The conservation of momentum isn’t rocket science, but you can’t get very far in space without understanding it.
Last week, a number of news outlets picked up on the story of an engine that theoretically shouldn’t work. But NASA tested it and published a (non-peer-reviewed) paper stating that the engine indeed produced some thrust. That made the news because of the laws this engine–dubbed the “Emdrive” (ElectroMagnetic drive) by British inventor Roger Shawyer–would break. NASA tested a version of this engine developed by American inventor Guido Fetta last year and found that it could produce thrust without any fuel. Needless to say, eliminating the vast reservoirs of fuel that we currently need to get objects into space would change the way we explore the cosmos.
To understand why a fuel-less engine is so seductive, we have to go back to rocket science. In classical physics, if you want to go faster, you have to push harder off something or have something push harder off of you. The engines that you’ve seen lift astronauts into space are propelled forward by throwing an explosion of fuel out behind them. Moving all that fuel out of the engine at high-speed thrusts the rocket ahead. Of course, the fuel we use is incredibly expensive and heavy, and is therefore a major limiting factor to how far we can go into the void. It’s a problem almost impossible to get around because momentum (the product of an object’s mass and velocity) is conserved–to change the velocity of a rocket hurtling through empty space, you have to change its mass (by blasting some out the back end). There, you’re part way to becoming a rocket scientist!
The beauty of discovering laws of the universe like the conservation of momentum is that they apply everywhere, in every situation we’ve ever known. Every kind of thrust we’ve thought up involves throwing mass from an engine at high speed. So when an engine comes along and claims that it has circumvented these laws, we owe it to ourselves to be skeptical. Here’s how the “impossible” Emdrive is supposed to work. The idea is that you can bounce microwaves around a specially-shaped chamber and the resulting forces inside, without any mass leaving the chamber, will produce a forward thrust. But, budding rocket scientist, we know that momentum is conserved. How can an object change velocity if neither the mass inside the chamber or the velocity of the chamber itself has changed? That’s the problem.
“It’s like sitting inside a car and making it roll forwards by pushing on the steering wheel,” writes John Baez, mathematical physicist U. C. Riverside in California. “Standard physics doesn’t allow this. Fetta didn’t claim to be using anything other than standard physics.”
Instead of adhering to the classical physics that governs all the rockets we know of, the Emdrive (or the updated “Cannae drive” Fetta developed and NASA tested) is supposed to be “a quantum vacuum plasma thruster.” In other words, microwaves are supposedly changing the momentum of virtual particles that pop into and out of our universe from nothing. Dude.
Pushing against “quantum plasma” is the explanation for the small thrust that NASA measured when they tested this impossible engine. If that is true, then the engine does not need any propellant–it changes the momentum of the chamber by slamming microwaves into particles that appear for free. But when you look a bit closer at the actual experiment, things start to break down.
John Timmer, writing in Ars Technica, notes something very important: the version of the Emdrive that was designed to produce nothing at all (the control experiment) inexplicably produced some thrust. John Baez read into the NASA report and found that the drive, which is supposed to run in a vacuum, wasn’t really tested in a vacuum. And while supporters of the engine are looking to exotic laws of physics to explain the numbers NASA found–much like the supposed “warp drive” a NASA engineer touted–that doesn’t mean that the physical laws we already know of no longer apply.
“The report talks about the ‘quantum vacuum virtual plasma,'” theoretical physicist at Caltech Sean Carrol told me in an email. “There is no such thing; it’s just made-up technobabble. In order for the system to do what they say it does, it would have to violate the conservation of momentum, which is no more possible in quantum mechanics than in classical mechanics.”
And this skepticism isn’t overturning particularly fantastic results either. The thrust figures that NASA noted are less than the force of a penny sitting in your hand.
“It’s extremely easy to imagine that the super-tiny effect they claim to see is just noise, rather than a dramatic breakdown of everything we know about physics,” Carrol told me.
It’s seductive to think about engines that use principles almost unknown to science. Who wouldn’t want something that could revolutionize the way we traverse the stars? But extraordinary claims require extraordinary proof, as Carl Sagan liked to say. We don’t have to throw out the laws of physics to uncover amazing things. We actually have ion engines! These machines use extreme electromagnetic fields to fire ions out the back of them, rocketing the whole apparatus forward (pictured above). NASA is already testing and implementing ion thrusters that could propel robotic missions to Jupiter’s icy moons at 200,000 miles per hour.
That’s right: “quantum vacuum virtual plasma” engines may not have any real science behind them, but NASA will have a TIE fighter pretty soon.
Kyle Hill is the Chief Science Officer of the Nerdist enterprise. Follow the continued geekery on Twitter @Sci_Phile.
IMAGE: NASA’s 2.3 kW NSTAR ion thruster for the Deep Space 1 spacecraft during a hot fire test at the Jet Propulsion Laboratory, NASA.