Bochner’s technique is
applied to the study of timelike vector fields on a Lorentzian manifold. A Lorentzian
Bochner integral formula is obtained; as a consequence, compact Ricci-flat Lorentzian
manifolds admitting a timelike conformal vector field are classified. Some
obstructions to the existence of timelike conformal vector fields and other conformal
symmetries are also given.