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Controversial invention
Inventor Roger Shawyer
Theory violation Conservation of momentum, Newton's Third Law

The EmDrive (or RF resonant cavity thruster) is a proposed spacecraft propulsion device invented by British aerospace engineer Roger Shawyer, who founded the company Satellite Propulsion Research Ltd (SPR) in the year 2000 to develop his invention.[1][2] The device uses a magnetron to produce microwaves which are directed into a metallic, fully enclosed conically tapered high Q resonant cavity with a greater area at one end of the device, and a dielectric resonator in front of the narrower end. The inventor claims that the device generates a directional thrust toward the narrow end of the tapered cavity. The device (engine) requires an electrical power source to produce its reflecting internal microwaves but does not have any moving parts or require any reaction mass as fuel. If proven to work as claimed, this technology could be used to propel vehicles intended for all forms of travel including ground travel, marine travel, sub-marine travel, airflight and spaceflight.[3][4][5][6][7][8]

The device, its mode of operation, and theories attempting to explain it are all controversial. As of 2015, there are still arguments about whether the EmDrive is genuinely a new propulsion device, or whether its experimental results are simply misinterpretations of spurious effects mixed with experimental errors. The proposed theories of its operation have all been criticized on the grounds they violate the conservation of momentum, a fundamental law of physics, though Shawyer asserts that EmDrive does not.[9]

Chinese researchers from the Northwestern Polytechnical University (NWPU) in Xi'an in 2010, published a report in their university's journal claiming to have confirmed the theory, and in 2012 they published a report stating that they had built and tested a device claiming to have replicated Shawyer's experiments, recording better results than Shawyer had claimed at even higher power levels,[6][10][11][12][13] though they were also clear that their work was still preliminary.

From 2014 a NASA evaluation group at Johnson Space Center have claimed replication at low power levels, measuring a directional thrust level in a vacuum, giving results apparently in accord with Shawyer's experiments and claims. However, these experiments and results have not yet been peer reviewed.[8][14]


DTI grant and first device[edit]

In the year 2001, Roger Shawyer founded Satellite Propulsion Research Ltd, in order to work on the EmDrive. The company was backed by a "Smart Award" grant from the now-defunct UK Department of Trade and Industry.[7] The DTI grant totalled £250,000, spread out over three years.[15] By December 2002, he was demonstrating a working prototype, reporting a total thrust of about 0.02 Newtons powered by an 850W magnetron.[16] It was later reported that the device could only operate for a few tens of seconds before the magnetron failed, due to overheating.[17] In 2003, Shawyer reported a move toward higher thrust, stating that "An extrapolation of the [EM Drive] theory has been carried out for high thrust engines and the concept appears feasible. Initial design work on a lift engine has been undertaken."[18]

Second device and New Scientist article[edit]

In October 2006, Shawyer conducted tests on a new water-cooled prototype, which increased thrust to 0.1 Newtons and ran on 300W of microwave power.[17] He planned to have the device ready to use in space by May 2009, and was considering making the resonant cavity a superconductor.[17]

After receiving criticism that no peer-reviewed publications on the subject had been made, Shawyer submitted a theory paper to New Scientist, a weekly popular science consumer magazine.[19] The EmDrive was featured on the cover of the 8 September 2006 issue of the magazine. The article portrayed the device as plausible, and emphasized the arguments of those who held that point of view.

Science fiction author Greg Egan, who holds a Bachelor of Science degree in Mathematics from the University of Western Australia, distributed a public letter stating that "a sensationalist bent and a lack of basic knowledge by its writers" made the magazine's coverage unreliable, sufficient "to constitute a real threat to the public understanding of science". In particular, Egan found himself "gobsmacked by the level of scientific illiteracy" in the magazine's coverage of the EmDrive, stating that New Scientist employed "meaningless double-talk" to obfuscate the relation of Shawyer's proposed space drive to the principle of conservation of momentum. Egan urged those reading his letter to write to New Scientist and pressure the magazine to raise its standards, instead of "squandering the opportunity that the magazine's circulation and prestige provides" for genuine science education. The letter was endorsed by mathematical physicist John C. Baez and posted on his blog.[20][21] Egan also recommended[20] that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics)[22] of Shawyer's paper.[19]

