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Thrust from Microwaves - The Relativity Drive
dfenstrate writes "The latest New Scientist has an article about an engine that exploits relativity and microwaves to generate thrust. There is a working prototype." From the article: "Roger Shawyer has developed an engine with no moving parts that he believes can replace rockets and make trains, planes and automobiles obsolete ... The device that has sparked their interest is an engine that generates thrust purely from electromagnetic radiation — microwaves to be precise — by exploiting the strange properties of relativity. It has no moving parts, and releases no exhaust or noxious emissions. Potentially, it could pack the punch of a rocket in a box the size of a suitcase. It could one day replace the engines on almost any spacecraft. More advanced versions might allow cars to lift from the ground and hover."
Because that would be a photon drive. And we already know how well those work - the amount of energy you need to input to get even a tiny amount of thrust out of them is astronomical (pun not intended). We've had the basic idea of light propulsion for at least fifty years, and it's been a major cornerstone of hard science fiction. But it just isn't workable with modern power generation.
You could describe either a photon drive, or it's passive counterpart, the light sail, as a "relativity drive", since they too operate on the oddities of conservation of momentum as it applies to light. Doesn't mean we're going to be using them in lieu of rockets anytime in the next few centuries.
Either this guy has found a revolutionary new way to build a photon drive (and I'm more than a little skeptical), or else the device doesn't actually work. I'm more optimistic about this than I am about the usual lot of crackpot science, since from TFA it sounds like this guy is applying good scientific procedures to his work (documenting, trying to get outside review), but I'm not exactly holding my breath.
Erotic is when you use a feather. Exotic is when you use the whole chicken.
I'm rolling on the floor laughing at that article, but have to remind myself that it's probably an ignorant reporter and (not necessarily) Shawyer.
"Since the microwave photons in the waveguide are travelling close to the speed of light"... no, the microwave photons ARE light and are, by definition, moving at the speed of light at that point. I'm not really weaseling -- 'c' is the speed of light in open vacuum and is the same thing for all photons, but a waveguide is only a few multiples of the photon's wavelength and various weird things (to us) happen. See also the (Shamir?) pressure you can get when you hold two conductive plates close together. Longer wavelengths can't exist between the plates but can exist outside of them so you get a very slight net force pushing the plates together.
"any attempt to resolve the forces they generate must take account of Einstein's special theory of relativity."... no, standard EM theory will suffice. (Well, you might need some QM in there, but definitely not special relativity.)
and my favorite
"by mounting it on a sensitive balance, he has shown that it generates about 16 millinewtons of thrust, using 1 kilowatt of electrical power."
Let that sink in. This is as much power as a hair dryer or stove element, and it generates 16 mN of thrust. Could it be, oh, Satan?! I mean, thermal?!
This is particularly ironic since the article referred to the discovery of light pressure earlier. Everyone knows those little bulbs with white and black fans that "demonstrate" this effect. What most people don't know is that it isn't a perfect vacuum in there and, gosh, the dark side gets slightly hotter than the white side. That means the gas heats up on one side, expanding, you know the rest. IIRC they spin leading with the white side. It should be the other way since you have twice as much momentum transfer to reflect light (white) than to simply absorb it (black).
(BTW, I agree 100% with everyone who's pointing out that the walls of the cavity account for the rest of 'thrust' and that the device will just sit there driving up your power bill.)
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
I'm very surprised that this is being reported on. There's nothing to this.
What's probably happening is that the microwaves are leaking out heating up one side of the thruster more than the other causing the air on that side to warm up and become bouyant which is whats creating the apparent thrust. I could make a lot more thrust with a 700 Watt fan than 88 millinewtons.
I'm starting to dispair over the state of science in this so called modern world when I see articles like this. Maybe next we could have an argument over whether sidereal or tropical based astrology is more accurate at predicting the future.
It was a really bad article. It was clearly a dodgy claim and you would think they would have an expert in the area totally vet the article, but alas no.
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There are some other worrying things in the article. For example, the author says...
What of the impact of such a device? On my journey home I have plenty of time to speculate. No need for wheels, no friction.
Yet it is precisely the friction between the wheels and road which make a car go forward. Friction with the car wheels is not bad, you need it. Friction with the air is bad, but not the wheels.
If I had do the EM Drive story, a story which sounds highly suspect, I would have looked at some critiques of similar schemes. Within a few minutes of searching I found similar "Reaction-less Drive" schemes which all turned out to be Oscillation drives. It's the same phenomena as when you move across the room in a swivel chair (without touching the floor) by shifting your body-weight around. When you do that you are exploiting the non-linear nature of friction between surfaces. A similar thing can happen with these reaction-less drives interacting with air, water or other surfaces. So it's quite possible that a prototype drive would appear to work. So I would have asked for some kind of proof that this was not an oscillation drive.
Another issue is that it's not clear that this Em Drive prototype has been tested in a vacuum. In one of the other articles on it, it says that the thrust only reaches the maximum after a few seconds. Now that sounds much more like a mechanical oscillation effect (building up to maximum amplitude) than a photon/microwave effect.
Some of what I have said here is re-posted from a discussion I had on the Elmurst Solutions Science forums. (http://www.elmhurstsolutions.co.uk/cgi-bin/yabb2
A photon's own reference frame? I didn't think you could consider things from the perspective of a photon and still achieve physical results. In a photon's frame of reference, it and all other photons would constantly be at rest, since they all move at the same speed. That doesn't make any sense, though, since photons always travel at the speed of light and can never rest.