Figure 1
Two Lorentz covariant vector valued functions associated with a field of a single source moving along the world-line \({\check{x}} (\tau )\). The point \(Q={\check{x}} (\tau (x))\) is the intersection of the world-line with the backward light cone with vertex at \(P=x\). The relative position null four-vector QP is \(r(x)=x-{\check{x}} (\tau (x))\), and \(w(\tau (x))\) is the four-velocity at the retarded time.
