User:Sparkyscience/Test2

A radio frequency (RF) resonant cavity thruster, also known as an EmDrive, is a controversial proposed type of electromagnetic thruster where an electromagnetic field inside the microwave cavity appears to produce thrust. The device does not expel any observed propellant and seems to produces a net reaction force, which would violate the conservation of momentum and Newton's third law. Several tests have observed an apparent thrust, but many theoretical physicists and commentators have labelled the device "impossible" and state the level of observed thrust may be due to measurement error.

Overview
Conventional rocket engines obey the conservation of momentum and Newton's third law by expelling propellant to move in the opposite direction. For example ships move masses of water, aircraft move masses of air, and rockets expel exhaust. A device that is a closed system with no external interaction that produces a net reaction force, would be a reactionless drive. Such a drive would violate the conservation of momentum and Newton's third law, creating a net force in a closed system is believed to be impossible, with many physicists labeling the idea pseudoscience. On the other hand, a drive that is capable of interacting with an external field might be able to form an open system, propellantless but not reactionless, like a sail catching and redirecting existing winds to move a ship. A device capable of doing this would be appealling for many reasons, primarily because of the possibility of supporting long voyages in space, where propellant is a primary limiting factor.

Roger Shawyer in 2001. He invented a design with a conical cavity, calling it the EmDrive. Guido Fetta later invented the Cannae Drive, with a pillbox-shaped cavity. Since 2008, a few groups of scientists have tested their own models, trying to confirm the observed thrust results of Shawyer and Fetta. Juan Yang at Xi'an's Northwestern Polytechnical University (NWPU) initially reported thrust from a model they built, but retracted her claims in 2016 after a measurement error was identified and an improved setup measured no significant thrust. In 2016 NASA's Eagleworks Laboratories, led by Harold White, reported a test of their own model had observed 40–100 μN of thrust from inputs of 40–80 W and published the results in the Journal of Propulsion and Power. In December 2016 the China Academy of Space Technology (CAST) announced the communication satellite division led by Yue Chen had tested several prototypes using an "experimental verification platform", observed thrust, and were now carrying out in-orbit verification of the results.

Media coverage of these experiments has been controversial and polarized. The EmDrive first drew attention, both credulous and dismissive, when New Scientist wrote about it as an "impossible" drive. Media outlets were later criticised 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, from both polarized sides.

Early history of microwave propulsion
A low-propellant space drive has long been a goal for space exploration, since the propellant is dead weight that must be lifted and accelerated with the ship all the way from launch until the moment it is used (see Tsiolkovsky rocket equation). Gravity assists, Solar sails, and beam-powered propulsion from a spacecraft-remote location such as the ground or in orbit, are useful because they allow a ship to gain speed without propellant. However, some of these methods do not work in deep space. Shining a light out of the ship provides a small force from radiation pressure, i.e., using photons as a form of propellant, but the force is far too weak (for a given amount of input power) to be useful in practice.

Conventional Electromagnetic propulsion designs which operate on the principle of reaction mass have been around since the start of the 20th century. In the 1960s, extensive research was conducted on two designs which emit high velocity ionized gases in similar ways: ion thrusters that convert propellant to ions and accelerate and eject them via electric potentials, and plasma thrusters that convert propellant to plasma ions and accelerate and eject them via plasma currents. In the latter, plasma can be generated from an intense source of microwave or other radio-frequency (RF) energy, and in combination with a resonant cavity, can be tuned to resonate at a precise frequency.

BAE's Project Greenglow
Soviet scientists were among the first to investigate microwave cavity thrust. More recently, the feasibility of the concept was investigated as part of BAE Systems' Project Greenglow, a small speculative research programme that funded research into the science behind novel propulsion systems. The program officially ran from 1997 to 2005, with its unofficial origins dating back as far as the 1970s.

From 2001 to 2003 BAE's Project Greenglow funded universities in Dundee and Strathclyde to investigate microwave cavity thrust

Founding of SPR
In the 1990's BAE Systems jointly owned Matra Marconi Space (MMS). Roger Shawyer was employed at MMS for 20 years a senior aerospace engineer and program manager; during this time of employment Shawyer was a consultant involved in the Galileo project. In 2000 MMS was merged with the space division of DaimlerChrysler Aerospace AG and Computadores Redes e Ingeniería SA to form Astrium, a joint venture between EADS and BAE Systems. Shawyer approached the then Technical Director of Astrium, Alvin Wilby, about his idea for the EmDrive asking if the company would be interested in patent coverage, Shawyer was sharply rebuked by Astrium and told "in no uncertain terms to drop it" with Wilbey stating:

I reviewed Roger’s work and concluded that both theory and experiment were fatally flawed. Roger was advised that the company had no interest in the device, did not wish to seek patent coverage, and in fact did not wish to be associated with it in any way.

Both Shawyer and Wilbey left Astrium in 2000, with the company becoming part of EADS Astrium in 2003.

