  We consider a departure from standard quantum field theory, constructed so as
to permit momentum eigenstates of both positive and negative energy. The
resulting theory is intriguing because it brings about the cancellation of
leading ultra-violet divergences and the absence of a zero-point energy. The
theory gives rise to tree-level source-to-source transition amplitudes that are
manifestly causal and consistent with standard S-matrix elements. It also leads
to the usual result for the oblique corrections to the standard electroweak
theory. Remarkably, the latter agreement relies on the breakdown of naive
perturbation theory due to resonance effects. It remains to be shown that there
are no problems with perturbative unitarity.
