Slightly more believable EM engine

[Cuddles]

http://www.newscientist.com/channel...tivity-drive-the-end-of-wings-and-wheels.html

Take a standard copper waveguide and close off both ends. Now create microwaves using a magnetron, a device found in every microwave oven. If you inject these microwaves into the cavity, the microwaves will bounce from one end of the cavity to the other. According to the principles outlined by Maxwell, this will produce a tiny force on the end walls. Now carefully match the size of the cavity to the wavelength of the microwaves and you create a chamber in which the microwaves resonate, allowing it to store large amounts of energy.

...snip...

You might think the forces on the end walls will cancel each other out, but Shawyer worked out that with a suitably shaped resonant cavity, wider at one end than the other, the radiation pressure exerted by the microwaves at the wide end would be higher than at the narrow one.

The basic claim is that a microwave cavity, of the kind found in particle accelerators, can provide thrust if properly designed. I found this quite interesting, but without seeing any more evidence I'm not sure whether to actually believe it or not. The main difference between this and other "clean" engines is that he seems to be working scientifically, and even has independent observers to verify the work. Not only that but he wants proper tests to see if it works, and is not asking for investors, which would seem the exact opposite of the usual claims in this area. Has anyone heard of Roger Shawyer or this idea before?

 

[casebro]

So, he thinks that electrons have enough momentum to make things move? Well, I know that a 50,000 watt transmitter should make 67 horsepower, or 2,000,000 ft/lbs/minute of torque, yet the antenna on a TAaaalll mast is only held up with a skinny wire. I know that the wave guide on a F-4 radar only needs tape to hold the joints together, so there can't be much 'momentum' to this "Wave Guide Motor" concept. My guess is that the EM force is vastly mutiplied in the typical electric motor, over the momentum of the electrons. Now, if he's thinking he may have a battery concept instead of a motor, he may be on the right track. The 'echo' of bouncing energy is how lasers work- the light is bouncing back and forth within a crystal until it finally gets powerful enough to leap out in one focused bolt. Perhaps he has a death ray on the drawing board?

 

[jmercer]

If it does produce thrust - even low thrust - it may be useful in space...

 

[Ririon]

If it does produce thrust - even low thrust - it may be useful in space...

That's all very neat, but let me be so arrogant as to suggest that either he has got his maths wrong, or the journalist is misquoting him.

We have microwaves bouncing back and forth in a cavity, yet they produce a net force on the cavity? Sorry, but my physics intuition is sounding the "check your calculations"-alarm. It actually sounds like the beginnings of a perpetual motion machine to me.

But this can all be built and tested in a vacuum-chamber at a reasonable cost.

Disclaimer:
All I know about this is from the OP.

(Sheeps?)

 

[Alkatran]

Sounds like turning the headlights on for a backward thrust.

 

[Ririon]

Sounds like turning the headlights on for a backward thrust.

...with mirrors in front of the headlights. Sounds like blowing on the sails of your sailboat...

 

[Verde]

So, he thinks that electrons have enough momentum to make things move?

Well, actually they do (albeit very small).
However, that is irrelevent here. As described, what is being injected into the cavity is an electromagnetic wave, or a bunch of photons, if you prefer, which has NO momentum.
The cavity is resonant because a standing wave is setup; however at any point the field is alternating at the rate of the applied frequency. The field will collapse as soon as the excitation is removed. Thus it is not even a storage device.

 

[Dilb]

Well, actually they do (albeit very small).
However, that is irrelevent here. As described, what is being injected into the cavity is an electromagnetic wave, or a bunch of photons, if you prefer, which has NO momentum.
The cavity is resonant because a standing wave is setup; however at any point the field is alternating at the rate of the applied frequency. The field will collapse as soon as the excitation is removed. Thus it is not even a storage device.

Individual photons have momentum. I can't imagine how they could have a net non-zero momentum in a cavity resonator, though.

 

[Verde]

Individual photons have momentum. I can't imagine how they could have a net non-zero momentum in a cavity resonator, though.

