But, does the right-hand side of Equation (1750) really transform as a contravariant 4-vector? This is not a trivial question, because volume integrals in 3-space are not, in general, Lorentz invariant due to the length contraction effect. However, the integral in Equation (1750) is not a straightforward volume integral, because the integrand is evaluated at the retarded time. In fact, the integral is best regarded as an integral over events in space-time. The events that enter the integral are those which intersect a spherical light wave launched from the event and evolved backwards in time. In other words, the events occur before the event , and have zero interval with respect to . It is clear that observers in all inertial frames will, at least, agree on which events are to be included in the integral, because both the interval between events, and the absolute order in which events occur, are invariant under a general Lorentz transformation.
We shall now demonstrate that all observers obtain the same value of for each elementary contribution to the integral. Suppose that and are two inertial frames in the standard configuration. Let unprimed and primed symbols denote corresponding quantities in and , respectively. Let us assign coordinates to , and to the retarded event for which and are evaluated. Using the standard Lorentz transformation, (1643)-(1646), the fact that the interval between events and is zero, and the fact that both and are negative, we obtain
We now know the transformation for . What about the transformation for ? We might be tempted to set , according to the usual length contraction rule. However, this is incorrect. The contraction by a factor only applies if the whole of the volume is measured at the same time, which is not the case in the present problem. Now, the dimensions of along the - and -axes are the same in both and , according to Equations (1643)-(1646). For the -dimension these equations give . The extremities of are measured at times differing by , where
Thus, is an invariant and, therefore, is a contravariant 4-vector. For linear transformations, such as a general Lorentz transformation, the result of adding 4-tensors evaluated at different 4-points is itself a 4-tensor. It follows that the right-hand side of Equation (1750) is indeed a contravariant 4-vector. Thus, this 4-vector equation can be properly regarded as the solution to the 4-vector wave equation (1739).