EmDrive

[FREEFALL1984]

What is an EmDrive and how can it seemingly defy the laws of physics by producing a force without the need for a reaction mass.

[Seret]

If something sounds too good to be true...

[Nuke]

its a device made by people who dont seem to understand how physics works, yet are convinced that their contraption is a breakthrough. it has been patented, and has been tested (and confirmed) by a shady lab in china. in other words its a hoax.

[StuntMax]

From Wikipedia: " the device generates thrust even though no detectable energy leaves the device."

Enough for me, bullnugget-o-meter jumped to 100% so hard that it bent the needle.

Edited July 31, 2014 by sal_vager
Profanity

[andrewas]

Reactionless drives always turn out to be either hoaxes or equivalent to photon drives. Either way, no cause for excitement.

[KSK]

Yeah, I filed the EmDrive in my mental 'extraordinary claims require extraordinary proof' drawer. It's getting quite dusty in there.

[TheGatesofLogic]

heh, people complain about this being bullnuggets but the wikipedia article is extremely misleading, in fact the name EmDrive is misleading in and of itself. The EmDrive concept is similar to the Wodward Drive concept in that it involves unequal forces on opposite ends of an oscillating system. However the EmDrive is not actually the result of electromagnetic resonation in a cavity but rather the relativistic effects of resonation in such a cavity. Furthermore the tapered waveguide under the conditions of special relativity allow for a substantial thrust force arising out of the open system consideration that must be applied when considering a relativistic speed resonant fluid (in this case microwave radiation). The theory behind the drive is actually quite solid and contrary to popular belief DOES NOT contradict conservation of momentum under Newtonian mechanics because Newtonian mechanics does not apply when considering an open system. All in all the theory is solid and has been thoroughly tested by a group of chinese researches using the same device the chinese space agency uses to test its ion thrusters.

Here is a link to the Chinese research study

and here is a link to the theory paper

The one foreseeable problem with the EmDrive concept is that it appears to violate 4-momentum, however 4-momentum fails to recognize any form of quantum relativity and as such it is questionable whether it is actually violated or not.

Edited July 31, 2014 by sal_vager
Profanity

[Idobox]

A drive that has a thrust without pushing anything (spewing microwave or infrared counts as something) contradicts the fundamental understanding of science.

Throwing technobabble and calculations does not mean the science is solid.

[TheGatesofLogic]

its a device made by people who dont seem to understand how physics works, yet are convinced that their contraption is a breakthrough. it has been patented, and has been tested (and confirmed) by a shady lab in china. in other words its a hoax.

The Northwestern Polytechnic Institute in China is VERY highly regarded. I would not doubt in any way that their findings are legitimate. Furthermore the theory paper surrounding the device is sound mathematically, which means that if it doesn't work we have to re consider some aspects of the physics that support its operation.

[TheGatesofLogic]

A drive that has a thrust without pushing anything (spewing microwave or infrared counts as something) contradicts the fundamental understanding of science.

Throwing technobabble and calculations does not mean the science is solid.

No it doesn't, modern physics cannot validate the conservation of momentum in relativistic scenarios

[TechnicalK3rbal]

From Wikipedia: " the device generates thrust even though no detectable energy leaves the device."

Enough for me, bulll****-o-meter jumped to 100% so hard that it bent the needle.

Or it just did this: http://i.imgur.com/90FDgjq.jpg

[TheGatesofLogic]

Would it be hard to ask for a legitimate discussion instead of random unbased denials? People claiming it doesn't work because they think the people making it don't understand physics is ultimately just popular hypocrisy.

[Red Iron Crown]

The paper was published in 2011, and it is claiming something that is a Big Deal in the physics/space travel world. Has any other lab been able to repeat their experiment with similar results?

[Ralathon]

heh, people complain about this being bull**** but the wikipedia article is extremely misleading, in fact the name EmDrive is misleading in and of itself. The EmDrive concept is similar to the Wodward Drive concept in that it involves unequal forces on opposite ends of an oscillating system. However the EmDrive is not actually the result of electromagnetic resonation in a cavity but rather the relativistic effects of resonation in such a cavity. Furthermore the tapered waveguide under the conditions of special relativity allow for a substantial thrust force arising out of the open system consideration that must be applied when considering a relativistic speed resonant fluid (in this case microwave radiation). The theory behind the drive is actually quite solid and contrary to popular belief DOES NOT contradict conservation of momentum under Newtonian mechanics because Newtonian mechanics does not apply when considering an open system. All in all the theory is solid and has been thoroughly tested by a group of chinese researches using the same device the chinese space agency uses to test its ion thrusters.

Here is a link to the Chinese research study

and here is a link to the theory paper

The one foreseeable problem with the EmDrive concept is that it appears to violate 4-momentum, however 4-momentum fails to recognize any form of quantum relativity and as such it is questionable whether it is actually violated or not.

