On the trace of the energy-momentum tensor of fields associated with particles of zero rest mass

Source abstract

The trace T of the metrical energy-momentum tensor T k l of fields associated with particles of zero rest mass may be zero, either identically or as a consequence of the field equations. This property of T kl is correlated here with the behaviour of the Lagrangian of the field under arbitrary conformal transformations. Certain classes of special fields are considered explicitly. It is shown in particular that T vanishes for all non-zero spin fields which correspond respectively to the two-component neutrino field or the photon field.