The Physics of the Impossible Drive – Michael Harrison

Around this time last year a series of small experiments run at Eagleworks—a NASA research and development lab—became big news. The subject of these experiments—the EmDrive and the Cannae Drive—are two independently invented “reactionless” propulsion systems.

Why are these experiments so exciting? The way that these drives may be working turns our understanding of physics on its head. As you might be familiar ,all chemical rockets (think the Apollo or fireworks) work on more-or-less the same principle: direct the rapidly expanding gasses from the combustion of fuel and an oxidizer through a nozzle. The focused hypersonic movement of the gas pushes the rocket in the opposite direction through the conservation of momentum (Newton’s Third Law). Other types of rockets, such as ion rockets (https://en.wikipedia.org/wiki/Ion_thruster), utilize the same principle by accelerating gasses to high speed using an electromagnetic field. Rockets that work in this manner are collectively known as reaction rockets.

Reaction rockets all share one critical drawback: they all must carry their own fuel. Fuel is heavy, I’ll let Randal Munroe of XKCD explain why this is a problem:

“If we want to launch a 65-kilogram spaceship, we need to burn around 90 kilograms of fuel. We load that fuel on board—and now our spaceship weighs 155 kilograms. A 155-kilogram spaceship requires 215 kilograms of fuel, so we load another 125 kilograms on board…

…fortunately, we’re saved from an infinite loop—where we add 1.3 kilograms for every 1 kilogram we add—by the fact that we don’t have to carry that fuel all the way up. We burn it as we go, so we get lighter and lighter, which means we need less and less fuel. But we do have to lift the fuel partway.”

For decades, long-term space exploration has been limited by the fuel problem and continues to represent a significant practical barrier. Overcoming the fuel problem would be a breakthrough of untold magnitude.

After all of that, what exactly is this reactionless drive? Chances are if you have read/watched/played any science fiction, a staple of faster-than-light space travel was a reactionless drive of some sort—an engine able to generate thrust without focusing the exhaust from the combustion of a propellant. Unfortunately, basic tenets of physics are not on our side. Remember Newton’s Third Law and the conservation of momentum? The same reason that a figure skater spins faster when they tuck in their legs is the same reason that rockets work at all. Removing the high-speed exhaust (and its mass) from the equation there is suddenly no force to move your rocket. Based on our current understanding of physics such an engine would indeed be an “impossible drive.”

With that, let’s look at the technologies causing the stir and what exactly they seem to be capable of. Before we begin I would like to remind you that we have very little certainty on any of the following; scientists are attempting to document and explain this unusual phenomenon.

Both the EmDrive and the Cannae Drive likely function on similar principles. You might have seen toys like this one that work on the principle that while photons have no relativistic mass, they do still possess momentum (start here for a discussion of this, or for a more formal treatment here). When the photons hit the wheel they transfer a small amount of energy, and with enough of them they overcome friction forces and turn the wheel. This action is known as radiation pressure, a principle loosely analogous to the water pressure that moves a turbine in a hydroelectric dam. The running hypothesis is that rapidly creating photons at specific frequencies in a specially designed chamber allows the particles to be focused out in one direction. Other hypotheses involve phrases like “quantum foam” and “space bubbles” that have not yet been entirely worked out into something cohesive. The inventor of the Cannae Drive claims that the particular shape of the internal cavity is critical to the design, although this has yet to be validated.

Both drive designs were independently tested by Eagleworks at the Johnson Space Center.

EmDrive test summary:

A test at 2500W of power during which a thrust of 750 millinewtons was measured by a Chinese team at the Chinese Northwestern Polytechnical University
A test at 50W of power during which a thrust of 50 micronewtons was measured by Eagleworks at the Johnson Space Center at ~760 Torr of pressure. (Summer 2014)
A test at 50 W of power during which a thrust of 50 micronewtons was measured by Eagleworks at the Johnson Space Center at ~5.0×10⁻⁶torr or pressure. (Early 2015)

Now the Cannae test summary. They did three trials with different experimental setups:

The device as the inventor designed it
The device as the inventor designed it without the slotting that the inventor claimed was critical
A control test that used the same energy, but without the cavity present in the design

Results summary:

Approximately 25 micronewtons of thrust at 50W
The same results as test #1, showing that at the very least, the slotting provided no benefit or detriment to the effect happening
No measurable thrust

Tests were conducted on an apparatus that could measure down to 10 micronewtons and in multiple directions. It is important to note that these tests were not conducted in a vacuum like the EmDrive tests.

As you can see, incredibly tiny amounts of thrust are being generated at these low power levels. A force of 50 micronewtons is approximately the amount of force required to launch a few ants into space. Tiny, but significant.

Normally, technology this early in its development cycle—especially one that is throwing a wrench into several well established principles of Newtonian physics—would be greeted by an enormous amount of skepticism. Thankfully this holds true: I have rarely seen more critical discussion about an emerging technology than I have about these reactionless drives. What is certain is that there is something going on here. Whether or not it does turn out to be a reactionless drive remains to be seen. Larger scale tests at a higher power will be completed this year and should provide more insight. A fourth experiment with the EmDrive (not listed above) was conducted with a device called a Michelson interferometer inside the chamber—a device that is capable of measuring changes in spacetime (a gross oversimplification). The device noted changes in the compression of spacetime within the chamber while the EmDrive was activated. If your reaction to that last sentence was anything like mine, this discussion might be interesting.

Should this technology pan out, the implications are enormous. Virtually every form of transportation (including cars, bikes, and planes) in addition to rocketry and space exploration can benefit greatly from the development of a power efficient reactionless drive. I have been following these experiments closely for the last year, and given that they have been turned over for dozens of reviews and still hold any amount of water a year later is exciting.

With developments like these we are one step closer to the world I dreamed of as a child. Outer space gets closer every day.