EmDrive: the mysterious propulsion technology that seems to defy physics

Even NASA doesn't know why it works

In 2001, a researcher named Roger Shawyer created something he called the EmDrive. He described it as a new space propulsion technology that can create thrust without propellant. It uses microwaves and electricity and he claimed it could theoretically push spacecraft through space without using any rocket fuel. If it really works, it will revolutionise space travel. We could get to Mars in 10 weeks without propellant.

Although Shawyer created a working model, the EmDrive wasn’t accepted by the scientific community. For it to work the way Shawyer describes it, the EmDrive must defy our current understanding of physics. He wasn’t the first person to make physics-defying claims so it’s no surprise that physicists didn’t take him seriously. Shawyer and his creation probably would have faded into obscurity if it wasn’t for the fact that 4 independent labs, including NASA’s Advanced Propulsion Physics Laboratory, have recreated the drive and reported that it does create thrust. They just can’t explain why.

Bouncing microwaves

The EmDrive is an electromagnet propulsion device, though it’s also known as a radio frequency resonant cavity thruster. A magnetron creates microwaves, which are bounced around inside an enclosed cavity that’s tapered to be a big cone. It’s designed so that thrust will be generated in the direction of the narrow end of the cone.

Energy is required in the form of electricity to run the magnetron but that’s all. There’s no fuel added and nothing is kicked out of one end to make the drive move. It seems to generate thrust all by itself without propellants. Thrusters that use electromagnetic propulsion isn’t new and there have been experiments on ion thrusters since the 1960s. These earlier concepts are known as “low-propellant” as they can generate force with very little propellant but that doesn’t upset any physicists. The EmDrive literally claims to do the impossible.


The most likely explanation for the thrust created by the EmDrive is that there’s a measuring mistake. The thrust is very tiny so perhaps scientists are seeing something that’s not really there. But more and more labs are recreating the experiments and making their measurements more precise. NASA has even got the EmDrive working in a vacuum to show that the thrust has nothing to do with air pressure. Over 8 experiments in 4 labs somehow keep finding that the EmDrive can generate thrust without any propellent. According to NASA:

“Test results indicate that the RF resonant cavity thruster design, which is unique as an electric propulsion device, is producing a force that is not attributable to any classical electromagnetic phenomenon.”

It can’t be overstated how controversial the EmDrive is. It makes no sense in light of physics. Think back to your lessons on Newton’s laws. The reason the scientific community isn’t willing to accept that the EmDrive really works is that it defies the conservation of momentum.

Spacecraft can’t move forward unless propellant is fired backwards or the spacecraft is pushed by something else. Most of our ships have used rocket fuel to get them going, and futuristic solar sails will reflect particles from the solar winds. You don’t just get movement from nowhere… unless it’s the EmDrive. When New Scientist published an article about the drive and said it was plausible, there was a backlash from physicists claiming sensationalism. Physicist Paul Friedlander wrote:

“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.”

The consensus is clear: Shawyer’s theory and experiment are flights of fancy and should be ignored. However, the independent labs are all claiming that the drive is doing something. Over time, the results are starting to look less and less like errors and more like an actual phenomenon that needs to be explained. It’s very plausible that thrust really is being created but all the researchers have overlooked some force that could explain it without violating the conservation of momentum.

But what if? If we have a drive that is genuinely creating thrust with no propellant, how could that be happening?

What about physics?

A number of hypotheses have emerged that assume there really is thrust being created without an obvious propellant. Many of these hypotheses are as controversial as the EmDrive itself. For example, physicists predict there are some particles that can simply pop into existence now and then throughout the universe. It could be that particles appearing inside the drive are being pushed out the back and nobody is realising. Like most of the hypotheses, they aren’t exactly easy to prove and sound far-fetched.

