EmDrive
Encyclopedia
EmDrive is the name of a spacecraft propulsion
system proposed, and reportedly developed, by Roger Shawyer. New Scientist
ran a cover story on EmDrive in its 8 September 2006 issue. The device is a magnetron with a specially shaped, fully enclosed tapering resonator cavity whose area is greater at one end. The inventor claims that the device generates thrust even though no detectable energy leaves the device. The inventor proposes to use it as a spacecraft propulsion
system that uses no fuel (other than electricity), and no reaction mass
.
On his homepage the inventor claims that independent peer-review is under way, but so far no publication is available.
The device's operation as described seemingly violates several basic laws of physics
, notably conservation of momentum, though the inventor insists to the contrary. John Costella, an expert in relativistic
electrodynamics describes the EmDrive as a 'fraud
'.
then at University College
London
, that involves forces created by reflecting microwaves between opposite walls of a cavity. The idea is to try to design a cavity in such a manner that force
s on one side are greater than the other.
The drive comprises a resonant cavity flooded with microwave
radiation. The radiation exerts radiation pressure
on the walls of the cavity, and normal Newtonian mechanics would of course indicate that, no matter what shape the cavity is, the forces exerted upon it from within must balance to zero.
However, Shawyer claims that relativistic effects
cause a cavity shaped like a truncated cone to experience a larger force against the large end than the small end, due to the group velocity
of the wave changing as the local diameter of the cavity varies.
The increased confinement of the tapered end of the cone leads to a higher effective propagation constant (phase velocity). It also leads to local reflections which account for the apparent force imbalance when considering only the end walls. However, since it is the phase of the light rather than the actual photons bouncing off the walls, each force acts quasi independently from another—much like in a ring laser gyroscope
where the beams act as if having an external frame of reference (which they have, since the speed of light
is constant). The same principle applies to the EmDrive.
No microwaves or anything else are allowed to leave the device. Since nothing leaves the drive for propulsive purposes an EmDrive can be classed as a reactionless drive
. The principle by which the EmDrive is supposed to operate seems to violate conservation of momentum. However, the emdrive website claims that the device is not reactionless because the force is created by a "reaction between the end plates of the waveguide and the Electromagnetic wave propagated within it." It is known that the physics equations describing microwaves, Maxwell's equations
, conserve momentum, and this would seem to cast doubt on Shawyer's derivation of a thrust effect. In his paper, Shawyer thus takes the following view: any thrust extracted from the device is directly withdrawn from the energy stored in the cavity (due to the Q reflections an average wave encounters when inserted into the cavity, the energy levels quickly build up). In other words: the apparent force on the wider diameter of the cone seems to lessen. The extent to which that happens perfectly matches the amount predicted by the law of conservation of momentum.
.
After receiving criticism that no peer-reviewed publications on the subject had been made, Mr. Shawyer submitted a theory paper to New Scientist (which is not a peer reviewed scientific journal
) Shawyer's paper was almost immediately challenged by Dr. John Costella, a theoretical physicist and electrical engineer who works for the Australian Department of Defence, whose Ph.D. is in relativistic electrodynamics, the field of physics that Mr. Shawyer relies on to support his theory. Notably Shawyer has since published a new theory paper (version 9.4) where the paragraph which Costella criticised is simply omitted.
In May 2007 Eureka magazine reported that a second unit has been built for demonstration purposes, weighing 100 kilograms, consuming 300 watts for microwave production (and an unspecified amount for ancillary purposes such as cooling) and producing 96.1 millinewtons of force during testing in October 2006. Unlike the prototype unit, which can only be run for short periods before burning out its magnetron, the demonstration unit can be run continuously.
The limiting factor for performance is claimed to be the Q factor
of the cavity, as microwave energy lost to heating the cavity reduces the field strength within, so Shawyer is experimenting with a cavity lined in a superconducting material that may produce Q factors sufficient to build a device capable of generating 30 newtons per watt.
These results have neither been reproduced by other scientists or engineers, nor have they been published in a peer-reviewed scientific journal.
