Strings in flat space and pp waves from N = 4 super Yang Mills

We explain how the string spectrum in flat space and pp-waves arises from the large-N limit, at fixed g(YM)(2), of U(N) N = 4 super Yang Mills. We reproduce the spectrum by summing a subset of the planar Feynman diagrams. We give a heuristic argument for why we can neglect other diagrams. We also discuss some other aspects of pp-waves and we present a matrix model associated to the DLCQ description of the maximally supersymmetric eleven dimensional pp-waves.