There have been many news stories saying that the EM Drive will solve almost all problems in interplanetary travel, permit low cost flying cars and who knows what else. Other stories say that it is flat out impossible and we shouldn't spend a single publicly funded research dollar on it. But I haven't seen a single article with the rather boring suggestion that perhaps in this case the research community has got it exactly right. That it's not a perpetual motion machine, doesn't deserve to be dismissed out of hand. But it's far too soon to justify huge research programs into it, even if it is a real effect. We just have to be patient and see how the experiment develops. So, here is a news story to say - that. In detail:

(You can also get this article as a kindle ebook)


There has been a lot of publicity for the original news stories. Headlines such as 'Impossible' rocket drive works and could get to Moon in four hours. And a lot stories debunking it, headlines such as  How to fool the world with bad science — Starts With A Bang!

This is the device that all the news is about. An asymmetrically shaped chamber. Radio frequency waves bounce around inside the cavity (specifically, microwaves like your microwave heater). The inventors claim that there is a net thrust, even though none of the waves escape the cavity, just through the way they bounce around inside it and the asymmetry of the cavity. With the direction of thrust depending on the orientation of the cavity.

So far the levels of thrusts involved are measured in micronewtons - if you hold one milligram in your hand, the downward force you feel is 9.8 micronewtons. Micronewton thrusters are used for fine position control of satellites. But the inventors claim that much higher levels of thrust will be possible in the future. 

But after the initial dramatic news stories about the device, and these stories debunking them with sometimes well known scientists debunking the experiments, you get a final wave of news stories debunking the debunkers. I think it is worth looking at these also, they deserve more publicity than they got. 

The early debunkers of the original stories, in their eagerness to downplay the dramatic media announcements, made several quite serious errors of understanding of what the claims were and how the experiments were conducted. These errors have been repeated over and over in the social media by people who share the news stories and quote the opinions of the debunking scientists.


First, the scientists who are doing the research are not responsible for all this media hype. 

It is true that some of the original inventors make extraordinarily bold claims. According to Robert Shawyer, inventor of the EM Drive, it will solve many of the world's problems.

Other experimenters though are just exploring it in an open ended way as an anomaly that needs to be investigated via the scientific method. With the implications, if any, to be discovered at a later date.

The critics often go the other way, going overboard by debunking things that the experimenters don't claim, or misunderstanding the methods of the experiments. 

I'd like to highlight a couple of these misunderstandings that have been repeated over and over - such as the misunderstanding about what they meant by a "null" experiment - and also some of the common objections.  such as the idea that it has to violate conservation of energy or momentum and the objections based on delayed response. These objections, as we will find out, are themselves based on misunderstandings of the claims and the experimental data, and I'll look into a famous example in the history of science of an advance in science that came about as a result of detailed investigation of an experiment that seemed on the face of it to violate both conservation of energy and conservation of momentum.


Some of the Eagleworks presentation was undoubtedly a bit "wowy", see for instance 49 minutes into the talk.

(NB the video is a bit technical, it may help to read this first for context if you want to give it a listen: 
The FACTS as we currently know them about the EmDrive and Cannae Drive)

However it may help to have a bit of background. That's their remit, to explore potential for advanced methods of propulsion for exploration of the solar system and beyond, even if the effects currently are tiny. 

"NASA/JSC is implementing an advanced propulsion physics laboratory, informally known as "Eagleworks", to pursue propulsion technologies necessary to enable human exploration of the solar system over the next 50 years, and enabling interstellar spaceflight by the end of the century.

... Recent work published by White [1] [2] [3] suggests that it may be possible to engineer spacetime creating conditions similar to what drives the expansion of the cosmos. Although the expected magnitude of the effect would be tiny, it may be a "Chicago pile" moment for this area of physics." NASA Announcement about setting up the Eagleworks Laboratories: Advanced Propulsion Physics Research

What you need then, to put these statements into context - is that they do research with a view to the very long term future,looking for tiny effects that maybe some decades from now may be useful new physics.

So I think the main thing there was that journalists probably got the idea from the presentation that they expected this imminently, especially combined with the things said by the inventors of the drives themselves. However their remit is to take the long view, to look at tiny effects that might lead to breakthroughs a few decades from now, and so what they said should be understood in that context.


This was one of the most publicized "mistakes" highlighted in articles like: How to fool the world with bad science — Starts With A Bang! and Page on which are shared over and over in social media discussions on this story.

Quoting from the abstract of the Eagleworks paper:

“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).”

On a first read you might think - well - why read any further - they have already said that it doesn't work? What is all the fuss about?

