The propagation 4-vector $k^{\mu}$ consists of the propagation 3-vector $\mathbf{k}$ as the space part, and the amplitude $\left\vert \mathbf{k}\right\vert =\omega/c$ as the time component:$$k^{\mu}:\left(  \omega/c,\mathbf{k}\right)  . \label{k}$$ The 4-vector $k^{\mu}$ defines the light cone and, according to the rule (\[e\]), it is self-orthogonal:$$k^{\mu}k_{\mu}=\left(  \omega/c\right)  ^{2}-\mathbf{k}^{2}=0, \label{l}$$ which is an important possibility in this non-Euclidean space.

The concept of transversality refers to the property of plane-wave fields expressed in a relativistic context as *covariant transversality*$$k^{\mu}A_{\mu}=0, \label{m}$$ in terms of the 4-potential $A^{\mu}$. In many textbooks on classical electromagnetic phenomena, transversality is defined as *geometrical transversality*$$\mathbf{k\cdot E}=0\text{ and }\mathbf{k\cdot B}=0, \label{n}$$ in terms of the electric and magnetic fields. It can be shown that covariant transversality infers geometrical transversality.

Terminology
-----------

Despite the conclusion in this paper that potentials are more basic than fields, it is not possible to avoid the use of the term field in a generic sense. For example, one important conclusion reached herein is that vector and scalar potentials provide more information than do electric and magnetic fields in the description of the effects of laser fields. In the preceding sentence, the term laser field is used generically to identify the radiation created by a laser, despite the 