This question is vaguely related to an earlier question, but is more focused.
In a paper, Electromagnetic Mass, Charge, and Spin, which apparently is not peer-reviewed, the author relates a "matter field" to a spinor representation of the electromagnetic field. The author states that field momentum of the "matter field" is an eigenvector of a field stress-energy tensor:
We also demonstrate that condition (3) is equivalent to the requirement that momentum density of the matter field 𝑃𝜇 is an eigenvector of the stress-energy tensor of the electromagnetic field. [page 4]
Unfortunately I haven't learned how to work with spinors yet, so am not able to check or understand the author's math.
A) In general, under what conditions would field momentum - or a component thereof - be an eigenvector of the stress-energy tensor? B) Why should (or should not) this paper be taken seriously?