Published online by Cambridge University Press: 09 April 2009
The trace T of the metrical energy-momentum tensor Tkl 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 Tkl 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.