Is String Theory a Theory of Strings

Recently a great deal of evidence has been found indicating that type IIA string theory compactified on K3 is equivalent to heterotic string theory compactified on T 4. Under the transformation which relates the two theories, the roles of fundamental and solitonic string solutions are interchanged. In this letter we show that there exists a solitonic membrane solution of the heterotic string theory which becomes a singular solution of the type IIA theory, and should therefore be interpreted as a fundamental membrane in the latter theory. We speculate upon the implications that the complete type IIA theory is a theory of membranes, as well as strings. September 1995 (Revised Version) Beyond the first quantized framework of the Polyakov path integral [1], our knowledge of string theory is sorely lacking. Determining a more fundamental formulation of the theory will be an essential step in fully addressing nonperturbative issues, such as supersymmetry breaking and selection of the vacuum. Recently there has been rapid progress into understanding the strong coupling dynamics of certain supersymmetric string theories [2]. These advances should provide new insights into the correct fundamental framework which must underly string theory. We take one step in this direction by presenting evidence