Here, I will demonstrate how M-theory implies that the string worldsheet quantum theory 'knows' all about four-dimensional physics. Start with the Dp-action:

4-dimensionality entails that the mass of a Dp-brane can be derived as:

and by T-dualizing in the

direction and factoring the dilaton, the dual is hence:

By matrix world-volume integral reduction on

the Polyakov action for the string is hence:

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