Science Forums 2

[Karnuvap]

At first I thought that this was just a spaceship with an open ended microwave cavity on the back thus microwaves out the back = thrust towards the front and thus acceleration (however slight) that would eventually build-up to pretty decent speeds.
But, on closer inspection, the cavity is sealed at the back too and it relies on the fallacious assumption that because the wall at the 'back' has a different surface area then the wall near the 'front' there would be more 'pressure' on the back than at the front. OK that is slightly an oversimplification of what they are trying to achieve and the reality of their theory is slightly more subtle.

The real secret to where they are bamboozling us lies in the expression "Group Velocity". This is not a real velocity that should be used in the subsequent equations. It is a mathematical construct to help solve the equations and not anything real. To give you a clue as to why group velocity isn't anything like real velocity - it is perfectly possible to have a group velocity faster than that of the light photons or waves that make it up (which, obviously, travel at the speed of light themselves).

So, unless something is emitted out of the back of your spacecraft it cannot accelerate.

Of course I might be wrong and it could be that the microwave photons are selectively destructively interfered at one end of the chamber (and, presumably) they constructively interfere at the other end. The destructively destroyed photons re-appear at infinity - or so my physics lecturer told me when I asked him where they would go if I shined two identical frequency lasers at a single point on a screen and then adjusted one of them so that it was precisely one half wavelength further away from the screen than the other. Where would the light go in this perfect destructive interference set-up? - they disappear and re-appear at infinity was his reply. I'd love to know what the real answer was.