You are quite correct. I was thinking of their mass. It's been a long day, and I shouldn't try and work and think at the same time.
However, as you say, there should be no net momentum. Perhaps there could be a tiny amount at the instant the wave is being initially setup, but over the course of one cycle the integrated net effect over the cavity surface should be zero.

 

[Ririon]

... Perhaps there could be a tiny amount at the instant the wave is being initially setup, but over the course of one cycle the integrated net effect over the cavity surface should be zero.

You'd think New Scientist would know that. I'm going to have to read the whole article, now...

 

[Tez]

You'd think New Scientist would know that.

New Scientist is the "news of the World" of science journalism, I would take anything they wrote with a really large grain of salt. Several times they have tried to make me sensationalize results to the point of having completely incorrect science, and when I've refused they havent published the article. Which is fine by me!

 

[DeviousB]

However, as you say, there should be no net momentum. Perhaps there could be a tiny amount at the instant the wave is being initially setup, but over the course of one cycle the integrated net effect over the cavity surface should be zero.

After reading the article, that was my objection too. A cross section of the cavity (a truncated cone) looks a bit like this...

 [FONT=Courier New]________[/FONT]
[FONT=Courier New]\      /[/FONT]
[FONT=Courier New] \    /[/FONT]
[FONT=Courier New]  \__/[/FONT]

The idea being that you get a greater force on the top surface than on the bottom, giving a non-0 resultant. My first thought was that there would also be a force on, and perpendicular to, the sides. As the sides slope, that force would resolve to a slight downwards force; at a guess, one the same size as the disparity between the top and bottom plates.

 

[MRC_Hans]

Well, let's assume that it works (I can't prove it doesn't):

So f...... what? OK; there just might be an opening for such a device in deep-space propulsion, but otherwise? Anybody here imagine that a magnetron runs on zero-point energy or something? No, it runs on good old electrical energy. And rather a lot of it. Perpetuum mobile? IF this thing works rest assured you will need to inject at least as much energy as you get out. Plus losses.

I'll bet you that a fan running on the same amount of power as the magnetron in question will produce considerably more thrust.

Hans

 

[Cuddles]

That's all very neat, but let me be so arrogant as to suggest that either he has got his maths wrong,

This is exactly what I thought, and what one of the scientists quoted in the article said, but without actually seeing any of the calculations I can't say for sure. I only metioned it at all because it seems he is doing real science with independent observers rather than just making wild claims. Sadly it's probably just another maths-gone-wrong, cold-fusion type thing, but would be interesting if it's true.

I should clarify one thing - he does not claim this is perpetual motion in any way and the way it is described makes it entirely clear that it never could be. The microwaves are injected into the cavity from a generator, they do not just sit there, external power is needed for anything to happen.

 

[Ririon]

This is exactly what I thought, and what one of the scientists quoted in the article said, but without actually seeing any of the calculations I can't say for sure. I only metioned it at all because it seems he is doing real science with independent observers rather than just making wild claims. Sadly it's probably just another maths-gone-wrong, cold-fusion type thing, but would be interesting if it's true.

I should clarify one thing - he does not claim this is perpetual motion in any way and the way it is described makes it entirely clear that it never could be. The microwaves are injected into the cavity from a generator, they do not just sit there, external power is needed for anything to happen.

Well, no PM machine. That's good at least. Why doesn't he just send the photons off one way and enjoy a tiny little bit of momentum the other way? How is he supposed to get more momentum out of a photon than the total momentum of the photon? (Which is what you get if you let it go on it's merry way.) My understanding is that this would just create an ever hotter resonant cavity and not much more.

(Sheeps? Really?)

 

[pmurray]

If the idea is to shoot photons out backward, then I suppose it might work. If the idea is some lifting-yourself-by-your-bootstraps notion, then I doubt it.