However, if you read the abstract and the paper carefully, it is clear that the  Null version wasn't a null test for the EM drive itself. It was a null test for a version without grooves, to test one of the inventors' claims that certain grooves put into the chamber were necessary. 

The experiment showed that not only were these grooves not necessary, they didn't make any difference to the amount of the effect. 

So it was a "null version" for the grooves claim, not for the basic drive hypothesis that RF waves bouncing inside an asymmetrical cavity can generate thrust. Both versions generated roughly the same measurable effect in the experiment, of apparently a very small amount of thrust.

The true "null" version was when they tried an RF load with no apparatus - and this, they report, generated no apparent thrust. 

All this is reported in the paper they link to. If you read the abstract only then you might come to the same conclusion as many of the debunkers. But after you read it, if you then read the article, you will find that the null experiment was just for the grooves claim, and the asymmetrical chamber hypothesis was tested separately and the asymmetrical chamber is what the experimenters say did have an effect in their experiments.

- so the authors of these debunking articles were just rather sloppy in the way they read the abstract and the paper. This tends to dent ones confidence in the debunkers, rather than in the original experimenters. If the debunkers could misunderstand such a basic point, one that is made clear in the paper, hard to see how it could be understood any other way - how much else have they misunderstood?

The authors of the Eagleworks paper also could have worded their abstract more carefully perhaps. But if  you've ever tried to write an abstract, you may know how hard it is to summarize your work in just a few sentences.


This also is often quoted and shared:

”CalTech physicist Sean Carroll, who we’ve spoken to previously about the feasibility of an EMDrive, echoes Davis’ sentiments.

“My insight is that the EMDrive is complete crap and a waste of time,” Carroll tells io9. “Right there in the abstract this paper says, ‘Our test campaign can not confirm or refute the claims of the EMDrive’, so I’m not sure what the news is. I’m going to spend my time thinking about ideas that don’t violate conservation of momentum.”

No, German Scientists Have Not Confirmed the “Impossible” EMDrive

And yes , if you read the conference paper carefully, it says clearly that it hasn't confirmed the drive, even in the abstract.

Direct Thrust Measurements of an EMDrive and Evaluation of Possible Side-Effects, and full paper available here (on the author's own university website).

That shows that the journalists for a lot of the newspaper reports didn't read the paper carefully. It's not the fault of its authors though that they were reported as confirming the EM drive when the abstract says nothing of the sort.

They make it clear that it is work in progress. 

The paper says that according to their measurements it behaved as expected in a vacuum - but they still have things to check by way of alternative explanations.  

On the other hand though, it doesn't say that they have found an alternative explanation yet. So, there is no way can this paper be taken as disproving the drive either.

It neither confirms nor disproves it. It's work in progress reporting some interesting interim results for other experimenters, and themselves, to follow up further to find out what the significance is, if anything. That's how science normally works.


Much also was made of the delayed onset of the effect reported in this paper and that it continued after the power was switched off. This seems to suggest it was caused by heat.

But - if you read the paper carefully again - you'll see that the experimenters looked carefully at the possibility of a thermal explanation. Doing things such as insulating the chamber so the external temperature increase was reduced to only 4C (had no effect on the amount of horizontal thrust). And they did spot thermal effects. But these operated vertically, which is what they expected, and so could be disentangled from the thrust effect which was horizontal.

And a delayed effect might be exactly what you'd expect if, for instance, the chamber was being "charged up" - like an increase in electric charge, or a pressure differential, or whatever it was. That's just an analogy there. If the thrust had turned on and off instantly, I think we might well have got skeptics saying that this instant turn on / off was a clear sign that, for instance, it was some issue in the electrical wiring and magnetism rather than a real effect.

So I don't think we should conclude that because of delayed onset, that it is not a real effect.


Also many say it is impossible because of conservation of energy or momentum. But new unexplained physics is very likely to appear to violate these laws. For instance if you didn't know about gravity, hadn't taken account of it in your physics, then whenever you drop something - that violates both conservation of energy and conservation of momentum. The object accelerates to the ground and then hits it, releasing energy. And does that without either apparently any energy supplied to it, or any momentum exchange.

Now we explain that in our physics by saying that there is a potential field and that it originally has potential energy. I.e. we add in a number that tells us how much energy it has. And when it drops, this number is reduced and converted to kinetic energy. (Let's stick to the simpler Newtonian mechanics approximation here rather than bring in the complexities of General Relativity, it's just for purposes of illustration).

And as for the momentum exchange, we say that when it accelerates towards the Earth, that the entire Earth also accelerates by a small amount towards the falling object - though there is no practical possibility of ever measuring this momentum change of the Earth itself. We just assume that it has to do this, by extrapolation from smaller scale experiments.

