Naive questions from someone who does not understand a lot about physics (to put it mildly): Did you just invent the perpetuum mobile, or did I misunderstand what you said? Or do I just not understand how this thing is supposed to work?
By special relativity you can't break the momentum conservation principle without breaking the energy conservation principle.
Somehow most people feel more comfortable dismissing the momentum conservation principle, but they are intertwined.
A carousel with Emdrives like the described in the previous comment should be a perpetual mobile and create energy if it's spinning in the right direction. (It will destroy energy in the reverse direction, so be careful.)
[Anyway, I think that the emdrive doesn't work and the conservation of energy and momentum are safe.]
However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up.
If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy.
A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion.
This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.
> reactionless thrust drives produce the same acceleration regardless of speed
A) Where is that in the paper? Or am I missing something elementary to this?
B) We don't even know if it produces the same level of thrust as speed increases. If it produces the same level of thrust across all speeds, then we have a problem, because as velocity increases, so does mass, and the same level of thrust would produce less acceleration over time, because of the increase in mass. Of course, that may only effect things at relativistic speeds.
Speed relative to what, exactly? There are no preferred reference frames. A true reactionless thruster could always treat itself as if it's initial speed was 0, and so it took 0.5J to accelerate by 1m/s.
Rockets do that too, but they get away with it because they stored the necessary extra energy into the kinetic energy of the fuel.
If there was a reactionless thruster that did, in fact, get less efficient at higher speeds, then that would also be world-shattering physics news, because then you could measure it's performance after accelerating in different directions and eventually get a true rest frame that is privileged over the rest of the reference frames out of it.
Put two emdrives on the ends of a rotating beam. Use the violation of conservation of energy/momentum to drive a generator. Infinite speed and energy are yours to command!
Driving the generator is going to rob the system of energy. That should be obvious, right? Nothing in the matter/energy equation is changing. This drive would just take electrical energy and convert it into kinetic energy.
That's the implication of the EM Drive. I didn't invent it, it's the very first thing that you think of once you violate the momentum conservation principle. This is why it's unlikely to be true, but at the same time an alchemists dream scenario.