The following month, the New Scientist editor addressed the ensuing controversy over the article stating that "[w]e should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious."[23]


Any apparently reactionless drive is treated with skepticism by the physics community because a truly reactionless drive would violate the law of conservation of momentum. Shawyer claims that his drive does not violate conservation of momentum and is not reactionless.[9] Shawyer has posted an updated theory paper (version 9.4) for the EmDrive.[24] Shawyer's paper includes the fundamental assertion underlying the theory: "[t]his force difference is supported by inspection of the classical Lorentz force equation F = q(E + νB). (1) If ν is replaced with the group velocity νg of the electromagnetic wave, then equation 1 illustrates that if vg1 is greater than vg2, then Fg1 should be expected to be greater than Fg2." This statement makes two assumptions which Shawyer does not substantiate and which may explain the discrepancy between Shawyer's predictions and those of conventional physics. First, Shawyer assumes that radiation pressure is the result of the Lorentz force acting on charged particles in the reflecting material. This is analyzed by Rothman and Boughn[25] who point out that the standard theory of radiation pressure is somewhat more complicated than the simplified analysis suggests. Second, Shawyer asserts that quantum energy is transferred at the group velocity, and thus momentum of the photon and the consequent radiation pressure must vary with group velocity. Photon momentum varies with phase velocity. Group velocity measures the rate of propagation of information. The phase velocity is constant throughout the frustum resonator, consequently radiation pressure would not be expected to produce unbalanced forces.

Various hypotheses and theories have been proposed explaining the underlying physics for how the EmDrive and related designs might be producing thrust. Shawyer claims that thrust is caused by a radiation pressure imbalance between the two faces of the cavity caused by the action of group velocity in different frames of reference within the framework of special relativity.[26] Yang from NWPU calculated the net force/thrust using classical electromagnetism.[12] Harold G. "Sonny" White, who investigates field propulsion at Eagleworks, NASA's Advanced Propulsion Physics Laboratory, speculated that such resonant cavities may operate by creating a virtual plasma toroid that could realize net thrust using magnetohydrodynamic forces acting upon quantum vacuum fluctuations.[27] Likewise, the paper describing the Eagleworks test of the Cannae drive referred to a possible interaction with a so-called "quantum vacuum virtual plasma".[14] This reference has been criticized by mathematical physicists John Baez and Sean M. Carroll because in the standard description of vacuum fluctuations, virtual particles do not behave as a plasma.[21][28][29]

Testing and replication claims[edit]

Static thrust tests[edit]

Shawyer has reported seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE.[15] As of 2015, no EmDrive has been tested in microgravity.

Shawyer speculated in 2006 that, with adequate funding, commercial terrestrial aircraft incorporating EmDrives as lift engines could be ready by 2020.[30][31] He proposed that very high Q superconducting resonant cavities could produce static specific thrusts of about 30 N/W, which is 3 tonnes-force of thrust per kilowatt of input power − "enough to lift a large car".[32]

Chinese Northwestern Polytechnical University (NWPU)[edit]

In 2008 a team of Chinese researchers led by Juan Yang (杨涓), professor of propulsion theory and engineering of aeronautics and astronautics at Northwestern Polytechnical University (NWPU) in Xi'an, claimed to have developed a valid electro-magnetic theory behind the Emdrive.[3][33] A demonstration version of the drive was built and tested under different cavity shapes and at higher power levels in 2010.[10] A maximum thrust of 720 mN was reported at 2,500 W of input power on an aerospace engine test stand usually used to precisely test spacecraft engines like ion drives.[7][11][12][13][34]

The editor of Wired magazine who covered the experimental results relating to reactionless drives reported that he received some comments from the Chinese researchers stating "the publicity was very unwelcome, especially any suggestion that there might be a military application"[5] and that Yang told him that "she is not able to discuss her work until more results are published".[7]

NASA/JSC Advanced Propulsion Physics Laboratory (Eagleworks)[edit]

A NASA team at the Advanced Propulsion Physics Laboratory (informally known as Eagleworks)[35] located at the Johnson Space Center (JSC) under the guidance of physicist Harold G. White is devoted to studying advanced propulsion systems that they hope to develop using quantum vacuum and spacetime engineering. The group has investigated a wide range of fringe proposals including the EmDrive, and related concepts listed below.

