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Gravity: Every consistent theory of quantum gravity must contain a massless spin 2 state, the graviton, whose interactions reduce at low energy to general relativity.
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Consistency: String theory is a consistent theory of quantum gravity, at least at the perturbative level.
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Grand Unification: String theory leads to gauge groups that are large enough to include the Standard Model of Particle Physics.
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Extra Dimensions: String theory requires a definite number of extra dimensions. The superstring theories live in 10 dimensions, 6 of which are small and curved, the remaining 4 are large and flat and make up the observable 4 dimensional spacetime.
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Supersymmetry: The superstring theories require spacetime supersymmetry, either as a manifest or as a spontaneously broken symmetry.
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Uniqueness: There are no free parameters, neither continuous nor discrete. There is a unique string theory.
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