, a weekly science magazine
. The article portrayed the device as plausible, and emphasized the arguments of those who held that point of view, although it did quote one engineer as saying "it's a load of bloody rubbish." The article included the following arguments from proponents of the theory:
New Scientist has drawn great criticism from the scientific community due to the uncritical treatment of EmDrive in its article. Science fiction
writer
Greg Egan
distributed a public letter stating that "a sensationalist bent and a lack of basic knowledge by its writers" was making the magazine's coverage sufficiently unreliable "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, where New Scientist allowed the publication of "meaningless double-talk" designed to bypass a fatal objection to Shawyer's proposed space drive, namely that it violates the 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 has also recommended that New Scientist publish Costella's refutation of Shawyer's theory paper.
is treated with skepticism by the physics community, since reactionless drives violate the well-established principle of the conservation of momentum, which has enormous experimental support. Shawyer claims that his drive does not violate the conservation of momentum.
Since there are no known phenomena that do not conserve energy, any calculation based on standard physical theory that predicts a violation of energy conservation almost certainly is in error. This is a non-controversial and fundamental fact regarding the mathematical structure of the theories, regardless of whether the theories themselves are or are not correct descriptions of the physical world. Accordingly, the results reported regarding the EmDrive, if true, would demonstrate that existing physical theory (or its application in engineering) is incorrect or incomplete.
The EmDrive has been compared to the previous Dean Drive
, in that an oscillatory motion is set up so that it has a different effect in each direction of the stroke, in the hope that momentum transfer will differ in each direction, except in this case the oscillations are said to be electromagnetic.
Conservation of momentum is also required and maintained in Maxwell's equations
, Newtonian mechanics, Special relativity
and quantum mechanics
(and their combination, quantum electrodynamics
), so this claim cannot be valid unless these well-established physical theories are false or can be otherwise explained in terms within these existing theories.
Shawyer's calculations may be in error. He may have incorrectly identified the forces on the sides of the waveguide. If an error is present, it is most likely that the 'thrust' is eliminated and the drive then cannot accelerate. Despite some criticism, Shawyer has released to the public a video purportedly demonstrating that his device works.
Any dispute will be settled when independent observations are able to conclude whether or not the machine works in the way it is claimed.
magazine reported that a team of Chinese researchers from Northwestern Polytechnical University
led by Yang Juan (杨涓) claims to have confirmed the theory behind the drive and are proceeding to build a demonstration version.
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
system proposed, and reportedly developed, by Roger Shawyer. New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...
ran a cover story on EmDrive in its 8 September 2006 issue. The device is a magnetron with a specially shaped, fully enclosed tapering resonator cavity whose area is greater at one end. The inventor claims that the device generates thrust even though no detectable energy leaves the device. The inventor proposes to use it as a spacecraft propulsion
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
system that uses no fuel (other than electricity), and no reaction mass
Working mass
Working mass is a mass against which a system operates in order to produce acceleration.In the case of a rocket, for example, the reaction mass is the fuel shot backwards to provide propulsion. All acceleration requires an exchange of momentum, which can be thought of as the "unit of movement"...
.
On his homepage the inventor claims that independent peer-review is under way, but so far no publication is available.
The device's operation as described seemingly violates several basic laws of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, notably conservation of momentum, though the inventor insists to the contrary. John Costella, an expert in relativistic
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
electrodynamics describes the EmDrive as a 'fraud
Fraud
In criminal law, a fraud is an intentional deception made for personal gain or to damage another individual; the related adjective is fraudulent. The specific legal definition varies by legal jurisdiction. Fraud is a crime, and also a civil law violation...
'.
Principle of operation
The device exploits an idea first suggested by Allen Cullen in the 1950s, an electrical engineerEngineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
then at University College
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...
London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
, that involves forces created by reflecting microwaves between opposite walls of a cavity. The idea is to try to design a cavity in such a manner that force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s on one side are greater than the other.
The drive comprises a resonant cavity flooded with microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radiation. The radiation exerts radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
on the walls of the cavity, and normal Newtonian mechanics would of course indicate that, no matter what shape the cavity is, the forces exerted upon it from within must balance to zero.
However, Shawyer claims that relativistic effects
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
cause a cavity shaped like a truncated cone to experience a larger force against the large end than the small end, due to the group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
of the wave changing as the local diameter of the cavity varies.
The increased confinement of the tapered end of the cone leads to a higher effective propagation constant (phase velocity). It also leads to local reflections which account for the apparent force imbalance when considering only the end walls. However, since it is the phase of the light rather than the actual photons bouncing off the walls, each force acts quasi independently from another—much like in a ring laser gyroscope
Ring laser gyroscope
A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...
where the beams act as if having an external frame of reference (which they have, since the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
is constant). The same principle applies to the EmDrive.
