To quote from the abstract: "any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine of th

To quote from the abstract: "any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine of the first kind, and thus should be excluded by the First Law of Thermodynamics" seems pretty categorical to me. The problem is, when you look at an object with a given velocity v, it has a kinetic energy proportional to v^2. To accelerate it from 0 to v, you must have put in all that kinetic energy (ie, v^2 energy). But reactionless drives give you velocity directly in proportion to energy. At some point, the v^2 and v lines will cross, and your object will have more kinetic energy then you put into it in the first place. For every unit of energy you pump into the drive, the velocity goes up by a unit, but since the kinetic energy of the object is proportional to v^2, at some point the kinetic energy you get out will be more than the electric energy you put in. The problem goes away with photon drives because that crossing point turns out to be at the speed of light. This is the argument the paper is making: If you can generate velocity directly from energy, you will eventually end up generating more energy than you started with.