fractal or foam at small scale. EXTRA dimensions do not seem likely to me at all. Just my personal view.

    You obviously have been thinking about new continuum models too, since you mention Moffat and Smolin. Both of them have explored new territory. (Smolin particularly has started or helped start a lot of very interesting ideas.) Neither of the two you mentioned is exactly representative of the main nonstring quantum gravity approaches. I could try to sketch out a map of the main nonstring QG approaches and current activity, if you want.

    The main groups of researchers are around people like Ashtekar and Rovelli, or younger leaders like Bojowald and Freidel. There is also very interesting work appearing around the approaches of Renate Loll and of Martin Reuter (but neither Loll nor Reuter papers get as many citations as those of the others I mentioned---people know Loll and Reuter stuff is interesting and they invite them to give talks at conferences and workshops, but as yet they don't have a large following. By contrast, scores of young postdoc researchers have joined the efforts headed by Ashtekar and Rovelli. And not only postdocs but others as well.)

    Some of the most active lines of research are things Smolin helped start---provided critical insights to get them going---but where he has moved on and is no longer himself active. It seems to me that he scouts things out where other people wouldn't necessarily think of going. then when they get there he as moved on. That is just a superficial thumbnail or cartoon sketch.

    Anyway, what we would both like to know, I assume, is where things are going. I could try to tell you what I think, how I would sum it up. Or you could read the recent papers of these people yourself (Ashtekar...Loll...Reuter....) and draw your own conclusions.
    Last edited: Nov 17, 2008
  4. Nov 17, 2008 #3
    Thanks Marcus, I'm not a professional physicist (more of a philosopher of science, etc.) so I will defer to you re citiations and thought leaders in physics. And I'm glad to hear your conclusions b/c they mirror what I would hope is indeed the current direction of physics.

    I have read Wilczek's book, but not much of his other work (it's on my list). I liked his excitement and eagerness to obtain LHC results, but he seems to mix a lot of ontological categories also - a problem that seems to be very common even for the must successful physicists. For example, he talks about gluons and quarks as the origin of (most) matter, because they are the constituents of neutrons and protons. And he talks about gluons and quarks being simply "ideas," that have never been directly observed, and thus supportive of Wheeler's "it from bit" notion. I'm very torn about It from Bit, at least insofar as Wheeler sketched it out, but I was pleased to see Wilczek revive the "ether," though in a different form than its 19th Century predecessor, of course. Wilczek describes his notion of the ether as a mix of virtual particles that form the basis for all matter/energy. I'm also intrigued by Reg Cahill's gebit network, which he also describes as a modern ether concept. Even Einstein admitted later in his career that some notion of an ether was compatible with - perhaps even required by - his general relativity theory.

    I look forward to the LHC results, but Moffat makes a good case in his book that the graviton, the last undetected boson in the Standard Model, will never be detected because it would require a collider the size of our galaxy to get the energy levels necessary. I certainly believe in the dictum to "never say never," but it does seem unlikely that gravity would be mediated by a particle (or even a field, as the Standard Model quantum field theory more accurately suggests).

    Anyway, a lot of thoughts to ponder. Thanks for your support re my skepticism of the notion of extra dimensions.
  5. Nov 17, 2008 #4


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    I'm glad to meet another person who has read Wilczek's book. Quite apart from the ontolological issues---one could say an ontological innocence that may be precisely what a creative physical intuition needs on occasion, to see existence with new eyes.

    Apart from those issues, I see the book as a possible turning point in public conceptions and expectations regarding fundamental physics. Three or four years ago teenagers would come here to PF having read Brian Greene and they would just assume that the ultimate reality of nature had something to do with strings. There would ensue a lot of bubbling enthusiasm and breathless questioning that simply took a Greene-ish picture for granted. I sense a change.

    The Wilczek is a book that appeals to the same appetite for imagining the fundamental texture of existence, but is somehow more honest, more realistic, closer to experimental reality. It scarcely mentions string at all. So the book's degree of popularity could indicate a kind of shift in the wind. That's my subjective impression.

