EmDrive

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

EmDrive (also RF resonant cavity thruster) is a proposed spacecraft propulsion device invented by British aerospace engineer Roger Shawyer, who develops prototypes at Satellite Propulsion Research Ltd (SPR),^[1] the company he created for that purpose in 2000.^[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 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 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 because they seem to 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, 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,^{[6][10][11][12][13]} 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.^[8][14]

In an article published on NASASpaceFlight.com^[15] José Rodal, Ph.D, Jeremiah Mullikin and Noel Munson report a successful vacuum test in which the drive created thrust.^[16]

Theory

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.^[17] 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 ν_g of the electromagnetic wave, then equation 1 illustrates that if v_g1 is greater than v_g2, then F_g1 should be expected to be greater than F_g2." 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^[18] 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.^[19] 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.^[20] 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][22][23]

Testing and replication claims

Static thrust tests

Shawyer has reported seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE.^[24] 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.^[25][26] 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".^[27]

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 a valid electro-magnetic theory behind the Emdrive.^[3][28] 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][29]}

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)

A NASA team at the Advanced Propulsion Physics Laboratory (informally known as Eagleworks)^[30] 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 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.^[31]

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.^[32] 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.^[33]

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.^[32]

Cannae drive

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,^[34] a company located in Pennsylvania, has filed two patent applications,^[35][36] and presented a paper at the same conference.^[37] 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;^[34] 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.^[31] 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.^[31] 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.^[31][38][39] 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.^[40]

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 popular science consumer magazine.^[41] 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.^[42][21] Egan also recommended^[42] that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics)^[43] of Shawyer's paper.^[41]

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."^[44]

See also

• Abraham–Minkowski controversy
• Beam-powered propulsion#Direct impulse
• Casimir effect
• Crookes radiometer
• Quantum vacuum plasma thruster
• Solar sail
• Woodward effect
• Interferometer experiment with an EmDrive

External links

• The FACTS as we currently know them about the EmDrive and Cannae Drive - subreddit thread
• So, about that EM drive... - History, Politics And Current Affairs forum (Space and Technology section) thread

References