Shawyer's "Electromagnetic Drive"

[sanman]

Hi,

I'm sure you've heard of this proposed concept by Robert Shawyer for an "electromagnetic drive" which is claimed to generate an unbalanced force inside of a resonant QED-cavity, thus causing acceleration that appears to violate Conservation of Momentum.

http://technology.newscientist.com/article/mg19125681.400

http://blog.wired.com/defense/2008/09/chinese-buildin.html

http://emdrive.com/faq.html

Now, we all know about the inviolability of Conservation of Momentum, but the claim being made is that there is supposed to be some kind of "loophole" for the small-scale quantum world.

The claim is that if the resonant cavity is asymmetric and tapered, that somehow there can be a net force acting against one side of it that is not balanced from the other side.
I can't picture this -- if you have a balloon that is asymmetrically shaped, it doesn't mean that the gas molecules will hit and impart more force on side than than on others. But I realize that a balloon is a macroscopic cavity, and not a small quantum-sized one.

Anybody can see in a macroscopic cavity diagram that even with a tapered end, there will be a component of force that would be in an axial direction, that would cancel out any imbalance or "thrust".
Point #3 in the FAQ site linked above attempts to boldly contradict this:

3.
Q. Why does the net force not get balanced out by the axial component of the sidewall force?
A. The net force is not balanced out by the axial component of the sidewall force because there is a highly non linear relationship between waveguide diameter and group velocity. (e.g. at cut off diameter, the group velocity is zero, the guide wavelength is infinity, but the diameter is clearly not zero.) The design of the cavity is such that the ratio of end wall forces is maximised, whilst the axial component of the sidewall force is reduced to a negligible value.


I'm not understanding what they're trying to assert, here.
Could anyone kindly debunk this above statement for me, so that I can understand the error in its reasoning? Are they saying that all dimensions/axes are not equal, to a wave inside a resonant cavity?

In regular macroscopic situations, when a particle or ray or wave hits a surface, it will reflect off with an angle of reflection that is equal to the angle of incidence, with the overall momentum conserved.
Does this rule somehow change when you go down to the tiny quantum scale?
The assertion quoted above seems to imply that wavelength confinement within a QED cavity can trump regular rules of reflection. (ie. if the diameter along the tapered end is too tight to accommodate the incident wave, then the surface of that tapered section will not reflect back the incident wave in a regular normal way)

But what does actually happen in such a small-scale situation?
What does happen to an oscillating wave that is striking a surface whose proximity to other confining surfaces is tighter than the wavelength itself?
The author seems to imply that some kind of "downconversion" of the incident wave occurs.
Is "downconversion" (a known phenomenon) then analogous to an "inelastic collision"?

In the quantum world, is it possible to selectively have inelastic collisions with just one side of a cavity?


Thanks for any serious replies.

[russ_watters]

Theoretically, it seems obviously flawed, as your balloon analogy implies. Experimentally, it is also obviously flawed, as he used one balance who'se resolution is a full half the measured thrust (he measured .002g on a scale with a precision of .001g) and another whose resolution is an order of magnitude larger!

Though he'd never let you know how long he's been failing (his website doesn't say), I'm pretty sure this guy haw been hocking his little copper paperweight for quite some time.

Editorially, that Newscientist article is just awful. It makes one wonder if magazines have a template for articles about crackpots. All the elements of such articles are the same, they basically just fill in the crackpot's name and the name of the device!

[atyy]

There is a quantum phenomenon called the Casimr effect which surprisingly has analogies to macroscopic thermodynamic phenomena, and has been called "vacuum pressure" or more accurately the "friction of vacuum". Because the analogies are possible, there are no quantum loopholes.
http://www.aip.org/pnu/2002/split/611-2.html
http://arxiv.org/abs/cond-mat/9711071

[sanman]

Hi russ, thanks for your reply.

