I gave it a quick read through, as electro-magnetic theory is something I teach, and I can say it makes no sense.It has the feel of something that makes sense, but makes a series of basic technical errors. To list a few: it obsesses about group velocity in a context where this doesn't really apply (group velocity is an age old source of confusion), it worries about the force on the end caps but ignores the force on the waveguide walls (which makes up for the difference), it states that there is no force at a dielectric boundary with no reflection (just wrong).
From a professional physicist's point of view, it is not hard to confuse oneself and get answers which make no sense. I do it all the time. The trick, and what we try to teach students, is to realize when you have something that makes no sense and try to find a different way of doing the problem to check your answer. Here, everything is classical.The "easy" way to solve the problem is to use the momentum of the electromagnetic field (Poynting vector) and measure it's flux outside the chamber (strict test of momentum conservation). This gives identically zero, QED. They basically don't understand the reaction of the wave against the angled walls of the waveguide.
While I admit physics is a very specialized and technical subject, this paper is at a Junior level undergraduate level. In this paper they conspicuously ignore the relevant tool (momentum of a wave) that any good physics major would use. For those of you with the background, look at chapters 8 & 9 of Griffith's Electrodynamics
