For years [Shawyer] has explored ways to confine microwaves inside waveguides, hollow tubes that trap radiation and direct it along their length. Take a standard copper waveguide and close off both ends. Now create microwaves using a magnetron, a device found in every microwave oven. If you inject these microwaves into the cavity, the microwaves will bounce from one end of the cavity to the other. According to the principles outlined by [physicist James Clerk Maxwell], this will produce a tiny force on the end walls. Now carefully match the size of the cavity to the wavelength of the microwaves and you create a chamber in which the microwaves resonate, allowing it to store large amounts of energy.Shawyer has received funding from the British government and has already built working prototypes of the thruster. His second prototype generates a push of only about 300 millinewtons, but Shawyer reckons he could achieve a thrust of 30,000 newtons—enough to "lift a large car"—if the waveguide cavity's walls are made into a superconductor so that energy from the microwaves isn't dissipated into heat. Engineers in Germany have already developed superconducting cavities as part of next-generation particle accelerators, and Shawyer hopes to have his own superconducting thruster ready within two years.
Since the microwave photons in the waveguide are travelling close to the speed of light, any attempt to resolve the forces they generate must take account of Einstein's special theory of relativity. This says that the microwaves move in their own frame of reference. In other words they move independently of the cavity - as if they are outside it. As a result, the microwaves themselves exert a push on the cavity.
In the meantime, his thrusters could be used in satellites as a replacement for conventional thrusters they use stay in orbit. Fuel for those thrusters makes up half of the satellites' launch weight, the New Scientist says, so a solar-powered electromagnetic drive could bring significant cost savings to the space industry. Shawyer expects to see his "emdrive" tested in space within two years.