Having left Astrium, in October 2000 Shawyer founded Satellite Propulsion Research Ltd (SPR) to develop his idea for the EmDrive. The idea was backed by a "Smart Award" granted in August 2001 from the UK Government's now defunct Department of Trade and Industry (DTI). The DTI hired John Spiller, an independent space engineer, to conduct a feasibility study. Richard Paris, a mathematics professor at the University of Abertay in Dundee and expert in magnetohydrodynamics, performed calculations that showed an asymmetric cavity has a higher group velocity of photons at the wide end than at the narrow end and, as a consequence a net force would be exerted. The study was completed in October 2002 following the completion of independent tests. The EmDrive was first made public in December 2002 when it appeared in Eureka Magazine. The article described a working prototype with a total thrust of about 0.02 newtons powered by an 850 W cavity magnetron. The device could operate for only a few dozen seconds before the magnetron failed, due to overheating.

In 2004, Shawyer gave an interview in The Engineer where he stated seven independent reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE had been completed and that a further UK government grant of £125,000 had been awarded on the condition of further independant tests as part of a three year £250,000 program  None of the independant tests were published in the scientific journals or were made publicly available, an issue which has been broadly criticized with the Royal Aeronautical Society’s Space Group Committee stating:

If the developers of such devices want to be taken more seriously, it’s incumbent upon them to allow greater scrutiny of their experiments or to prove, beyond reasonable doubt, that they have something that works. We note the reluctance of the proponents to publish detailed results which is a barrier to acceptance by the scientific community"

In 2005 Shawyer sought to publish an EmDrive paper in the Royal Aeronautical Society's The Aeronautical Journal but was rejected. Shawyer later published a paper in the Journal of the British Interplanetary Society.

New Scientist controversy
In September 2006 New Scientist published a feature article shown on the front page titled "Relativity Drive: The End of Wings and Wheels?" about Shawyer and the EmDrive. The article drew strong criticism for failing to make clear how contentious the invention is, and failed to mention the device would appear to violate the conservation of momentum. Australian science fiction writer Greg Egan published an open letter "A Plea to Save New Scientist" which was endorsed by theoretical physicist John Baez who reposted it on his blog. Following a number of letters from the public the then editor of New Scientist, Jeremy Webb, posted a note acknowledging that the article "did not make clear enough how controversial Roger Shawyer’s engine is" or report "that several physicists declined to comment on the device because they thought it too contentious" but defending the decision to publish stating "should New Scientist should have covered this story at all? The answer is a resounding yes... Shawyer has experimental data that has convinced peer reviewers that he is onto something. He believes he can explain his machine's behaviour in terms of existing physical laws, which is what the theorists contest."

The controversy prompted Alan Duncan, the MP for Rutland and Melton, to formally ask Margaret Hodge, the then Secretary of State for Trade and Industry, how much public money Shawyer had received and confirm from which budget the funds had been taken, Hodge responded in writing stating that thus far two grants totaling £125,100 had been made available following "independent tests and evaluation" and that "Both grants were awarded against the criteria of the DTI’s Smart scheme that was designed to help fund pioneering and risky R&D projects in small and medium enterprises. Highly qualified technical experts and academics carried out an assessment on behalf of the Department"

Despite controversy, the SPR continued to be backed by DTI and a number of private investors. In October 2006, Shawyer conducted tests on a new water-cooled prototype and said that it had increased thrust. He planned to have the device ready to use in space by May 2009 and was considering making the resonant cavity a superconductor. In 2007 Shawyer revealed details of SPR's development plans to an IEE chapter meeting in Portsmouth, to an audience that included a number of Shawyer's ex-colleagues. David Hall from EADS Astrium made a presentation discussing the use of microwave technology to gently to divert Near-Earth object or asteroids that may be in danger of colliding with the planet. He said shooting down up such a threat, for example with nuclear weapons, was not a viable option, as the parts would still be likely to hit the Earth. Hall stated a superconducting EmDrive might be a possible option when looking at finding ways of nudging asteroids into a safe trajectory. A 1kW engine would require 24kW to keep it cool and shifting the asteroid would take somewhere in the order of 10 years, depending on its size.

Boeing incident
In 2007, the UK Department of Trade and Industry granted SPR an export licence to Boeing in the US. In 2008 Shawyer was invited to visit Boeing’s Phantom Works; later on 10th December Shawyer was invited by the US Air Force (USAF) to a meeting in The Pentagon to present on the EmDrive. The meeting was chaired by the head of the National Security Space Office (NSSO), and had representatives from USAF, DARPA and NASA in attendance. Colonel, M.V. "Coyote" Smith, the Chief of Future Concepts "Dream Works" at the NSSO commented:

The potential is so great if I did not bring this to the attention of the scientific community inside the US that works inside our space program I would have been fired. Thats just absolutely the type of technology that we have to track down, these revolutionary breakthroughs. Now, all of the physicists disclaimed it, but the ironic thing is when I took it to the engineering community, they didn't care why it worked. They were just interested that it worked.

In 2009 Boeing confirmed they wanted to license the technology. The UK Ministry of Defence agreed to a technology transfer, and SPR designed, built and tested a thruster for use on a test satellite. According to Shawyer, the 10-month contract was completed by July 2010 and the thruster, giving 18 grams of thrust, was transferred to Boeing. Boeing did not however license the technology and communication stopped. Questioned on the matter in 2012, a Boeing representative confirmed that Boeing Phantom Works had previously explored exotic forms of space propulsion, including Shawyer's EmDrive, but such work has since ceased. Boeing stated that "Phantom Works is not working with Mr. Shawyer”. adding in a further statement that:

Boeing has no current plans for EmDrive technology. Boeing will continue to monitor [propellantless drive] development and proof of concept testing by other entities, including NASA.

