The 'Impossible' EM Drive Being Tested By NASA May Finally Be Explained (technologyreview.com)

MarkWhittington writes: The EmDrive, the so-called "impossible" space drive that uses no propellant, has roiled the aerospace world for the past several years ever since it was proposed by British aerospace engineer Robert Shawyer. In essence, the claim advanced by Shawyer and others is that if you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea, noting correctly that such a thing would violate physical laws. However, organizations as prestigious as NASA have replicated the same results, that prototypes of the EmDrive produces thrust. How does one reconcile the experimental results with the apparent scientific impossibility? MIT Technology Review suggested a reason why.

Comments

Re:Thanks, Summary

[Narcocide]

Short version: photons seem to have inertial mass after all.

Re:Thanks, Summary

[guruevi]

Not at all that. From what I can read from the (equally bad) article, the claimed effect may be due to the (currently unobserved) Unruh effect. The Unruh effect is a hypothetical black body radiation observed by an accelerating observer. Basically, if you were accelerating in reference to a "stationary" observer, you observe yourself heating up (very, very slightly) while the "stationary" observer would not see this heat.

Re:FTFY summary

[myowntrueself]

Second, if you want to appeal to Noether's theorem, note that the theorem refers to smooth manifolds. If space is quantized, then Noether's theorem doesn't apply (despite being true). It's possible that Noether's theorem will break down at small scales. (If space is smooth, ie *not* quantized, then the true location of any particle is a [mathematical] real number with infinite entropy and it's action is non-computable. Not that having a non-computable universe is a problem, but...)

Theres something about this that reminds me of Zenos paradoxes.

The Eleatics had the idea that they could logically prove that reality is nothing like anything we can imagine.

First, if you assume that space or time are discrete, you are led to paradox. So they can't be discrete.
Second, if you assume that space or time are continuous, you are led to paradox. So they can't be continuous.
So space and time can be neither continuous nor discrete, nor can they be both continuous and discrete.
What is left? Nothing we can imagine. There appears to be no other option than continuous or discrete.
Therefore time and space, reality, must be something unimaginable.

Yes, there were people thinking like this a very very long time ago.

Re:Thanks, Summary

[Anonymous Coward]

Photons do not have rest mass, but they do carry both energy and momentum.

A photon imparts a force and thus may transfer momentum onto a reflecting mirror. The Mirror has mass and is thus subject to this quantized acceleration.

Also. The idea that the speed of light in a vacuum varies in a electromagnetic cavity is largely accepted as part of Quantum Electrodynamics.

This is closely related the Casimir force that developed between two uncharged metal plates. Empty space is filled with a continuum of virtual electromagnetic modes ( field fluctuation of every size). As the plates come together more and more long wave modes are excluded from the vacuum between the plates... but not "outside" , thus Vacuum between the plates is literally more empty, and the plates experience an attractive force. This is crazy.. but it had been actually measured.
Now....
In accelerating reference frames some of the virtual modes of the vacuum are converted into real modes this is the"Unruh radiation"... thus space looks like a heat bath when your are accelerating. it is not so clear how time in a heat bath becomes the generator of momentum, and inertial mass, unless General Relativity makes the Radiation bath An-isotropic ( not the same in all directions). thus the radiation pressure form the thermal radiation retards acceleration, acting exactly like inertial mass..... Hmmmm

so if you try to impart momentum on a small element embedded withing a larger cavity. The smallest change in momentum allowed is now a function of the size of the cavity , and lowest frequency mode that the cavity will support......

This is right out of an area in physics call Quantum Cavity Electrodynamics ( Google it).... Cool idea There may be something to it... Hell, Why it does NOT work may also be just as interesting. I like it! It been a long time since the Physics degree.

Part of the paper seems like nonsense

[mbkennel]

It looks like nonsense because it treats photons as if they were Newtonian particles and with ignorance of Maxwell's equations and relativity.

Start with section 2. It treats photons as particles with some momentum m*v. I mean, what? That's just wrong. Photons are relativistic p = E/c and quantum mechanical, E = 2\pi hbar f.

I mean take a look at this:

"Normally, of course, photons are not supposed to have inertial mass in this way,
but here this is assumed. It is not clear what the size of this mass is, but it is
clear for example that light inside a mirrored box produces a kind of inertial mass
for the box. "

So in orthodox physics, photons are not supposed to have inertial mass, but also in orthodox physics light makes inertial mass and it's clear that it's so.

The second statement, about light inside a mirrored box, is so because of relativity and the assertion of the equivalence principle. Electromagnetic fields are part of the stress energy tensor (following Maxwell) which feeds into the source term of general relativity. So yes, there is some sort of inertial contribution, but in fact it can be computed pretty exactly, and it's extraordinarily tiny, and really mostly related to the energy density of the EM field.