Even with the photons shooting out the back: lets say I take a torch and shine it off the back of my spaceship. If a photon has X joules of energy in it, it cannot transfer those joules into the forward motion of my spaceship, because it carries those joules away with itself - right? Yet, we know that the photon *does* have momentum and must therfore provide some kickback that will accelerate my ship - correct?

So we have a contradiction - conservation of energy vs conservation of momentum.

The key is the acelleration. My ship accelerates forward, which means that the photons coming off the back of my ship will be red-shifted! red-shifted photons have less energy, that energy has to go somewhere, and it goes into the KE of my ship.

So: the amount of energy being extracted from the photons will relate pretty much directly to the amount of acelleration you get. The more red-shift, the more energy that is being extracted as forward motion. It follows that a photon engine is less and less efficient in proportion to the amount of mass it is trying to push, but more efficient in relation to it's size.

 

[Zombified]

EM fields certainly carry momentum, even in classical Maxwell electrodynamics without the quantum notion of photons.

Unfortunately for the engine at hand, the momentum is rather small. The easiest way to get at it is via SR, where we have E²=m²c⁴+p²c². Plug in p=0 and you get Einstein's famous formula. Plug in m=0 and you get the formula for photon energy and momentum: E=pc, which means p = E/c.

How much momentum do you get if you power your microwave cavity at 1 million watts for 1 second? Well, that's 10⁶ J / 3x10⁸ m/s = 0.0033 kg m / sec. The force exerted for that second is therefore 0.0033 newtons.

That's not a lot of force for an awful lot of energy.

 

[Yllanes]

It looks like New Scientist is going down the drain... Here's a plea to save the magazine, motivated by the article about Shawyer's magical drive.

 

[Yllanes]

New Scientist has answered the criticisms about Shawyer's article with a blog post.

If you haven't followed this, the summary is that New Scientist published an idea for a 'new EM drive' which would violate conservation of momentum. Basically, the thing was akin to bouncing a football thousands of times inside your car to make it accelerate. This prompted science fiction writer Greg Egan to issue a plea to save the magazine from itself (follow the link for a discussion on a physics blog). He also sent a letter to the director. New Scientist counter-attacked by posting Shawyer's theory article (pdf file). This was even more preposterous and only made things worse: Egan commented that his letter had been 'a bit too charitable, because I had not yet read his “theory paper”'. Now they have the blog entry, defending their publication of the article. And a new discussion has been started at the n-Category café (continuation of the one I linked before).

New Scientist editor's note:

Editor's note

It is a fair criticism that New Scientist did not make clear enough how controversial Roger Shawyer’s engine is. We should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious.

But should New Scientist should have covered this story at all? The answer is a resounding yes: it is, after all, an ideas magazine. That means writing about hypotheses as well as theories. [bolding mine]

And let’s not forget that Shawyer has experimental data that has convinced peer reviewers that he is onto something. He believes he can explain his machine's behaviour in terms of existing physical laws, which is what the theorists contest.

The great thing is that Shawyer’s ideas are testable. If he succeeds in getting his machine flown in space, we will know soon enough if it is ground-breaking device or a mere flight of fancy.

Jeremy Webb, Editor, New Scientist

Uh, uh. Does this kind of reasoning ring any bells?

 

[DeviousB]

Shawyer's theory has been trapped, skinned, skewed and set over hot coals more than once (a good example is http://www.assassinationscience.com/johncostella/shawyerfraud.pdf ).

It's also worth noting Shawyer's explanation of how his drive doesn't violate COM.

As the engine accelerates, momentum is lost by the electromagnetic wave and gained by the spacecraft, thus satisfying the conservation of momentum.

IIRC a photon's momentum is h/λ. So (given that Planck's constant is, well, constant), to lose momentum a microwave would have to increase in wavelength.

I) A fundamental principle of his device is that it is that the waveguide is resonant with the microwaves. A change of wavelength would destroy this resonance and Q values of 50k or more should be impossible.

II) This change of wavelength shift should be easily observed in the apparatus without the expense of sending it into space. Hell, I could test for it with two lengths of wire and a LED!