Does this space drive break the law of physics?

There's been quite a bit of coverage on news services these last few days of a new technology that appears to violate the laws of physics and that could revolutionise space travel. I've read a few of these and found the explanations a bit confusing. Discussing it with people over dinner last night also suggested that just there's a bit of confusion as to just why it is so significant. So I thought I would try and explain it in simple terms.

The technology is called the EmDrive. It's been in development by British company SPR Ltd for several years amid much scepticism. It's hit the headlines this month because NASA engineers, apparently, have studied it, confirmed it works as advertised and published their findings in a conference paper. However they have not, reports say, explained exactly how it works, simply that it does work.

This has produced headlines like this one "‘Impossible’ space drive tested by NASA foretells future of deep-space travel"" The story went on to say: "NASA has conducted long-awaited experiments to prove that the fabled space drive, capable of generating its own thrust and breaking a fundamental law of physics, works. If the find survives fresh scrutiny, space ship construction will be revolutionised."

So, why is this so exciting and why does it appear to violate the laws of physics? Here's the problem. You're sat in your spaceship, stationary out in space. The laws of physics say that if you want to move forward you have to chuck something out the back end. The more you can throw and the faster you can throw it the faster you will go forward.

You can have an infinite supply of energy with which to throw stuff, but if you ain't got anything to throw you're going nowhere. This creates another problem. The mass you want to throw out the back later has to move forward with you initially. So the further and faster you want to go the more mass you need to carry and the more you need to throw out the back (or the faster you need to throw it) to even get started. It's a case of diminishing returns.

This is why you need a massive space rocket to launch a small satellite. (In this case the source of energy, the rocket fuel, also provides the mass that gets shoved out the back to move the rocket forward.)

This state of affairs is formally expressed as the law of conservation of momentum. It states that, in a closed system such as a spaceship in space, the product of mass x velocity remains constant. Put simply if you want to get a mass of 1kg moving one way at 10km/minute you need 1kg moving in the opposite direction at 10km/minute, or 100gms at 100k/minute. And that 100gms must come from the spaceship.

What the EmDrive claims to have done, according to its web site, is to "convert electrical energy directly into thrust" that is move the object forward without throwing anything out the back end. "Thrust is produced by the amplification of the radiation pressure of an electromagnetic wave propagated through a resonant waveguide assembly."

At first blush this would seem to be in conflict with the law of conservation of momentum. SPR claims this is not the case. According to the FAQs on its web site "The EmDrive does not violate any known law of physics. ... The EmDrive cannot violate the conservation of momentum. The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection. The momentum gained by the EmDrive plus the momentum lost by the electromagnetic wave equals zero. The direction and acceleration that is measured, when the EmDrive is tested on a dynamic test rig, comply with Newton's laws and confirm that the law of conservation of momentum is satisfied.

I have no idea what that means. But I guess eventually this will turn out to be either the breakthrough of the century, or the con of the century.