EM Drive: Real Technology? Magic? Hokum?

Blog Post created by benz on May 10, 2017

  As technologists, we need ways to tell the difference

Over the past few months I’ve been hearing more about a propulsion technology called an EM Drive or EMDrive or, more descriptively, a “resonant cavity thruster.” As a technology that uses high-power microwaves, this sort of thing should be right in line with our expertise. However, that doesn’t seem to be the case because the potential validity of this technique may be more in the domain of physicists—or mystics!

Before powering ahead, let me state my premise: What interests me most is how one might approach claims such as this, especially when a conclusion does not seem clear. In this case, our knowledge of microwave energy and associated phenomena does not seem to be much help, so we’ll have to look to other guides.

First, let’s consider the EM drive. Briefly, it consists of an enclosed conductive cavity in the form of a truncated cone (i.e., a frustum). Microwave energy is fed into the cavity, and some claim a net thrust is produced. It’s only a very small amount of thrust, but it’s claimed to be produced without a reaction mass. This is very much different than established technology such as ion thrusters, which use electric energy to accelerate particles. The diagram below shows the basics.

Diagram of EM drive, showing mechanical configuration, magnetron input to the chamber, and supposed forces that result in a net thrust

This general arrangement of an EM drive mechanism indicates net radiation force and the resulting thrust from the action of microwaves in an enclosed, conductive truncated cone. (image from Wikipedia)

The diagram is clear enough, plus it’s the first time I’ve had the chance to use the word “frustum.” Unfortunately, one thing the diagram and associated explanations seem to lack is a model—quantitative or qualitative, scientific or engineering—that clearly explains how this technology actually works. Some propose the action of “quantum vacuum virtual particles” as an explanation, but that seems pretty hand-wavy to me.

Plenty of arguments, pro and con, are articulated online, and I won’t go into them here. Physicists and experimentalists far smarter than me weigh in, and they are not all in agreement. For example, a paper from experimenters at NASA’s Johnson Space Center has been peer-reviewed and published. Dig in if you’re interested, and make up your own mind.

I’m among those who, after reading about the EM drive, immediately thought “extraordinary claims require extraordinary evidence.” (Carl Sagan made that dictum famous, but I was delighted to learn that it dates back 200 years to our old friend Laplace.) While it may work better as a guideline than a rigid belief, it’s an excellent starting point when drama is high. The evidence in this case is hardly extraordinary, with a measured thrust of only about a micronewton per watt. It’s devilishly hard to reduce experimental uncertainties enough to reliably measure something that small.

I’m also not the first to suspect that this runs afoul of Newton’s third law and the conservation of momentum. A good way to evaluate an astonishing claim is to test it against fundamental principles such as this, and a reaction mass is conspicuously absent. Those who used this fundamentalist approach to question faster-than-light neutrinos were vindicated in good time.

It’s tempting to dismiss the whole thing, but there is still that NASA paper, and the laws of another prominent scientific thinker, Arthur C. Clarke. I’ve previously quoted his third law: “Any sufficiently advanced technology is indistinguishable from magic.” One could certainly claim that this microwave thruster is just technology more advanced than I can understand. Maybe.

Perhaps Clark’s first law is more relevant, and more sobering: “When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.”

I’m neither distinguished nor a scientist, but I am a technologist, and perhaps a bit long in the tooth. I should follow Clarke’s excellent example and maintain a mind that is both open and skeptical.