“…axions are potentially detectable through their weak coupling to electromagnetism…” -Aaron Chou

We know, from hundreds of years of experience with the laws of physics, that momentum is strictly conserved, and therefore a reactionless drive is impossible. What’s not impossible is an engine that has a reaction that’s simply invisible, or otherwise undetectable to us. This has been seen in experiments involving neutrinos, but NASA’s impossible space engine, the EMdrive, offers another possibility: a dark matter reaction.

Image credit: ESO/L. Calçada, of the illustration of the dark matter halo surrounding the luminous disk of our galaxy.

You see, one of the leading candidates for dark matter is the axion, an ultra-light, massive, abundant particle that would couple to microwave photons under the right conditions. While ADMX, the axion dark matter experiment, looks for this coupling in a microwave cavity, it’s come up empty so far. Could the tinkerer who invented the EMdrive have accidentally stumbled upon dark matter instead?

The surface magnetic field of an active EMdrive, during the NASA test. Image credit: NASA Spaceflight forums, via Chris Bergin.

It seems like this speculation hinges on quantitative questions: What is the local axion DM density? What is the axion-photon scattering cross-section (as opposed to photon-axion mixing)? What is the photon density in the EMDrive cavity?

I can very roughly estimate the latter. The average density of dark matter is about 5 GeV/cm^3 (don’t you love mixing units!), based on the very approximate ISM density of 1 hydrogen atom per cm^3, and DM being five times normal matter. The axion mass is estimated at a fraction of an meV (“milli” not “mega”). So the number density in the EM drive (and everywhere else) is something times 10^12/cm^3.

That’s surprisingly non-negligible, actually. If those were regular atoms, they would be at a pressure of microbars or less, which is typical of high vacuum systems, but not super extreme.

The cross-section is more problematic. A 2014 paper (https://arxiv.org/abs/1402.4937) calculates cross-sections around 10^-29 cm^2 (10^-2 mb) for typical interactions, but enhanced up to 10^-17 cm^2 (10 Mb !!) in a resonant cavity. I don’t do axion physics, so I don’t have a really good sense for how realistic this might be, but let’s proceed…

If the EM drive is the perfect magical cavity for axion-photon interaction, then we can combine the density and cross-section to say that photons would have a mean free path around 10^5 cm (1 km), so we might expect 1 in 1000 photons in a 1 m cavity to interact with an axion.

The figure above says they put 100 W of power into the cavity. A conversion rate of 1e-3 would mean something like 0.1 W of “disappearing” power, which would be immediately noticeable, I think.

It seems like this speculation hinges on quantitative questions: What is the local axion DM density? What is the axion-photon scattering cross-section (as opposed to photon-axion mixing)? What is the photon density in the EMDrive cavity?

I can very roughly estimate the latter….

Or, you could just put the axion detector and the EM drive in the same room. That way nobody can claim that the EM drive is in a high DM density area while the axion detector isn’t.

Though honestly, I would see that more as assuaging the public than a scientifically justified test. ADMX is in Washington (State). The EM drive experiment occurred in Arizona. There is nothing about the DM hypothesis that would lead us to believe DM densities should vary by a lot on length scales of mere thousands of miles. Ethan has already written about the relevant subject: without a strong interaction between DM particles, they can’t clump. So the density shouldn’t be that heterogeneous on such small scales. Realistically then, this is an experiment we shouldn’t have to do. Realistically, any upper limit on axion DM density set by the ADMX experiment already applies to the EM drive experiment. But if it helps with convincing the EM drive defenders that it can’t be a DM effect, its probably worth doing.

Michael:
Your density estimate of 5GEV per cc assumes the DM is distributed the same as the ISM. I strongly suspect the ISM is much more strongly concentrated in the galactic plane than the DM. So the local DM to ISM ration is probably smaller than than the generation ratio of matter to DM. Perhaps by one or two orders of magnitude?

Now assuming DM particles have velocities of a couple hundred KM/sec, we could estimate the mass flux of DM that
flows throw the EM chamber. On average what change in DM velocity would be required to generate the observed force? Is that reasonable? What is the energy represented by this velocity/momentum change?

“Highly speculative possibility” …yeah, that’s one way of putting it very very mildly. I have another much shorter term, it actually IS a form of actual ‘dark matter’, but it doesn’t sound very scientific, smells bad, makes great rose fertilizer, and would probably get your dander up.
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Before even more imagined invisible undetectable things get involved in this …this endeavor, with even fancier math, WHY is DM is even being connected to the ‘not even understood’ EM drive at all other than ‘no clue’? Would not perhaps, I don’t know, maybe the EM field be the place to start looking…before all the imaginary, sorry, highly speculated possibilities? Any chances this could be evidence for a physical presence of a charge field as opposed to …stuff that no one can actually detect except by inference at astronomical distances and in sci-fi novels?
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HOW is this even considered a way to figure things out? Combine several “highly speculative possibilities” into a daisy chain? Undetectable X proves the presence of Unobservable Y? You literally have professed no detectable causes or effects in order to determine something else’s undetectable causes and effects. Good luck with that.
I also wasn’t aware DM could be detected at all directly (which would also logically be required), that it is merely inferred over large areas of space due to otherwise unexplained gravitational effects that are being attributed to the actually undetected ‘theorized stuff’ acting on matter to explain the observation according to present gravitational ‘theory’, with the BIG assumption that the current gravitational ‘theory’ is actually correct. All that aside, how would they know how to discern DM in a LITTLE area small as the EM drive itself? 1.) Steal Underwear. 2.) ?
3.)Profit. Ok. I get it now.
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Please throw another WAG on the fire and keep digging.
I’ve recently been informed I just ‘don’t understand how science is done’ and how my skepticism is bad for the cause.
Never mind all that, I’ve been scolded.
No doubt some great scientific discovery is just about to be made and I’m very entertained, I brought plenty of popcorn… and an Occam straight edge.

