Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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 Direct links to the article may not work as it seems to have been archived.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.

share|cite|improve this question

migrated from 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. – rjzii 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
up vote 29 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 ${\rm gm s^{-1}}$ of momentum, which in radiation can only be carried by $3 \times 10^5$ J (multiply by c), so you need 30,000 Watts of energy to push with mN force, or at least a million Watts for 80 mN. 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 $300 \ {\rm cm^3}$ of air (1 gram) to 1 m/s every second, or to get 80 mN, accelerate $1 {\rm m^3}$ of air (3000 g) to 0.2 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.

EDIT: The initial tests were at atmospheric pressure. To test the fan hypothesis, an easy way is to vary the pressure, another easy way is to put dust in the air to see the air-currents. The experimenters didn't do any of this (or at least didn't publish it if they did), instead, they ran the device inside a vacuum chamber but at ambient pressure after putting it through a vacuum cycle to simulate space. This is not a vacuum test, but it can mislead one on a first read.

In response to criticism of this faux-vacuum test, they did a second test in a real vacuum. This time, they used a torsion pendulum to find a teeny-tiny thrust of no relation to the first purported thrust. The second run in vacuum has completely different effects, possibly due to interactions between charge building up on the device and metallic components of the torsion pendulum, possibly due to deliberate misreporting by these folks, who didn't bother to explain what was going on in the first experiments they hyped up. Since they didn't bother to do a any systematic analysis of the effect on the first run, to vary air-pressure, look at air flows with dust, whatever, or if they did this they didn't bother to admit their initial error, this is not particularly honest experimental work, and there's not much point in talking about it any more. These folks are simply wasting people's time.

share|cite|improve this answer
Unfortunately you did not read the papers. The EMdrive while appearing as a closed system, is in fact an open system when taking relativistic effects and the 'vacuum' medium into account. In effect the vehicle may well propel itself without the expulsion of a traditional propellant. – BAR Jun 1 '15 at 18:32
@BAR: I do not need to read anything to know that this claim is false. The vacuum is a unique state, and to produce thrust, you need to produce something going the other way, either air or radiation, and radiation is ruled out as I explained. The experiments are fraudulent, and your comment is gullible. – Ron Maimon Jun 1 '15 at 21:29
@VolkerSiegel: No, NASA claims it does work, which means I have more smarts than all the NASA idiots put together. – Ron Maimon Jun 2 '15 at 15:45
@BAR: Solid scientific knowledge has this property. I don't need to pick up a pencil to prove the things I am saying, I understand the theory. The gullible folks at NASA don't, so one can only feel pity for them. The claim is theoretically unsound, but it is an experimental claim, so one must look at the experiments. The experiments are also unsound, so there is no basis for this claim, and it is simply fraudulent. – Ron Maimon Jun 3 '15 at 14:17
Shaw wrote a paper on the theory; I think he's sincere, but like a lot of electrical engineers arrogantly deluded, and doesn't understand the criticisms of professional physicists. I understand where he's coming from because I've done exactly the same, only to realize my stupid mistakes upon spending a year reading around it. Laithwaite was a professor of electrical engineering, convinced that gyroscopes held the key to weight loss: Eric Laithwaite Gyro Propulsion 1994 UK – John McVirgo Jun 4 '15 at 0:51

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.

share|cite|improve this answer
No free energy here - conservation laws are preserved. In an ideal case it is 100% efficient but no overunity. – BAR Jun 2 '15 at 19:34
@BAR: Failure of conservation of momentum implies failure of conservation of energy in a moving frame, because energy and momentum mix up together under boosting even nonrelativistically. – Ron Maimon Jun 6 '15 at 12:07

No. 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 why do we measure force? Possibly currents in the waveguide walls induce currents in the metal support structure which creates small magnetic forces between them.

share|cite|improve this answer

protected by Qmechanic Nov 2 '13 at 17:07

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.