Gluing two manifolds M 1 and M 2 with a common boundary S yields a closed manifold M . Extending to formal linear combinations x = Σa i M i yields a sesquilinear pairing p = , with values in (formal linear combinations of) closed manifolds. Topological quantum field theory (TQFT) represents this universal pairing p onto a finite dimensional quotient pairing q with values in C which in physically motivated cases is positive definite. To see if such a "unitary" TQFT can potentially detect any nontrivial x, we ask if x, x = 0 whenever x = 0. If this is the case, we call the pairing p positive. The question arises for each dimension d = 0, 1, 2, . . .. We find p(d) positive for d = 0, 1, and 2 and not positive for d = 4. We conjecture that p(3) is also positive. Similar questions may be phrased for (manifold, submanifold) pairs and manifolds with other additional structure. The results in dimension 4 imply that unitary TQFTs cannot distinguish homotopy equivalent simply connected 4-manifolds, nor can they distinguish smoothly s-cobordant 4-manifolds. This may illuminate the difficulties that have been met by several authors in their attempts to formulate unitary TQFTs for d = 3 + 1. There is a further physical implication of this paper. Whereas 3-dimensional ChernSimons theory appears to be well-encoded within 2-dimensional quantum physics, e.g. in the fractional quantum Hall effect, Donaldson-Seiberg-Witten theory cannot be captured by a 3-dimensional quantum system. The positivity of the physical Hilbert spaces means they cannot see null vectors of the universal pairing; such vectors must map to zero.
AMS Classification numbers Primary: 57R56, 53D45Secondary: 57R80, 57N05, 57N10, 57N12, 57N13