For the past several years, a few corners of the Internet have sporadically lit up with excitement about a new propulsion system, which I'll call the WTF-thruster. The zombie incarnation of the EM-drive has all the best features of a new technology: it generally violates well-established physical principles, there is a badly outlined suggestion for how it might work, and the data that ostensibly demonstrates that it does work is both sparse and inadequately explained.

The buzz returned this week, as the group behind the EM-drive has published a paper describing tests of its operation.

Before getting into the paper, let me step back a bit to set the scene. I am not automatically rejecting the authors' results. I am not even rejecting the possibility that this study may hint at a new physics. I am saying that before I will take that possibility seriously, I have to be convinced that the data cannot be explained by the current laws of physics. And currently, I am not convinced. In fact I am very frustrated by the lack of detail.

The idea

The idea behind the WTF-thruster is so poorly explained that I need to use some guesswork here. It seems that it's related to a plasma thruster; one needs to provide an electromagnetic field and accelerate charged particles. The back-action, provided by accelerating the charged particles, accelerates the thruster. This is an excellent idea, and I think everyone should have at least two in their home.

However, the WTF-thruster goes two steps further: first, there is no place to shoot the charged particle out of the thruster, and second there are no charged particles in the thruster in the first place. In a Chopra-esk kind of way, the Universe cooperates and provides the fuel free of charge.

What do I mean by that? The idea comes from quantum vacuum fluctuations. Vacuum is not nothing. Instead, it's filled with virtual particles that can become real under the right circumstances. Indeed, there are many measurable phenomena that are explained by virtual particles becoming momentarily real, doing something, and vanishing back into the imagination of the Universe.

It's kind of like borrowing your neighbor's lawn mower: you can ask, but maybe they'll say no. Or you can wait until they're out, grab it, mow your grass, top up the tank, and return the lawn mower. Your neighbor can still detect the borrowed lawn mower by the smell of recently cut grass, even though the lawn mower—making gentle ticking noises as it cools—has not apparently moved. OK, that's a bad analogy, but the fact is that virtual particles can make their presence felt in the real world.

The WTF-thruster uses a mechanism to make these particles real and then accelerates them, after which they vanish. In the short time that they are accelerated, however, they generate thrust via back-action.

On some level, this makes a small degree of sense, but then you get to the details. There is nothing to suggest that a big hollow copper cone with a bit of plastic at one end and almost enough microwave radiation to heat a slice of bread is conducive to extending the life of virtual particles.

Is there more detail?

The reason that these particles are only momentarily real is that the particles are created in pairs that annihilate each other almost immediately, so the virtual universe giveth some energy and immediately yanks it back.

Under unusual circumstances, you can steal from the virtual universe. At the event horizon of a black hole, for instance, a pair of particles may be created in such a way that they are on either side of the event horizon. One escapes, carrying energy (called Hawking radiation), and one falls into a black hole. Sadly, as dense as the copper used in this device may be, there are no black holes involved in the experiment.

You can also suppress vacuum fluctuations to generate a force. Two conducting plates, separated by a certain distance, restrict which fluctuations are allowed between the plates. Outside, there are no such restrictions. The imbalance between the fluctuations inside and outside the plate provides a net force that pushes the two plates together. Again, this is all well explained by current physical theories.

But the experiment performed by these researchers isn't. No, the WTF-thruster seems to operate on the principle that these momentarily created particles are easy to separate (needed to avoid that paired annihilation issue), because at the boundary of a bit of plastic, the density of vacuum fluctuations changes and therefore... um, "physics" happens?

Unfortunately, that's all I can possibly say about the physical basis for this device. The paper mentions at one point that the authors have a model that predicts how much thrust should be generated for a given set of conditions. But there is no reference to that model. An Internet search yields a YouTube channel (which I did not peruse) and a NASA report. The NASA report has some concepts that hint at a model, but nothing beyond that (certainly nothing predictive) and no references to more detailed information.

Surely the experiment reveals all, right?

The WTF-thruster consists of a closed microwave cavity that has a cut-off conical shape. The necessity for the conical shape is not explained. A bit of plastic is placed at the narrow end. When microwaves are emitted into the cavity, thrust is produced.

Since the amount of thrust is very small, the whole thing is mounted onto a torsion pendulum, which rotates slightly when the microwave is turned on. The amount of rotation was measured, and that measurement was used to determine the amount of thrust. This is the same experiment that these researchers have used previously, with small refinements to how things are mounted and connected to reduce spurious signals.

Despite having a setup that has been pretty much operating for years, how many data points are in the paper? Eighteen. Now, if this were a really time-consuming experiment, I wouldn't let that bother me. Hell, some synchrotron experiments have only a single data point. But this is clearly not a time-limited experiment.