DARPA has acknowledged talks about the EmDrive took place. When questioned regarding the EmDrive the USAF replied:

The Air Force is aware of EmDrive and the ongoing research but is simply watching to see if and when the technology becomes viable. We could not find any evidence that the AF has an investment in or a licensing agreement for the technology.

In 2013 and 2014, Shawyer presented further ideas for 'second-generation' EmDrive designs and applications, at the annual International Astronautical Congress. A peer reviewed paper based on his 2014 presentation was published in Acta Astronautica in 2015. It describes a hybrid single stage to orbit (SSTO) spaceplane that takes off and lands in the same manner a Boeing X37-B, while using a superconducting EmDrive to provides de-orbit braking and maneuverability in space.

In April 2015 the USAF’s Rapid Capabilities Office (RCO), which manages the X-37B programme, announced it would be "investigating an experimental propulsion system on the X-37B"; this was later clarified to be tests involving a Hall thruster with the USAF stating “The resulting data will be used to validate and improve Hall thruster and environmental modeling capabilities, which enhance the ability to extrapolate ground test results to actual on-orbit performance” In March 2016 the BBC reported that Boeing had apparently licensed its own version of the EmDrive. In November 2016 the International Business Times reported that the U.S. government was testing a version of the EmDrive on the Boeing X-37B and that the Chinese government has made plans to incorporate the EmDrive on its orbital space laboratory Tiangong-2. This report was followed by the December 2016 announcement by China Academy of Space Technology (CAST) that it had tested several EmDrive prototypes using an "experimental verification platform", observed thrust, and were now carrying out "in-orbit verification". In February 2017, the USAF’s RCO was asked if the electric thruster was an EmDrive and the RCO stated it was not.

In October 2016, a UK patent application describing a new superconducting EmDrive was published, followed by a first international version. Shortly thereafter Shawyer unveiled the creation of Universal Propulsion Ltd., a new company aimed to develop and commercialise such thrusters, as a joint venture with Gilo Industries Group, a small UK aerospace company designing and selling paramotors and the Parajet Skycar.

Fetta's Cannae drive
The Cannae Drive (formerly Q-drive), another engine designed to generate propulsion from a resonant cavity without propellant, is another implementation of this idea. Its cavity is also asymmetric, but relatively flat rather than a truncated cone. It was designed by Fetta in 2006 and has been 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 they would run for 6 months to observe how it functions in space.

In 2011, Fetta tested a superconducting version of the Cannae drive. 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 drive was energized by sending 10.5 watt power pulses of RF power into the resonant cavity, there was, as predicted, a reduction in compressive force on the load cells consistent with thrust of 8-10 mN.

They have been posted on their inventors' websites.

In August 2016, Cannae announced plans to launch its thruster on a 6U cubesat which they would run for 6 months to observe how it functions in space. Cannae has formed a company called Theseus for the venture and partnered with LAI International and SpaceQuest Ltd. to launch the satellite. No launch date has yet been announced.

NASA Eagleworks
In 2011 NASA and the Johnson Space Center implemented the Advanced Propulsion Physics Laboratory, informally known as "Eagleworks", to "pursue propulsion technologies necessary to enable human exploration of the solar system over the next 50 years, and enabling interstellar spaceflight by the end of the century." Eagleworks largely continues the line work set undertaken by NASA's Breakthrough Propulsion Physics Program which ended in 2003. Eagleworks is headed by Harold White and has investigated ideas for a wide range of ideas including Alcubierre drives, drives that interact with the quantum vacuum, and RF resonant cavity thrusters.

“While research into theoretical faster-than-light travel from a team at the Johnson Space Center has created headlines, this is a conceptual investigation. The agency is committed to the priorities and investments identified by the NASA Strategic Space Technology Investment Plan. Through these investments, NASA will develop the capabilities necessary to send humans further into space than ever before, including missions in the proving ground of space, like the asteroid redirect mission, and eventually Mars.”

White has had a team at NASA known as the, or Eagleworks Laboratories, which is devoted to studying exotic propulsion concepts. The group

In 2014, the group began testing resonant cavity thrusters of their own design and sharing some of their results. In November 2016, they published their first peer-reviewed paper on this work, in the Journal of Propulsion and Power.

In July 2014, Eagleworks reported tentative positive results for evaluating a tapered RF resonant cavity. Testing was performed using a low-thrust torsion pendulum able to detect force at the micronewton level within a sealed but unevacuated vacuum chamber (the RF power amplifier used an electrolytic capacitor unable to operate in a hard vacuum). The experimenters recorded directional thrust immediately upon application of power.

Their first tests of this tapered cavity were conducted at very low power (2% of Shawyer's 2002 experiment). A net mean thrust over five runs was measured at 91.2 µN at 17 W of input power. The experiment was criticized for its small data set and for not having been conducted in vacuum, to eliminate thermal air currents.

