Thanks for the response. I do not find the idea of making a choice to set the function ##\xi ## to zero too troubling, but making such a choice feels somewhat artificial (at least when making this choice directly). Would it be possible to exclude such a term for more physical reasons, such as...
I am trying to show that the four-potential is a four-vector when working in the Lorenz gauge. In this gauge, we have ## \Box A^{\mu} = 4\pi J^{\mu}##. If we perform a Lorentz transformation, we can show that ## \Box A'^{\mu} = \Box \Lambda_{\nu}^{\mu} A^{\nu}##. From what I have seen, people...