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Why have strings?

  1. Jan 9, 2005 #1
    Could any of you clever people answer me this.. Whenever I have read a 'popular' book on string theory, I get the impression that the strings themselves aren't nessecary. They are described as a vibrating string that wraps around a folded dimension, and so say an electron is particular type of vibration on a string. Saying there is a string, just leads to questions like 'whats the string made of?' So, why does there 'need' to be a string, can it not be described as just a vibrating folded dimension?

    (obviously I'd prefer a laymans explaination:) If is comes down to 'it's required by the math and can't work without them' I'd be happy with that.)
  2. jcsd
  3. Jan 9, 2005 #2
    Well as to "what are the strings made of?" they are made of pure energy I believe and as to "why strings?" thats just the fundamental object that came out of the equations when string theory was born.
  4. Jan 9, 2005 #3


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    They tried out the basic string equation, at first in an attempt to explain quark confinement, and then as a general theory. They found that when they added the extra dimensions they got a consistent theory (no infinities so no renormalization needed!) and that it had a particle in it which acted like a graviton. That was a great discovery and it set off the "feeding frenzy" among theorists, which with various ups and downs through the years is still going on.

    I believe there is a theorem that any consistent particle theory of gravity will be some variety of string theory.
  5. Jan 9, 2005 #4
    Strings have not produced the mass that they hoped for. So apart from the spin 2 particle of the graviton, strings seem to be a bit arbitrary. Has it been proven that gravitons cannot come from higher dimensional objects such as vibrating membranes or volumes? Thanks.
  6. Jan 9, 2005 #5


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    I tacitly included branes in string theory. The first branes were discovered in bosonic string theory. I didn't specify any other offshoots either, like AdS/CFT.
  7. Jan 9, 2005 #6
    I am also trying to understand these concepts, and do not yet have the math to look at the source materials, but maybe I can throw in an idea or two.

    First, a string would be a one dimensional brane, so it seems to me. There is no conflict then between string theory and brane theory, in that sense, since branes can be thought of as composed of strings. Volumes and higher dimensional structures would be decomposable into strings, but to understand the complex interactions we ordinarily observe, such as mass, charge, angular momentum, we should have to look at how strings interrelate to each other.

    Then, as to what a string is made of, there is what seems to me to be an elegant answer, but unfortunately it involves a rather estensive paradigm shift and is not likely to be accepted as physics by this generation of physicists, if it ever finds any favor at all. Bare bones: fundamental spacetime units on the Planck scale which are expanding from every point. Matter as we normally think of it exists at a larger scale that the spacetime units, so it is possibly not meaningful (?) to ask what branes and strings are made of....certainly not energy, since that is also a dimensional entity.

    The key to this question, so I believe, will be found when we learn to accomodate higher dimensionalities than 3space + 1time into our vision.

    I am playing around with building a website but am still a pretty rude newbie at it, but if you can put up with my fumbling attempts, please be welcome to visit.


    Be well,

    Last edited by a moderator: Jan 9, 2005
  8. Jan 9, 2005 #7
    What bothers me is that Strings only beg the question. There seems to be some structure to strings that only makes one wonder how and why such an embedding in higher dimensional space should emerge from space alone. So they cannot be fundamental because there is something that is causing such structure to become embedded in the higher dimensions. So I am more attracted to structures that may be explained as part of the structure of spacetime itself. I'm thinking of hypersurfaces that form a collective boundary to spacetime, or perhaps genus holes. These are feature of the spacetime itself and not something embedded in spacetime that beg the question as to why they are preferred over other types of possible structure.
  9. Jan 10, 2005 #8
    Thanks fdr the replies, however I'm still not 100%:)

    The question was, why have strings at all? It seems to me, the only properties that strings/branes possess is that they a)wrap around 1 or many folded dimensions and b) vibrate. Is it not simpler (ie occams razor) to just say the folded dimensions vibrate and the particles we see are a manifestions of these vibrations? (to put it another way, that matter/energy is itself the extra dimensions)

    Much of the math would be the same I'd imagine and it doesn't beg the question as much as the proposed string. And picturing the big bang is much simpler, ie we have dimesions that may have been in balance to start off, but now 4 are extended and expanding while several more are folded. Pointing to a break in symmetry of some kind.

