Yesterday, Wired UK had geeks everywhere salivating at the prospect of a purely electrical space thruster system—one that should be impossible based on what we know about classical physics. The article notes that NASA engineers have now tested one of these devices and found that it appears to produce thrust without using any fuel. Although there are ways that non-classical physics can make things work, there are enough red flags raised by material in the initial report that the news should be greeted very skeptically.

The limit that space thrusters face is purely classical: to push something along at a higher velocity, you need to push against something else. So typical thrusters push against the mass of the burning fuel that they're explosively expelling behind them. Even the most sophisticated, efficient thrusters—ion drives—act as particle accelerators that shoot ions out in the opposite direction of the way they're accelerating. As a result, any form of thrust that we've used requires that the spacecraft carry some mass that then gets shot out from the spacecraft. This adds weight to the launch vehicle and sets a finite limit on how much propulsion can be generated during the spacecraft's lifetime. Which is a bit frustrating, given that the high-efficiency solar panels on many spacecraft can give them a surplus of energy. It's just not energy we can convert into thrust—or at least we think we can't convert it into thrust.

The Wired UK article details how a variety of groups have suggested that it might be possible to use electricity to generate some thrust via a decidedly non-classical mechanism. The device involves a radio frequency resonant cavity, which takes microwaves as an input and uses them to create an oscillating electrical field. These cavities are used in particle accelerators like the Large Hadron Collider, where the electric field helps boost the energy of the circulating ions. At the LHC, however, they actually provide some ions for the resonant cavity to push around, which would be equivalent to supplying this thruster with fuel. Yet the backers of this device suggest it's pushing against the swarm of virtual particles that quantum mechanics indicates are constantly popping in and out of existence in empty space.

When we hear terms like virtual particles and microwave resonators, our first thoughts turn to Chris Lee, our resident physics expert. We pointed Lee at the Wired UK article and set him loose. One of his first stops was at the webpages of a company that the article suggested was developing a similar thruster. In looking for details, Lee quickly headed for the company's "theory" page, which links to three pages on theory, mathematical modeling, and experimental results. All three links were dead. "Two of the three proof of concept pages 404 as well," Lee noted. The NASA report wasn't much help either, since it only delved into the practicalities of testing one of the devices. At first glance, however, it looked quite interesting, stating, "Approximately 30-50 micro-Newtons of thrust were recorded from an electric propulsion test article." If you push a bit further, it looks much less promising:

Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed to produce thrust, while the other did not (with the latter being referred to as the "null" test article).

In other words, the negative control in the experiment worked. Which means that the experiment as a whole tells you nothing. Clearly, the device (even when disabled) appears to produce a force. But Lee suggested a variety of ways that this could happen and indicated that there are ways to monitor the device's operation to see if any of them play a role (for example, he suggested that a mass imbalance of as little as 3 mg could account for the small force the NASA researchers found. "All in all," Lee concluded, "it will take a lot more information before we can judge whether the thrust is really a thrust or not."