No microwaves or anything else are allowed to leave the device. Since nothing leaves the drive for propulsive purposes an EmDrive can be classed as a reactionless drive
Reactionless drive
A reactionless drive is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion...
. The principle by which the EmDrive is supposed to operate seems to violate conservation of momentum. However, the emdrive website claims that the device is not reactionless because the force is created by a "reaction between the end plates of the waveguide and the Electromagnetic wave propagated within it." It is known that the physics equations describing microwaves, Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
, conserve momentum, and this would seem to cast doubt on Shawyer's derivation of a thrust effect. In his paper, Shawyer thus takes the following view: any thrust extracted from the device is directly withdrawn from the energy stored in the cavity (due to the Q reflections an average wave encounters when inserted into the cavity, the energy levels quickly build up). In other words: the apparent force on the wider diameter of the cone seems to lessen. The extent to which that happens perfectly matches the amount predicted by the law of conservation of momentum.
Criticism
The EmDrive was the cover story for the 8 September 2006 issue of New ScientistNew Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...
.
After receiving criticism that no peer-reviewed publications on the subject had been made, Mr. Shawyer submitted a theory paper to New Scientist (which is not a peer reviewed scientific journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...
) Shawyer's paper was almost immediately challenged by Dr. John Costella, a theoretical physicist and electrical engineer who works for the Australian Department of Defence, whose Ph.D. is in relativistic electrodynamics, the field of physics that Mr. Shawyer relies on to support his theory. Notably Shawyer has since published a new theory paper (version 9.4) where the paragraph which Costella criticised is simply omitted.
Prototypes and tests
In September 2006 New Scientist reported that Shawyer had constructed a prototype unit weighing 9 kilograms that consumes 700 watts of power and produces 88 millinewtons of force.In May 2007 Eureka magazine reported that a second unit has been built for demonstration purposes, weighing 100 kilograms, consuming 300 watts for microwave production (and an unspecified amount for ancillary purposes such as cooling) and producing 96.1 millinewtons of force during testing in October 2006. Unlike the prototype unit, which can only be run for short periods before burning out its magnetron, the demonstration unit can be run continuously.
The limiting factor for performance is claimed to be the Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
of the cavity, as microwave energy lost to heating the cavity reduces the field strength within, so Shawyer is experimenting with a cavity lined in a superconducting material that may produce Q factors sufficient to build a device capable of generating 30 newtons per watt.
These results have neither been reproduced by other scientists or engineers, nor have they been published in a peer-reviewed scientific journal.
New Scientist article
The EmDrive was featured on the cover of the 8 September 2006 issue of New ScientistNew Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...
, a weekly science magazine
Science Magazine
Science Magazine was a half-hour television show produced by the Canadian Broadcasting Corporation from 1975 to 1979.The show was hosted by geneticist David Suzuki, who previously hosted the daytime youth programme Suzuki On Science...
. The article portrayed the device as plausible, and emphasized the arguments of those who held that point of view, although it did quote one engineer as saying "it's a load of bloody rubbish." The article included the following arguments from proponents of the theory:
- With a grant from the UK government's Department of Trade and Industry of £250,000, (actually two grants; one a feasibility study of £45,000 and a second of £81,000 to build a demonstration engine - source SPR Ltd.) a commercial regulatory and support agency, Shawyer has built two prototypes that reportedly produce 16 mN and 300 mN of thrust respectively; each using 1 kW of electrical power. A condition of the funding was independent analysis, which was recently completed by John Spiller who says "The thruster's design is practical and could be adapted fairly easily to work in outer space". Shawyer claims that he has been visited by representatives from China and the US Air Force, but ESA has not yet shown much interest. He estimates that his design could save the aerospace community $15 billion over the next ten years.
- Engineers in Germany have created superconducting resonators (for use in particle accelerators) with Q values of several billion, which Shawyer claims would equate to a thrust of 30 kN per kilowatt, "enough to lift a large car". Shawyer states that the thrusters work best while stationary relative to their thrust.
New Scientist has drawn great criticism from the scientific community due to the uncritical treatment of EmDrive in its article. Science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
writer
Writer
A writer is a person who produces literature, such as novels, short stories, plays, screenplays, poetry, or other literary art. Skilled writers are able to use language to portray ideas and images....
