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String theory states that the oscillations of little strings are responsible for all the particles in and the evolution of the universe. The specific type of particle created by a string depends on the energy and the shape of its vibrations. However, in order for a particle to have a definite identity, the string would need to have a definite energy and a definite rate of change. Since the Heisenberg uncertainty principle forbids this, strings would constantly be a random, uncertain jumble and particles could never retain their identities.