> Someone had a nice explanation of this over on Reddit but I don't have a link handy. Basically if momentum is not conserved then you can build kinetic energy without bound on finite power or something like that.
Are you referring to this reddit comment by myself?
https://reddit.com/r/spaceflight/comments/34ay1c/nasa_resear...
What you say is true but I don't see why people make assumption 1. Does anomalous thrust tell you it can accelerate linearly to power?
The thrust doesn't need to be a linear function of input power for this argument to work. As long as you can get some constant thrust for some constant power input, kinetic energy will eventually exceed the energy put into the system.
True. But how do we know thrust doesn't go down as velocity increases? I feel like we don't have that information yet.
Maybe, but velocity is relative. So that would mean that the drive is pushing against something which has its own rest frame, relative to which velocity can be measured.
It's not clear what that rest frame would be. Virtual particles in the quantum vacuum are often mentioned together with the emDrive, but those particles are thought to be frameless.
I'll admit that it's not entirely implausible to have a special rest frame like that. We already know that the cosmic microwave background has a rest frame, and this is something that you would start to feel in the form of frontal radiation pressure as you gain speed relative to the CMB.
Awesome, thanks for the info. While I wouldn't be that surprised if the EmDrive works by undiscovered spooky action at a distance, if it works by violating conservation of momentum that would be bonkers.
Undiscovered spooky action at a distance to what? Even assuming you can shuttle momentum around from particle A to particle C that is substantially distant, what exactly are particles C? Does the EM drive choose the distant particles to transfer momentum to on the basis of what would be the most convenient for the experimenter? What is the mechanism by which putting microwaves in a funny-shaped container creates a momentum-transfering bond from that container to unknown other masses? Does this totally unexplained effect have, you know, a range? If so, is that range remotely realistic? If so, we don't actually have a reaction-mass-less drive, do we?
For bonus points, answer all these questions in ways that do not create a privileged frame of reference and thus break special relativity.
E-Cat or anything like it (LENR) would mean new nuclear physics -- some way of bypassing the Coulomb barrier at low energies via some novel probably quantum mechanism. But fundamentally it's not violating anything as foundational as a conservation law. We know the energy is there via E=mc^2 and all. We just don't currently know of any way to get at that energy at sub-plasma temperatures and pressures (and for the proposed fusion reaction in the E-Cat, by no known physical means). LENR would require new physics but would not break existing physics.
I do recall reading some interesting stuff about how some of the gas giant planets seem a bit warmer than they should be, and that if LENR were a thing it would provide a possible explanation for some anomalous heating observed in astronomical observations.