The microwave was pulsed for about 40 seconds, and an entire data run seems to take about 200 seconds. Allowing five minutes between measurements, it should have been possible to record 12 data points for the same settings every hour. Indeed, although the researchers have numerous variables at their hands to change between experiments, they only play with one. In previous papers, they played with two, but still this limited exploration and limited data is really disheartening.

Then there's the error analysis: the authors estimate many measurement uncertainties so that each thrust measurement has an uncertainty of about ten percent. That sounds brilliant, right? Except the authors ignore the main uncertainties. In one experiment at 60 Watts of microwave power, the authors measure thrust of 128 microNewtons, while all three data points for 80 Watts of microwave power have thrusts of less than 120 microNewtons. Indeed, the thrust at 60 Watts for all data overlaps pretty much perfectly for all data taken at 80 Watts. They can only claim a slope by turning the power down to 40 Watts, where they do consistently measure less thrust.

In other words, you apparently can't get more than 120 microNewtons of thrust out of this machine. Why? The paper doesn't speculate on that question at all.

The more important point is that the individual uncertainties in their instrumentation don't account for the variation in the thrust that they measure, which is a very strong hint that there is an uncontrolled experimental parameter playing havoc with their measurements.

Don't expect to be able to figure out what it might be, though, because all manner of details are missing from the experimental description. For instance, the authors describe how it is important to tune the microwave frequency to a particular cavity resonance very precisely. However, the all-important details of showing the adjacent modes and a comparison between operating in one mode compared to another was not shown. I later discovered this in another paper, where an instrument screen shot is compared to a calculation of the cavity resonances (they don't match). So overall, I'm not even convinced that the authors are exciting the microwave cavity the way they think they are.

Indeed, it's difficult to determine if the amplifier was operated correctly: according to the researchers' data on the cavity, the bandwidth (and I'm doing this by eye) is about 40kHz, so if the time it takes to ramp the amplifier from "off" to "full power" is less than 25 microseconds, there is going to be a substantial amount of reflected power in that first moment. But no rise-time is given, and the plots of microwave power as a function of time indicate a number of transients. It's not possible to tell if this is deliberate or if it's the microwave cavity reflecting power.

Then there is the pendulum itself. The authors have applied damping and only measure when there is no seismic activity. They've checked for magnetic and electrostatic effects, but they don't report the resonance frequency of the pendulum. This is the data that is needed to appropriately interpret the graphs.

The use of a pendulum also suggests the sort of experiment that would, again, amplify the signal. Since the pendulum has a resonance frequency, the authors could have used that as a filter. As you modulate the microwave amplifier's power, the thrust (and any thermal effects) would also be modulated. But thermal effects are subject to a time constant that smears out the oscillation. So as the modulation frequency sweeps through the resonance frequency of the torsion pendulum, the amplitude of motion should greatly increase. However, the thermal response will be averaged over the whole cycle and disappear (well, mostly).

I know that every engineer and physicist in the world knows this technique, so the fact that it wasn't used here tells us how fragile these results really are.

So where is the thrust coming from?

And yet the authors report net thrust. Where does it come from? The simple answer is "I don't know." It could simply be random noise and that there is absolutely nothing to explain. Yet the authors have a pair of papers that consistently show a positive result (there is also one paper from a second group that failed to replicate the result). I suspect that if the thrust isn't noise, it is due to a thermal effect that is unaccounted for. That is, something is radiating in some unexpected way and generating thrust. My suspicion is that it is the copper cavity (most people suspect the amplifier and its enormous heat sink, but I like a dark horse).

Unfortunately, the author's analysis of thermal effects consisted of taking infrared images and reporting an average temperature of various components. But temperature is one of the hardest things to measure with accuracy and precision; considering that they are measuring displacements of just a few micrometers, it's kind of important to get it right. This is brought home by the fact that the known thermal displacements are already larger than those they attribute to thrust.

In the end, we can't conclude that this is a null result, nor can we excitedly say that it works. The sad truth is that this paper is not much better than the researchers' last one, and it doesn't actually have enough detail to let us fully evaluate the data. Nor does the paper have enough data to allow a conclusion in the absence of a model. And despite mention of a model in the paper, any model that exists is very well hidden.

What's more, the discussion section is not very encouraging. It's essentially a laundry list of strange things that happen because of vacuum fluctuations, coupled to—for no reason that I can discern—Bohm's pilot wave interpretation of quantum mechanics. Whatever Bohm's failings might be, I'm pretty sure he didn't deserve that.

I know that I sound a bit flippant and dismissive of this work. I promise you that I went into this paper determined to be skeptical but positive. Unfortunately, all of my positive thoughts drained through the gaping holes in the paper, leaving me at skeptical and exasperated.

Journal of Propulsion and Power, 2016, DOI: 10.2514/1.B36120