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What are strings made of?

  1. Nov 15, 2003 #1
    At one time, I read where strings were vibrating bits of space-time. In "The Elegant Universe" TV program, they were described as vibrating bits of energy. In "The Elegant Universe" book they are described as consisting of fundemental "string stuff" and that questioning their composition really has no meaning.

    OK... so now I'm (even more) confused.
  2. jcsd
  3. Nov 15, 2003 #2
    The book description is the closest to the truth. They are certainly not vibrating "bits of spacetime"; I don't even know what that would mean. They aren't "bits of energy", either; they have energy.

    Strings aren't made up of anything more fundamental; they are the fundamental building blocks --- everything is made up of them. Strings have a size and shape and a tension, and that's about it for classical physical properties (neglecting technicalities like conformal fields, Chan-Paton factors, etc.).
  4. Nov 15, 2003 #3
    Yeah, "vibrating bits of spacetime or energy" never made sense to me either.

    The string's (incredibly huge) tension is an interesting property however. Tension in a classical string is created by the electromagnetic force. What is the force creating tension in a superstring?

    Is there another fundamental force? It seems that the vibration of a superstring would involve either the oscillation of this force or the oscillation of the Calabi-Yau space dimensions.
  5. Nov 15, 2003 #4
    There isn't any; tension is fundamental.

    The vibration of a string is pretty much just like the vibration of a classical string, postulating the "tension" as an elementary property of the string. String vibrations aren't due to any oscillation of space; they occur even in flat background spacetimes.
  6. Nov 15, 2003 #5
    Strings could consist of probability space, rather than spacetime.
  7. Nov 16, 2003 #6
    Tension is a force. Force is the gradient of a scalar potential field. Perhaps this is where the "background" potential comes in. Now, they suppose that the tension is constant along the string. But that seem more of a stipulation than a derivation.

    See comments in the thread:

    diff EQ on strings, check out the math
  8. Nov 16, 2003 #7
    Doesn't your definition of tension (with regard to strings) necessarily have to rely on the way in which the term is used?
    For example, tension can be looked at as coming from without or from within the thing... it can be a force applied or a force inherent, the act of movement or the condition of the already moved or moving...

    Or - and I think this is probably the closest to the norm where strings are concerned - it can be a measure of that which something already contains, i.e., condition of stretch, tautness, elongation, position, measure of vibration or even balance.

    And sometimes, it's only sematics... we all know what we're talking about; it's just getting that concept across to someone else, right?
  9. Nov 16, 2003 #8

    The classical definition of tension that I'm thinking of is the internal force within something that act against a set of external forces working to pull the thing apart. For example it's the EM force providing tension in my guitar string that prevents it's breaking from the pulling force the guitar and I exert on it.

    Which raises the question: can superstrings break under their tension and what happens if they do? The sound my guitar makes when I break a string isn't very musical.

    This seems to be the key point. The string tension force would appear to be the fundamental force since it is from the actions of this force that gives rise to all the other forces.
  10. Nov 16, 2003 #9
    Re: tension

    Strings have have a uniform tension (perturbatively), but they can break.

    There isn't a "string tension force" in the sense that there is, say, an electromagnetic force: there is no force field permeating space. Tension is just one physical property of a string, like its length.
  11. Nov 16, 2003 #10
    "String stuff"?? Shades of Sagan! Remember "star stuff" from Cosmos? "We are made of star stuff." So, I guess star stuff is made of string stuff.


    Ed Witten at Santa Barbara in 1996 answered the question as follows:

    (an approximation)
    In our theory, all matter is explained in terms of strings. Without a better theory, it makes no sense to ask then what a string is. It will probably take another half century to understand the present theory in a sensible way.

    Here, you can listen to his whole talk and look at his overheads.

    UCSB KITP public lecture "Duality, Spacetime and Quantum Mechanics" --->
  12. Nov 16, 2003 #11
    Re: Re: What are strings made of?

    i m not quite sure how to interpret that statement either, but i sometimes say it too.

    the reason is that i consider the geometry of spacetime to be a coherent state of gravitons. gravitons are stringy excitations, therefore spacetime is somehow a coherent state of string.

    in this sense, i can think of a string as a bit of spacetime, no? i, like you, am not quite sure what to make of the statement. so this mean that spacetime is no longer a manifold? that certainly doesn t appear from the math.

    what do you think?
  13. Nov 16, 2003 #12
    Re: tension

    Since you can't experimentally 'pluck' strings, like you can a guitar string, you have to look at them indirectly.

