||This article may be too technical for most readers to understand. (August 2014)|
||The neutrality of this article is disputed. (August 2014)|
EmDrive (also RF resonant cavity thruster) is a proposed spacecraft propulsion device invented by British aerospace engineer Roger J. Shawyer, who develops prototypes at Satellite Propulsion Research Ltd (SPR), the company he created for that purpose in 2000. New Scientist ran a cover story on EmDrive in its 8 September 2006 issue. 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 the large 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 a breakthrough in all forms of travel including ground travel, marine travel, sub-marine travel, airflight and spaceflight.
The device, its mode of operation, and theories attempting to explain it are all controversial. As of 2014[update], there are still arguments about whether the EmDrive is genuinely a new propulsion device, or its experimental results are simply misinterpretations of spurious effects mixed with experimental errors. The proposed theories of its operation have all been criticized because they seem to violate the conservation of momentum, a fundamental law of physics, though the inventor asserts that it does not.
Chinese researchers from the Northwestern Polytechnical University (NWPU) in Xi'an in 2010, built and tested their own device based upon Shawyer's design, claiming to have replicated Shawyer's experiments, recording better results than Shawyer had claimed at even higher power levels,  though they were also clear that their work was still preliminary. Then at the Johnson Space Center in 2014 a NASA evaluation group also claimed replication at low power levels, measuring a directional thrust level in accord with Shawyer's experiments and claims.
Static thrust tests
Shawyer, in 2006, claimed that, with adequate funding, commercial terrestrial aircrafts incorporating EmDrives as lift engines could be ready by 2020. He proposed that very high Q superconducting resonant cavities could produce static specific thrusts of about 30 kN/kW, which is 3 tonnes of thrust per kilowatt of input power − "enough to lift a large car".
New Scientist article
After receiving criticism that no peer-reviewed publications on the subject had been made, Shawyer submitted a theory paper to New Scientist, a weekly science magazine which is not a peer-reviewed scientific journal. The EmDrive was consequently 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 writer Greg Egan 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, and alleged 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. Egan also recommended that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics) of Shawyer's paper.
The following month, the New Scientist editor addressed the ensuing controversy over the article stating that "We 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."
Chinese Northwestern Polytechnical University (NWPU)
In 2008, Wired magazine reported that 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 valid electro-magnetic theory behind the Emdrive. Their demonstration version of the drive was built and tested with design modifications and at higher power levels in 2010, with considerably better results than prior testings. A maximum thrust of 720 mN was measured at 2500 W of input power on an aerospace engine test stand usually used to precisely test spacecraft engines like ion drives.
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" and that Yang told him that "she is not able to discuss her work until more results are published".
NASA/JSC Advanced Propulsion Physics Laboratory (Eagleworks)
A NASA team at the Advanced Propulsion Physics Laboratory (informally known as Eagleworks) 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)
In July 2014, the group reported on an evaluation of a RF resonant tapered cavity similar to Shawyer's EmDrive, with positive results. Testing was performed on a low-thrust torsion pendulum that is capable of detecting force at a single-digit micronewton level, within a sealed stainless steel vacuum chamber, but at ambient atmospheric pressure, because the RF power amplifier used an electrolytic capacitor not capable of operating in a hard vacuum.
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.
The experiment was criticized for not having been performed in a vacuum because of the realized vulnerability of some of its minor components and connections. It was believed that this could potentially allow air currents to compromise results. This criticism was partly allayed, however, by experimenters recording that the directional thrust measured began and ended immediately upon the application and disconnect of power, not leaving enough time for air currents to develop or dissipate. The researchers plan to replace incompatible components so that full-power vacuum testing can be later conducted. 
In the coming months, Eagleworks plans to upgrade their equipment to higher power levels, use vacuum-capable RF amplifiers with power ranges of up to 125 W, (a small less-powerful prototype compared to the Chinese version tested at 2,500 Watts) and design a new tapered cavity analytically determined to be in the 0.1 N/kW region. Then, the test article will be shipped to other laboratories for independent verification and validation of the technology, at Glenn Research Center, the Jet Propulsion Laboratory and the Johns Hopkins University Applied Physics Laboratory.
The same NASA test campaign also evaluated another unconventional test device known as the Cannae drive (formerly Q-drive) invented by Guido P. Fetta, which also has an asymmetric but flatter resonant cavity. Fetta is the CEO of Cannae LLC, a company located in Pennsylvania, who filed two patent applications and presented a paper at the same conference. 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," according to Shawyer, which he says "degrades the Q resonance factor of the device and hence the level of thrust that can be obtained".
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; and a "null" test article lacking those radial slots. Both drives were equipped with an internal dielectric. 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. Like the EmDrive tests, these took place at atmospheric pressure, not in a vacuum.
According to the technical report, both Cannae drives (the standard and the supposed null-test device) produced about the same net thrust. The experimental control measured zero thrust as expected. 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. In the complete paper, Eagleworks concluded, however, that the test results proved that "thrust production was not dependent upon slotting".
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. Shawyer has published an updated theory paper (version 9.4) for the EmDrive.
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. Yang from NWPU calculated the net force/thrust using classical electromagnetism. 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. Likewise, the paper describing the Eagleworks test of the Cannae drive referred to a possible interaction with "quantum vacuum virtual plasma". 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.
All of these explanations and hypotheses are equally unsatisfying to most physicists concerning the closed-system design and reported directional thrust, as it relates to the conservation of momentum.
- Abraham–Minkowski controversy
- Woodward effect
- Quantum vacuum plasma thruster
- Solar sail
- Beam-powered propulsion#Direct impulse
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