There’s been a lot of chatter on these here intertubes about the “NASA”1 validation of a new propulsion mechanism called the Cannae drive, which wouldn’t need to carry reaction mass with it into space2. It supposedly works by pushing against the “quantum vacuum virtual plasma” which sounds like something Gene Roddenberry might have typed at 2:00 am on the day of filming. The original Wired UK article spurred reactions from the physics crowd, which led Wired to post a rebuttal to the objections. I haven’t seen a lot of counter-rebuttals, and now that the (non-peer reviewed) conference paper is out3, we can finally start digging into what they did and did not see.

(Edit: Here are some great G+ posts by John Baez (Link 1, Link 2), who lent his voice to the articles linked above, but also wrote his own fantastic pieces. Thanks /u/Saivo)

There are several things to unpack when talking about this experiment.

What does the paper actually show? Are the objections raised by physicists about the experiment valid?

What is conservation of momentum, and why does this cause physicists to doubt the Cannae drive measurements?

Is the authors’ explanation of how the drive works valid? What is the “quantum vacuum virtual plasma”?

I’ll be writing a series of posts to answer these questions to the best of my ability. Keep an eye out for updates.

Update! The momentum article is up now!

Update! A short relativity proof that argues against the feasibility drive is up now!

But first, let’s talk about the experiment as laid out in the paper.

What was tested:

There were, in fact, three drives tested: two types of the Cannae drive and a tapered cavity. Each Cannae drive looks from the outside like a deranged vase with a tube coming out both the top and the bottom. The difference lies in the bulb of the vase — in one, suitably called the “slotted” device, slots were cut in only the bottom half. Cannae himself hypothesized that this asymmetry would cause the driving force. The other was identical except that slots were not cut. This is the “null drive” referred to in the other articles. It isn’t really a control; it only tests whether the slots are necessary. You can see the side view diagrams, generated with my earth-shaking Inkscape skills, which show the three devices in the test chamber.

All of these have a resonance cavity in the center. In a resonance cavity, the incoming radio waves from the source bounce back and forth a lot before leaving one side or the other, adding together as they do so — the sensor is there to allow adjustments to the power or frequency. This alone shouldn’t produce any thrust. There has to be a difference in something moving to the left or the right to produce a thrust. That something could just be radio waves — if more radio waves bounced back to the right, there would be a thrust to the left — but that wouldn’t be anything more than a complicated solar sail4. A simple calculation5 shows that the maximum thrust the photon radiation pressure of the radio waves themselves could be is 0.2 micronewtons, much less than the reported force.

The assertion in the paper is that the electromagnetic field of the radio waves confined in the pipes adjacent to the resonator is stronger on the right side than the left, and this is somehow responsible for the thrust.

The only device control is in the form of a 50 ohm resistor. More on that later.

The test:

The total force delivered by these drives at low power is minute, so a very sensitive experiment is necessary. Fortunately, that’s what the team appears to regularly do. The measurement device is called a torsion pendulum. In essence, it is like a super-sensitive spring scale on vertical rods. Just like a spring scale shows you how much force of bananas you’ve added by how far the spring stretches, if the rods of the torsion pendulum are twisted by a force, lasers measure how much they move. This is calibrated to give a force, measured in the physics-y units of Newtons.

One way to test whether it was the drive making the force, as opposed to a hiccup from the testing environment, is to flip the drive over and try it again. If it’s legit, the force direction should flip, too, but the amount of force should be about the same. The experimenters did just that, but only once for each drive.

The results:

In each case, between 20 – 30 Watts of power were used (enough power to run a fan or charge four cell phones simultaneously), and 50 micronewtons of force were generated (enough to levitate a couple of mosquitoes that weren’t already flying) in the expected directions. The flip test also seemed alright, if under-sampled — the force was of similar strength in the opposite direction.

Is that a good amount of force? Well, it isn’t great, but it’s not as bad as it might seem on the face of it. First of all, 50 micronewtons doesn’t do anyone any good, but 20 Watts is an incredibly low power to drive a real system. It could scale up with the application of more power. For example, a shuttle’s engine, after the solid-fuel stage, burns at a rate of 30 billion Watts, a billion times more power than in the Cannae experiment. However, that rocket also delivers a force of 5 meganewtons, one hundred billion times more force than the Cannae.

But, this drive isn’t meant to replace a rocket to get a satellite into space. This is meant to replace the drive you’d use on something that was already in space. A small force over a long period of time could have the same effect as a large force over a small period of time. Without having to drag extra reaction mass around or burn out, it could work for much longer and achieve very high speeds in space.

The Control:

A… resistor? A 50 ohm resistor? Not even inside a pipe like those attached to the resonant cavities of the Cannae drives. I suppose this resistor matched the impedance of the resonant cavity/waveguide test objects, but it has nothing else in common. And I suppose this shows that the radio waves aren’t affecting the torsion pendulum of the measurement device, but it leaves something to be desired. Generally, you’d like a control which is as close to your test objects as possible, but without the element you consider critical. The “null” device was a decent control to the assertion that the slots would be necessary, but only if they can validate the force measurements generally.

The Outcome

So far, the experiment seems reasonable, except for a poorly considered control. Why all the hullabaloo? Because the mechanism for generating the force is, let’s just say, uncertain. As it stands, we must either question the experiment’s results, or we must question the Law of Conservation of Momentum and our understanding of the quantum vacuum. We have a lot of evidence in support of those last two, the authors must present a lot more evidence in support of their results before they can gain much ground with the community at large. That is, other labs (and not commercial ones, like EmDrive) must validate the results. That’s science.

So, what about that Conservation of Momentum or the quantum vacuum dealy? What do we know about them, and what, if anything, do they have to do with this experiment?

You’ll just have to wait for my updates.

Update! The momentum article is up now!

Edit: I’m commenting on a Reddit thread with my brand new account. This is proof that /u/gildthetruth is me.

Edit 2: No, I’m not. My account is too new. I’ve lurked for a couple of years without an account and made one today to comment. Anyway, just listen to /u/kleinergruenerkaktus.

1 It was a small team among NASA’s giant organization. While this is what they do for a living, they aren’t speaking for NASA.

2 It would still require energy. If they claimed it didn’t require energy, no one would be paying any attention at all. For more on how rockets use reaction mass to generate lift, see my upcoming article on conservation of momentum.

3 Though not public. Since this is a federally funded agency, the research should be public eventually, as I understand it.

4 As long as the RF source remained external. For these drives, though, the source travels with the drive, which wouldn’t work as a solar sail any more than you can point a fan at the sail of your boat and hope to move.

5 They used about 30 Watts of power in the radio frequency signal. Each photon that hits a sail would impart an impulse of 2*p, where p is the magnitude of the momentum of the photon (assuming a single reflection isn’t enough to accelerate the sail much). The momentum of a photon is related to its energy by E = pc, where c is the speed of light. Since power P is energy over time, P = dE/dt = dp/dt *c = F*c, because F = dp/dt is the momentum form of Newton’s 2nd law. Thus F = 2*P/c = 2* 30W/3E8 m/s = 0.2 micronewtons.