    Swartz and Scherk used the postulated properties of a graviton and its messenger particle to calculate that the particle's transmitted force is inversely proportional to its string tension. So you have the direct relationship of 1/(2pa') where a' is alpha prime and is equal to the square of the string length scale.

    And since the graviton is so weak, the tension is enormous (actually the Planck tension or around 1039 tons...) And this huge tension means the string contracts to the Planck length (very very tiny...)

    Also, high tension means high energy string. So a string's energy is determined by two things: its vibration and its tension.

    So, if you have tension directly related to both length and vibration of a string, it becomes an inate part of the string's nature and not some outside 'input.' So maybe the analogy of a guitar string is inaccurate, since you have to pluck a guitar string to make it vibrate, as well as having to first string it up to give it the proper tension...
  14. Nov 16, 2003 #13
    The string tension “force” sort of reminds me of a pre-Einstein description of a force, which got me thinking.

    My guitar string is essentially held together by the exchanges of photons between the atoms of the metal. If I were to cut one end of the string while it were under tension, the other end wouldn’t “know” it until a later time determined by the speed of light.

    Since the tension in a superstring is fundamental and is not transmitted by a boson limited to the speed of light, when a string breaks is there a limitation as to how fast that information is transmitted across the entire string?
  15. Nov 16, 2003 #14
    I see a rubber band (elastic) on my table. I am amazed about i's level of activity. Maybe it's due to the air pressure in Europe but it doesn't move or osccilates at all. The rubber band lays still. Like in a relaxed state.
    Now on the most basic Witten level this must be different? Amazing! It's almost like magic.
    Is it also different in the States?
    Last edited: Nov 16, 2003
  16. Nov 16, 2003 #15
    If you postulate a relativistic string, which is what is done in string theory, then influences cannot propagate faster than c.

    In fact, effects on a string propagate at a speed

    v = \sqrt{\frac{\tau}{\mu}}

    where τ is the tension and μ=m/L is the mass per unit length. But in string theory, the tension is equal to the energy per unit length, so

    [tex]\tau = E/L = mc^2/L = \mu c^2[/tex]

    and thus

    [tex]v = \sqrt{\frac{\mu c^2}{\mu}} = \sqrt{c^2} = c[/tex]
  17. Nov 16, 2003 #16
    Re: Re: What are strings made of?

    And how do they get energy? For a mobile phone?

    So they ARE?
    That's it?
  18. Nov 16, 2003 #17
    the whole thing becomes a lot less mystifying if instead of calling it "tension", you call it mass per unit length.

    a particle has mass (which labels its irrep of the Poincaré group), so the string (which is going to replace the particle) should have a mass too, and a mass per unit length.
    Last edited: Nov 16, 2003
  19. Nov 16, 2003 #18
    So - as Ambitwistor said - "strings aren't made up of anything more fundamental" there was mass from the beginning. Nice
  20. Nov 16, 2003 #19
    Now Herringbone I think this is one of the best - fundamental questions - on this forum about string.
    I could ask also 'How strings are created?', but that's even more fundamental magic.
    After 35 years ST the experts don't know. They even say that's a question for half of the 21 Century. See any logic?
  21. Nov 16, 2003 #20
    Well, I agree that the guitar might not be the best analogy but after all the pop representations of string theory its kind of hard not to go there.

    And aside from the obvious difference of open vs closed loops and fundamental string stuff, I can see some analogies, albeit extremely superficial. Now of course we're talking about one of those perfect guitars you find in physics books where there are no frictional losses, etc...

    You don't actually have to pluck a guitar string to make it vibrate. Just increasing the tension quickly will start it vibrating as the string length decreases. This could be considered analogous to the energy of the Big Bang (?) causing strings under their enormous tension to shrink down to be contained by the curled dimensions which could have given them their initial "input".

    The analogy is that as I add energy into my guitar string by turning the tuning peg, I'm causing the string to "shrink" down to be constrained by the space defined by the guitar's body which also causes the string to vibrate.

    With no losses the guitar string would indefinitely continue to produce a spectrum of vibrations determined by the shape of it's guitar "space" as well as the length and tension of it's strings just as a superstring produces a spectrum of vibrations determined by the string's length and tension as well as the shape of the tiny Calabi-Yau space that constrains it.
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