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Relativistic origin of Hertz-form and extended Hertz-form equations for Maxwell theory of electromagnetism

We show explicitly that the Hertz-form Maxwell's equations and their extensions can be obtained from the non-relativistic expansion of Lorentz transformation of Maxwell's equations. The explicit expression for the parameter $α$ in the extended Hertz-form equations can be derived from such a non-relativistic expansion. The extended Hertz-form equations, which do not preserve Galilean invariance, origin from Lorentz transformation of Maxwell's equations and differ from the Galilean-transformed Maxwell equations (the original Hertz equations) by the relative sign differences between the two $α$ terms etc. Especially, the $α$ parameter is of relativistic origin. The superluminal behavior illustrated by the D'Alembert equation from the extended Hertz-form equations should be removed by including all subleading contributions in the $v/c$ expansion, although such a superluminal behavior will not occur in the vacuum because $α=0$. We should note that in the Hertz form and extended Hertz form equations, the electromagnetic fields should take the forms $ \vec{\mathcal{E}}(x)=\vec{E}(Λ^{-1}x)$ and $ \vec{\mathcal{B}}(x)=\vec{B}(Λ^{-1}x)$. Such a choice of description for the fields is different from the ordinary one with $\vec{E}(x)$ and $\vec{B}(x)$, which are well known to satisfy the ordinary Maxwell's equations. The descriptions of electromagnetic phenomena using the function set $\{\vec{\mathcal{E}}(x),\vec{\mathcal{B}}(x)\}$ and the function set $(\vec{E}(x),\vec{B}(x))$ are equivalent, with the $\{\vec{\mathcal{E}}(x),\vec{\mathcal{B}}(x)\}$ description satisfying the extended Hertz-form Maxwell's equations in the low speed approximation. The solution of (extended) Hertz-form Maxwell's equations describe the traveling wave form electromagnetic field.