The Curious Link Between the Fly-By Anomaly and the “Impossible” EmDrive

April 21, 2016 2:26 AM

The EmDrive (previously 1, 2) is still getting attention from the scientific community. MIT Technology Review sums it up: The Curious Link Between the Fly-By Anomaly and the “Impossible” EmDrive Thruster
posted by Harald74 (39 comments total)

That's very interesting... but this thing with photons having mass - how is it not a total deal-breaker? McCulloch seems to explain it in this article (pdf):

Normally, of course, photons are not supposed to have mass in this way, but supposing we consider this? We assume the inertial mass of the microwave photons (whatever its absolute value) is affected by MiHsC, but instead of the horizon being the far-off and spherically symmetric Hubble horizon as before, the horizon is now made by the asymmetric walls of the cavity.

Does it mean that photons acquire mass only in this environment and slow down (the speed of light becomes lower)?
posted by hat_eater at 2:51 AM on April 21, 2016

i don't want to sound disparaging but, wow McCulloch's research is a rabbit-hole of fringe physics: casimir effect, MoND/alt-dark matter, space anomalies, mysterious propulsion devices...

the problem is that, outside of claims which are obviously impossible (like say, ;) creating a propulsion device by emitting microwaves in a metal cone) it's kind of hard to be definitive on whether a theory is reasonable or not without making the effort to understand the theory... and if you have the technical skills to do that, why would you commit yourself to doing it? So, if you are a physicist or mathematician reading this stuff, you tend to first look for professional qualifications, then for obvious mistakes or impossibilities, and finally have to say either: "I have a PhD and this is crazy" (argument from authority) or "Dunno, maybe, why don't you research it and tell me..."

But, if photons have rest mass and inertia is quantized, sure, why not build a space drive with microwaves in a cone...
posted by ennui.bz at 3:12 AM on April 21, 2016 [4 favorites]

Note: "photons have rest mass" reads to me like "up is down" but you know, sure, the "more things under heaven and earth..." and all that...
posted by ennui.bz at 3:16 AM on April 21, 2016 [1 favorite]

It's doing something, possibly even working as intended, but no one knows why.

Still.
posted by Slackermagee at 3:25 AM on April 21, 2016

"Crucially, McCulloch’s theory makes two testable predictions."

That's a good start! Go test them.
(also woo for crazy science coming out of my alma mater!)
posted by Just this guy, y'know at 3:26 AM on April 21, 2016 [1 favorite]

Strikes me as nonsense (and indeed the community has been ignoring stuff from McCulloch for quite a while now). Unruh radiation is incredibly weak. And if it inertia were caused by some coupling to radiation, why doesn't mass correlate with how strongly something interacts with radiation?

It's unfortunate this is reported so uncritically.
posted by edd at 3:54 AM on April 21, 2016 [1 favorite]

Unruh radiation is incredibly weak.

as far as I can tell from lazily googling, there isn't any definitive evidence for Unruh "radiation" although the Unruh "effect" (blackbody radiation observed from an accelerated reference frame?) appears to be real???

Since my days in astrophysics grad school, I've always wanted to come up with a crank theory for the observed kinematic data that didn't involve positing some new form of ubiquitous but incredibly weakly-interacting matter AKA "dark matter" but instead involved some more fundamental physics principles, maybe something involving statistical thermodynamics and general relativity... but I was never smart enough. So, I'm sympathetic but...

It's unfortunate this is reported so uncritically.

science reporting is so uniformly terrible it's kind of hard to make this criticism. personally, i think it's kind of fun. I'd never heard of the Unruh effect before...
posted by ennui.bz at 4:13 AM on April 21, 2016

The Unruh effect/radiation is very similar to Hawking radiation. Like it, it's the sort of thing that would be very hard to observe, but you'd be properly surprised if it weren't real as it is so well founded in accepted theories.
posted by edd at 4:16 AM on April 21, 2016 [2 favorites]

My Conversation with McCulloch (not mine, some guy on Reddit)
posted by Just this guy, y'know at 4:17 AM on April 21, 2016 [3 favorites]

I'm not a qualified physicist, scientist, or the like, but I do love a good theory, and I'm jumping in, starting at the blog. Thanks for what is, at minimum, a fun diversion from life's troubles, and at an admittedly unlikely best, getting in on the ground floor of an interesting new physics.
posted by MikeWarot at 4:18 AM on April 21, 2016

Any claim that this device works has to address two huge problems. First, and least odd: this is a free energy device. Have you measured net energy gain?

Far weirder, this machine implies that the universe does not have translational symmetry. This is so deeply, foundationally odd that it has to be addressed. McCulloch doesn't seem to understand this implication, which reduces his credibility to zero.