And - sometimes physicists do observe apparent violations of these laws. For instance in the process of beta decay, then there is an apparent violation of both these laws. This was explained by the hypothesis of a neutrino. Pauli proposed this in a letter in 1930 (though here he calls it a "neutron" - the particle now called a neutron wasn't discovered until a few years later):

Wolfgang Pauli, who proposed the "neutron" (now called neutrino) as a "desperate remedy" to preserve conservation of energy during beta decay in 1930.
"Dear radioactive ladies and gentlemen,

As the bearer of these lines [...] will explain more exactly, considering the 'false' statistics of N-14and Li-6 nuclei, as well as the continuous β-spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the energy theorem. Namely [there is] the possibility that there could exist in the nuclei electrically neutral particles that I wish to call neutrons,[nb 2] which have spin 1/2 and obey the exclusion principle, and additionally differ from light quanta in that they do not travel with the velocity of light: The mass of the neutron must be of the same order of magnitude as the electron mass and, in any case, not larger than 0.01 proton mass. The continuous β-spectrum would then become understandable by the assumption that in β decay a neutron is emitted together with the electron, in such a way that the sum of the energies of neutron and electron is constant."

Pauli's letter (wikipedia)

The neutrino itself was not discovered until much later. It was finally detected in 1956, by beta capture, about a quarter of a century after Pauli's original suggestion as a "desperate remedy". And it was a long time before the lower energy neutrinos from the sun were discovered (in the 1960s).

Though it is easy to create neutrinos, they interact only weakly with matter and a single neutrino can easily pass through the entire Earth without interacting with any matter. So they can only be detected in very large numbers. And they can carry away momentum, and they can lead to apparent violations of conservation of energy.

Which doesn't mean that the EM drive has to create neutrinos. But there could be many other particles that are even more weakly interacting than neutrinos. Or other effects that we don't understand yet that can carry away momentum and balance up the energy equations.


The one thing that is impossible is a perpetual motion machine. If you find a system that returns to its original state precisely, with a net output of energy - then there is no way to make that consistent with conservation of energy or momentum. That's because any numbers you attach to assign "potential energy" to the system will be the same at the beginning and the end of the experiment, so you there is no way to do that to make the numbers add up.

But that's not the situation here. There's a net input of power leading to the thrust, so you don't have the same state at the beginning and end of the experiment, so it is not a perpetual motion machine.


Irrespective of any arguments about conservation of energy or momentum you need to start with the data first. Whatever you see, you need to describe it and understand it. You can't dismiss data just because it doesn't fit your scientific pre-conceptions.

Isaac Asimov has a fun story about that, a physicist who wakes up one day to find that he can levitate. And he can't find any explanation of what is happening, and has a lot of problems trying to get anyone to investigate it because he is the only person able to levitate. (This story is called "Belief", published in Astounding Science Fiction in 1953, and various anthologies since then, details, see Asimov's Published Stories (wikipedia)).

Why is it that this never happens in real life? Could it? If not, why not? Physics can't answer questions like that.

Which doesn't mean this EM drive is a violation of these laws. It may conserve momentum by new particles - or by turning the ideas of a "virtual plasma" in some way into something acceptable as new physics. Or in some other way not thought of yet.

But - what if we did find something that seemed to violate these laws? Well - you still have to investigate that also, as good scientists. It's not scientific to be like the characters in Asimov's story and refuse to look at the experimental data because it doesn't fit your views about how science "should" work. Experimental data always comes first in science.


In a normal spacecraft - most of the energy goes into the exhaust, not into moving the spacecraft, if it has high velocity exhaust. 

For instance, an artificial scenario, to use as a thought experiment - suppose you have a friend who keeps pace with you in another spacecraft who will hand over whatever exhaust mass you need whenever you need it, but won't supply you with extra energy to fire it?

Then it would make much more sense to fire 1 kg at 1 meter per second than 1 mg at 1000 meters per second. Both give your spacecraft the same delta v, but the 1 mg requires a thousand times as much energy to fire it for the same delta v. (Energy required is half the mass times the velocity squared).

It would make even more sense to fire one ton at a thousandth of a meter per second, which would require a thousandth of the amount of energy needed for one kilogram at one meter per second. 

So in this artificial situation, with limited energy, and with unlimited exhaust mass available from a friend like that, you'd take on board as much fuel as they can supply you, and fire it at the lowest exhaust velocities you can to achieve the desired thrust, and the energy requirements would then be very small.

So an ion thruster would make no sense at all if the exhaust mass was unlimited. 

