Absence of periodic solutions to scale invariant classical field theories

It is shown that in 3 + 1 Minkowski space classical field theories with energy-momentum θ_μν(x,t) satisfying θ₀₀(x,t) ? 0 and θ_μ^μ(x,t) = 0 have no finite energy periodic (or static) solutions. In particular this rules out classical glueball solutions. to Yang-Mills theories with compact gauge groups.