The group announced a plan to upgrade their equipment to higher power levels, to use vacuum-capable RF amplifiers with power ranges of up to 125 W, and to design a new tapered cavity that could be in the 0.1 N/kW range. The test article was to be subject to independent verification and validation at Glenn Research Center, the Jet Propulsion Laboratory and the Johns Hopkins University Applied Physics Laboratory. , this validation has not happened.

In 2015, Paul March from Eagleworks made new results public, measured with a torsional pendulum in a hard vacuum: about 50 µN with 50 W of input power at 5.0×10−6 torr. The new RF power amplifiers were said to be made for hard vacuum, but failed rapidly due to internal corona discharges. Without funding to replace or upgrade them, measurements were scarce for a time.

They conducted further experiments in vacuum, a set of 18 observations with 40-80W of input power. They published the results in the American Institute of Aeronautics and Astronautics's peer-reviewed Journal of Propulsion and Power, under the title "Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum". This was released online in November 2016, with print publication in December. The study said that the system was "consistently performing with a thrust-to-power ratio of 1.2±0.1mN/kW", and enumerated many potential sources of error.

The paper suggested that pilot-wave theory (a controversial, non-mainstream deterministic interpretation of quantum mechanics) could explain how the device produces thrust. Commenters pointed out that just because a study reporting consistent thrust was published with peer-review does not necessarily mean that the drive functions as claimed. Physicist Ethan Siegal commented on the paper, saying that the drive most likely does not violate conservation of momentum as this would "make physics fall apart" but rather that there is something else going on. He said that "Whether it’s new physics [or] the effect’s cause simply hasn’t been determined yet, more and better experiments will be the ultimate arbiter". Physicist Chris Lee was very critical of the work, saying that the paper had a small data set and a number of missing details he described as 'gaping holes'. Electrical Engineer George Hathaway analyzed and criticized the scientific method described in the paper.

White's 2014 tests also evaluated two Cannae drive prototypes. One had radial slots engraved along the bottom rim of the resonant cavity interior, as required by Fetta's theory to produce thrust; another "null" test article lacked those radial slots. Both drives were equipped with an internal dielectric. A third test article, the experimental control, had an RF load but no resonant cavity interior. These tests took place at atmospheric pressure.

About the same net thrust was reported for both the device with radial slots and the device without slots. Thrust was not reported for the experimental control. Some considered the positive result for the non-slotted device 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 how thrust was produced by the device. In the complete paper, however, Eagleworks concluded that the test results proved that "thrust production was not dependent upon slotting".

Dresden University of Technology
In July 2015 an aerospace research group at the Dresden University of Technology (TUD) under Martin Tajmar reported results for an evaluation of an RF resonant tapered cavity similar to the EmDrive. Testing was performed first on a knife-edge beam balance able to detect force at the micronewton level, atop an antivibration granite table at ambient air pressure; then on a torsion pendulum with a force resolution of 0.1 mN, inside a vacuum chamber at ambient air pressure and in a hard vacuum at 4e-6 mbar.

They used a conventional ISM band 2.45 GHz 700 W oven magnetron, and a small cavity with a low Q factor (20 in vacuum tests). They observed small positive thrusts in the positive direction and negative thrusts in the negative direction, of about 20 µN in a hard vacuum. However, when they rotated the cavity upwards as a "null" configuration, they observed an anomalous thrust of hundreds of micronewtons, significantly larger than the expected result of zero thrust. This indicated a strong source of noise which they could not identify. This led them to conclude that they could not confirm or refute claims about such a thruster. At the time they considered future experiments with better magnetic shielding, other vacuum tests and improved cavities with higher Q factors.

Eric W. Davis, a physicist at the Institute for Advanced Studies at Austin, noted "The experiment is quite detailed but no theoretical account for momentum violation is given by Tajmar, which will cause peer reviews and technical journal editors to reject his paper should it be submitted to any of the peer-review physics and aerospace journals."

Northwestern Polytechnical University
In 2008, a team of Chinese researchers led by Juan Yang (杨涓), a professor of propulsion and astronautical engineering at the Northwestern Polytechnical University (NWPU), developed their own prototype resonant cavity thruster, publishing a report in their university's journal on the theory behind such devices. In 2012 they measured thrust from their prototype, however in 2014 they found this had been an experimental error. A second, improved prototype did not produce any measured thrust.

Juan Yang (杨涓), Northwestern Polytechnical University (NWPU) in Xi'an, China, said that they had developed a valid electro-magnetic theory behind a microwave resonant cavity thruster. A demonstration version of the drive was built and tested with different cavity shapes and at higher power levels in 2010. Using an aerospace engine test stand usually used to precisely test spacecraft engines like ion drives,  they reported a maximum thrust of 720 mN at 2,500 W of input power. Yang noted that her results were tentative, and said she "[was] not able to discuss her work until more results are published". This positive result was over 100x more thrust per input power than any other experiment, and inspired other groups to try to replicate their work.

