Eh... technically, there is reaction mass being ejected, it’s just that you don’t have to bring your reaction mass with you and it’s essentially endless. This is a pseudo-reactionless drive, really, and probably as close as we’re going to get until we figure out more physics.
What reaction mass is ejected? Perhaps some is, but none has been identified, and with the basic physics in doubt, it seems impossible to say so with genuine certainty.
IMO it’s a BS drive. Focused microwaves excite sub-atomic particles within the drive chamber creating a relative pressure so to speak. The excited particles exit the drive producing a very small, yet measurable thrust.
Basically, they are heating sub-atomic particles which exit the drive in a specific direction.
The quantity of sub-atomic particles ejected must be of sufficient mass to propel a craft. I.e., thousands of pounds of these sub-atomic particles -which are of exceedingly small mass, would be needed to move a craft.
As reactive particles are ejected how will the reaction state within the chamber be maintained within the parameters of the thrust effect?
How will the sub-atomic (fuel) particles be replenished at a rate which does not change the reaction state within the drive chamber?