[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.