News about the EmDrive has been buzzing again recently because a paper has been published with another controversial hypothesis but it’s one of the most interesting so far. The author, Dr Mike McCulloch of Plymouth University, claims that the drive’s thrust might mean we need a new theory of inertia and he has suggestions. His idea is controversial, but it’s also testable. That’s what makes it worth considering. Also, if true, it could explain the thrust without violating the conservation of momentum.

The hypothesis (MiHsC) seems counter-intuitive at first glance because it suggests that momentum increases as the thruster moves, but his model is a surprisingly good fit for the results and some other observations already made. His model involves inertia, which is about how massive objects resist being moved or accelerated to an extent. Imagine a planet hurtling through space at high speeds. It’s really difficult to stop it or even slow it down considerably or in other words it has a lot of inertia. Physicists have appreciated intertia for centuries but it isn’t clear why intertia actually exists. What causes it to happen?

Unruh radiation

McCulloch proposes that inertia can be explained by the Unruh radiation predicted by Einstein’s theory of general relativity. Basically, the faster you move, the warmer things get for you. McCulloch’s idea is that inertia is just Unruh waves putting pressure on accelerating objects.

We don’t notice this phenomenon at the everyday speeds we think of as normal, but at tiny accelerations the wavelengths of Unruh radiation are so long that they don’t even fit within the observable universe. The result? At low accelerations, inertia is forced to use only whole-level Unruh wavelength values.

The jumping from one whole wavelength value to another during low acceleration can explain an intriguing mystery: the Flyby Anomaly. When spacecraft accelerate around planets, they end up faster than we expect and nobody can figure out where they get the extra boost from. Bizarrely, the speed of spacecraft appears to increase in small “jumps”, which fits McCulloch’s model well. He thinks the unaccountable boost comes from radiation pressure from Unruh waves. The model has also been proposed to explain the rotation of galaxies without the need for dark matter.

McCulloch believes the Unruh radiation could also explain why the EmDrive works. The coned shape of the drive’s enclosed cavity means that Unruh waves are different sizes at the different ends of the cavity. If his ideas about inertia being caused by Unruh radiation are true, we should expect photons bouncing around inside to have their inertia changed, with stronger radiation pressure in one direction than the other. Because momentum is always conserved, which is what physicists have been saying all along, thrust is generated.

McCulloch’s model explains some anomalies and fits some experimental data but why should we care? Well, for starters it’s entirely testable. There are a number of ways to test his hypothesis and the most obvious is simply to redesign the EmDrive. McCulloch has suggested making the length of the EmDrive’s cavity the same as the diameter of the smaller end. If he’s right, Unruh radiation will fit better at the small instead instead and the thrust will be reversed. It’s a fairly straightforward test and hopefully we’ll know what happens soon.

If it works?

The obvious application of the EmDrive if it really works is in space travel, which would become fast and efficient. A journey from Earth to Mars could take just a few months and use no fuel. If the drive really does continue to accelerate, we could send spacecraft to other stars faster than we ever thought possible. Thrust with no propellant could even lead to transport technologies used on Earth. Think of all those flying cars in sci-fi films.

The problem is still the explanation. If you were in charge of a space organisation, would you spend millions to build and launch spacecraft with EmDrives if you don’t even know exactly how they work? There’s a very good chance that the drives don’t work and that people are mistaken. Indeed, that’s the consensus. If this will change when McCulloch’s hypothesis is tested remains to be seen but before we can travel to other stars without using fuel, we’ll need to know what the hell we’re actually doing.

Perhaps the EmDrive is doing nothing special. Or perhaps it’s breaking physics and we need to rewrite the textbooks. Or perhaps momentum is conserved still but there’s a form of radiation nobody considered that could explain the thrust and other anomalies associated with inertia. With so many labs interested in the drive, we’ll soon have an answer. Step one: let’s be really sure about these test results.

Physicists rightly still think the EmDrive is a load of nonsense, and it probably is. But wouldn’t it be nice to be wrong?

Main image © SPR Ltd.