In a 2014 follow-up experiment (published in 2016), Yang could not reproduce the 2010 observation and suggested it was due to experimental error. In that experiment they refined their experimental setup, using a three-wire torsion pendulum to measure thrust, and tested two different power setups. In one trial, the power system was outside the cavity, and they observed a "thrust" of 8-10 mN. In a second trial, the power system was within the cavity, and they measured no such thrust. Instead they observed an insignificant thrust below their noise threshold of 3 mN, fluctuating between ±0.7 mN with a measurement uncertainty of 80%, with 230 W of input power. They concluded that they were unable to measure significant thrust; that "thrust" measured when using external power sources (as in their 2010 experiment) could be noise; and that it was important to use self-contained power systems for these experiments, and more sensitive pendulums with lower torsional stiffness.

China Academy of Space Technology
At the China Academy of Space Technology, Yue Chen filed several patent applications in 2016 describing various RF 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, Yue Chen told a reporter at China's Science and Technology Daily that his team was testing an EmDrive in orbit, and that they had been funding research in the area for five years. Chen noted that their prototype's thrust was at the "micronewton to millinewton level", which would have to be scaled up to at least 100–1000 millinewtons for a chance of conclusive experimental results. Despite this, he said his goal was to complete validation of the drive, and then to make such technology available in the field of satellite engineering "as quickly as possible".

Russian devices
In 2007 the Russian Research Institute of Space Systems launched an experimental micro-satellite called Yubileiny (Jubilee) with a "non-traditional" engine which, according to Director Valery Mesnshikov, functions without ejecting reaction mass. However, it was later stated that "further developments" were needed and nothing further appears to be been published on Russian reactionless drives.

Controversy
The plausibility of thrusters that emit no propellant, such as the EmDrive are controversial, primarily because their operation would violate the conservation of momentum.

"Quantum vacuum virtual plasma" is something you'd say if you failed a course in quantum field theory and then smoked too much weed. There's no such thing as "virtual plasma". If you want to report experimental results that seem to violate the known laws of physics, fine. But it doesn't help your credibility to make up goofy pseudo-explanations.

My insight is that the EmDrive is complete crap and a waste of time...Right there in the abstract this paper says, "Our test campaign can not confirm or refute the claims of the EMDrive", so I’m not sure what the news is. I’m going to spend my time thinking about ideas that don’t violate conservation of momentum.

A true zero-propellant drive is widely believed to be impossible, but if it existed, it could potentially be used for travel in many environments including deep space. Thus, such drives are a popular concept in science fiction, and their improbability contributes to enthusiasm for exploring such designs.

John C. Baez, a mathematical physicist at the University of California, Riverside, and Australian science-fiction writer Greg Egan said that as of 2006 the positive results reported by Shawyer were likely misinterpretations of experimental errors.

Physicists Eric W. Davis at the Institute for Advanced Studies in Austin, and Sean M. Carroll at the California Institute of Technology, said in 2015 that the thrust measured in both the Dresden University experiments and in earlier Eagleworks publications were indicative of thermal effect errors.

Harold White's 2014 conference paper suggested that resonant cavity thrusters could work by transferring momentum to the "quantum vacuum virtual plasma." 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 can't be used for propulsion.

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. Science fiction author 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". 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 mathematical physicist John C. Baez and posted on his blog. New Scientist editor Jeremy Webb responded to critics, stating:

It is a fair criticism that New Scientist did not make clear enough how controversial Roger Shawyer’s engine is. 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... The great thing is that Shawyer's ideas are testable. If he succeeds in getting his machine flown in space, we will know soon enough if it is ground-breaking device or a mere flight of fancy.

New Scientist also published a letter from the former technical director of EADS Astrium, who stated: "I reviewed Roger’s work and concluded that both theory and experiment were fatally flawed. Roger was advised that the company had no interest in the device, did not wish to seek patent coverage and in fact did not wish to be associated with it in any way", and a letter from physicist Paul Friedlander, who stated "As I read it, I, like the thousands of other physicists who will have read it, immediately realised that this was impossible as described. Physicists are trained to use certain fundamental principles to analyse a problem and this claim clearly flouted one of them... The Shawyer drive is as impossible as perpetual motion. Relativistic conservation of momentum has been understood for a century and dictates that if nothing emerges from Shawyer’s device then its centre of mass will not accelerate. It is likely that Shawyer has used an approximation somewhere in his calculations that would have been reasonable if he hadn’t then multiplied the result by 50,000. The reason physicists value principles such as conservation of momentum is that they act as a reality check against errors of this kind."

Media
To say Shawyer’s research is controversial is an understatement. I will be the first to admit that I am a writer and not a scientist. While I don’t understand the physics involved, I do understand controversy and EmDrive is controversy on steroids.

Conservation of momentum
The EmDrive appears to violate the conservation of momentum which states any interaction cannot have a net force; a consequence of the conservation of momentum is Newton's third law where for every action there is an equal and opposite reaction. The conservation of momentum is a symmetry of nature. Symmetries are mathematical transformations that leave the general form of equations unchanged and are of prime importance in physics: symmetries give rise to all conservation laws or laws of physics and are closely connected with the hypothesis that certain physical properties cannot be detected and are unobservable. The relationship between symmetries and conservation rules can be rigorously proven by the Noether theorem. For example, time translation symmetry gives rise to the conservation of energy and implies absolute time is unobservable, while invariance under an arbitrary spatial translation (i.e. translational symmetry) gives rise to the conservation of momentum and implies that absolute position in space is unobservable, or merely a relative quantity. In mathematics, if the equations governing a field, such as the vacuum field described by Maxwell's equations or by quantum electrodynamics (QED), are invariant under spatial translation, the field conserves momentum, and the equations are called homogenous. If the equations are homogeous the system provides nothing for an object to push against. Symmetries give rise to the geometry or properties of spacetime.

