Particle physics experiments

Don’t be surprised if EmDrive experiences never work


Every few years, the “EmDrive”, a proposed method of generating rocket thrust without any exhaust, hits the headlines. Each time, everyone wonders: could that be it? Could this be the technological leap to revolutionize space flights?

Don’t hold your breath.

Here is the basic idea behind the EmDrive. You get a room with a funky shape (usually narrower at one end than the other). You let a bunch of microwave radiation bounce around inside. As a result for some – hum – controversial experiments, the device begins to move, without any emission from the reader. This goes against the usual rocket setup, which requires the expulsion of a thruster to push a spacecraft.

Supporters of EmDrive claim that it represents a revolution in physics and technology, a big step for humanity that frees us from the shackles of… well, of needing a thruster. Of course, the claimed thrust is insanely small – not even enough to push a piece of paper – but anything other than 0 would be a big deal.

Big deal indeed. Assuming the EmDrive works as advertised, it completely violates known physics. And not just to a small extent – it breaks one of the most important and fundamental aspects of physics: conservation of momentum.

Things cannot start to move on their own. From literally everything we know about physics, the EmDrive simply cannot go alone. It has to push something in the opposite direction, or react to something else, in order to generate a push.

Retaining momentum isn’t just a good idea. It highlights almost all of modern physics. Everything from quantum field theory to general relativity are really just expressions of conservation of momentum in specialized contexts. The conservation of momentum has been experimentally confirmed countless times in everything from high-energy particle colliders to the expansion of the universe itself.

Yes, of course, conservation of momentum could being broken in an exotic new case that we’ve never encountered before. But it’s unlikely to show up as a tiny push in an EmDrive, where the observed push is so small that there are many other more plausible explanations: microwave leakage, reactions with the field. magnetic earth, bad calibration, etc.

One day we may need to refresh our understanding of conservation of momentum. But not today.