No, NASA has not verified an impossible space drive!

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 body’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 craft’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.  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 spacecraft’s motion in response to the escaping propellent is termed a ‘reaction’. A rocket motor is a reaction engine (or “drive” in science fiction parlance).

Momentum conservation is predicted by Isaac Newton’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 rocket’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 “loopholes” in existing physics) permitting a “reactionless drive” which would run solely on electric power without carrying any messy and bulky propellant ? A spacecraft with a reactionless thruster would be a space enthusiast’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.

Shawyer’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 EmDrive’s magnetron emits microwaves which bounce around inside the cavity pushing against its sides. According to Shawyer, thanks to the cavity’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 concept’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.  Shawyer has created a company (with the help of a £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.


Shawyer’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 Shawyer’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 is  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,  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 Shawyer’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. Yang’s published data suggests the EmDrive passed its tests with flying colours, but she has not convinced many others to revisit Shawyer’s brainchild (see updates at end of article).

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 “background as a sales and marketing executive with more than 20 years of experience in the chemical, pharmaceutical and food ingredient industries”, 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 website  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 NASA’s advanced propulsion thinktank, the Eagleworks Laboratories, tested Cannae drive and “tapered cavity” devices with interesting results. These were published in the paper Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum. The experimenters describe how they placed drive units on a torsion pendulum capable of detecting thrusts  “at a single-digit micronewton level” in a stainless steel vacuum chamber.  When the Cannae devices were supplied with around 30 watts of power, the tests measured them to generate 30-50 micro-Newtons of thrust. These are  fantastically small forces, equivalent perhaps to the weight of a sand grain, measuring them alone is an achievement as the environment is full of noise (such as the footsteps of passersby) that could swamp this signal .

The experiments with the tapered cavity device (which is not called an EmDrive in the paper) found that “the presence of some sort of dielectric RF resonator in the thrust chambers” was essential to observe a thrust from the device. When it worked the authors saw an average thrust of 91.2 micro-Newtons generated for an input power of about 17 watts. This means this device has a “thrust to power ratio” of 5.3 micro-Newtons per watt, this statistic is rather esoteric , but it will be important later.

After describing the experiments and their results, the paper suggests refinements to both the authors’ techniques and their equipment for further investigation. The team’s paper ends by discussing in detail possible human space missions to the moons of Mars and Saturn that would be possible if a reactionless drive based on improved scaled up versions of their test articles were to be used. To space buffs and science fiction fans (and I am both) these projected voyages are a mouth-watering prospect.

The report suggests an operational reactionless thruster to take human explorers on round trips to Saturn's moons such as icy Enceladus. (Image credit: NASA)

The report suggests an operational reactionless thruster could take human explorers on round trips to Saturn’s moons such as icy Enceladus. (Image credit: NASA)


Despite the tiny measured thrusts, this is a startling announcement. The NASA researchers seem to have found a flaw in a centuries old central dogma of science, opening the possibility of a wonderful new era of interplanetary travel. This seems to be news worthy of the attention it is receiving. Sadly it is not as simple as that. In fact I am rather dubious and here are my reasons to be sceptical.

  • The researchers describe the vacuum chamber used in the experiment in a lot of detail (in  the section “II. Thrust Measurement System Torsion Pendulum” of their paper), yet the testing was not actually conducted in a vacuum, rather with the vacuum chamber “door closed but at ambient atmospheric pressure”. This was because the capacitors used in the test devices could not survive vacuum conditions, I presume this was a last minute discovery but the test programme went ahead regardless. The section “VI. Summary and Forward Work” recommends that future tests be performed in a vacuum. Not performing these tests in a vacuum is a serious blow to the experiment’s credibility. The slightest air current could interfere with so slight a measurement.  I originally suggested that the electrical current fed to the drive device was generating heat which caused convectional air currents, moving the device on its pendulum. The paper seems to indicate thrust occurs instantly when the power is applied and drops immediately to zero when the power is cut off. That seems to suggest the device is not effected by self-generated convectional currents.(UPDATE: in February 2015 one of the Eagleworks team, Paul March, reported the tests have now been repeated in a vacuum obtaining measured thrusts of about 50 micronewtons, March says if they can obtained thrusts of  at least 100 micronewtons there will be attempt to replicate these results at NASA’s Glenn Research Center.)
  • The research team also tested a Cannae device designed to accept electrical power but not to function as thrust-generating unit. To make it inoperable it was manufactured without the slots its inventor believes to be essential for its operation.Yet the team measured a force generated from this device too! This non-functional device was not an experimental control, instead the researchers also tested an RF load with no functioning components -a resistor – and indeed measured zero thrust for that test. It is extremely odd that a device designed by its creator to be inoperable “works” just as well as “functional” devices.
  • The team suggest this is not actually reactionless propulsion (indicating that they know how outrageous this would be) but rather momentum is being transferred “via the quantum vacuum virtual plasma”. This sounds profoundly impressive but it is also scarily like Star Trek-style technobabble. To the best of my knowledge quantum mechanics predicts that all space is permeated by “sea” of virtual particles but I have never seen this described as “plasma” before. It is also intriguing that this hypothesis has absolutely no common ground with how Shawyer claims his EmDrive should work. Shawyer says an EmDrive must be accelerated by an external force while the measurement takes place- so according to him the NASA experimenters should not have seen a thrust from their stationary devices!
  • Harold White, a team member, has, shall I say, form in presenting his team’s research in a prematurely positive way.
  • Most damning in my opinion, is the reported “thrust to power ratio” of 5.3 micro-Newtons per watt. Say the device was not a closed cavity after all and instead just squirted out microwaves. As mentioned earlier, the microwave beam would actually act a rocket exhaust. You can calculate the thrust to power ratio of such a beam, it comes out as 3.3 nano-Newtons per watt . This very low efficiency is a consequence of physical law and is the best that can ever be achieved. Yet the NASA team claim to have observed an efficiency about 1500 times greater! This is seems impossible.

While I’m at it, can I also clarify some misconceptions about this technology:

  • It is not an anti-gravity device
  • It is not in any way based on the work of Nikola Tesla
  • It is not based on “flying saucer” technology
  • It doesn’t prove “Einstein was wrong” (on reflection, actually it does)
  • It has nothing to do with “Electric Universe” Theory (don’t even ask!)

I would love this to be real, as it would be the greatest step forward in space travel ever, sadly over the years I have seen so many such steps come, go and disappear without a trace. Once again I am sorry to throw cold water on so exciting a story but in short, the concept of reactionless propulsion is still as impossible as it has ever been. NASA has not overturned Newtonian dynamics.  A small-scale research project inside NASA has tested a device based on exotic, if not fringe, science, claimed to see anomalous results and placed these forward for scrutiny. Perhaps more research will show this to be nothing real or even verify these findings with exciting results. Let’s wait and see.

UPDATES: In July 2015 researchers in Germany reported further inconclusive tests on a EMdrive style device. Although some excited reports have claimed this proves the device’s validity, the authors claim their test do not confirm or refute this.

In 2016 Yang et al. apparently published a paper reporting that a greatly improved experimental setup failed to observe thrust from their devices.

In November 2016 White et al. published a paper Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum describing a test program with their devices in a vacuum, reporting 9 measurements of thrusts in the 30-119 micro-Newton range and proposing this effect to be a consequence of  pilot-wave theory, a unconventional interpretation of quantum effects. I am eager to see if these results can be replicated by other researchers.

(Article by Colin Johnston, Science Education Director)

(Last update 4 November 2015)