Every few years the “EmDrive”, a proposed method for generating rocket thrust without exhaust fumes, makes headlines. Everyone asks themselves every time: Could that be? Could this be the leap in technology to revolutionize space travel?
Don’t hold your breath.
Here is the basic idea behind the EmDrive. You take a chamber with a funky shape (usually narrower on one end than the other). They let a lot of microwave radiation bounce around in it. After a few – um – controversial experiments, the device begins to move without any emissions from the drive. This contradicts the common rocket structure that requires the ejection of propellant to push a spacecraft around.
Proponents of EmDrive claim that it is a revolution in physics and technology, a huge leap for humanity that will free us from the shackles of … well, needing fuel. Sure, the claimed thrust is incredibly small – not even enough to squeeze a piece of paper – but anything but 0 would be a big deal.
Big deal indeed. Assuming the EmDrive works as advertised, it completely violates known physics. And not just on a small scale – it breaks one of the most important, fundamental cornerstones of physics: conservation of momentum.
Things cannot move on their own. For literally everything we know about physics, the EmDrive can’t go by itself. It has to push something out in the opposite direction or react to something else in order to generate thrust.
Conservation of momentum isn’t just a cool idea. It underlines almost the entirety of modern physics. Everything from quantum field theory to general relativity is really just an expression of conservation of momentum in special contexts. Conservation of momentum has been experimentally confirmed countless times in everything from high-energy particle accelerators to the expansion of the universe itself.
Yes, sure, conservation of momentum could to be broken into a new, exotic case that we’ve never seen before. However, it is unlikely to show up as a tiny thrust in an EmDrive where the observed thrust is so small that there are many other, more plausible explanations: microwave leakage, reactions with the Earth’s magnetic field, miscalibration, etc.
We may one day find that we need to update our understanding of conservation of momentum. But not today.