In theoretical physics, M-theory is an extension of string theory in which 11 dimensions of spacetime are identified as seven higher-dimensions plus the four common dimensions (11D st = 7 hd + 4D). Proponents believe that the 11-dimensional theory unites all five 10 dimensional string theories and supersedes them. Though a full description of the theory is not known, the low-entropy dynamics are known to be supergravity interacting with 2- and 5-dimensional membranes.