Positive-energy spectrum condition ($\mathrm{spec}{P}^{\mu }\subseteq {\overline{V}}_{+}$)

The joint energy-momentum spectrum lies in the closed forward light cone. In axiomatic quantum field theory, this condition permits analytic continuation of Wightman functions in suitable complex spacetime domains. Together with Lorentz invariance, microcausality and the positivity of the physical state space, it is one of the hypotheses behind the CPT theorem.