But because you have potentially huge amounts of energy via e = mc^2, and because of the impossibility of carrying enough fuel to fire it at low exhaust velocity, it makes sense for an ion thruster to fire small amounts at high velocity even though it is a much more inefficient way of propelling a spacecraft.


So the big question would seem to be, where does the mass come from for the momentum change? 


If it comes from conversion of energy to mass, then - it's really not that much different from an ion thruster in how it works. 

You have your RTG, it supplies energy to your "reactionless drive" by converting mass to energy, and your thruster then converts some of that energy back to mass, and the rest it uses to accelerate that mass. And so you would be using mass, but indirectly, using some of the mass of the RTG as it decays. 

If you use solar power then similarly, you are using the incoming solar photons as energy (which were originally the result of a mass to energy conversion in the sun), then converting some of them back to mass, and then using that mass for propulsion. In that case it isn't depleting any mass from your spacecraft.

Both of these might be an efficient way of exploring the solar system, It's not violating any laws of physics. It seems a kind of roundabout way of doing things, why not just accelerate fuel that you take with you like the ion thruster does? It might not have quite the savings they expect when you take that into account, but still might be worth doing. 

It might be more efficient than solar sailing, when using energy from the sun, because the energy to mass conversion lets you fire the reaction mass away at a much lower relative velocity.

But does this idea work? A calculation using the relativistic energy momentum relation shows that if the particles it emits have positive rest mass, then the momentum is actually less than if it emits photons. See the FAQ.

So, actually a photon rocket is optimal, surprisingly, by the energy momentum equation. The reason that ordinary rockets don't use photons is because we have no easy way to convert rocket fuel mass directly into energy. If we had total conversion of mass to energy, all our rockets would be photon thrusters as it would make most sense to convert any excess rest mass into energetic photons, and use those for thrust.

But as we'll see in the FAQ, the EM thruster is a couple of orders of magnitude too powerful for a photon thruster, given the amount of energy supplied. There are a couple of rather wayout ways we could get around it, but basically it doesn't seem too plausible that it is just converting energy to matter and using that for thrust.


On the other hand if you are picking up the mass somehow from your surroundings as you travel - well - that's like the friend supplying you with mass as the travel alongside you - there would need to be a lot more explanation, and more gaps to fill, to explain how you manage to get mass supplied to you that's traveling at the same speed as you. 

The easiest way to understand this - without bringing in problematical ideas of a virtual plasma - might be that the mass actually isn't moving at the same speed as your spacecraft.

There might then be some energy loss needed to bring this mass up to the speed of your spacecraft before you fire it out as exhaust - nevertheless it might be worth doing, just as for a Bussard ram jet.

So - maybe the "vacuum state" actually has an inertia, and it is actually some kind of a plasma which is,say, at rest relative to the Big Bang for instance. That doesn't violate any laws of physics because we already have a "preferred rest frame" locally which you can determine by finding out whether or not you are accelerating relative to the three degree background. If the "vacuum state" in some way gets locked into a particular structure in the big bang and then just spreads out, it might have a preferred rest frame in that sense perhaps. Of course very much hand waving here, it's going to be new physics after all.

So then the efficiency of the motor would depend on how fast you are traveling relative to the three degree background radiation. That would be sort of like using the three degree background radiation itself as fuel. 

Or might be that it is using some other component that is distributed in space that we don't know about. Maybe picking up and accelerating WIMP like particles from what seems to be a vacuum for instance. 

It might be stationary relative to our galaxy for instance - or orbiting the center of the galaxy at the same speed of the sun - or stationary relative to the sun  or even orbiting the sun at the same speed as the planets, could be many possibilities there.

So with either of those ideas, you'd expect then that there would be some penalty there as you travel faster and faster relative to the source "material" whatever it is. And if you happen to be at rest relative to it, then you could pick up and accelerate as much mass as you like with almost no penalty.

You might notice that experimentally, perhaps, if the drive works just a bit better depending on your velocity relative to the three degree background, or any of these other potential sources - which might vary depending on the time of year, and time of day also for that matter.

Here I'm not at all attempting to give an alternative theory for the drive. It is just to show by way of examples that it doesn't immediately and obviously have to violate conservation of energy and momentum. 


The idea that it might involve some kind of weakly interacting unknown new particle is not my own idea.

It is one of the suggestions made in the Eagleworks talk, which also surveyed and touched on some other more outlandish ideas for how it might work without violating the conservation laws, such as that it might involve a low level of warping of space time (which would result in propulsion without any momentum exchange at all, like the theoretical Alcubierre "warp" drive)


In all of this, I think the chance that the drive actually does work is low, and expect that, like the faster than light neutrinos, that they will find an alternative explanation eventually.

