The short, short life of the EM drive

        The EM drive, a.k.a. the Resonant Cavity Thruster, made its début in 2001 as a theoretical design by Roger Shawyer. The device is a closed hollow metal frustrum within which resonant microwaves are generated by a magnetron. The difference between the areas of the two ends of the frustrum, according to the theory, gives rise to a net thrust toward the smaller end, without anything being ejected. So it is potentially a low-thrust rocket engine requiring no fuel.

        Positive test results from the Northwestern Polytechnical University in Xi'an, China, reported in 2008, were retracted in 2016 as experimental error. Nevertheless some people, including some at NASA's Advanced Propulsion Physics Laboratory, continued to report thrust in the 50 µN range.

        At the Technical University of Dresden, Martin Tajmar and G. Fiedler built a version of the EM drive inside a vacuum chamber evacuated to ~10^-7 millibars. The device was rigidly attached to a torsion balance whose deflection was measured by a laser interferometer with sufficient sensitivity that a deflection produced by a thrust as low as 1 µN could be measured. Tajmar & Fiedler reported preliminary results at the 51st AIAA/SAE/ASEE Joint Propulsion Conference, AIAA Propulsion and Energy Forum, in 2015. The paper was titled Direct Thrust Measurements of an EMDrive and Evaluation of Possible Side-Effects. From the Abstract:

"[W]e observed thrusts of +/- 20 µN however also in directions that should produce no thrust. ...Our test campaign can not confirm or refute the claims of the EMDrive but intends to independently assess possible side-effects in the measurement methods used so far."

        I, and many other commenters, knew there was something very dodgy about the device when we read that the control experiment—a run when the device was rotated so that thrust along its axis could not possibly deflect the torsion balance—produced better "thrust" than when the device was oriented optimally. Well, we knew that already because this thing cannot possibly work as claimed. It violates Newton's third law of motion. As I wrote at that time, if this works then sitting in your car and pushing on the dashboard will work, too.

The kibosh
        Now, the Dresden lab has put the almost-final kibosh on the EM thingy. On 16th May, at a Space Propulsion conference in Seville, Spain, they presented The SpaceDrive Project – First Results on EMDrive and Mach-Effect Thrusters by Martin Tajmar, Matthias Kößling, Marcel Weikert and Maxime Monette. Stripped down to telegram length, Tajmar et al. reported that they think they've found the source of the experimental error. They remembered from Electrical Engineering 101 that an electrical conductor carrying current in the presence of a magnetic field experiences a force orthogonal to both the electrical current and the magnetic field. This is the principle that makes my coffee grinder, and your vacuum cleaner, and every other electric motor in the whole world, go round and round.

        Is there an electric conductor carrying current in this experiment? Yes, the cable supplying the magnetron, which runs along the torsion arm. Is there a magnetic field present? Yes, the good old field from Mother Earth that makes compasses work. Back-of-an-envelope calculation confirms that forces in the same range as those observed could well be the result of that current (2 amps) and that magnetic field (~48 µTesla at an angle of 70°). Assuming the length of the cable is 15 cm.,

F = 2x48x0.15 sin 70 =13.52 µN^{note 1}

        I hope Sarah Knapton enjoyed her breakfast of words a few days ago. She's the Science Editor of the London Daily Telegraph who wrote, on 28th July 2015, 'Impossible' rocket drive works and could get to Moon in four hours.

Physics 1, Mike Bara 0
        Knapton isn't the only one eating words. Mike Bara, the world-famous denier of evolution, climate change, and the entirety of physics, loves the EM. So much so that he wrote a section about it in his book about "The Secret Space Program." Strange, because there's nothing secret about the device and its connection to space is limited to extravagant claims. Bara wrote of the original Xi'an announcement "the results were astounding," not realizing that the results had already been retracted. He also wrote that the device falsifies Einsteinian physics, not understanding that it's Newton, not Einstein, whose ideas would have to be wrong if this device works as claimed.

         In September 2010, in a promo interview for his appalling book The Choice, Bara said "I’m completely confident that I can prove there’s no such thing as the laws of physics." In the book itself, he wrote (page 67) "most mainstream physicists are blithering idiots." After the 2015 announcement by Tajmar & Fiedler, Bara tweeted  "Aaaaannnddd just like that the "laws" of physics go out the window.."

        Look around, Mike. I think you'll find that the laws of physics just flew right back in.

See also:
NASA’s EM-drive is a magnetic WTF-thruster. Test reveals that the magic space unicorns pushing the EM-drive are magnetic fields. --Chris Lee in Ars Technica, 5/21/2018.

Scott Manley reports on Youtube, with many of the key illustrations and diagrams from the new Dresden paper.

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[1] This is known in electrical theory as the Lorentz force, and it's partly responsible for vibration and "hum" in AC conductors carrying substantial current.