I'm sorry, but as someone who studies Applied Physics that second paper is utterly hilarious. He's confusing group velocity with momentum and then tries to use the Lorentz force equation to explain it. That's not only not right, that's not even wrong!

It's the kind of paper I'd expect from someone who knows some impressive terms and the absolute basics behind a lot of disciplines but utterly misunderstands how these things hook together. Both thanks to the content and the layout.

[TheGatesofLogic]

Here is a more recent paper concerning the system, perhaps this is more relevant. This one was presented at the International Astronautical Congress in 2013. Edited May 9, 2014 by TheGatesofLogic

[Ralathon]

Here is a more recent paper concerning the system, perhaps this is more relevant.

That one is even better. They fail to take into account that the speed of light is constant during their Doppler calculations. That way they get double values for their Doppler shift on one side and zeros on the other side, no wonder their energy starts to disappear.

The problem they're trying to solve there is actually a very common problem given during special relativity courses. They completely botched it, something that shows to me that they're probably victims of the Dunning-Kruger effect.

[N_las]

Both "papers" by Roger Shawyer are complete nonsense. He ignores the radiation pressure at the side walls of the cavity: http://imgur.com/be7Qyza

It is interessting. The chinese team is aware of this error. Look in the paper at figure 1: http://www.emdrive.com/yang-juan-paper-2012.pdf

They even say in the text, that the whole surface has to be included in the calculations. But mysterioulsy, they don't show these calculations. They claim in the paper: "The theoretical calculations and experimental results for the direction of thrust were identical." But the never show these "theoretical calculations".

[Skyler4856]

Wow, that diagram is hilarious!

[KSK]

No it doesn't, modern physics cannot validate the conservation of momentum in relativistic scenarios

Uh, modern physics can do that just fine, so far as I know. Apart from anything else it's what solar sails rely on and spacecraft using solar sails have actually flown so it's not just a nice piece of theory.

[Z-Man]

I'm sorry, but as someone who studies Applied Physics that second paper is utterly hilarious. He's confusing group velocity with momentum and then tries to use the Lorentz force equation to explain it. That's not only not right, that's not even wrong!

Actually, that he is using the group velocity for the momentum calculation is the most sensible part of it all. He is not saying they are equal, just using that they are proportional. That is what you have to do... if you want to abstract away the reason why the group velocity is different from the vacuum speed of light. Here, of course, the reason for the change are the side walls, and ignoring them is a mistake.

What baffles me most about the paper is that he is using Special Relativity, so I assume he considers it valid, yet the results are manifestly not covariant with thrust direction depending on the current absolute velocity.

As for the Chinese experiments, they are feeding the resonator with a wave guide and the actual microwave source is fixed away from the resonator. A lot can happen there that has nothing to do with propellant-less propulsion.

[Kinglet]

If there is a nonsense word and drive behind it, it is obviously a hoax.

[KOCOUR]

If there is a nonsense word and drive behind it, it is obviously a hoax.

EM as "ElectroMagnetic" is not an nonsense word.

And using electromagnetic field for propeling isnt imposible you just need to do it right. For example u can use a group of coils and group of magnetes, or two groups of coils to change electric energy to magnetic field and then to rotational momentum and if you atach a wheel or propeler....................the miracolous device is called Electricla engine

But of cours you cant use it for spacetravel as there atre no roads or air in space.

:-D

[TheGatesofLogic]

Uh, modern physics can do that just fine, so far as I know. Apart from anything else it's what solar sails rely on and spacecraft using solar sails have actually flown so it's not just a nice piece of theory.

That works on a quantum basis, not a relativistic basis

[KSK]

That works on a quantum basis, not a relativistic basis

Both actually but that shouldn't come as a surprise given that solar sails rely on momentum transfer due to reflection of photons. And yes - quantum mechanics is consistent with special relativity. For example, see the Dirac equation.

Briefly, photons have zero rest mass (by definition they travel at the speed of light and so from Relativity they must have zero rest mass). Since momentum (p) is equal to mass (m) multiplied by velocity (in this case the speed of light or c), then one might expect photons with zero rest mass to have zero momentum.

However, photons have energy and so from Einstein's equation (E=mc2), they do have relativistic mass. From the Planck equation, e=hf, where f = frequency and h = Planck's constant. Substituting into Einstein's equation we get hf=mc2 and therefore m = hf/c2.

The momentum of a photon (mass x velocity) is therefore chf/c2 or simply hf/c. Assuming a perfect reflector exactly perpendicular to the direction of travel of the photon, that photon will transfer 2hf/c units of momentum to the sail.

[shynung]

But of cours you cant use it for spacetravel as there atre no roads or air in space.

:-D

One could use electromagnetic coils to hurl ferromagnetic propellant at high speeds. This is called a mass driver, and it is possible to use it to propel spacecraft.

If properly configured, it could also thrust against a magnetic field, but this type only works near planets/stars possessing a strong magnetic field around it.

Edited May 13, 2014 by shynung