Introduction

Assertion: hidden within every non-Abelian gauge theory, even within the weak and strong nuclear interactions, is a theory of Quantum Gravity.

This is one implication of AdS/CFT duality. It was discovered by a circuitous route, involving in particular the relation between black branes and D-branes in string theory. It is an interesting exercise, however, to first try to find a path from gauge theory to gravity as directly as possible. Thus let us imagine that we know a bit about gauge theory and a bit about gravity but nothing about string theory, and ask, how are we to make sense of the assertion?

One possibility that comes to mind is that the spin-two graviton might arise as a composite of two spin-one gauge bosons. This interesting idea would seem to be rigorously excluded by a no-go theorem of Weinberg & Witten. The Weinberg–Witten theorem appears to assume nothing more than the existence of a Lorentz-covariant energy momentum tensor, which indeed holds in gauge theory. The theorem does forbid a wide range of possibilities, but (as with several other beautiful and powerful no-go theorems) it has at least one hidden assumption that seems so trivial as to escape notice, but which later developments show to be unnecessary.