RF resonant tapered cavity thruster (EmDrive)[edit]

In July 2014, the group reported positive results for an evaluation of a RF resonant tapered cavity similar to Shawyer's EmDrive.[14] Testing was performed using a low-thrust torsion pendulum capable of detecting force at the micronewton level within a sealed but not evacuated vacuum chamber; the RF power amplifier used an electrolytic capacitor not capable of operating in a hard vacuum.[14] The experimenters recorded directional thrust immediately upon application of power.

NASA's tests of this tapered RF resonant cavity were conducted at very low power (2% of Shawyer's 2002 experiment and 0.7% of the Chinese 2010 experiment), but a net mean thrust over five runs was measured at 91.2 µN at 17 W of input power. A net peak thrust was recorded at 116 µN at the same power level.[14]

The experiment was criticized for not having been conducted under vacuum, which would have eliminated thermal air currents. The researchers plan to replace vacuum-incompatible components.[36]

In the paper, Eagleworks announced a plan to upgrade their equipment to higher power levels, use vacuum-capable RF amplifiers with power ranges of up to 125 W, and design a new tapered cavity analytically determined to be in the 0.1 N/kW region. The test article will be subjected to independent verification and validation at Glenn Research Center, the Jet Propulsion Laboratory, and the Johns Hopkins University Applied Physics Laboratory.[14]

Six months later, early 2015, Paul March from Eagleworks made new results public, claiming positive experimental force measurements with a torsional pendulum in a hard vacuum: about 50 µN with 50 W of input power at 5.0×10−6 torr, and new null-thrust tests.[37] The new RF power amplifiers were said to be made for hard vacuum, but still fail rapidly due to internal corona discharges, with not enough funding to replace or upgrade them, so measurements are still scarce and need improvement before a new report can be published.[38]

Glenn Research Center offered to replicate the experiment in a hard vacuum when Eagleworks manage to reach 100 µN of thrust, because the GRC thrust stand can only measure down to 50 µN.[37]

Cannae drive[edit]

The same NASA test campaign evaluated a similar unconventional test device known as the Cannae drive (formerly Q-drive)[14] invented by Guido P. Fetta. Its cavity is also asymmetric, but is flatter than that of the EmDrive. Fetta is the CEO of Cannae LLC,[39] a company located in Pennsylvania, has filed two patent applications,[40][41] and presented a paper at the same conference.[42] Shawyer stated that the Cannae drive "operates along similar lines to EmDrive, except that its thrust is derived from a reduced reflection coefficient at one end plate," which he says "degrades the Q resonance factor of the device and hence the level of thrust that can be obtained".[8]

Eagleworks tested two versions of the Cannae drive: one device with radial slots engraved along the bottom rim of the resonant cavity interior, as required by Fetta's theory to produce thrust;[39] and a "null" test article lacking those radial slots. Both drives were equipped with an internal dielectric.[14] The null test device was not intended to be the experimental control. The control device was a third test article involving an RF load but without the resonant cavity interior.[36] Like the EmDrive tests, these took place at atmospheric pressure, not in a vacuum.

About the same net thrust was reported for both the standard and the null test devices. The experimental control without a resonant cavity interior measured zero thrust as expected.[36] Some considered the positive result for the non-slotted article as indicating a possible flaw in the experiment, as the null test device had been expected to produce less or no thrust based upon Fetta's theory of the designed mechanics.[36][43][44] In the complete paper, Eagleworks concluded, however, that the test results proved that "thrust production was not dependent upon slotting".[14] As pointed out by Baez, the fact that the results were not dependent of the slotting, which was claimed to be necessary for thrust according to the inventor, should be seen as an invalidation of the device.[21]

Fetta had tested a superconducting version of the "Q-drive" or Cannae drive on 13 January 2011 several years prior to the Eagleworks test campaign. The RF resonant cavity was suspended inside a liquid helium-filled dewar. The weight of the cavity was monitored by load cells. Fetta theorized that when the device was activated and produced upward thrust, the load cells would detect the thrust as change in weight. When the Cannae drive was energized by sending 10.5 watt power pulses of 1047.335 MHz RF power into the resonant cavity there was a reduction in compressive force on the load cells consistent with thrust of 8-10 mN. The results have not been published in the scientific literature, but were posted on Cannae LLC's website.[45]