Greg Egan
Greg Egan
Greg Egan is an Australian science fiction author.Egan published his first work in 1983. He specialises in hard science fiction stories with mathematical and quantum ontology themes, including the nature of consciousness...
distributed a public letter stating that "a sensationalist bent and a lack of basic knowledge by its writers" was making the magazine's coverage sufficiently unreliable "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, where New Scientist allowed the publication of "meaningless double-talk" designed to bypass a fatal objection to Shawyer's proposed space drive, namely that it violates the 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
John C. Baez
John Carlos Baez is an American mathematical physicist and a professor of mathematics at the University of California, Riverside in Riverside, California. He is known for his work on spin foams in loop quantum gravity. More recently, his research has focused on applications of higher categories...
and posted on his blog
Blog
A blog is a type of website or part of a website supposed to be updated with new content from time to time. Blogs are usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in...
.
Egan has also recommended that New Scientist publish Costella's refutation of Shawyer's theory paper.
Analysis
Any claim of a reactionless driveReactionless drive
A reactionless drive is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion...
is treated with skepticism by the physics community, since reactionless drives violate the well-established principle of the conservation of momentum, which has enormous experimental support. Shawyer claims that his drive does not violate the conservation of momentum.
Since there are no known phenomena that do not conserve energy, any calculation based on standard physical theory that predicts a violation of energy conservation almost certainly is in error. This is a non-controversial and fundamental fact regarding the mathematical structure of the theories, regardless of whether the theories themselves are or are not correct descriptions of the physical world. Accordingly, the results reported regarding the EmDrive, if true, would demonstrate that existing physical theory (or its application in engineering) is incorrect or incomplete.
The EmDrive has been compared to the previous Dean Drive
Dean drive
The Dean drive is a device intended to be a reactionless thruster that was invented by Norman L. Dean. Dean claimed that it was able to generate a uni-directional force, in violation of Newton's Third Law of Motion. Such a violation is generally considered to be impossible in physics...
, in that an oscillatory motion is set up so that it has a different effect in each direction of the stroke, in the hope that momentum transfer will differ in each direction, except in this case the oscillations are said to be electromagnetic.
Conservation of momentum is also required and maintained in Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
, Newtonian mechanics, Special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
and quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
(and their combination, quantum electrodynamics
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
), so this claim cannot be valid unless these well-established physical theories are false or can be otherwise explained in terms within these existing theories.
Shawyer's calculations may be in error. He may have incorrectly identified the forces on the sides of the waveguide. If an error is present, it is most likely that the 'thrust' is eliminated and the drive then cannot accelerate. Despite some criticism, Shawyer has released to the public a video purportedly demonstrating that his device works.
Any dispute will be settled when independent observations are able to conclude whether or not the machine works in the way it is claimed.
Chinese interest
In September 2008, WiredWired (magazine)
Wired is a full-color monthly American magazine and on-line periodical, published since January 1993, that reports on how new and developing technology affects culture, the economy, and politics...
magazine reported that a team of Chinese researchers from Northwestern Polytechnical University
Northwestern Polytechnical University
Northwestern Polytechnical University is a University located in 127#, Youyi West Road, Xi'an, Shaanxi, People's Republic of China.-History:...
led by Yang Juan (杨涓) claims to have confirmed the theory behind the drive and are proceeding to build a demonstration version.
See also
- Abraham-Minkowski controversyAbraham-Minkowski controversyThe Abraham–Minkowski controversy is a physics debate concerning electromagnetic momentum within dielectric media. The preponderance of evidence in the debate suggests that the Abraham equation is correct, but some investigators disagree....
- Dean driveDean driveThe Dean drive is a device intended to be a reactionless thruster that was invented by Norman L. Dean. Dean claimed that it was able to generate a uni-directional force, in violation of Newton's Third Law of Motion. Such a violation is generally considered to be impossible in physics...
- MomentumMomentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
- Reactionless driveReactionless driveA reactionless drive is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion...
Other sources
- http://www.wired.com/dangerroom/2009/10/impossible-drive-designers-dream-flying-cars-stealth-missiles/
External links
- YouTube: EmDrive Video of EMDrive in action
- Satellite Propulsion Research, Ltd. (SPR), the website of Roger Shawyer