The EmDrive is actually less physically plausible than the E-Cat if you get into it, since reactionless propulsion is identical to perpetual motion and/or infinite energy. 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. The only way the EmDrive could work is if either (a) we are really wrong about some fundamental stuff, or (b) it's not actually reactionless but is pushing against something "exotic" like dark matter or dark energy.

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.

> 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.

I think I'll wait for more conclusive results before starting work on these questions ;)

At least the EmDrive is legitimately difficult to verify. It's claimed to produce tiny amounts of thrust. That requires extreme care to measure and makes it hard to see if it works as claimed.

The e-cat, on the other hand, is claimed to be a megawatt nuclear reactor. Verifying it should be easy: let it run and see if it produces power. Let it run long enough to rule out trickery like hidden batteries. If it does what the creator says it does, it should pass this test easily.

But for some reason they won't do this. They run all sorts of other weird tests but won't just demonstrate power production. To me, this makes it obvious that the "some reason" is because it doesn't work. If it worked there wouldn't be any question.

The biggest problem I have with all of these claims is the rigor of the tests they conduct. They never seem to use standard equipment that is appropriate for the effect they are trying to measure. Always they have a device that needs 1kW that has a 50kW 3phase power meter hooked up to it, or other techniques that are odd and make you wonder what they are trying to hide (thermal camera measuring temp of 1 side, nothing for the other to ensure heating is uniform). If you can afford to point a thermal camera at it, you can afford the cheapest of thermocouples to help prove the other side isn't room temperature.

For the EM drive, I would be surprised if they didn't report some trust after dumping large amounts of power into a box due to the million different things that can produce a misleading result (metallurgical defect in cavity causes heating to not match model, etc). If they can convince someone to try it in orbit, it would be a different story. Thanks to cube sats, this wouldn't be that expensive, <$500k to give it a go (launch 2 1U cube sats, one with drive and one without, see if separation changes). The bonus for the EM Drive folks is that several Nasa sites offered to independently test it when they can produce enough thrust for them to measure accurately, so if they do see something, its worth it to give it a go in orbit. They have yet to deliver such a device though.

OK that makes sense. Probably this reaction is common among people who know a lot about the domain and have criticized research with less serious problems before?

But to be fair, I see a lot of far-out claims about the capabilities of computers, the NSA, and so on. While I'm not going to encourage unfounded speculation about the stuff I'm more familiar with, it isn't unexpected and I guess it can make for a lively discussion.

I do want to see someone try the EmDrive in orbit.