See also[edit]


  1. ^ "Satellite Propulsion Research". Aerospace Member Directory. ADS Group. 
  2. ^ "EmDrive.com". Satellite Propulsion Research Ltd (SPR) web site. Roger Shawyer / SPR Ltd. 
  3. ^ a b Hambling, David (24 September 2008). "Chinese Say They're Building 'Impossible' Space Drive". Wired. Wired. 
  4. ^ Hambling, David (2 October 2008). "Video: 'Impossible' Space Drive In Action?". Wired. Wired. 
  5. ^ a b Hambling, David (29 October 2009). "'Impossible' Device Could Propel Flying Cars, Stealth Missiles". WIred. Wired. 
  6. ^ a b Hambling, David (5 November 2012). "Propellentless Space Propulsion Research Continues". Aviation Week & Space Technology. 
  7. ^ a b c d Hambling, David (6 February 2013). "EmDrive: China's radical new space drive". Wired UK. Wired UK. 
  8. ^ a b c Hambling, David (31 July 2014). "Nasa validates 'impossible' space drive". Wired UK. Wired UK. Retrieved 31 July 2014. 
  9. ^ a b "EmDrive FAQ". SPR Ltd. Retrieved 2011-07-24. 
  10. ^ a b YANG, Juan; YANG, Le; ZHU, Yu; MA, Nan (6 December 2010). "Applying Method of Reference 2 to Effectively Calculating Performance of Microwave Radiation Thruster" (PDF). Journal of Northwestern Polytechnical University 28 (6): 807–813. 
  11. ^ a b Yang, Juan; Wang, Yu-Quan; Li, Peng-Fei; Wang, Yang; Wang, Yun-Min; Ma, Yan-Jie (2012). "Net thrust measurement of propellantless microwave thrusters" (PDF). Acta Physica Sinica (in Chinese) (Chinese Physical Society) 61 (11). doi:10.7498/aps.61.110301.  edit
  12. ^ a b c Yang, Juan; Wang, Yu-Quan; Ma, Yan-Jie; Li, Peng-Fei; Yang, Le; Wang, Yang; He, Guo-Qiang (May 2013). "Prediction and experimental measurement of the electromagnetic thrust generated by a microwave thruster system" (PDF). Chinese Physics B (IOP Publishing) 22 (5): 050301. doi:10.1088/1674-1056/22/5/050301.  edit
  13. ^ a b Feng, S.; Juan, Y.; Ming-Jie, T. (September 2014). "Resonance experiment on a microwave resonator system" (PDF). Acta Physica Sinica (in Chinese) (Chinese Physical Society) 63 (15): 154103. doi:10.7498/aps.63.154103.  edit
  14. ^ a b c d e f g h i Brady, David A.; White, Harold G.; March, Paul; Lawrence, James T.; Davies, Franck J. (30 July 2014). Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum. 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference. American Institute of Aeronautics and Astronautics. doi:10.2514/6.2014-4029. Retrieved 31 July 2014. Lay summary (PDF)NASA (30 July 2014). 
  15. ^ a b Fisher, Richard (5 November 2004). "Defying gravity: UK team claims engine based on microwaves could revolutionise spacecraft propulsion". The Engineer (London) 293 (7663): 8. 
  16. ^ Tom Shelley (12 Dec 2002). "A force for space with no reaction". Eureka Magazine. Retrieved 4 May 2015. 
  17. ^ a b c Tom Shelley (14 May 2007). "No-propellant drive prepares for space and beyond". Eureka Magazine. Retrieved 4 May 2015. 
  18. ^ Tom Shelley (14 Aug 2003). "Driving to the future". Retrieved 4 May 2015. 
  19. ^ a b Shawyer, Roger (September 2006). "A Theory of Microwave Propulsion for Spacecraft (Theory paper v.9.3)" (PDF). New Scientist. 
  20. ^ a b Egan, Greg (19 September 2006). Baez, John C., ed. "A Plea to Save New Scientist". The n-Category Café (a group blog on math, physics and philosophy). 
  21. ^ a b c Powell, Corey S. (6 August 2014). "Did NASA Validate an "Impossible" Space Drive? In a Word, No.". Discover. Retrieved 6 August 2014. 
  22. ^ Costella, John P. (2006). "Why Shawyer's 'electromagnetic relativity drive' is a fraud" (PDF). John Costella’s home page. 