In certain select cases symmetries can be broken. In these well-tested and verified instances where matter appears to violate conservation laws, the apparent non-conservation is in reality an interaction with the vacuum so that overall symmetry in the system is restored. It can be shown that based on the assumption of homogeneous electric and magnetic fields it is impossible for the EmDrive, or any other device, to extract a net momentum transfer from either a classical or quantum vacuum. An often cited example of apparent nonconservation of momentum is the Casimir effect; in the standard case where two parallel plates come together, energy can be extracted from the vacuum, but the plates move in opposite directions, so no net momentum is extracted and moreover energy must be put into the system in order to take the plates apart again. The magnitude and direction of the Casimir effect depends on the size, geometry and topology of the cavity and field. Extraction of a net momentum "from nothing" has been postulated in an inhomogeneous vacuum but this remains highly controversial as it will violate Lorentz invariance. If the EmDrive were able to induce an instability in the vacuum it may be able to produce a net force in this way, Both Harold White's   and Mike MucCulloh's theory of how the EmDrive works rely on these asymmetric or dynamical Casimir effects. However, if these vacuum forces are indeed present, they are expected to be exceptionally tiny based on our current understanding; So even assuming this mechanism is present, these forces are unlikely to explain the level of observed thrust. In the unlikely event that the observed force is not an experimental error, a positive result is believed to be indicative of new physics

Hypotheses
Various attempts have been made to come up with a potential mechanism the for apparent thrust. However, to date, there is no acceptance or consensus over how, if at all, these cavities produce thrust. With the Royal Aeronautical Society commenting:

The issue with all of these ‘thrusters’ is the lack of theoretical background to them, with each proponent claiming their own to be obvious and sound, yet with none of them agreeing.

Attempts to explain the thrust fall into three rough categories: The simplest and most likely explanation is that any thrust detected is due to experimental error or noise. While some experimental observations have led to a fundamentally new understanding of physics, such as the anomalous precession of Mercury which saw Newtonian gravity overturned by Einstein's general relativity, or the Wu experiment which was the first observation of a broken symmetry, more often then not, anomalous results which would fundamentally change our understanding of physics are eventually explained as sources of error present in the experimental set up, famous examples of which include N-rays, faster-than-light neutrinos, the Pioneer anomaly and cold fusion.
 * The thrust observed isn't real and is the result of measurement error. Most theoretical scientists who have looked at the EmDrive believe this to be the case.
 * There is a change in the electromagnetic or vacuum field, such that the change in the momentum of the field balances the change in momentum of the thruster.
 * There is an exhaust that isn't being measured.

Noise or experimental error
The simplest and most likely explanation is that any thrust detected is due to experimental error or noise. In all of the experiments set up, a very large amount of energy goes into generating a tiny amount of thrust. The strongest early result, from Yang's group in China, was later reported to be caused by a large experimental error.

The 2016 NASA Eagleworks paper went through about a year of peer review involving five expert “referees” instead of the more typical two referees. Peer review does not mean the results or observations are true, only that the referees looked at the experiment, results and interpetation and found it to be sound and sensible. Brice Cassenti, a professor at the University of Connecticut and expert in advanced propulsion, spoke to one of the referees who was asked to review the paper: Cassenti said the referee did not believe the results point to any new physics, but that the results were puzzling enough to publish. Cassenti believes that there is a mundane explanation for the results but the probability of the results being valid is slim but not zero.

The NASA Eagleworks paper was published in the Journal of Propulsion and Power. Marc Millis and Eric Davies who ran NASA's previous advanced propulsion project, the Breakthrough Propulsion Physics Program have commented that while Eagleworks have used techniques that would be acceptable for checking the electric propulsion of Hall thrusters, the tests are not sufficient to demonstrate that any new physics effect exists.

The NASA paper discusses nine possible sources of experimental error. Harold White, the lead NASA scientist, argues that most of those error sources were eliminated "fairly definitively" but that further work was needed to address possible thermal effects stating "We’re still potentially even in the mode of definitively eliminat[ing] all false positives.”

Shift in center of gravity due to thermal effects
The largest error source is believed to come from the thermal expansion of the thruster's heat sink; as it expands this would lead to a change in the centre of gravity causing the resonant cavity to move. The authors of the NASA Eagleworks paper attempted to model the thermal effect on the overall displacement by using a superposition of the displacements caused by "thermal effects" and “impulsive thrust” with White saying "That was the thing we worked the hardest to understand and put in a box". Despite these efforts, White's team were unable to exhaustively account for the thermal expansion in full. In an interview with Aerospace America White comments that "although maybe we put a little bit of a pencil mark through [the error]... they are certainly not black-Sharpie-crossed-out.”

