What are the fundamental degrees of freedom of M-theory?
At present, it is unclear as to what are the fundamental physical degrees of fre edom underlying M-theory. Most of what we understand about M-theory is based on the facts that it has eleven-dimensional supergravity as its low energy limit and, via compactification on a circle, can be related to type IIA string theory. While the string of the type IIA theory was at one time regarded as the fundamental obje ct, it is now clear that this is not the basic degree of freedom underpinning M-theory as a whole. Of course, one may take the point of view tha t there is no truly fundamental physical degree of freedom but rather that, in different regions of the M-theory moduli space, different degrees of freedom appear to be fundamental . An interesting approach to M-theory, which may resolve at least some of these conceptual issues, is Matrix theory: a supersymmetric quantum mechanics of N-by-N matrix degrees of freedom (see, e.g., this paper for a recent review). Matrix theory was originally con
