EmDrive
The EmDrive is a controversial device first proposed in 2001, purported by its inventors to be a reactionless drive. While no mechanism for operation was proposed, this would violate the law of conservation of momentum and other laws of physics. The concept has at times been referred to as a resonant cavity thruster. The idea is generally considered by physicists to be pseudoscience.
Neither person who claims to have invented it committed to details about it beyond showing prototypes they have built. While the lack of a published design or mechanism makes it hard to say whether a given object is an example of an EmDrive, over the years prototypes based on its public descriptions have been constructed and tested.
In 2016, Harold White's group at NASA observed a small apparent thrust from one such test, however subsequent studies suggested this was a measurement error caused by thermal gradients. In 2018 and 2021, Martin Tajmar's group at the Dresden University of Technology replicated and refuted White's results, observing apparent thrusts similar to those measured by his team, and then made them disappear again when measured using point suspension.
No other published experiment measured apparent thrust greater than the experiment's margin of error. Tajmar's group published three papers in 2021 claiming that all published results showing thrust had been false positives, explaining each by outside forces. They concluded, "Our measurements refute all EmDrive claims by at least 3 orders of magnitude."
History and controversy
s operate by expelling propellant, which acts as a reaction mass and which produces thrust per Newton's third law of motion. All designs for electromagnetic propulsion operate on the principle of reaction mass. A hypothetical drive which did not expel propellant in order to produce a reaction force, providing thrust while being a closed system with no external interaction, would be a reactionless drive, violating the conservation of momentum and Newton's third law. Reactionless drives, like other forms of perpetual motion, do not exist in nature, and claims that a drive is reactionless are considered by physicists to be pseudoscience.The first design of a resonant cavity thruster claiming to be a reactionless drive was by Roger Shawyer in 2001. He called his conical design an "EmDrive", and claimed that it produced thrust in the direction of the base of the cone. Guido Fetta later built a "Cannae Drive", based in part on Shawyer's concept, using a pillbox-shaped cavity.
Since 2008, a few physicists have tested their own models, trying to reproduce the results claimed by Shawyer and Fetta. Juan Yang at Xi'an's Northwestern Polytechnical University was unable to reproducibly measure thrust from their models, over the course of 4 years. In 2016, Harold White's group at NASA's Advanced Propulsion Physics Laboratory reported in the Journal of Propulsion and Power that a test of their own model had observed a small thrust. In late 2016, Yue Chen of the communication satellite division of the China Academy of Space Technology, said his team had tested prototypes, and would conduct in-orbit tests to determine if they could observe thrust. Martin Tajmar's group at the Dresden University of Technology started testing prototypes in 2015, and by 2021 concluded that observations of thrust were false positives, reporting in the CEAS Space Journal they had refuted all EmDrive claims by "at least 3 orders of magnitude".
Media coverage and responses
Media coverage of experiments using these designs has been polarized. The EmDrive first drew attention, both credulous and dismissive, when New Scientist wrote about it as an "impossible" drive in 2006. Media outlets were later criticized for misleading claims that a resonant cavity thruster had been "validated by NASA" following White's first tentative test reports in 2014. Scientists have continued to note the lack of unbiased coverage.In 2006, responding to the New Scientist piece, mathematical physicist John C. Baez at the University of California, Riverside, and Australian science-fiction writer Greg Egan, said the positive results reported by Shawyer were likely misinterpretations of experimental errors.
In 2014, White's first conference paper suggested that resonant cavity thrusters could work by transferring momentum to the "quantum vacuum virtual plasma", a new term he coined. Baez and Carroll criticized this explanation, because in the standard description of vacuum fluctuations, virtual particles do not behave as a plasma; Carroll also noted that the quantum vacuum has no "rest frame", providing nothing to push against, so it cannot be used for propulsion. In the same way, physicists James F. Woodward and Heidi Fearn published two papers showing that electron−positron virtual pairs of the quantum vacuum, discussed by White as a potential virtual plasma propellant, could not account for thrust in any isolated, closed electromagnetic system such as a quantum vacuum thruster.
In 2015, physicists Eric W. Davis at the Institute for Advanced Studies in Austin and Sean M. Carroll at the California Institute of Technology concluded that the thrust measurements reported in papers by both Tajmar and White were indicative of thermal effect errors.
