[twinkletwinkle]

The EmDrive claimed to violate conservation of momentum. To extend your analogy, instead of shining the light out the back of your spacecraft, you shined it inside the spacecraft at the back wall. It bounced around and came out as net positive thrust. Hence the extreme skepticism.

[gascan]

Ok, if you shine it on the back wall one might expect the radiation pressure balances out and there is zero net force.

What happens if you used a waveguide to turn it around 180 degrees to the front wall? Photons have no mass, so turning it in a waveguide would result in zero force on the waveguide right?

[GW150914]

Photons have momentum and energy.

[gascan]

Right, but if you bend their trajectory, does the device doing the bending experience any net force?

Edit: just learned light cannot be bent by electric or magnetic fields, I must have been thinking of radiation particles.

[GW150914]

You can absorb light and emit new photons, but the only way to bend their trajectory is bending spacetime with gravity. Needless to say, the EMDrive doesn’t contain a black hole.

[ben_w]

It will exert a force. I don’t know how waveguides work, but momentum is conserved (and photons have it) so it must exert a force.