The Impossible Space Engine is Back

Here we go again.

I didn’t really want to write this post. I’m reluctant to give further visibility to something that’s already overly sensationalised. I’m also a little scared of wading back into this controversy, especially since others have already done fantastic jobs, but against my better judgement, here we are. The last time I wrote about this, in an admittedly hastily-written article for The Conversation, I was accused of everything from basic incompetence to being an agent of the Illuminati, working to suppress this wondrous new discovery from humanity for the benefit of the New World Order. (We had the accuser killed, of course.)

The infamous EMDrive, the propellant-less engine that’s the brainchild of British engineer Roger Shawyer, is back in the news this week because a paper from NASA’s Eagleworks team has passed peer review and will be published in the AIAA Journal of Propulsion and Power. It’s already available online here. (NB: This work was done by a small NASA sub-team which the IBT report has since been shut down and as far as I’m aware, NASA as an entity has not endorsed the results. I’m saying this not to disparage the people behind the work, but to caution that applying the weight and kudos of the label ‘NASA’ to this work is, as Corey Powell says, a ‘gross oversimplification’.)

Let’s back up a second and discuss what it is. The EMDrive, stripped back to its basics, works on the idea that reflecting microwaves around a conical cavity leads to the production of a force, despite not emitting any from of propellant or needing any material fuel aside from a microwave source, and thus leads to a highly efficient form of engine. If it worked, this would be great, but it’s not that simple.

Straight off the bat, the core of the idea ostensibly contradicts the Conservation of Momentum, a law of physics related to Newton’s Third Law and one that, more rigorously, stems from Noether’s Theorem under the condition that free space be homogeneous. There are some exotic suggestions as to how the EMDrive might operate without violating this law, but to the best of my knowledge they’re all fringe hypotheses that have little support.

That being said, however much we capitalise their names and call them ‘laws’, the laws of physics aren’t immutable and really just represent our best guess as to how the universe works. There could be more going on than we know of – in fact, there almost certainly is. The laws of physics could potentially be wrong or incomplete, the exotic extensions could be correct or there could be loopholes in the standard theories. But if something is going to challenge one of the best-tested, most fundamental laws of physics, it has to make an extremely good argument for it.

Which brings us back to the EMDrive. The latest paper on the topic has just passed peer review, the process designed to weed out bad science and ensure only solid science is published.

Does this mean that the EMDrive works?


What it means is, as far as I can tell, that the latest experiments have measured a persistent thrust that cannot yet be explained. Were I a betting man, I’d be prepared to wager a great deal that more accurate experiments will determine this anomalous thrust to be caused by some experimental error that so far hasn’t been accounted for. In the new paper, the team themselves give a detailed discussion of possible sources of error and acknowledge the possibility that it might be a thermal effect (and in my last article, that’s what I said about it then too).

Experimentally, it’s hard to measure zero thrust. There is always some sort of experimental error. Electrical noise in the wires, thermal noise, air pressure and for something as sensitive as this, vibrational and potentially even gravitational effects could play a role in generating an anomalous non-zero thrust. The experimental team have really had their work cut out trying to eliminate all such possibilities, and it looks like they’ve given this a great deal of thought. However as long as there remain credible sources of error and as long as we are lacking a rigorous theoretical understanding of how such a device could operate, the conclusion that it’s actually working, against all odds, remains premature.

It’s important to be clear here that when I talk about errors, I’m not for a second suggesting the experimental team aren’t doing their jobs properly – ‘error’ doesn’t mean ‘mistake’ on their part. Experiments are fiendishly difficult and it’s incredibly hard to eliminate all possible causes of false signals. All we know for sure is that the team have tested the device and measured a thrust. Either the laws of physics are incomplete, wrong, or there’s something unaccounted for going on with the experiment that’s leading to a false reading. The simplest solution is the latter.

The team suggest some possible explanations for what they observe – and there are various other papers out there with other suggestions too – however at this point there is still no rigorous theoretical understanding of any effects that could cause this anomalous thrust. The pilot wave approach (a non-standard version of quantum theory which I’m a big fan of) obliquely appealed to in the latest paper makes no predictions to support the EMDrive, despite claims in the media suggesting otherwise, and the ‘quasi-classical’ (i.e. not fully quantum mechanical) numerical calculations of a single hydrogen atom which they refer to do not support a leap to talk of ‘pushing against the quantum vacuum’, something which, with all due respect, makes very little sense. Their theoretical suggestions are muddled and incoherent, and there remains no solid basis to believe the EMDrive can work as suggested.

tl;dr – there is still no reason yet to believe that the observed thrust is anything other than an experimental error not fully controlled for.

If that ever changes, I’ll happily admit to being wrong. I’d love it if the EMDrive turned out to work – it’d mean we’d learn something new about physics and having a clean, efficient space engine would be fantastic. But I don’t think we’re there yet.

Now if you’ll excuse me, I have to go alert my Illuminati buddies that I’ve done my part for the cause.

(For the sense of humour impaired, that was a joke…)

Dr Steven J Thomson




3 thoughts on “The Impossible Space Engine is Back

  1. The idea has been universally panned of course. Another good example here: .

    Panned since as so many pseudosciences they rely on non-existing evidence of conveniently weak effects. If you make extraordinary claim you must have extraordinary evidence. But if you can’t reject the null, give or take an experimental error, you have nothing. I like the description of “idea zombies”, which in Feynman’s analogy would be “cargo cults of ideas”.

    It is of course practically impossible to drown out the noise of the Internet, or even get facts promoted to the top of a search list. And cargo cults/pseudosciences can be studied as general phenomena. Responses to them could possibly be more usefully seen as preserving sanity among onlookers. (And I note with satisfaction that the group has separated from NASA based on ‘irreconcilable differences’, they felt underappreciated I take it, perhaps NASA thought they were wasting their money on nothing useful.)

    Which lead me up to the point in the analysis I am going to respond to. 🙂 If the nitwits of the work relied on handwaving pilot wave theory around, I would put that down as another problem for them. I don’t claim to be an expert on these things, but I have read – and not checked – the claim that PW theory is Galilean instead of Poincaré or in other words does not respect relativity as a constraint. (Or as symmetry, seeing the title of the blog). So its claim to fame should be a s a toy model, not as theory in competition with other quantum mechanical theories as long as testing can’t narrow the field further.


    As an aside, I don’t think current understanding of Noether’s theorems as they are embodied in our universe is likely to be wrong. When the LHC observed a “near-enough-standard” Higgs it put the final rivet into the steely glinting tower of Wilczek’s Core Theory for everyday physics, and the test itself was fantastically complex. (Tens of free parameters.) The remaining exotic physics (dark matter, inflation/dark energy, black holes) must be of higher energies than LHC could cover.


    1. Thank you for this fantastic and insightful comment. I hadn’t seen the ArsTechnica post so I’m really glad you’ve highlighted it here – it’s an excellent, far more detailed look at the situation. (Frankly, I wish I’d written it!)


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