The method of accounting for thermal effects has been criticized by Millis and Davies. While the Eagleworks paper attempts to account for thermal effects, Millis and Davies highlight that there is a lack of both mathematical and empirical detail to justify the exact assumptions regarding thermal effects. For example, they do not provide data on temperature measurement over time compared to the device's displacement. The paper includes a graphical chart but it is based an a priori assumption on what the shapes of both the “impulsive thrust” and "thermal effects" should be and how those signals will superimpose. The model assumes all noise to be thermal and does not include consideration of other effects such as interaction with the chamber wall, power lead forces and tilting. Because the model of thrust in the Eagleworks paper is not fully explained by empirical justification, the model is ultimately subjective and data from the Eagleworks test can then be interpreted in more than one way. The Eagleworks test therefore does not conclusively show a thrust effect, but cannot rule it out either.

Future experiments are planned to run on a type of apparatus called a Cavendish balance. In such a setup, the EmDrive would be able to rotate out to much larger angular displacements, so that the force of the thrust (if present) would completely dominate over any possible thermal effects. Testing a device in space would also eliminate the centre of gravity issue.

Interaction with the vacuum chamber’s wall
Another source of error arises from the possible electromagnetic interaction with the vacuum chamber’s wall. The Eagleworks team argue that any wall interaction could only be the result of a wellformed resonance coupling between the device and wall, because of the high frequency, the chances of this happening would be highly dependant on the device's geometry. As the components get warmer due to thermal expansion the device's geometry changes and this shifts the resonance of the truncated cone. In order to counter this effect and keep the system in optimal resonance conditions, the Eagleworks team use a phase-locked loop system (PLL). The use of a PLL means it is not likely that the device can establish and maintain an effective external RF resonance with the wall.

Power cabling
Another source of error was a Lorentz force arising from power leads. In order to eliminate interface from cable forces tests by experimenters have made use of Galinstan liquid metal screws which should eliminate such forces. However error sources can still arrise from magnetic damping: In the NASA paper it is not stated if the connections are all coaxially aligned with the stand’s rotation axis, which would be required to minimize errors from Lorentz forces, and the paper gives no data from equivalent tests with power into a dummy load so these influences can be compared with those seen in the test run. Dummy tests performed by Martin Tajmar from Dresden University of Technology found magnetic interaction of the power feeding lines the most important possible error source in their test (this may not be the case in NASA test due to differences in the power supply). When Tajmar replaced the galinstan liquid metal damping with oil fluid damping and then modelled the thrust effect again, they found the thrust effect to be well within the resolution of experimental error.

Other error sources
Other possible sources of error include air currents, tilting of the thrust stand, vibration, electrostatic interaction, outgassing and a photon rocket force all of which are unlikely to contribute significantly to the level of observed thrust in the Eagleworks paper.

Radiation pressure
Shawyer has suggested that thrust from a frustum cavity is caused by a radiation pressure imbalance between the two faces of the cavity.

The equations used to calculate the guide wavelengths in the static thrust equation are very non-linear. This is exploited in the design of the resonator to maximise the ratio of end plate forces, while minimising the axial component of the side wall force. This results in a net force that produces motion in accordance with Newton’s laws.

He gave a presentation on this at the International Astronautical Congress 2014, later publishing it in the peer-reviewed Acta Astronautica. In it he wrote, In an EmDrive engine, microwave energy is converted to mechanical force according to the thrust equation, derived from the basic radiation pressure equation: F= 2 P0 / c. Shawyer's thrust equation, derived from Allen Cullen's equations, is given by:

$$ F = \frac{2 P_0}{c} \ Q_u \left(\frac{\lambda_0}{\lambda_{g1}} - \frac{\lambda_0}{\lambda_{g2}}\right) \left(1 - \frac{\lambda_0^2}{\lambda_{g1} \lambda_{g2}} \right)^{-1} $$

where $$F$$ is the force, $$P_0$$ is the incident power, $$c$$ is the speed of light, $$Q_u$$ is the unloaded Q factor of the cavity, $$\lambda_0$$ is the wavelength of the microwaves in free space, and $$\lambda_{g1}$$ and $$\lambda_{g2}$$ are the wavelengths at the end of the largest and smallest cross-section, respectively.

Shawyer insists the EmDrive is an open system. However, physicists point out that relying only on special relativity, without emitting anything and with no interaction with an outside field or matter, makes his drive a closed system. Since the two end plates are part of the thruster and the microwaves are trapped inside the cavity, standard Einstein–Maxwell equations and the conservation of momentum show no effective thrust can occur due to any force on the cavity caused by internal electromagnetic energy.

Vacuum energy
Harold White, the lead scientist in the NASA investigations, suggested in 2014 that their model could be an example of a quantum vacuum thruster (QVT). This is a theoretical system that would use magnetohydrodynamics to generate thrust, similar to conventional plasma thrusters, only using the fleeting vacuum quantum fluctuations of the zero-point field as an extremely low-density plasma.

White's 2016 paper states that pilot-wave theories, non-mainstream interpretations of quantum mechanics, may help explain how QVTs could "push off of the quantum vacuum and preserve the laws of conservation of energy and conservation of momentum.".