Simply put, if this thing works then the universe would appear different depending on which direction you looked at it. Since this is categorically not the case, why should I consider this to me anything more than a measurement error?
posted by Combat Wombat at 4:22 AM on April 21, 2016 [2 favorites]

The Unruh effect/radiation is very similar to Hawking radiation. Like it, it's the sort of thing that would be very hard to observe, but you'd be properly surprised if it is so well founded in accepted theories.

ok, after more googling it seems like the Unruh effect is just looking at a vacuum state in QFT from an accelerated reference frame? i'm still not clear what the story is observationally...

i took a long detour out of physics into differential geometry and never made it to QFT... shows my ignorance.
posted by ennui.bz at 4:33 AM on April 21, 2016

Does 'em drive' = 'jet packs' ? Because if no then I'm out.
posted by From Bklyn at 4:47 AM on April 21, 2016 [1 favorite]

Simply put, if this thing works then the universe would appear different depending on which direction you looked at it.

Can you expound on this? I neither understand this requirement, nor how it relates to the data being collected.
posted by meinvt at 4:50 AM on April 21, 2016 [1 favorite]

To be pedantic, looking the same in different directions would be rotational, not translational invariance.

Anyway yes, it seems nutty to explain this profoundly symmetry breaking claimed effect with a theory so wonderfully symmetric.
posted by edd at 5:04 AM on April 21, 2016 [2 favorites]

Fascinating article. I don't remotely have the physics to assess it, but if it's true that multiple (presumably very careful) attempts to reproduce the "Em drive" have succeeded, then it's pretty clear that something is going on. This particular attempt to explain it may or may not be valid, but it'll be very interesting to follow the discussion.
posted by Zonker at 5:33 AM on April 21, 2016 [1 favorite]

As nice as it is to understand this phenomenon I'm all for skipping right to building practical applications for it. I guess that puts me in the "Jet Pack" camp.
posted by Mr.Encyclopedia at 5:33 AM on April 21, 2016 [1 favorite]

At very small accelerations, the wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia can take only certain whole-wavelength values and so jumps from one value to the next.

This statement confuses me. The size of the observable universe is a function of how old the universe is. How could a photon be affected by this?
posted by justkevin at 5:37 AM on April 21, 2016

There's a lot of crazy going round, but ultimately this is a thing you can test.
The Earth bound tests are showing thrust, so do more tests. If you can build and test it independently and make it work repeatedly then you can try and understand it.
You can build a test rig and take it to space.

If it doesn't work then we can stop trying to understand it.
posted by Just this guy, y'know at 6:08 AM on April 21, 2016 [7 favorites]

Any claim that this device works has to address two huge problems. First, and least odd: this is a free energy device. Have you measured net energy gain?


It's not clear to me that that's the case. In the experiments, there is clearly energy going into the device, in the form of microwaves. Could you explain why you think there's a net energy gain?
posted by newdaddy at 6:38 AM on April 21, 2016 [1 favorite]

Newdaddy: consider it in the rest frame of the device. You've put energy in which either stays there bouncing around the cavity or seemingly vanishes. If it stays, an outside observer would see you've accelerated but the energy to accelerate you has appeared out of nowhere. If it vanishes where's it gone? From your perspective sitting on the drive you've not gained kinetic energy and there's no propellant you've pushed out one end to balance the books.

It's more obvious to consider the equally problematic violation of conservation of momentum, but if you've violated conservation of momentum there's going to be disagreements about energy too from one frame or another.
posted by edd at 7:02 AM on April 21, 2016 [1 favorite]

slashdot says it's bullshit.
posted by quonsar II: smock fishpants and the temple of foon at 7:15 AM on April 21, 2016 [1 favorite]

The fun thing about this for me as a lay person is that it isn't an obvious scam, like the e-cat guy. There's no secret sauce here, no huckster insisting on eye brow raising testing protocols. Just a seemingly crazy device that shouldn't work...but maybe it does?

99.99999% sure it's just measurement error, but if it's not the universe opens up to us. (And physics will need a fundamental re-writing)
posted by Eddie Mars at 7:17 AM on April 21, 2016

So the fundamental problem is that if you can move without pushing anything, then you have to break conservation of energy in most reference frames, even in the classical limit. Because everyone needs to measure the same change in energy of the system.

So if you have this magic reactionless system, from the reference frame of your starting velocity, sure, you added some energy and now have some velocity and thus some kinetic energy. But from the reference frame of the ending state, or say someone floating by, that your reactionless system has taken this energy, but instead of adding kinetic energy to you, it has removed it, making you stationary (instead of moving backwards). Energy has just vanished! It has not gone into heat or anything due to inefficiency, but just gone away entirely.

And this isn't in some weird, outlandish limit of high energies where you have to worry about special or general relativity, or small scales where quantum physics might complicate things and throw off your intuition - this is claimed to be occurring at ordinary, human scales.

So basically, the possibility of pushing on nothing to go somewhere breaks our understanding of reality so badly, it would have to be noticeable in just everyday life, no fancy instruments needed.
posted by Zalzidrax at 7:18 AM on April 21, 2016 [2 favorites]

Well, OK, I see what you're saying, but if we concede that some of the input microwave energy 'goes away' (and surely some of it does go away, as heat) but then we also have this unexpected and unexplained acceleration, doesn't there exist at least the possibility that the acceleration is the result of the conversion somehow of some of the input energy? It's clear that if no microwaves are applied, the acceleration doesn't happen (or doesn't persist when they are turned off.)