In May 2018, researchers from the Institute of Aerospace Engineering at Technische Universität Dresden, Germany, concluded that the dominant effect underlying the apparent thrust could be clearly identified as an artifact caused by Earth's magnetic field interacting with power cables in the chamber, a result that other experts agree with.
In March 2021, Tajmar's group published a definitive analysis of their own past experiments and those of others, showing that all could be explained by and reproduced via outside forces, refuting all EmDrive claims.
When power flows into the EmDrive, the engine warms up. This also causes the fastening elements on the scale to warp, causing the scale to move to a new zero point. We were able to prevent that in an improved structure. Our measurements refute all EmDrive claims by at least 3 orders of magnitude.
Designs and prototypes
EmDrive
In 2001, Shawyer founded Satellite Propulsion Research Ltd, to work on the EmDrive, which he said used a resonant cavity to produce thrust without propellant. The company was backed by a SMART award grant from the UK Department of Trade and Industry. In December 2002, he loosely described a prototype which he alleged had produced a thrust of powered by an 850 W cavity magnetron. He reported that the device could operate for only a few dozen seconds before the magnetron failed from overheating, however details were never published or replicated.Second device and ''New Scientist'' article
In October 2006, Shawyer claimed to have conducted tests on a new water-cooled prototype with increased thrust. He reported plans to have the device ready to use in space by May 2009 and to make the resonant cavity a superconductor, neither of which materialized.New Scientist magazine featured the EmDrive on the cover of 8 September 2006 issue. The article portrayed the device as plausible and emphasized the arguments of those who held that point of view. Egan, a popular science fiction author, 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". Especially, Egan said he was "gobsmacked by the level of scientific illiteracy" in the magazine's coverage, alleging that it used "meaningless double-talk" to obfuscate the problem of conservation of momentum. The letter was endorsed by Baez and posted on his blog. New Scientist editor Jeremy Webb responded to critics: New Scientist also published a letter from the former technical director of EADS Astrium: A letter from physicist Paul Friedlander:
Later work
In 2007, the UK Department of Trade and Industry granted SPR an export license to Boeing in the US. According to Shawyer, in December 2008 he was invited to present on the EmDrive, and in 2009 Boeing expressed interest in it, at which point he stated that SPR built a thruster which produced 18 grams of thrust, and sent it to Boeing. Boeing did not license the technology and communication stopped. In 2012, a Boeing representative confirmed that Boeing Phantom Works used to explore exotic forms of space propulsion, including Shawyer's drive, but such work later ceased. They confirmed that "Phantom Works is not working with Mr. Shawyer", nor pursuing those explorations.In 2014, Shawyer presented ideas for 'second-generation' EmDrive designs and applications at the annual International Astronautical Congress. A paper based on his presentation was published in Acta Astronautica in 2015. While no functional prototype of the first-generation drive had yet been produced, and no detailed schematic of a new device was provided, it loosely described models for a superconducting resonant cavity and for thrusters with multiple cavities.
In 2016, Shawyer filed further patents and launched a new company, Universal Propulsion Ltd., as a joint venture with Gilo Industries Group, a small UK aerospace company.
Cannae and other drives
The Cannae Drive is another implementation of this idea, with a relatively flat cavity. It was designed by Guido Fetta in 2006 and promoted within the US through his company, Cannae LLC, since 2011. In 2016, Fetta announced plans to eventually launch a CubeSat satellite containing a version of the Cannae Drive, which would run for 6 months to observe how it functions in space. No followup was published.In China, researchers working under Yang at NWPU built a resonant cavity thruster in 2008, and tested it for a number of years. A 2012 report claimed they had observed thrust, but in 2014 they found it to have been an experimental error. A second, improved prototype did not produce any measured thrust.
At the China Academy of Space Technology, Yue Chen filed several patent applications in 2016 describing various radio frequency resonant cavity thruster designs. These included a method for stacking several short resonant cavities to improve thrust, and a design with a cavity that was a semicylinder instead of a frustum. That December, Chen announced that CAST would conduct tests on a resonant cavity thruster in orbit, without specifying what design was used. In an interview on CCTV in September 2017, Chen showed some testing of a flat cylindrical device, corresponding to the patent describing stacked short cavities with internal diaphragms.