Why is spacetime Lorentzian?

We expand on the idea that the spacetime signature should be treated as a dynamical degree of freedom in quantum field theory. It has been argued that the probability distribution for the signature, induced by massless free fields, is peaked at the Lorentzian value uniquely in D=4 dimensions. This argument is reviewed, and certain consistency constraints on the generalized signature (i.e., the tangent-space metric η_ab(x)=diag[e^iθ(x),1,1,1] are derived. It is shown that only one dynamical "Wick angle" θ(x) can be introduced in the generalized signature, and the magnitude of fluctuations away from the Lorentzian signature δθ=π-θ is estimated to be of order (l_P/R)³, where l_P is the Planck length, and R is the length scale of the Universe. For massless fields, the case of D=2 dimensions and the case of supersymmetry are degenerate, in the sense that no signature is preferred. Mass effects lift this degeneracy, and we show that a dynamical origin of the Lorentzian signature is also possible for (broken) supersymmetry theories in D=6 dimensions, in addition to the more general nonsupersymmetric case in D=4 dimensions.