Every few years the “EmDrive”, a proposed method of generating rocket thrust without any exhaust, hits the news. Each time, everyone asks: could this be it? Could this be the technological leap to revolutionize spaceflight?
Don’t hold your breath.
Here’s the basic idea behind the EmDrive. You take a chamber with a funky shape (usually narrower on one end than the other). You let a bunch of microwave radiation bounce around inside it. Accordingly to some – ahem – controversial experiments, the device begins to move, with no emission from the drive at all. This runs counter to the usual rocket setup, which requires the expulsion of a propellent to push a spacecraft around.
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Proponents of the EmDrive claim that it represents a revolution in physics and technology, a great leap for mankind that frees us from the shackles of…well, needing a propellent. Sure, the claimed thrust is incredibly tiny – not even enough to push a piece of paper – but anything other than 0 would be a big deal.
Big deal indeed. Assuming that the EmDrive works as advertised, it completely violates known physics. And not just in a small way – it breaks one of the most important, fundamental, cornerstone aspects of physics: conservation of momentum.
Things can’t just start moving on their own. According to literally everything we know about physics, the EmDrive can’t just go all by itself. It has to push something out in the opposite direction, or react to something else, in order to generate thrust.
Momentum conservation isn’t just a cool idea. It underscores almost the entirety of modern physics. Everything from quantum field theory to general relativity are really just expressions of conservation of momentum in specialized contexts. Conservation of momentum has been experimentally confirmed countless times in everything from high-energy particle colliders to the expansion of the universe itself.
Yeah, sure, momentum conservation could be broken in a new, exotic case that we haven’t encountered before. But it’s unlikely to show up as a tiny thrust in an EmDrive, where the observed thrust is so small that there many other, more plausible, explanations: microwave leakage, reactions with the Earth’s magnetic field, miscalibration, etc.
We may someday find that we need to update our understanding of momentum conservation. But not today.