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String Theory vs. Superstring theory

  1. Nov 7, 2006 #1
    What's the difference between string theory and superstring theory? Are they two different names for the same thing? Which one came first? Which one is more popular?
     
  2. jcsd
  3. Nov 7, 2006 #2

    selfAdjoint

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    String theory is the generic name for the whole field. It began with Bosonic string theory which was not supersymmetric, and had two shortcomings, it had tachyons in it, faster than light particles that would destabilize the vacuum, and it didn't do fermions, or matter particles, but only bosons, or force carriers. Its shining virtue was that one of the force carriers it did have was identifiable as a graviton, the first time a particle theory had produced a valid account of one.


    It was soon discovered that adding the property called supersymmetry solved both the tachyon and the fermion problems, and modern string theory is almost entirely superstrings. It has greatly proliferated, including many things like branes and dualities so that the modern theory is much more detailed and mathematical than the original bosonic theory, which was already more detailed and mathematical than previous particle theories.
     
  4. Nov 7, 2006 #3
    Supersting theory is more popular because it solves some fundamental problems of the original string theory and creates a symmetry between fermions and bosons.
     
  5. Feb 5, 2011 #4
    But may I know how Supersymmetry was combined with the string theory?
     
  6. Feb 5, 2011 #5
    Do you know the concept in relativity of a world-line? This is a line in space-time which records the history of a point particle - all the locations in space-time that it ever occupies.

    The history of a string is a surface in space-time, called a "world-sheet". There are also space and time directions inside the world-sheet. So the world-sheet is like a little two-dimensional space-time, embedded in the n-dimensional space-time that the string is moving through. In the larger space-time, every point on the world-sheet has an n-dimensional position vector. So it's as if there are n fields on the string - on the world-sheet - corresponding to the space-time coordinates that the string is moving through.

    When you go to quantum theory, these n fields turn out to be bosonic. But for supersymmetry, you need fermionic fields as well. So there are two ways to achieve supersymmetry for the string. You can postulate extra fermionic fields that are inside the string - defined only on the world-sheet. Or you can postulate extra fermionic coordinates of space-time, alongside the normal space-time coordinates (which will then show up as fermions when you adopt the world-sheet perspective). The superstring can be defined either way - the first way is called Ramond-Neveu-Schwarz formalism, the second way is Green-Schwarz formalism.

    You can see Nima Arkani-Hamed talking about extra fermionic coordinates (he calls them "quantum dimensions") in the second half of this recent video. As he says, it's not like ordinary space dimensions, because it's as if there are only two positions available. You might see the analogy with fermions and bosons here. You can have an arbitrary number of bosons on top of each other - that's analogous to the way that you can keep going in a bosonic direction of space - but fermions exclude each other, and so in each location you only have 0 fermions or 1 fermion.

    I should emphasize that these "fermionic dimensions of space" are different from and additional to the usual extra dimensions that you hear about in string theory. You could argue that they are just a formal trick to unify the mathematical treatment of the world-sheet fermions with the bosonic space-time dimensions. The superstring has 10 space-time dimensions plus some fermionic coordinates; the purely bosonic string has 26 space-time dimensions, and I have long been interested in the idea that you might be able to get the superstring from the bosonic string in some way, so that the fermionic dimensions really are derived from 16 extra bosonic dimensions, but no-one knows how to do this and it's not a mainstream idea.
     
    Last edited: Feb 5, 2011
  7. Feb 5, 2011 #6
    Thank you!
     
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