    To provide a reality check, I am tracking the book's Amazon salesrank---relative to the five most popular string books whatever they happen to be at the moment---as a ratio here:

    So far only Fra and myself have registered our guesses as to how well the book is going to do. I'd be delighted if you would visit that thread and join us, if you wish.
    Last edited: Nov 17, 2008
  6. Nov 17, 2008 #5
    marcus, I have to tell you this: the way you liberally use all caps and bold reminds me of Time Cube a little bit (I do the same thing myself with liberal use of italics) but it's hilarious because instead of nonsensical Time Cube pseudo-science the content of what you write is articulate stuff about mainstream science with lots of really helpful links mixed in usually, awesome stuff.
  7. Nov 17, 2008 #6
    Marcus, I've never been one to blindly accept authority. I'm actually quite pleased by folks like Moffat and Cahill who are not shy in saying they think string theory may simply be wrong, and also (more controversially) that Einsteinian relativity may be wrong (Moffat saying his theory subsumes Einstein just as Einstein subsumed Newton, and Cahill saying simply that Einstein got it wrong in initially rejecting absolute motion, based on numerous experiments supporting absolute motion). This all comes back to the ether, which I conceptualize as the basis for all matter and space, but also the basis for absolute motion. In one of Einstein's more interesting intellectual turns, he hinted at this possibility also, according to Isaacson's biography:

    "[Einstein] had to face the possibility that general relativity did not necessarily eliminate the concept of absolute motion, at least with respect to the metric of spacetime.
    It was not exactly a retreat, nor was it a return to the ninetheenth-cetury concept of the ether. But it was a more conservative way of looking at the universe, and it represented a break from the radicalism of Mach that Einstein had once embraced.
    This clearly made Einstein uncomfortable." (P. 320).

    And here's a direct quote along the same lines (from a 1920 speech delivered at Leiden University, Lorentz's institution):

    "To deny ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view... Besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real ... The conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical wave theory of light ... According to the general theory of relativity, space is endowed with physical qualities; in this sense, there exists an ether. Space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any spacetime intervals in the physical sense. But this ether may not be thought of as endowed with the qualities of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."
  8. Nov 17, 2008 #7


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    Captain Q,
    I'm delighted with your response. Also you've been helping with your questions and replys to keep some of the cosmo threads going in a productive direction. Thanks. Hmmmm maybe I will try using italics more, as you say you do. May look better than bold.

    You say
    I probably agree with the thrust of this but you could say it simpler. Lots of people suspect that string is on the wrong track.
    If you just want a way to say "I'm skeptical of the stringy approaches. I doubt all those extra dimensions have much to do with nature." then you could just say that directly without mentioning any names, or you could equally well say

    "I think people like Renate Loll and Abhay Ashtekar and Laurent Freidel and Martin Reuter and [twenty other people plus all their graduate students] are probably right in doubting string validity and pursuing other directions."

    The fact that you chose Smolin and Moffat says something more but I'm not clear what. You are not just saying that you doubt string research will get anywhere or do anything much going forward, and that people who doubt string validity are probably right. You are suggesting that you think the alternative paths pointed out by Smolin and by Moffat are especially worth exploring, among what has recently become somewhat of a booming nonstring alternatives business.

    I'm not criticising this extra message, what I want is for it to be clear and not inconspicuously hitchhike in with the other message, so to speak.

    I guess the point is that the title of the thread is about extra spatial dimensions. Lots and lots of brilliant creative people doubt there is much promise in extra spatial. Lots of exciting work is in progress.

    What's puzzling me is where did your choice of Smolin and Moffat from amongst the whole field come from?
    Last edited: Nov 17, 2008
  9. Nov 17, 2008 #8
    Marcus, I mention Smolin and Moffat because they have written recent books that are critical of string theory - Smolin's directly and Moffat's less directly. I am not convinced that loop quantum gravity (Smolin's key endeavor) is the answer, largely because it assumes that general relativity is correct. Moffat's suggested alternative, Modified Gravity, seems promising and he makes quite a good case for it in his new book Reinventing Gravity. He compares it to GR and Modified Newtonian Dynamics (MOND) primarily, suggesting ways in which it can explain existing data better than either GR or MOND, and also suggesting ways in which MOND and MOG may be more definitively distinguished with new experiments.

    Similarly, Reg Cahill is someone who is working actively on alternatives to GR and string theory - though he is much less well-known and is apparently considered a little kooky by a lot of physicists. If you haven't read his work, I'd be curious what you think, in particular about his convictions about evidence in favor of absolute motion.

    You're right that this discussion is now far afield from the initial post about extra dimensions. Perhaps a new post about the ether and its revival by various theorists is appropriate?
  10. Nov 17, 2008 #9


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    Right! I understand much better now.

    Not to worry. Discussions here are often wide ranging. It doesn't cause difficulty as long as everybody understands and is satisfied. As long as there are only 2 or 3 people conversing I think you can do pretty much as you please. Talk about what you want. Split off and start another thread if you want, etc.

    I'm not a moderator so I can't actually say what's ok or not, but that's my impression.
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