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In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some claims that New Scientist was engaging in pseudo-science.

Since the original article the inventor claims that a "Technology Transfer contract with a major US aerospace company was successfully completed", and that papers have been published by Professor Yang Juan of The North Western Polytechnical University, Xi'an, China. 2

Furthermore, it was reported in Wired magazine that the Chinese were going to attempt to build the device.

Assuming that the inventor is operating in good faith and that the device actually works, is there another explanation of the claimed resulting propulsion?

1. Direct links to the article may not work as it seems to have been archived.
2. The abstracts provided on the EmDrive website claim that they are Chinese language journals which makes them very difficult to chase down and verify.

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migrated from skeptics.stackexchange.com Apr 13 '12 at 18:16

This question came from our site for scientific skepticism.

100kg unit producing 96 milinewtons of thrust? I wouldn't call that "working". –  vartec Apr 10 '12 at 13:54
@vartec - Depends upon the applications, if we are talking about applications in space then that might be enough over a long enough peirod of time. The HiPEP only produced 460 - 670 mN in the pre-prototype testing. –  rob Apr 10 '12 at 14:15
This belongs on Physics and it's very unlikely to get a decent answer here, in my opinion. Do you want me to migrate? –  Sklivvz Apr 10 '12 at 21:43
The second part of your question would almost certainly get an answer on Physics (essentially, "no," with explanation). The first part, I'm not sure about. I think it'd be on topic for us, but there is a chance nobody on the site would be able to answer it. I will say that it would be helpful to split this into two separate posts, one for each part of the question, if it is migrated. –  David Z Apr 11 '12 at 19:30
Actually I am a mod on Physics - I figured I could reply here since the discussion would benefit from being public. –  David Z Apr 11 '12 at 19:42

5 Answers 5

up vote 8 down vote accepted

It is impossible to generate momentum in a closed object without emitting something, so the drive is either not generating thrust, or throwing something backwards. There is no doubt about this.

Assuming that the thrust measurement is accurate, that something could be radiation. This explanation is exceedingly unlikely, since to get mN of radiation pressure you need an enormous amount of energy, since in 1s you get 1 gm/s of momentum, which in radiation can only be carried by 3*10^5 J (multiply by c), so you need 30,000 Watts of energy to push with mN force, or at least a million Watts for 80mN. So it's not radiation.

But a leaky microwave cavity can heat the water-vapor in the air around the object, and the heat can lead to a current of air away from the object. With a air current, you can produce mN thrusts from a relatively small amount of energy, and with a barely noticible breeze. To get mN force, you need to accelerate 300 cm^3 of air (1 gram) to 1 m/s every second, or to get 80mN, accelerate 1 m^3 of air (3000 g) to .2 m/s (barely perceptible) and this can be done with a hot-cold thermal gradient behind the device which is hard to notice. If the thrust measurements are not in error, this is the certain cause.

So at best, Shawyer has invented a very inefficient and expensive fan.

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I'm guessing your explanation still holds given the new claims by the Chinese researchers right? –  FrenchKheldar Nov 7 '12 at 3:59
This seems a very reasonable explanation, and it could also easily be tested by its inventor by putting the whole apparatus in a vacuum and seeing if it works even there. –  Ferdinando Randisi Nov 7 '12 at 13:17
It will not work in a vacuum, that is certain. It should produce thrust proportional to ambient air pressure, since it's blowing air back. This is kind of a no-brainer. –  Ron Maimon Nov 7 '12 at 17:43
I see several mentions of microwaves, so it seems like the power consumption argument is the proper debunking. They claim to bounce the radiation between reflectors. This would work if one reflector was stationary (indeed, this is a viable concept), but both are firmly on board the device. –  AlanSE Nov 30 '12 at 14:36

There is an alternative explanation published in Grav. & Cosmol. 19 (2013) 201, also available in arXiv:1302.5690. It is related to possible strong grav effects mediated by scalar fields with source in the EM fields.

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Dear user32117, not to conflict with Phys.SE self-citation policy, please spell out explicitly in your answer if you are in any way associated with the author of the paper arXiv:1302.5690. –  Qmechanic Nov 2 '13 at 17:17

Shawyer's "analysis" is a mess, incoherent and deeply confused about fundamental aspects of relativity: he mixes up frames, assumes a universal rest frame, etc. The EmDrive supposedly works best when "stationary relative to the thrust", whatever that means, and Shawyer goes on to suggest using it for levitating vehicles with some kind of conventional propulsion for driving them forward: he apparently believes there is something special about gravitational acceleration.

According to his latest paper, the EmDrive supposedly acts as an electric motor, consuming energy when accelerating and producing it when decelerating. However, a deceleration is just an acceleration in a particular direction, so if it worked, the EmDrive could operate as an infinite energy machine just sitting on one end in a gravity field or while producing thrust for a spacecraft.

So to answer the question in the title: "No." As for other explanations of the observed propulsion, there aren't many details of the measurement procedures or results. There are videos of an EmDrive test on a rotating platform, but there's numerous pieces of equipment that may contain fans, thick power cables going to the equipment that may apply torques, and even a laptop with a hard drive that may be spinning up or down. (And on top of everything else, the whole thing's apparently rotating in the wrong direction.) If this rig is typical of his testing methodology, it's probably safe to chalk up the rest to bad measurements.

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In special relativity, 4 momentum is exactly conserved. The first component of 4 momentum is total mass/energy, but the next 3 are given by:

p = m*γ(v)*v

m is the invariant mass, how much inertia it has when you are moving at the same velocity of it.

This is Newton except now momentum is a non-linear function of velocity. Nonlinearity does not change anything. Mass and momentum still are constant (ignoring leaks), making γ(v)*v, and thus the center-of-mass velocity v, constant.

So how could we measure force? Possibly currents in the waveguide walls induce currents in the metal support structure which creates small magnetic forces between them.

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Well, the Wiki link you supplied also links to a paper:

Why Shawyer’s ‘electromagnetic relativity drive’ is a fraud


Note the PhD in relativistic electrodynamics compared to Shawer's C.Eng MIEE.

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Yes, I have read that paper; however, the paper doesn't provide any sort of explanation to address any actual measured force which is why I'm looking for an explanation with the assumption that force is being measured as opposed to an explanation as to why the inventors explanation is likely wrong. –  rob Apr 13 '12 at 19:07

protected by Qmechanic Nov 2 '13 at 17:07

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