IANAPhysicist, but the 'consider it from the rest frame of the device' bit seems unreasonable to me. From the rest frame of an automobile, you keep burning fuel but you've not gained kinetic energy. Could you explain how this example differs?
posted by newdaddy at 7:20 AM on April 21, 2016

In the car, you've burnt fuel applying a force to the car but you've applied an equal and opposite force to the Earth as well. In any inertial frame you choose it all adds up ok and energy gets conserved (as well as momentum). But this thing doesn't apply an equal and opposite force to anything as it has no exhaust or anything to push against, so it doesn't add up.
posted by edd at 7:33 AM on April 21, 2016 [1 favorite]

But the energy doesn't stay in the cavity. If you put a pulse of microwave RF into a cavity, then it bounces around for a bit then converts ultimately into thermal energy due to various effects that I'm sure I can't describe fully but are perfectly understood by people who know more EM than me (which is most people who know EM). If you can take account of all those conversions and show they add up to all the EM input, then sure - if there's left-over momentum then you've got a free energy machine. But I don't think that's what's being claimed. (Shawyer's Emdrive does depend on having a high q for the cavity, which is a measure of, among other things, how long a pulse would bounce around inside it. ISTR he posits building stuff with extremely high q (which is hard) that would have much higher thrusts, although there is even more weirdness along that path...)

I'm absolutely loving this. I both believe it works and believe it can't, and I'm still not sure about the experimental results. And I wouldn't treat the theories of McCulloch as anything other than entertainment at this point If it does work, then we perhaps have our space propeller and everything changes in a wonderful way. If it doesn't, then it should become apparent reasonably quickly. I don't like the high level of noise around the reporting - TR used to be much better at it, but had a chance of policy some while ago and now does a lot more gee-whizz uncritical stuff which annoys the hell out of me.

I do have a friend who's a mathematician at Plymouth University (or Plymouth Poly, as cynical old farts still unfairly refer to it), so I'll see if he knows what the buzz is on campus. It's not unknown for universitiies to harbous weird cranks (sometimes entire departments of them), which I find strangely conforting

But it's good fun, it's clearly not a scam, it involves my two favourite things - EM physics and space - and it's now connected with Devon. What could be finer?
posted by Devonian at 7:34 AM on April 21, 2016 [3 favorites]

I should probably clarify that it's not really right to take the frame sitting on the engine while it accelerates and say you've not gained KE. You should really take the frame where the engine finishes up stationary, and note that it lost kinetic energy to finish up stationary, but that the microwaves inside have to be consistent with both that and a frame where it starts stationary and finishes up moving. The energy content inside can't both go up and down.
posted by edd at 7:35 AM on April 21, 2016

> slashdot says it's bullshit.

OMG, another time traveler (this one from just 10 years ago) has shown up in the thread.
posted by benito.strauss at 9:05 AM on April 21, 2016 [7 favorites]

Well, "there's no way this could ever work" is a long way away from "this doesn't work."
posted by newdaddy at 10:10 AM on April 21, 2016

So, the difference between this and, say, a light sail is that the photons that transfer energy into a light sail *have* to come from outside the sail's frame of reference? Or do I have that wrong?
posted by rhamphorhynchus at 10:28 AM on April 21, 2016

Metafilter: slashdot says it's bullshit.
posted by wavy at 10:49 AM on April 21, 2016 [6 favorites]

Clearly it's pushing on some quintessential substance that we can't detect. I vote we call it "Aether".
posted by Mr.Encyclopedia at 11:18 AM on April 21, 2016 [4 favorites]

The r/EmDrive link Just this guy, y'know posted up there is 10X better than the Technology Review article. It was posted by userid crackpot_killer. :)
posted by bukvich at 11:39 AM on April 21, 2016 [1 favorite]

At this stage the most critical point is the claim, in the article, that there are 6 independent replications of the experiment. Is this true, or not?

Second, they should stop calling it a propulsion device, because all that does is trigger motivated reasoning biases. At most it's a puzzling observation and should accordingly thought of as a physics puzzle.
posted by polymodus at 12:33 PM on April 21, 2016 [1 favorite]

So, the difference between this and, say, a light sail is that the photons that transfer energy into a light sail *have* to come from outside the sail's frame of reference? Or do I have that wrong?


Photons have mass, and whether they're bouncing off a solar sail or being emitted directly (like with the Pioneer anomaly) they'll produce a non-zero acceleration even if you stick to nice simple Newtonian mechanics. The EmDrive claims to be getting a bit more oomph than EM leakage or thermal radiation could account for though.
posted by figurant at 12:43 PM on April 21, 2016

Photons have mass

Photons have momentum not mass. The momentum of a photon is based on its frequency.
posted by delicious-luncheon at 1:27 PM on April 21, 2016 [3 favorites]

If photons have momentum, wouldn't that mean generating a bunch of microwaves and shooting them out the back of a spacecraft would require the spacecraft to gain momentum as well?
posted by jamincan at 12:03 PM on April 23, 2016

jamincan: yes, that would work. The problem with this thing is the microwaves are supposed to be stuck inside not shooting out the back.
posted by edd at 2:40 AM on April 24, 2016 [1 favorite]