I have created an entry-for-inclusion

in order to collect notation and terminology for plane (relativistic) waves (as a glorified “related concepts”-section); and so I am including this now into the relevant entries.

In the course of this I created stubs for *speed of light*, *Compton wavelength* and *dispersion relation* redirecting *energy-momentum relation*, but no substantial content yet in any of these stub entries.

I’ll go next and harmonize *Fourier transform* with this.

(Previously I had had the ambition there to stick with the convention where the “$2\pi$” is in the exponents, instead of in the global prefactors; but I feel now that this is just too different from established practice, and I’ll change it.)

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