A radio frequency (RF) resonant cavity thruster is a proposed type of electromagnetic thruster. Unlike conventional electromagnetic thrusters, a resonant cavity thruster uses no reaction mass and emits no directional radiation. Their design principles are not supported by prevailing scientific theories, apparently violating the law of conservation of momentum; as a result they are controversial.
Aerospace engineer Roger Shawyer designed the EmDrive in 2001 and has persistently promoted the idea through his company, Satellite Propulsion Research. Chemical engineer Guido Fetta designed the Cannae Drive, based on similar principles.
If they are found to work as claimed, providing thrust without consuming a propellant would revolutionise many propulsion applications.
Imagine something like the impulse drive in Star Trek. The Star Trek impulse drive may have been described in technical manual as a nuclear fusion powered drive using something like regular rocketry, but the Star Trek technical manual wanted to assist suspension of disbelief. In the Star Trek Stories Impulse drive could provide acceleration up to any limit desired by the writers and have it offset by antigravity. Also, there was generally no limit on fuel resources being expended. Therefore, the impulse drive in Star Trek per how it was used in the movies and books was more akin to a reactionless drive which provides excellent propulsion so long as the system has power. Impulse drive and EMdrive both still have speed of light (relativistic) limitations.
You would not have antigravity but whatever power level you could achieve would have constant acceleration up to relativistic limits. Limitations would be based on the energy density and energy levels of your power source.
Cannae drive is preparing for another demonstration of our superconducting thruster technology in May. Here is a picture of our assembled thruster and test apparatus. The thruster is under vacuum and ready for cryogenic operation.
New Superconducting Cannae device demo
Various teams around the world have begun to build their own versions of the EmDrive and put them through their paces. And to everyone’s surprise, they’ve begun to reproduce Shawyer’s results. The EmDrive, it seems, really does produce thrust. In total, six independent experiments have backed Shawyer’s original claims.
How to explain the seeming violation of conservation of momentum from EMdrive and Cannae drive ? Quantized Momentum
Mike McCulloch at Plymouth University explanation is based on a new theory of inertia that makes startling predictions about the way objects move under very small accelerations.
Inertia is the resistance of all massive objects to changes in motion or accelerations. In modern physics, inertia is treated as a fundamental property of massive objects subjected to an acceleration. Indeed, mass can be thought of as a measure of inertia. But why inertia exists at all has puzzled scientists for centuries.
McCulloch’s idea is that inertia arises from an effect predicted by general relativity called Unruh radiation. This is the notion that an accelerating object experiences black body radiation. In other words, the universe warms up when you accelerate.
According to McCulloch, inertia is simply the pressure the Unruh radiation exerts on an accelerating body.
That’s hard to test at the accelerations we normally observe on Earth. But things get interesting when the accelerations involved are smaller and the wavelength of Unruh radiation gets larger.
At very small accelerations, the wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia can take only certain whole-wavelength values and so jumps from one value to the next. In other words, inertia must quantized at small accelerations.
McCulloch says there is observational evidence for this in the form of the famous fly by anomalies. These are the strange jumps in momentum observed in some spacecraft as they fly past Earth toward other planets.
Testing this effect more carefully on Earth is hard because the accelerations involved are so small. But one way to make it easier would be to reduce the size of allowed wavelengths of Unruh radiation. “This is what the EmDrive may be doing,” says McCulloch.
The idea is that if photons have an inertial mass, they must experience inertia when they reflect. But the Unruh radiation in this case is tiny. So small in fact that it can interact with its immediate environment. In the case of the EmDrive, this is the truncated cone.
The cone allows Unruh radiation of a certain size at the large end but only a smaller wavelength at the other end. So the inertia of photons inside the cavity must change as they bounce back and forth. And to conserve momentum, this must generate a thrust.
McCulloch puts this theory to the test by using it to predict the forces it must generate. The precise calculations are complex because of the three-dimensional nature of the problem, but his approximate results match the order of magnitude of thrust in all the experiments done so far.
Crucially, McCulloch’s theory makes two testable predictions. The first is that placing a dielectric inside the cavity should enhance the effectiveness of the thruster.
The second is that changing the dimensions of the cavity can reverse the direction of the thrust. That would happen when the Unruh radiation better matches the size of the narrow end than the large end. Changing the frequency of the photons inside the cavity could achieve a similar effect.
McCulloch says there is some evidence that exactly this happens. “This thrust reversal may have been seen in recent NASA experiments,” he says.
MiHsC suggests that the thrust can be increased by increasing the input power, the Q factor, or using a dielectric. As a direct test MiHsC predicts that the thrust can be reversed by making the length L equal to the width of the narrow end.
It has been shown that truncated cone-shaped cavities with microwaves resonating within them move slightly towards their narrow ends (the emdrive). Standard physics has no explanation for this and an error has not yet been found. It is shown here that this effect can be predicted by assuming that the inertial mass of the photons in the cavity is caused by Unruh radiation, whose wavelengths must fit exactly within the cavity, using a theory already applied successfully to astrophysical anomalies such as galaxy rotation where the Unruh waves have to fit within the Hubble scale. In the emdrive this means that more Unruh waves are allowed at the wide end, leading to a greater inertial mass for the photons there, and to conserve momentum the cavity must move towards its narrow end, as observed. The model predicts thrusts of: 3.8, 149, 7.3, 0.23, 0.57, 0.11, 0.64 and 0.02 mN compared with the observed thrusts of: 16, 147, 9, 0.09, 0.05, 0.06, 0.03, and 0.02 mN and predicts that if the axial length is equal to the diameter of the small end of the cavity, the thrust should be reversed.
SOURCES- Cannae, Wikipedia, Arxiv, Technology Review