I also wasn’t aware DM could be detected at all directly (which would also logically be required), that it is merely inferred over large areas of space due to otherwise unexplained gravitational effects that are being attributed to the actually undetected ‘theorized stuff’ acting on matter to explain the observation according to present gravitational ‘theory’

That’s how science is done, CFT: you infer an unobserved force, particle, or law from observed effects. Then you test that inference through further indirect observations – and direct ones if you can, but if you can’t, the endeavor is still called science. What matters for science is whether the inference can be empirically tested, not whether the inferred thing can be directly observed.

Having said that, I fully agree with you that invoking DM in this case is fairly speculative. I guess its possible that its providing the thrust, but like Ethan, I think its far more likely that the EM drive just doesn’t work as advertised.

eric, PJ has truly inspired me.
I propose the EM drive works according to bullshit. You see, people make shit up all the time, in such large quantities in fact that it permeates all known space and time, but in general forms denser concentrations around places where there are higher potentials for semi-intelligent life to be wasting both space and time engaging in bullshit. As a side note, scientists should definitely be looking out for higher concentrations of bullshit near heavily polluted planets indicating extraterrestrial civilizations at least as full of it as ours, also indicating excellent opportunities for us to engage in future cultural exchanges of shouting matches about sports teams and deep metaphysical debates over how real WWF really is. Since bullshit is mostly opaque and hard to make sense of, it is also the perfect primary component for DarkMatter as well (this would really explain so much!) The beauty of this idea, is that BS/DM (hmm, that almost sounds bad) is completely and utterly invisible to the gullible and mentally naïve as well as being virtually unlimited in supply so future star travelers will not have to worry about running out this wonderful self renewing energy source as they fart around the universe at highly improbable speeds.
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I humbly suggest my bullshit/darkmatter/EM drive proposal is a win-win opportunity for lucrative personal financial gain and the abject flattery and manipulation of all mankind, as it brings us uncomfortably closer together as a species in doing something we can actually do really well without trying to hard.

You’ve mentioned money and financial gains around awhile now, in comments to this post as well as on previous ones. And when you talk about physicists and establishement, and money…I don’t know if you get your info in movies or in universities? Because according to statistics in US, a physicist on average earns between $5-8k per month. That’s about the same as a police officer or detective earns.

If you want to b.s. around.. at least do some checking first.

This has no bearing on the EMdrive, but has on painting a picture of scientists as a whole being a unscurpulous, money/profit driven, greedy hags who care more about the grant and lining their pockets then doing science. Well guess what… if they cared anything about profit or money, they wouldn’t have chosen a life in astrophysics or HEP physics in the first place.

“You’ve mentioned money and financial gains around awhile now, in comments to this post as well as on previous ones.”

Because CFT sees that “his” money is going to science and he doesn’t like what they’re looking at, therefore money MUST be a bad thing.

Of course, money in private industry isn’t any proof that there’s a massive fraud taking place in commerce worldwide, because he’s one of those dumb neoconservative neocapitalists who think that the only place you have competence in government is where they’re scamming people, and the only place where the right things can EVER be done is the “free market”.

DM interacts gravitationally, so should be trapped inside black holes. It doesn’t interact in other ways, so DM “atoms” should have huge orbits – on a galactic scale or larger. It seems there might be a lower density of DM around a galactic BH, since it has the ability to clear out DM that intersects its event horizon.

So perhaps we can detect variations in orbital periods around huge BHs?

Ethan covered this in an older post, here. There are objectively higher densities of DM around the galactic center (than there are out here, where Earth is), but the density relative to the density of normal matter is actually much smaller than out here, because the normal matter gets denser much faster as you go into the center. Basically, because it collides, scatters, and clumps – DM doesn’t.

If I’m understanding that correctly, we should actually expect the areas around galactic center to have celestial mechanics more like what would be predicted in models without DM. However, that’s still a predicted difference, so maybe we can test and observe it.

Why do you keep referring to the EM drive as “NASA’s” EM Drive? The drive was invented by Roger Shawyer and various prototypes have been built by Shawyer himself as well as by others. NASA’s only involvement to date has been to test another prototype of Shawyers design. That hardly makes it theirs does it?

Why assume DM is being emitted, when we already know that photons are being emitted, and photons carry momentum? Back of envelope estimates of the radiation flux from a 100C surface gives a thrust estimate of a few mN per square meter, similar to what was observed.

So hang some thermocouples on the EM drive, and do some corrections for biased surface temperature and photon flux.

It’s not assuming that dark matter is being emitted. The hypothesis is that (a) photons are being emitted, some fraction of which are then (b) striking the dark matter that is in the cavity (because it’s everywhere); and then (c) striking the inside wall of the device, causing it to move.

This may be unlikely, but it doesn’t require the drive to be emitting dark matter.