    Whilst I know that the extra dimesions where invented to make string theory work, has anyone try removing the strings from the theory?
  10. Jan 10, 2005 #9
    Perhaps the difficulty is one of language. A dimension in physics is not really something which can vibrate.... for example length is a dimension. An object which posseses the dimension length could vibrate, but you wouldn't say length itself vibrates. At least not when talking about the effects of matter such as mass and so on. Does mass vibrate? Well yes, an object that has mass as a dimension could vibrate, but it isn't the quality of mass itself that is said to vibrate, is it? A mile could go uphill or downhill, but mile itself as a concept would not be said to go up or down.

    The word 'dimension' is often used loosely, especially in a new age sense, as if it were a place that may or may not contain other worlds, other people, and so on. But in physics, altho sometimes used more or less loosely, the word really only means a quality which can be measured. Foot squared is a two dimensional quantity, but no one so far as I know lives in a place called foot squared, nor even square meter.

    I don't want to trivialize the question. A dimension is an abstract concept, not something which can be acted upon. Some objects behave as if they were two dimensional, as a shadow behaves on a wall, or as a wave behaves on the surface of a pond. Really this is just a convenience we use in describing the behavior of the object.

    So a string is a one dimensional object, which has the quality of length, but not of thickness or of width. It does obviously also possess the quality of duration, in that it must be said to persist in time. A point particle like an electron also persists in time, but has no spatial dimension. A string would also be like that. The most fundamental structures seem to me to be units of space and units of time, which exist as dimensions but do not exist in the sense that we usually think of as material.

    Hope this clears things up some.

    Be well,

  11. Jan 10, 2005 #10
    Space-time can vibrate, as a result of the GTR. Are the folded dimensions of string theory viewed as static and not obeying relativity? I'm reaching here I know. But it doesn't seem unreasonable, to me at least, to suggest that a dimension is capable of 'vibrating' or at the very least having some intrinsic value or property similar to vibration.
    Last edited: Jan 10, 2005
  12. Jan 10, 2005 #11


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    All the mainstream string physics is done in a fixed, static, flat, background Minkowski space of 1+n dimensions, where n can be 25 for bosonic strings, 9 for superstrings, or 10 for M theory. n is the number of spacelike dimensions and the extra one is time. Compacting the dimensions doesn't make them wiggle; they have to be compacted on a manifold that is locally flat, such as a torus or Calabi-Yau manifold.
  13. Jan 10, 2005 #12
    Hooray, now that is the first time I have seen this layed out for examination! I am very pleased. I think we are making progress in understanding. Anyway I feel that I am understanding better than before.

    Strings are so very small that a flat, Minkowski space is a good model for local relationships. Strings exist in a space that is so small that it is not meaningful to speak of curvature. That is the value of working in natural units of Planck length, which are irreducible. Then any bending has to take place between the locally flat spaces of the strings, triangles, tets, and other simplices. Then we can speak of angles of bending and so on.

    Thanks all.

  14. Jan 11, 2005 #13
    I've not seen anything written yet that would make 'strings' themselves nessecary, only nessecary within the framework of string theory itself, which is circular.
    Strings only have a few properties, I honestly haven't seen a reason why these properties must be part of the string and not the higher dimensions that string theory points towards.
    Just because string theory is based on static flat minkowski space, doesn't imo make 'strings' nessercary. Indeed, afaik it's one of the problems of string-theory (ie background dependence).
    I am, however, over my head when talking about these issues, so I can't really take the idea further. I apreciate your comments though.

    It makes alot of sense to me, as a layman, to think about the energy and matter around us as only a byproduct of interaction between expanded and collapsed dimesions. When looking at it this way, it becomes simplier to view quantum behavior (because the wave/particle is only a manifestion of the underlying interaction) and also negates the need to 'create' energy from nothing in the bigbang.
    Heh, maybe I'll look into when I actually know what the hell I'm thinking about :smile: .
  15. Jan 11, 2005 #14

    Check out the above link, if you are interested in an introductory link on string theory (part 1,2 and 3). What is it ? How was it born ? What are strings ???
    There are also some links included to some nices sites that are reliable as an informationsource

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