Resonant Cavity Thrusters


EmDrive. Q drive. Cannae drive. All of these are a class of devices known as resonant cavity thrusters, and for the past few years they’ve been popping up in the media from time to time.

What’s the big idea?

Take an ordinary household magnetron (you’ll find one in every microwave oven). Use it to pump microwave radiation into a resonant chamber of a certain shape (a metal box that’s wide at one end and narrow at the other.)

Turn it on and voilà: even though nothing is emitted from the system, you’ll detect a very small anomalous force pushing the chamber.

Do they work?

Probably not. A closed system that can generate thrust in a vacuum without expending propellant violates the law of conservation of momentum. This is a law of physics so fundamental that it has been almost indisputable since Isaac Newton’s time.

On the other hand, a few experiments have not been able to rule out an anomalous thrust.

If they do work, how do they work?

Most explanations are handwavey and invoke quantum this-or-that. Physics is not my field, and the formulas in the EmDrive paper are very speculative, describing the way the thrust varies with velocity. As far as I know, all of the tests so far have been stationary.

At least one paper attempts to explain the phenomenon using quantized inertia, and provides a formula for calculating the thrust produced by the device:

F = - \frac{6PQL}{c}(\frac{1}{L+4w_{s}} - \frac{1}{L+4w_{b}})

The formula fits (to within an order of magnitude) with several observations, so I’ve created an EmDrive calculator based on this formula for anyone who wants to play with the numbers.

Why is this idea so appealing?

Devices like this are known as “reactionless drives”, and they’re such a common idea that the Atomic Rocket site has an entire page dedicated to them. (Spoilers: they violate the laws of physics and don’t work.)

Reactionless drives like the EmDrive are an appealing idea because they would let us build spacecraft that need no propellant. All they need is a working power source, and they can accelerate and decelerate indefinitely.

What would happen if the EmDrive really does work?

NASA and other space agencies would be ecstatic. Interplanetary travel — and possibly even interstellar travel — becomes much more feasible. A spacecraft equipped with an EmDrive would need only a power source, such as a nuclear reactor. It would not need to carry large amounts of propellant for acceleration and deceleration.

Meanwhile, physicists on Earth would have a problem. Conservation of momentum is a consequence of Noether’s theorem, which relates conservation laws to physical symmetries.

According to the theorem, momentum is conserved because physics is symmetric under linear translation. That is, an experiment works the same whether I perform it at my desk, or three feet over at the next desk, or 2 million light-years away in the Andromeda galaxy. If that goes out the window, a large chunk of modern physics goes with it.

EmDrive — potentially opening up the universe to us, at the expense of destroying our entire understanding of it.

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