It has been recently proposed that at each infinite distance limit in the moduli space of quantum gravity a perturbative description emerges with fundamental degrees of freedom given by those infinite towers of states whose typical mass scale is parametrically not larger than the ultraviolet cutoff, identified with the species scale. This proposal is applied to the familiar ten-dimensional type IIA and IIB superstring theories, when considering the limit of infinite string coupling. For type IIB, the light towers of states are given by excitations of the $D1$-brane, as expected from self-duality. Instead, for type IIA at strong coupling, which is dual to M-theory on $S^1$, we make the observation that the emergent degrees of freedom are bound states of transversal $M2$- and $M5$-branes with Kaluza-Klein momentum along the circle. We speculate on the interpretation of the necessity of including all these states for a putative quantum formulation of M-theory.

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• Field & string theory models & techniques
• M-theory
• String dualities
• String phenomenology
• Strings & branes