Is the EM-Drive a Closed System?

Since its disclosure, the so-called EM-Drive, an apparently reactionless electromagnetic thruster conceived by Roger Shawyer, has simultaneously caused wide scepticism, related to the physical principles that may allow its functioning, and understandable enthusiasm, by virtue of the astonishing scenarios potentially offered by such a device. On the one hand, thrust without exhaust is de facto impossible, unless we deny the Law of Action-Reaction, whose evident violation would result in acknowledging the concrete need for a new Physics; on the other hand, it would appear that opportunely shaped resonant cavities, when fuelled with microwaves, deliver a certain thrust, apparently without a detectable exhaust. In this paper, instead of discussing the validity of the various tests to date independently carried out, speculating about misleading side-effects or inadequate instrumental precision, we simply suppose that the thrust may be an actual phenomenon. Therefore, we try to provide a qualitative explanation to the functioning of the alleged reactionless device, by resorting to a theory elsewhere proposed and herein briefly discussed.