Well, ordinarily one doesn't want to give such claims a second thought. But there have been cases of inventions which show quantum phenomena superceding known laws that we hold to be inviolable at the macroscopic scale. For instance, some fellow came up with the idea of a "quantum afterburner" which would convert waste heat into energy, thus exceeding classical Carnot efficiency limits. I'm sure you've heard of this. There was no skeptical reaction to his claims, because his proofs seem quite solid.

I'll ignore the experimental portion of Shawyer's claims and just focus on the theory.
A macroscopic balloon may still have inelastic collisions happening out of the many inside of it, but not in a way that favors one side of the balloon overall. The distribution of all such collisions would be normalized, due to the huge size differential between the balloon/cavity and the confined gas molecules.

In a tiny-scale quantum resonant cavity, we lose that size differential between the cavity and the electromagnetic wave it's confining. So quantum effects/processes come into play or become relevant. "Downconversion" -- if possible in this situation -- can become relevant. I just want to understand whether quantum mechanics can really be used to somehow supercede a law that we normally hold to be inviolable. I'd at least like to know what the fellow has failed to take into account.

For instance, if a photon/wave down-converts, then where does its momentum go? Probably into an electron or electrons, which gain the momentum/energy lost by the photon. So could this resonant cavity end up producing a stream of energetic electrons, so that if these are included in the net summation would give a net momentum change of zero?

[atyy]

Conservation of momentum holds in the quantum world. When physicists found an apparent violation of momentum conservation, they postulated that an unknown particle called the "neutrino" was taking away the apparently missing momentum. The neutrino was experimentally detected later, indicating that momentum conservation remains valid in the quantum world.
http://education.jlab.org/glossary/neutrino.html

[sanman]

True enough, Conservation of Momentum holds in the quantum world, but do "closed systems" hold in the quantum world?

It's one thing to talk about momentum being conserved in a closed system when describing a macroscopic situation, but can you truly have a closed system when there are quantum processes such as tunneling, etc?

And what about inelastic collisions? We know that inelastic collisions don't violate Conservation of Momentum, even though a superficial observation may not immediately reveal the inelastic conversions of momentum.

http://en.wikipedia.org/wiki/Compton_scattering


Let me give another analogy. It has recently been found that some nanomaterials can act as a "heat diode" allowing heat to flow in one direction across the nanomaterial, but not flow back the other way. This is accepted as a consequence of the nano-engineered structures that only allow phonons to move one way:

http://www.technologyreview.com/Infotech/17822/?a=f

I'm wondering if a nano-sized cavity could enable some kind of asymmetric transfer of momentum, by having an asymmetric shape.
The resonator used by Shawyer isn't nano-sized, of course. But what if it was?

[Vanadium 50]

Quantum tunneling does not violate any conservation laws.

What Shawyer has done is strung a lot of scientific-sounding words together in an attempt to explain a device that, as asyy points out, does not actually produce significant thrust. For example "QED resonant cavity". QED is a theory - a mathematical framework for performing calculations. A resonant cavity is an object. What does "QED resonant cavity" even mean? (Can you have a "linear algebra resonant cavity"? It makes just as much sense, and sounds way cooler!)

It's been two years since the New Scientist article. At the time, Shawyer was building a version that would levitate a car. Where is this levitator?

I guess it's up to each of us to decide how much of our time we want to spend looking at crackpottery. Personally, I feel there's enough fresh crackpottery out there that I don't have to go looking for two year old stuff.

[sanman]

Hi Van, thanks for your reply.

Well, from what I can see, this thing shouldn't produce any thrust at all.
Saying that it does not produce significant thrust is like saying that Conservation of Momentum was not significantly violated. Well, hopefully his Chinese collaborators would be able to debunk the concept, as even debunking can be worth the effort.

[russ_watters]

Well, from what I can see, this thing shouldn't produce any thrust at all.
Saying that it does not produce significant thrust is like saying that Conservation of Momentum was not significantly violated. Well, hopefully his Chinese collaborators would be able to debunk the concept, as even debunking can be worth the effort. Well let me be more specific then: This device does not produce any thrust and his own tests confirm that. His measurements are just not capable of enough precision to end up with a reading of exactly zero (no measuring device can be expected to).