I have no understanding of physics, but given that 70% of the known mass isn't measurable, couldn't something like the EMDrive "just" (lol) send dark matter one way, and get momentum the other way?
This is probably stupid to anybody that knows about physics, but I had to ask :)
We don't think that there's very much dark matter in our immediate vicinity. If there were, we'd probably have an easier time detecting it. Even though there's (according to theory) a lot of it in the universe, it clumps much less strongly than regular matter, so it's diffuse.
And the characteristic of dark matter is that it interacts very weakly or not at all to electromagnetism. It would be very odd if it interacted so strongly with the EM drive that the EM drive could generate noticable thrust from such a diffuse medium.
Finally, dark matter, if it has inertia, should have an inertial frame of reference. That frame is unlikely to be "at rest relative to the surface of the Earth." Therefore, if the EM drive is pushing off dark matter, we'd probably expect it to work more strongly in some directions than others. Think of it as a sail in a ghostly wind.
>we'd probably expect it to work more strongly in some directions than others.
According to the paper, the EM drive did actually quit working when rotated. However the dark matter wind seems less likely of an explanation than the Earth's magnetic field. ;)
The Earth is changing orientation in the galaxy as it spins, around itself and the Sun. Unless it was done exactly once, this experiment was performed at many orientations relative to the galaxy.
That's a perfectly astute guess, and it sounds more plausible than the original theory behind the EM drive. However photons in a cavity, at these energies, are unlikely to be producing many particles - the effect you're describing is known and can happen (not even with dark matter, photons can directly produce regular matter and antimatter), but we have a good idea of how much is produced and it's not enough to thrust anywhere. (It would be easier just to shoot the photons out the back of the rocket - they're massless and therefore carry momentum with the highest possible energy efficiency.)
Although much more plausible than reactionless thrust, and I don't think your comment deserves downvotes, this is still highly unlikely. Raattgift posted a good comment explaining why in the last big EmDrive discussion:
This is probably stupid to anybody that knows about physics, but I had to ask :)