News spread quickly this week that a propulsion system said to violate the laws of physics had been validated by no less an authority than NASA itself. Could a fuel-less space drive propelled by microwave thrust –something once deemed impossible – actually be possible? We spoke with some experts. Let's just say they're skeptical.


Above: Spaceship concept art by L.A.-based artist Cuba Lee

Earlier this week, Wired reported on an unusual engine designed and tested by researchers at NASA's Johnson Space Center. Dubbed the "Cannae drive," the propulsion system is similar to the so-called EmDrive, a "reactionless" engine proposed years ago by British engineer Roger Shawyer and popularized in a 2006 writeup in New Scientist. Both space drives are designed to convert electric power into thrust by bouncing microwaves around in a closed container, thereby eliminating the need for onboard propellant. The concept has been roundly criticized for appearing to violate the law of conservation of momentum.


And yet, Chinese scientists claimed last year to have built an EmDrive capable of producing thrust. Now, in a paper presented at last week's 50th Joint Propulsion Conference in Cleveland, Ohio, a team of NASA researchers claims its Cannae drive has also achieved thrust. Hence Wired's headline: "Nasa validates 'impossible' space drive."

But according to Drexel University physicist Dave Goldberg (io9's friendly neighborhood "Ask a Physicist" columnist), it's important to keep three things in mind, when claiming that NASA has validated the Cannae drive's "quantum vacuum virtual plasma," the hand-wavy concept by which the engine is purported to exert thrust:

A team of NASA researchers ≠ NASA, the organization. NASA is not a monolithic entity. It has hundreds of research groups that do their own work. Extraordinary claims (for instance, that the Cannae drive violating conservation of momentum) require extraordinary evidence. We simply don't have the data to make a judgment here yet.

CalTech physicist Sean Carroll, who, earlier this week, equated the notion of "propulsive momentum transfer via the quantum vacuum virtual plasma" to "nonsensical sub-Star-Trek level technobabble," had more to add, telling io9:

The business about "quantum vacuum virtual plasma" (the physics of which they "won't address" in this paper) is complete bullshit. There is a quantum vacuum, but it's nothing like a plasma. [The researchers] hook up a gizmo with all sorts of electromagnetic fields fluctuating around, then claim to measure an extremely tiny thrust (about the weight of a single grain of sand), which occurs even for the test article that wasn't supposed to produce any thrust at all.

Carroll's final point – that the researchers measured thrust not only when the drive was configured to produce it, but also when set up to do nothing at all – may be the most important takeaway of all. It's a point Mika McKinnon expands upon in her explanation below.