ah.clem and infodragon write:
This story from a Wired article: NASA is a major player in space science, so when a team from the agency this week presents evidence that "impossible" microwave thrusters seem to work, something strange is definitely going on. Either the results are completely wrong, or Nasa has confirmed a major breakthrough in space propulsion. British scientist Roger Shawyer has been trying to interest people in his EmDrive for some years through his company SPR Ltd. Shawyer claims the EmDrive converts electric power into thrust, without the need for any propellant by bouncing microwaves around in a closed container. He has built a number of demonstration systems, but critics reject his relativity-based theory and insist that, according to the law of conservation of momentum, it cannot work.
NASA states... "Test results indicate that the RF resonant cavity thruster design, which is unique as an electric propulsion device, is producing a force that is not attributable to any classical electromagnetic phenomenon and therefore is potentially demonstrating an interaction with the quantum vacuum virtual plasma"
While I think there are many other things that could have generated the measured force (pushing against the air in the chamber? It wasn't a vacuum. Maybe it created an electromagnet and pushed against earth's magnetic field?), if it does work it still conserves momentum. They think it was pushing against the vacuum quantum plasma, the virtual particles that continuously pop in and out of existence even in vacuum. If so, the virtual particles become the propellent, accelerated away from the drive, providing thrust. Momentum is conserved.