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Question
Hi,

There has been a lot of publicity about this EmDrive:-

https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster#Second_device_and_New_Scientist_article


http://www.emdrive.com

http://www.wired.co.uk/news/archive/2014-07/31/nasa-validates-impossible-space-d
What do you think of the scientific plausibility of this drive. Is this nonsense or does it  actually work?

Answer
The Wired magazine article shouldn't be taken seriously (they can't even get the face that NASA is an acronym and not spelled "Nasa").  However, I looked into your other references and watched the test video.  The problem with these measurements are that they're small forces (0.1N when the rig weighs almost 10,000 times as much) compared to the weights of the measurement devices.  They also don't really conserve momentum.

I've seen a lot of exciting science go down in flames when the evidence was tenuous, mostly when people followed up on crumbs of exciting hope while ignoring failed tests (which are still valid).  Air wafting through what looks like a radiator in the test video on emdrive.com alone could easily be responsible for forces in the mN range, which these are.  If they had video with a $250 Flir One thermal camera, I might dismiss that one.  They're also attached with wires for power (high current).  I've held wires that had significant electromagnetic forces on them from local fields.

In short, they haven't nearly eliminated all the possible sources of thrust they could have in order to justify even a cubesat-scale test (though that is kinda the only way this will get resolved).  I think there might be more of a chance of using the Sun's magnetic field and the interstellar magnetic field to propel spacecraft with this scale of propulsion than there would be with these devices (they haven't seemed to eliminate the Earth's magnetic field as a source of error, either).

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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