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The S-matrix of superstring theory will have finitely many massive particles which decouple in the asymptotic future/past and massless particles like gravitons, photons and dilatons. Typically, there will be an infinite number of massless "soft quanta". In the far future/past, all of these dressed particles will decouple and essentially travel along straight worldlines. So, we can draw a spatial boundary enclosing all the massive dressed particles but none of the massless particles. In the far future/past, that is. According to the holographic principle of quantum gravity, the massive particles would then be completely determined by the massless particles. Everything about them can be deduced in principle from the massless particles. This isn't as implausible as it may seem because we have infinitely many soft quanta.

''Does this mean the entire concept of an S-matrix has to be overthrown in superstring theory?''

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The entire concept of an S-matrix is already overthrown in the same way in quantum electrodynamics. A possible resolution in both cases is to have external classical fields at infinity that can absorb the infinite number of soft photons, but the details of this procedure are no longer fasionable, and I don't know how it works. – Ron Maimon Sep 29 '11 at 4:02

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