In recent headlines, a supposed EM drive, or RF resonant cavity thruster, has been reported to produce thrust despite the apparent lack of any propellant. The drive is purported to work by pushing microwaves through a cone, thereby creating more force on the back end, pushing the object along. Naturally, the scientific community is skeptical that such a device can even exist. Popular Mechanics is blaming populism for the hype (is that irony?). Forbes insists "Physics Falls Apart If The EMdrive Works". The conservation of momentum seems violated by such a theory; there needs to be something of mass moving in the opposite direction to produce thrust. Yet, they, Eagleworks, claim to have something that produces thrust without expelling mass in the opposite direction. Are they lying? Does it work?


3 Answers 3


From the EagleWorks paper:

The test campaign included a null thrust test effort to identify any mundane sources of impulsive thrust; however, none were identified.

They are not lying, they did detect thrust, but the source of that thrust remains a mystery. The fact that the EM drive would violate Newton's third law, one of the most established principles of physics makes this highly suspicious. Notice that they do not claim that the thrust could not be caused by experimental error (proving a negative is impossible) but that they simply were not able to detect one.

The paper does suggest an alternative theory (pilot-wave), but that in itself is not fully accepted by the scientific community (which doesn't mean it's wrong!).

In conclusion, thrust was measured, but it's not clear if the thrust is caused by experimental error or by currently ill understood quantum effects. The burden of proof lies with the proponents of the EM drive.


Currently, the answer actually seem to be yes!

It was previously reported that radio-frequency (RF) resonant cavities generated anomalous thrust on a low-thrust torsion pendulum [1,2] in spite of the apparent lack of a propellant or other medium with which to exchange momentum. It is shown here that a dielectrically loaded, tapered RF test article excited in the transverse magnetic 212 (TM212) mode (see Fig. 1) at 1937 MHz is capable of consistently generating force at a thrust-to-power level of 1.2±0.1  mN/kW1.2±0.1  mN/kW with the force directed to the narrow end under vacuum conditions.

In a simple explanation, a model of the technology was suspended on a pendulum in a vacuum and was able to produce thrust. In comparison of how much thrust, the Hall-effect thruster produces about 50 times more, but the EM drive seems to produce thrust nonetheless and future models may be able to produce something more comparable. Source

It is shocking and certainly does seem to be a violation of physics as we currently understand it, however, there are a few theoretical explanations. For example:

The team [Eagleworks] also offers a hypothesis about how the drive actually works without contradicting the laws of physics: "[The] supporting physics model used to derive a force based on operating conditions in the test article can be categorised as a nonlocal hidden-variable theory, or pilot-wave theory for short." ...

"If a medium is capable of supporting acoustic oscillations, this means that the internal constituents were capable of interacting and exchanging momentum," the team writes.

"If the vacuum is indeed mutable and degradable as was explored, then it might be possible to do/extract work on/from the vacuum, and thereby be possible to push off of the quantum vacuum and preserve the laws of conservation of energy and conservation of momentum."

I am not a physicist, but that explanation sounds like nonsense, and the scientific community remains skeptical. A very common criticism centers around uncertain instrumentation, and therefore bad measurements. For example:

The more important point is that the individual uncertainties in their instrumentation don't account for the variation in the thrust that they measure, which is a very strong hint that there is an uncontrolled experimental parameter playing havoc with their measurements.

Some also criticize that there are so few data points, which is an indication of lazy experimentation. For example:

Indeed, although the researchers have numerous variables at their hands to change between experiments, they only play with one. In previous papers, they played with two, but still this limited exploration and limited data is really disheartening.

Conversely, some will likely and eagerly try to reproduce the experiment's results. The above paper from the AIAA, if the findings are legitimate, marks what may be the biggest discovery of our century; the promise is too good to pass up for some.

  • 5
    That's an overly charitable reading of that paper, to be frank. From §III: "The test campaign included a null thrust test effort of three tests performed at vacuum at 80 W to try and identify any mundane sources of impulsive thrust; none were identified". Most working physicists, I would argue, will tend to view this with the lens of "if they try harder then they will identify such a mundane source of thrust".
    – E. P.
    Commented Nov 25, 2016 at 22:45
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    It's important not to forget that the 'laws' of physics are descriptive, not prescriptive. That is, they are our best efforts to describe how the universe works, rather than dictating how it should work. Relativity, quantum mechanics, and more all broke what were thought to be established laws of physics. Maybe this does too. Seems like the easiest way to test would be to put one in a satellite and turn it on. If the satellite passes the orbit of Mars and is still accelerating... well, time to re-write a few laws :-)
    – jamesqf
    Commented Nov 26, 2016 at 5:21
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    The recent results will almost certainly turn out to be experimental error of some kind. Conservation of momentum is backed by lots of experiments ranging over lots of circumstances, whereas this drive is backed by one experiment. However that doesn't mean it should be written off: challenges to the status quo are how science progresses. So we should absolutely carry on experimenting. But don't put any bets on it turning out to be real. Commented Nov 26, 2016 at 14:09
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    @JonathonWisnoski The claimed thrust is orders of magnitude above the ideal photon-drive thrust. That is the paper quotes about 1.2 mN/kW, when a photon drive should get 3µN/kW. The paper by White et.al. makes an attempt to cover all the obvious sources of systematic error, which is the first time we're really see that, but ... well, we've seen the millennium announced before. Physicist taken as a group will be assuming a subtler experimental issue. Commented Nov 26, 2016 at 23:42
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    The group responsible for this has not behaved very well. They have made a lot of claims in the press about their amazing drive, but published VERY few details to refereed journals. What they have "published" has been to internet forums by and large. What physicists have managed to get their hands on has been full of experimental concerns. None of this means that the result can't be good, but it means that there should be extreme skepticism. Much more so than has been seen on much of the internet, which has been overly enthusiastic IMO.
    – KAI
    Commented Nov 29, 2016 at 0:37

The answer is no.

The paper does not claim that the thrust comes without "propellant", and it includes a possible physical interpretation of the results not violating Newton's third law (emphasis mine):

If the vacuum is indeed mutable and degradable as was explored, then it might be possible to do/extract work on/from the vacuum, and thereby be possible to push off of the quantum vacuum and preserve the laws of conservation of energy and conservation of momentum. It is proposed that the tapered RF test article pushes off of quantum vacuum fluctuations, and the thruster generates a volumetric body force and moves in one direction while a wake is established in the quantum vacuum that moves in the other direction.

This explanation relies on the Bohmian interpretation of quantum mechanics. In practice, the authors are (partially) trying to substantiate that interpretation with this experiment. Note that this is mostly an interpretation of accepted physics, and not a novel theory.

Regarding whether the thrust is effectively measured, the experiment does take a number of precautions to make sure they eliminate some bias, but it still shows a huge error margin of about 8%:

Thrust data from forward, reverse, and null suggested that the system was consistently performing with a thrust-to-power ratio of 1.2 ± 0.1 mN/kW.

We need to see if the results can be consistently replicated in the future before drawing conclusions.

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    and don't forge that the EM drive needs electrical power to work, so energy is indeed injected, which is generated somewhere else. The idea that an EM drive requires no power is just as false as the idea that an electric car requires no fuel.
    – jwenting
    Commented Jan 2, 2017 at 8:07

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