Could the engine of our science-fiction dreams be on the horizon?
The EmDrive is the brainchild of microwave engineer Roger Shawyer, who after 40 years with aerospace giants Sperry, BAE Systems and Astrium set out to pursue a remarkable idea: a thruster that can produce a force without any exhaust. The problem with rockets is that they need a reactive mass – something to throw away – so if you’re going to Mars you have to take all the propellants you’re going to need for the whole flight with you. Shawyer’s design is controversial, but it all comes down to one point of contention. The thruster consists of a closed tapering box, and microwaves beamed into the box bounce back and forth between the ends, with every bounce producing a small force. The ends are also shaped so the microwaves are focused away from the walls.
This radiation pressure is accepted – it’s how solar sails work – but now things get counterintuitive. People have trouble getting away from the image of a gas trapped inside the thruster. If it were gas molecules in there, you couldn’t focus them to prevent them hitting the walls. Even if you could, you’d just get a high pressure on the small end and a low pressure on the big end that balances out. Radiation pressure is different, however, because electromagnetic radiation moves at light speed. This means the force produced per bounce depends on the average speed of the microwaves in the box. If half the microwaves are going in each direction, they are moving at the speed of light, but the average speed is zero – and this group speed can be manipulated. If the width of the small end is smaller than the wavelength of the microwaves, it reduces the group speed relative to the big end, so the big end makes a bigger force.
The final step that Shawyer maintains, and the one that causes controversy, is that because the microwaves move at light speed, they must be considered under Einstein’s theory of relativity, where each end of the box has its own valid point of view. This would mean that, as far as the universe is concerned, the thruster is an open system and able to produce thrust. A good analogy is that the trapped microwaves provide an anchor point to push off space itself.
So far, Shawyer has created thrusters made of copper that appear to confirm the theory, and since his initial work a group at Northwestern Polytechnical University in China has been carrying out independent trials that also agree with his research. But the sheer audacity of a propellant-free thruster has kept a barrage of scepticism coming. Copper thrusters could have a massive market in satellite-manoeuvring, but they only produce 20 grams of thrust per kilowatt of power put in, and this is because energy is lost with every bounce. But if the thruster were made from a superconductor, no energy would be lost
