Note on the quantum mechanics of M-Theory

We observe that the existence of black holes limits the extent to which M-T heory (or indeed any quantum theory of gravity) can be described by convent ional quantum mechanics. Although there is no contradiction with the fundam ental properties of quantum mechanics, one can prove that expectation value s of Heisenberg operators at fixed times cannot exist in an ordinary asympt otic Lorentz frame. Only operators whose matrix elements between the vacuum and energy eigenstates with energy greater than the Planck scale are artif icially cut off, can have conventional Green's functions. This implies a Pl anck scale cutoff on the possible localization of measurements in time. A s imilar behavior arises also in "little string theories". We argue that conv entional quantum mechanics in light cone time is compatible with the proper ties of black holes if there are more than four non-compact at dimensions, and also with the properties of "little string theories". We contrast these observations with what is known about M-Theory in asymptotically Anti-de S itter spacetimes.