June 2014 saw excited reports that NASA was working on a faster than light warp drive starship. Astonishingly, weeks later we are being told that NASA has also successfully tested a device which could push along a space vehicle without consuming any propellant. If true, this would be an astonishing discovery, not only violating laws which are cornerstones of science but also possibly allowing easy access to the worlds of the Solar System. But are these latest reports correct?
(For the latest developments in this affair see NASA’s Space Drive: the Plot Thickens (link).
In physics, momentum is a quantity obtained by multiplying a bodyÁ€™s mass and velocity (velocity is not just speed, it is speed in a set direction- an important distinction). Both theory and centuries of practice indicate that momentum is conserved; essentially meaning that it is never created or destroyed. Let me illustrate this with a pertinent example.
Imagine a spacecraft floating in empty space. Inside it are tanks of propellant, say liquid hydrogen and oxygen, and a rocket motor. When the craftÁ€™s motor is turned on, the hydrogen and oxygen are burned together in the combustion chamber, creating hot gases which are allowed to escape at very high speed out a nozzle, pushing the space craft forward. á Looking more closely, every second the motor operates, a relatively small mass of gas is emitted at high speed out of the back of the spacecraft as the exhaust. A small mass of gas multiplied by a high speed rearward yields a significant momentum in that direction. To balance the books (conserve momentum), the spacecraft must move with an equal and opposite momentum, so it shoots forward (its mass will be greater than the gas in the exhaust, so its velocity will be lower, but the spacecraft’s velocity will keep building up as long as the rocket motor is fed propellant. The spacecraftÁ€™s motion in response to the escaping propellent is termed a Á€˜reactionÁ€™. A rocket motor is a reaction engine (or “drive” in science fiction parlance).
Momentum conservation is predicted by Isaac NewtonÁ€™s laws of motion (and in modified form Einsteinian relativity) and is observed throughout science and utilised in engineering all the way from collisions of subatomic particles to launching probes to the planets.
However, rockets are clumsy and inefficient; to accelerate to meaningful speeds vast quantities of propellant must be carried and consumed. Perhaps 90% of a rocketÁ€™s mass at launch is propellant, perhaps only 10% structure and payload. This is a sad fact, meaning rockets to send missions into to space must always be behemoths, suggesting space travel will forever be difficult and expensive. What if there was an easier way? Could there be entirely new physics (or Á€œloopholesÁ€ in existing physics) permitting a Á€œreactionless driveÁ€ which would run solely on electric power without carrying any messy and bulky propellant ? A spacecraft with a reactionless thruster would be a space enthusiastÁ€™s dream, rising silently into the sky without the sound and fury of a rocket launch, permitting a probe or even a spacecraft with a human crew to roam the planets. Unfortunately this seems impossible. Yet some disagree.
Dozens if not hundreds of concepts for reactionless drives have been proposed, the vast majority being the fantasies of science fiction authors or crackpots or the lies of scammers. However, this is not always the case. Roger J. Shawyer , a British aerospace engineer with impeccable professional qualifications has proposed a device he calls an EmDrive.
ShawyerÁ€™s EmDrive thruster is a magnetron, a microwave generator, inside a specially shaped, tapering resonant cavity whose area is greater at one end. Both ends of the cavity are sealed. Essentially an EmDrive unit is a metal can with a microwave source inside. When it is turned on, the EmDriveÁ€™s magnetron emits microwaves which bounce around inside the cavity pushing against its sides. According to Shawyer, thanks to the cavityÁ€™s shape there is a slight imbalance in the pressure exerted by the microwaves which manifests as a thrust, hence the thruster moves without emitting any exhaust. An alternative name for the concept is RF resonant cavity thruster.
Electricity is apparently being turned directly into thrust in defiance of the conservation of momentum law. Shawyer believes that his conceptÁ€™s behaviour is permitted under Einstein relativity (hence the device is actually called a “relativity drive” by some) and he insists it obeys Newton’s laws and conserves momentum. He has written highly mathematical papers to justify this and claims to have successfully tested prototypes. Eureka magazine’s website has a video of an EmDrive being demonstrated.á Shawyer has created a company (with the help of a á¸45 000 grant from the UK’s Department of Trade and Industry) to develop this technology. Shawyer has shared his beliefs on the theory and potential of his device in a series of videos.
ShawyerÁ€™s proposal has received some positive coverage in engineering journals and websites but not from many scientific publications (apart from New Scientist, which positively gushed enthusiasm). The science community has been largely reluctant to repeat ShawyerÁ€™s research because his theoretical justification sounds frankly absurd. All electromagnetic waves, such as microwaves, possess momentum. This means that a beam of microwaves does indeed exert thrust and you could actually make a grossly inefficient rocket based on the principle of an exhaust of microwaves alone (it would in fact be a form of photon rocket), but that is not what Shawyer claims to have invented. The microwaves are trapped in his device, and do not escape as an exhaust, making it reactionless.
Take one of those little RC helicopters you can fly indoors. Imagine getting an incredibly light-weight cardboard box, putting the helicopter inside and sealing the lid before turning the helicopter on. Will the box rise into the air thanks to the spinning rotor inside? This isá comparable to what Shawyer claims his device does.
Since I wrote the above paragraph I have read Shawyer’s document A Note on the Principles of EmDrive force measurement, which muddies the waters considerably. In it Shawyer claims his device cannot generate thrust when at rest, instead it must be in accelerating motion. If correct this means that you cannot measure an EmDrive’s thrust by it placing in on a balance (Shawyer explicitly states this), instead it must be accelerated by an external force while the measurement takes place. This is both inconvenient for experimenters and really odd physically.
The physics community mostly believes Shawyer is profoundly mistaken (laying my cards on the table,á I would agree with this opinion). However if a prototype were to be tested in space conditions and work as advertised then physicists, scenting a Nobel prize, would really pay attention.
Other experimenters have indeed attempted to duplicate ShawyerÁ€™s research. The Boeing aerospace company has investigated Shawyer’s technology but this does not seem to have led anywhere. Juan Yang , a professor of propulsion theory and engineering of aeronautics and astronautics at Northwestern Polytechnical University (NWPU) in Xi’an, China, claimed to have tested a high power EmDrive on a rocket motor test rig in 2010. YangÁ€™s published data suggests the EmDrive passed its tests with flying colours, but she has not convinced many others to revisit ShawyerÁ€™s brainchild.
Another inventor, Guido P. Fetta has suggested a similar device to the EmDrive that he has called the Cannae drive (confusingly also known as the Q-drive). Fetta, with a Á€œbackground as a sales and marketing executive with more than 20 years of experience in the chemical, pharmaceutical and food ingredient industriesÁ€, owns a company called Cannae LLC to exploit his research. Although the Cannae device is also essentially a metal can with a microwave source inside some report that it is intended to operate under entirely different principles to the EmDrive perhaps exploiting quantum mechanics to violate the laws of classical physics. I cannot verify this as the Cannae websiteá has nothing to say about it.
The Cannae device is a thick disc-shaped resonant cavity with radial slots in one inside face, according to its inventor these are vital to produce an internal force imbalance leading to an external thrust. I recommend everyone read the patent for the Cannae device which discusses how it could be applied for “energy harvesting”, suggesting Fetta believes he has also invented a free-energy device. This makes the concept self-confessed nonsense.
Bringing the story up to date, in 2013-14 a team from NASAÁ€™s advanced propulsion thinktank, the