Quantized inertia
A paper in EPL by Mike McCulloch, a Lecturer in Geomatics at Plymouth University, describes a possible method in which thrust from resonant cavities can be predicted using McCulloch's controversial theory of quantization of inertia, the "Modified Inertia Hubble-scale Casimir effect" (MiHsC). While this model allows the device to create thrust without breaking Newton's third law, it assumes that Unruh radiation is real, and requires the speed of light to change within the microwave cavity. This change in the speed of light is contrary to the central tenet of special relativity. Unlike some other hypotheses used to explain the device, McCulloch's hypothesis is testable and McCulloch has suggested building a cavity where the length of the cavity is the same as the diameter of the small end, to cause the Unruh radiation to fit better in the small end, resulting in a reversal of thrust.

Photon leakage
Scientists in Finland have proposed a possible explanation of this phenomenon involving the propagation of microwave photons leaking from the closed metal cavity and thereby producing an exhaust momentum, satisfying the classical action-reaction principle. This explanation relies on the wave-particle duality of electromagnetic radiation, postulating that the stochastic phases of the microwaves will (with some probability) result in destructive interference between microwaves which cancels their electromagnetic fields but allows continued propagation of the microwave photon pairs, generating net thrust consistent with the impulse-momentum theorem depending on the asymmetric shape of the cavity.

The observed thrust of experimental results has been argued to exceed the maximum efficiency of a perfectly collimated photon rocket, comprised between 3.33 and 6.67 µN/kW. However, the paper follows on White's idea of a degradable quantum vacuum for effective pair production, and Lewis's original concept of the photon which would be the conserved entity of nature, not its carried energy: The authors argue that the environment modifies photon energy and that pairing of photons within the electromagnetic energy density gradient of a resonant cavity would cause a shift down in energy, and the loss of electromagnetic potential becomes available for thrust, so according to the authors the level of energy of the paired photon when it escapes the cavity and the associated thrust efficiency remain an open question. The authors also argue that the cavity walls become transparent for the photon pair when it forms; as it has no associated electromagnetic field, it escapes the cavity to sparser surroundings.

Mach effect
A group of scientists led by James F. Woodward claim general relativity allows propellantless propulsion using a Mach effect. In a fully Machian general relativity theory like the Hoyle–Narlikar theory of gravity, inertia is a physical gravitational interaction of matter with the rest of the mass-energy in the universe, through an action at a distance instantaneous radiative reaction field. In the theory, a mass changing effect suitable for propulsion emerges from the general equation of motion.

The RF resonant cavity thruster would act as a capacitor where surface currents propagate inside the cavity on the conic wall, between the two end plates; electromagnetic resonant modes create electric charges on each end plate; a Mach effect is triggered by Lorentz forces from surface currents on the conic wall; and a thrust force arise in the RF cavity, due to the variation of the electromagnetic density from evanescent waves inside the skin layer. When a polymer insert is placed asymmetrically in the cavity, its dielectric properties result in greater asymmetry, while decreasing the cavity Q factor. The cavity's acceleration is a function of all the above factors, and the model can explain the acceleration of the cavity with and without a dielectric.

Like the slingshot maneuver of a spacecraft exchanging momentum with a planet, a Mach effect gravity-assist drive is an open system where momentum is conserved when accounting for gravitational interaction. The thruster, acting as an "impulse engine" would then be propellantless, but not reactionless. In more speculative developments, the second term present in Woodward's transient mass equation indicates the mass fluctuations could theoretically become largely negative, producing exotic matter suitable for an Alcubierre warp drive.

Warp field


It has been suggested that time-varying electromagnetic energy density could produce a local gradient in the gravitational potential (a distortion or warping of spacetime, sometimes called "warp fields"), as in the theoretical Alcubierre drive or diametric drive. Warp fields have never been observed, however they could potentially be tested using interferometry. White developed the White–Juday warp-field interferometer to attempt to detect such fields over short distances. His team used one to test a symmetric resonant cavity in 2013, and observed small anomalous effects. However the effects have yet to be replicated on an asymmetric cavity, or in a vacuum to prevent interference from the heating of surrounding air.

Physicist Fernando Minotti, building on work by Matt Visser, estimated the forces on asymmetric electromagnetic resonant cavities using a particular scalar–tensor theory of the Brans–Dicke type, an alternate framework for describing gravity that competes with general relativity. In Minotti's model, thrust results from gravitational forces on the cavity walls, with some scalar coupling field providing an effective negative energy source. Minotti suggested that this model implies the direction of the force produced by a resonant cavity would be dependent on its resonant mode, and the thrust magnitude would increase with the thickness and mass of the cavity walls.

However, Minotti noted that the scalar–tensor theory he used is not accepted by the majority of the scientific community, and that his linear model also erroneously predicts large gravitational effects due to the Earth's magnetic field which do not exist. He hypothesized that some nonlinear version of his model might provide a framework which does not predict such unreal effects.

After Yang retracted her previous high power results, Minotti completed a revaluation of the scalar-tensor theory fixing some inconsistencies. The equations are now derived without ad hoc conditions and Maxwell's equations are obtained in the weak field approximation with no unreal prediction.