So relativity sometimes, but not other times? WTF?

And if the non-standard theory that inertia comes from matter interacting with Unruh radiation, how exactly does that work with photons? Photons don't interact with photons. Zero cross section until the point that they are so energetic they can pop out electron/positron pairs from the vacuum, which is so far not an experimentally accessible regime.

Presumably the idea is that the Unruh radiation inside the cavity is quantized in a particular way different from free space, but wouldn't that mean that inertia of (presumably charged) particles inside that cavity would be altered? But he was talking about the non-sensical 'inertial mass' of the photons themselves. WTF?

I don't mind non-standard theories and their exploration at all, but it's necessary to be clear which standard axioms are being rejected and which others are preserved, and follow that consistently. I just saw very unclear physics.

Re:Probaly not Uhruh radiation

[michelcolman]

Why would it be a problem?

Maybe "the speed of light" is a misnomer, c is just a maximum speed defined by the universe, and photons travel at speeds that are extremely close to this cosmic speed limit. It would then be perfectly possible for them to have a tiny amount of mass. No laws break down (at least not in GR), you just have to replace Einstein's flashlights by hypothetical devices that transmit information slightly faster than light, with precisely the speed c.

Turns out "faster than light" is possible after all (but not faster than c).

Re:Great summary

[dbIII]

The link isn't really worth a summary, but I'll try.

Given a variable speed of light and a different theory of momentum maybe it could all make sense.


Would you bother to even click on the link if that's all it leads to?

Very Serious Flaws

[Roger W Moore]

Short version: photons seem to have inertial mass after all.

...which raises some very serious questions such as why do we always observe photons as having the same speed regardless of frequency? In addition the proposed mechanism means that the quantization of inertia depends on the size of the universe. If this effect is observable today then shortly after the Big Bang the effect would have been incredibly huge due to the far, far smaller size of the universe. This raises serious questions bout the effect on nucleosynthesis etc. which Big Bang models without this physics appear to get right.

You cannot just rewrite fundamental physics to fix one issue without also looking at the implications of your theory for other predictions which is it likely to change. Worse it seems that nobody has tested these drives for the emission of charged particles. A far, far simpler explanation is that this drive works by electron emission. There are a variety of way this can work which all work in a vacuum but whic would unfortunately not work in space where you are electrically isolated and would eventually build up a counter charge and cause the thrust to reduce to zero over time. This all uses established fundamental physics so it would be nice to see this ruled out BEFORE coming up with crazy new physics. It might be less exciting but it is better science.

Re:Very Serious Flaws

[Immerman]

Actually, we would have removed a host of variables, and introduced virtually no new ones - it's *incredibly* difficult to apply electric power to something without it interacting electromagnetically with nearby objects, including your measurement apparatus. In orbit you've pushed the nearest objects many miles away, and need only really worry about interaction with the Earth's magnetic field. And orbital dynamics are well enough understood that measuring even small changes in velocity can be done extremely accurately.

>If this is ever going to be a serious drive we have to understand how it works.
Why? We use gravity all the time with no understanding of how it works. Most of early medicine worked fine without any understanding of how it works - even animals routinely ingest medicinal herbs and minerals when ill. Ditto metallurgy - people were forging ever stronger swords for centuries without the slightest real understanding of *how* their techniques made the metal stronger, they simply relied on trial and error to find new techniques that worked better.

Having a theoretical understanding of how something works allows you to optimize it far more rapidly, but if you have discovered an already useful phenomena, there's absolutely no reason not to harness it immediately - all you need is a practical understanding of how to create it, not a theory of the principles upon which it works.

As for an orbital test of an EM drive - the measured thrusts (if not due to experimental error) are more than sufficient to raise a simple probe to a much higher orbit over the course of, say, a year long test. And *nothing* else is going to have that effect without also raising the orbits of pretty much everything else as well - we have a huge control group of satellites already present. Even a few weeks would likely be enough to determine if the drive was actually generating thrust, and probably even if that thrust is too great to be explained by photon rocketry or ion-drive effects.

Now, you could certainly argue that the expense of putting such a test probe into orbit isn't justified by the current evidence, but frankly given the implications if it worked, I rather suspect that if you just built the damned thing, as small as possible, Elon Musk would be happy to deliver it to orbit for free using some of the excess capacity available on a typical launch. Or at worst, at the incremental cost of the additional fuel, etc required, which is what, in the $10-$100 per pound range?

Re:Great summary

[gweihir]

I read the original article on the "Exceptionally Magic" drive way back. I have not bothered to read anything else since.

Re:Amounts

[tinkerton]

Only, I have to walk that back. It actually generates a good amount of thrust. Now I'm impressed.

Re:Quantized inertia?

[Anonymous Coward]

"There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here,
it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened."

- Douglas Adams, The Restaurant at the End of the Universe