[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.