It turns out that the four components of the potential four-momentum are not independent, but are subject to the condition \[\frac{\partial Q_{x}}{\partial x}+\frac{\partial Q_{y}}{\partial y}+\frac{\...It turns out that the four components of the potential four-momentum are not independent, but are subject to the condition \[\frac{\partial Q_{x}}{\partial x}+\frac{\partial Q_{y}}{\partial y}+\frac{\partial Q_{z}}{\partial z}+\frac{1}{c^{2}} \frac{\partial U}{\partial t}=0 \label{14.16}\] This is called the Lorenz condition. The physical meaning of this condition will become clear when we study electromagnetism.