But it's been my experience so far that the writings of the debunkers of this experiment are often of a low quality scientifically as I just outlined above. 

This is not too surprising as these are scientists who are not at all expert in this topic area  and scientists are only human and often make embarrassing mistakes when they make statements outside their main areas of expertise. 

And the mistakes were subtle ones. Such as the misunderstanding about the nature of the "null" device. I'm not at all sure I'd have spotted them myself, I found them as a result of reading some of the spate of less publicized news stories debunking the debunkers that were published immediately after the debunking stories.

So - I sympathize with the experimenters. They don't deserve to be debunked in this harsh way by people who haven't taken the time to properly familiarize themselves with what they are doing.

Though it is also natural for the debunkers to want to debunk something that seems to violate the basic laws of physics. It's not often you encounter something that seems to violate conservation of momentum as a physicist, and when you do it is usually an error - and it's only rare that observations of apparent violations lead to new physics.

Which doesn't mean I think that the EM drive is real. I've no idea.

But I think it is intriguing enough to follow up until we either find that explanation - or in the more surprising and interesting case - confirm it.

And - as so often happens with things like this, I expect that they will find that it is experimental error. I suggest this for no other reason than because that's what usually happens with these things.

But every now and again out of all these tests and experiments, then you get one or two that end up revolutionizing science. And you wouldn't get those revolutions in science if it weren't for the tenacious experimenters that keep working at their experiments even when everyone else tells them to stop. 

If they do find that it is experimental error - well - that's a forward step in science also, to find a source of an error that is at present so baffling that nobody has yet been able to explain it.


Even if it turns out to be a real effect, then without knowing how it works, then it's impossible for anyone to say for sure what the implications will be. Robert Shawyer has said he will soon create new versions of the device able to generate thrusts of order of newtons, not just micronewtons and has made many optimistic predictions. If he succeeds then it would have many benefits probably.

But - just based on the history of dramatic scientific breakthrough announcements by inventors, and dramatic new science results - with no disrespect intended to him - it could as easily turn out to be results of experimental error like the faster than light neutrinos. Or it could be a small scale effect, perhaps based on known physics, that never gets turned into a useful device. Or it may be in between the two, and result in a device that is sometimes useful in particular circumstances, like an ion thruster. Or in the more interesting case where it involves new physics, or new principles, it could be indirect evidence for something interesting, but still not result in a new space drive or hover cars.


So, yes, I do think that the experiments are being unfairly downplayed by a few scientists and journalists. Also unfairly overplayed by others :).

I wouldn't go so far as to say they are being downplayed by the scientific community as a whole however. Scientists are investigating it carefully as the recent German experiments show. I think they are giving it an appropriate level of attention.

At the same time both the experiments and the debunkers are being way overhyped by the media. And in a situation like this, emotions run high, and some of the scientists who get involved, who are usually not experts in this topic area and not the same scientists who are tasked with doing the experiments or reviewing them - being only human, are liable to make many mistakes in their eagerness to either support or debunk the experiments.

But it's not going to be settled in this way through internet discussions in various forums, and social media or through dramatic news stories. We will only find the answer by continuing this process of careful experimentation. And unlike the examples of the likes of Spock in movies - in the real world scientific progress is often a gradual and slow process. The rest of us will just have to be patient and see what happens. 

Spock with Kirk and McCoy on the bridge of the Enterprise in 2267 (Star Trek fictional future universe). In the movies, we are used to scientists such as Spock making a few observations and then immediately announcing groundbreaking new physics. The reality is often much slower, involving experiments that push the limit of what we can measure, and can take years or decades sometimes to reach a final resolution.

And along the way you get many intermediate results that advance understanding a little without yet resolving the question of whether there is a real effect or not, or what exactly it does if it is real. We just have to be patient and await the results of this research. We probably won't solve this by internet discussions or "Spock like" sudden illuminations of insight based on minimal data.


This article originates in my answer to a quora question: Is the significance and/or creditability of the Eagleworks EM Drive hard vacuum tests being unfairly downplayed by the scientific community?

See also the earlier quora question: How does the EmDrive warp space?

And this Reddit discussion from last year which gives good background data on the two drive ideas (there are actually two different suggested drives, the EM drive and the Cannae Drive, that are similar in design): The FACTS as we currently know them about the EmDrive and Cannae Drive • /r/Futurology

And the EM drive subreddit


Do you want to download this as an ebook, to read on your kindle, or kindle app (available for most operating systems)?

You can get it here:

EM Drive - Let's Wait for the Data :) (Amazon)