Researchers in China say that they've successfully managed to test an engine that runs on electricity, requires no propellant and produces no exhaust. It's called the EmDrive, and it's able to convert microwave energy directly into thrust inside a sealed chamber. Oh, it's totally silent and highly efficient, too. If it seems too good to be true, well, you're not the only one who feels that way. But the researchers have a prototype that apparently works, and they've just published a paper detailing it.
Before we get into the mechanics of the EmDrive, let's give it some context. An engine is something that converts energy into motion. There are lots and lots of different ways of doing this, but let's take a look at a very simple one: a rocket engine. With a rocket engine, you've got a big tube with some explosive stuff (propellant) in it. When you launch the rocket and the propellant combusts, it tries to expand in all directions at once, but because it's stuck in a tube, it can only make it out the back. This means that the force that explosion exerts on the tube is symmetrical in every direction except for the back and the front, and as the propellant escapes out the back, the rocket gets pushed forward.
The basic principle here is that there's a reaction going on: stuff you don't care about (the exhaust) going one way pushes something that you do care about (the rocket) the other way. The reason that this works is a fundamental law of physics: Newton's Third Law, to be exact, which says that all forces exist in pairs. It also works because of the Law of Conservation of Momentum, which says that in a closed system, total momentum (which is the product of both mass and velocity) is constant. In other words, you can't have a rocket that moves forward without something moving backwards, and you also can't have a rocket that moves forward if you seal up the back end.
This is why the engine that scientists in China are claming to have successfully tested is so crazy: According to the fundamental laws of physics, the universe should not allow it to work.
The basic operating principle of this engine was conceptualized by a British dude named Roger Shawyer several decades ago, and has been in the news over the last few years (including on DVICE) as Shawyer built prototypes. The EmDrive itself is simply a microwave resonating cavity in the form of a closed, truncated cone. You fire up a big electrically-powered microwave generator and start beaming microwaves inside this thing, and the microwaves bounce around all over the place, exerting radiation pressure on the inside of the cavity.
Here's a diagram we found on New Scientist:
Now, if we believe those pesky laws of physics, we'd say that the EmDrive won't be able to move anywhere, because the energy imparted by the microwaves bouncing off of the inside of the EmDrive will cancel out. There's no "exhaust."
However, if we believe Shawyer, the EmDrive is in fact able to extert a small amount of thrust that propels it towards the large side of the cone. Shawyer says this happens because inside the resonating cavity, the velocity of the microwaves changes significantly as the cavity diameter varies. The velocity changes enough, in fact, to exert a larger force on the larger end of the cavity, and a smaller force on the smaller end of the cavity, resulting in net thrust. Shawyer argues that this isn't a violation of anything, because you have to treat the electromagnetic wave itself as acting in a different reference frame than the cavity, in accordance with Einstein's Special Theory of Relativity, meaning that the system is actually not closed.
A lot of people are skeptical about a lot of the physics here, and Shawyer has been called a nutcase by some people with extensive math and phyics backgrounds, but here's a video of the EmDrive purportedly moving itself on a test stand from back in 2007:
The EmDrive is contained inside that box-like thing, and that's part of what makes this thing so crazy: since it's reactionless and doesn't have to breathe in air or exhaust anything, you can completely seal it up and it'll still work quite happily, thrusting away inside whatever you've stuffed it into. Put one in your sock drawer, and it'll start trying to lift your dresser. Swallow one, and it'll start trying to lift you. Heck, seal it up in a block of concrete, and as long as you feed it enough electric power, you'll have a block of concrete that you can make levitate.
We want to reiterate that there are apparently a lot of very good reasons why a drive like this absolutely shouldn't work, and the general consensus of the scientific community is that Shawyer's results are caused by something else and that the drive doesn't function like he thinks it does (or at all). Boeing's Phantom Works took a look at one of Shawyer's prototypes and didn't pursue it, and when that sort of thing happens, it's usually a pretty good sign that the thing doesn't function. If it functioned even a little tiny bit, you'd have to think that a company like Boeing would be all over it, right?
Still, researchers in China have developed Shawyer's design over the last several years. Yang Juan, Professor of Propulsion Theory and Engineering of Aeronautics and Astronautics at the Northwestern Polytechnic University in Xi'an, just published a paper entitled "Net thrust measurement of propellantless microwave thruster." In the paper, Yang describes an iteration of the EmDrive that's able to generate 72 grams of thrust with 2,500 watts of electricity. It doesn't sound like a huge amount, but if you compare it to the hands-down most efficient spacecraft engine we've got right now (where efficiency is at an absolute premium), an ion thruster, the Chinese EmDrive gets you four times as much thrust from half as much power without sucking down any fuel at all. Yeah, you need electricity, but electricity is cheap in space and cheaper on the ground. Anyway, you can read the paper here, and if you can make conclusive heads or tails of it, please do us all a favor and explain it in the comments.
As skeptical as we are, let's just pretend for a second that the EmDrive is proven to work. What's the future like? Well, Shawyer says that with a superconducting cavity, the EmDrive could eventually be boosted to produce three tons of lift from just one kilowatt of input power. You could put them on anything you wanted to counteract gravity, while using conventional engines to provide high-impulse thrust. This would mean flying cars, it would mean highly efficient aircraft, it would mean cheap cargo to space. Gravity would cease to be a factor, requiring only renewable energy to counteract. It would be a fundamental and mind-blowing paradigm shift in all aspects of transportation.
We want to reiterate, again, that we have to side with physics on this one. The EmDrive is an extraordinary claim, and is going require extraordinary evidence and testing and demonstrations and we're absolutely not there yet. We're nowhere close. We've got some claims, some pictures of hardware, some YouTube video, and a paper published in China — and that's it. We need to see these things mailed to NASA and JPL and some major universities with no strings attached, and we need to see them perform there, and we need to see them getting built from scratch and working as advertised, and none of that is going to happen soon. The best we can reasonably hope for is a demonstration of the Chinese prototype at an aerospace conference sometime this year.
Now, having said all that, it's definitely hard to not get just a little bit excited about the potential here. I mean, even in science fiction reactionless drives are totally science fiction, and that's what we live for around here: science fiction becoming reality.