  23. ^ Webb, Jeremy (3 October 2006). "Emdrive on trial". New Scientist Publisher's blog. 
  24. ^ Shawyer, Roger (March 2007). "A Theory of Microwave Propulsion for Spacecraft (Theory paper v.9.4)" (PDF). SPR Ltd. 
  25. ^ Rothman, Tony; Boughn, Stephen. "The Lorentz force and the radiation pressure of light" (PDF). 
  26. ^ Shawyer, Roger (29 September – 3 October 2008). Microwave Propulsion - Progress in the EmDrive Programme (PDF). 59th International Astronautical Congress (IAC 2008). Glasgow, U.K.: International Astronautical Federation. 
  27. ^ Harold "Sonny" White (2013). "Eagleworks Laboratories WARP FIELD PHYSICS" (PDF). NASA Technical Reports Server (NTRS). NASA. 
  28. ^ Baez, John. "The incredible shrinking force". Google Plus. Retrieved 6 August 2014. 
  29. ^ Gonzalez, Robert T. "Don't Get Too Excited About NASA's New Miracle Engine". io9. Retrieved 6 August 2014. The business about "quantum vacuum virtual plasma" (the physics of which they "won't address" in this paper) is complete bullshit. There is a quantum vacuum, but it's nothing like a plasma. 
  30. ^ Fisher, Richard (1 September 2006). "Microwave engine gets a boost". The Engineer (London). 
  31. ^ "80 Ton Lifter Possible in 6 Years: Interview with EmDrive Inventor". BTE Blog. BuildTheEnterprise. 27 February 2013. 
  32. ^ "Fly by light". Contact Center Solutions, TMCnet. Technology Marketing Corporation (TMC). 8 September 2006. 
  33. ^ ZHU, Yu; YANG, Juan; MA, Nan (September 2008). "The Performance Analysis of Microwave Thrust Without Propellant Based On The Quantum Theory". Journal of Astronautics (in Chinese) 29 (5): 1612–1615. 
  34. ^ YANG, Juan et al. "Figure 4: Different microwave output power range thrust measurement results. Output power ranging from 300-2500W." (PDF). 
  35. ^ White, Harold; March, Paul; Nehemiah, Williams; O'Neill, William (5 December 2011). Eagleworks Laboratories: Advanced Propulsion Physics Research. NASA Technical Reports Server (NTRS) (Technical report) (NASA). JSC-CN-25207. 
  36. ^ a b c d "No, NASA has not verified an impossible space drive!". Armagh Planetarium. Retrieved 2014-08-07. 
  37. ^ a b Wang, Brian (6 February 2015). "Update on EMDrive work at NASA Eagleworks". NextBigFuture. 
  38. ^ Wang, Brian (7 February 2015). "NASA Emdrive experiments have force measurements while the device is in a hard vacuum". NextBigFuture. 
  39. ^ a b "Cannae Drive". Cannae LLC website. Retrieved 31 July 2014. 
  40. ^ WO application 2007089284, Fetta, Guido Paul, "Resonating cavity propulsion system", published 2007-11-15, assigned to Fetta, Guido Paul 
  41. ^ US application 2014013724, Fetta, Guido P., "Electromagnetic thruster", published 2014-01-16, assigned to Cannae LLC 
  42. ^ Fetta, Guido P. (30 August 2014). Numerical and Experimental Results for a Novel Propulsion Technology Requiring no On-Board Propellant. 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference. American Institute of Aeronautics and Astronautics. doi:10.2514/6.2014-3853. Retrieved 31 July 2014. 
  43. ^ Nelsen, Eleanor (31 July 2014). "Improbable Thruster Seems to Work by Violating Known Laws of Physics". Nova. PBS. Retrieved 1 August 2014. 
  44. ^ Timmer, John (1 August 2014). "Don’t buy stock in impossible space drives just yet". Ars Technica. Ars Technica. Retrieved 2 August 2014. 
  45. ^ Page is no longer available, but an archived version as of 2 November 2012 is available at archive.org: www.cannae.com/proof-of-concept/experimental-results (retrieved 11 February 2015)