The EM Drive is the most important advance in space propulsion since rocket fuel itself — just so long as it isn’t a big, fat mistake.

It’s being hailed as a next-generation electric space thruster that requires no fuel, but its apparent ability to generate thrust has defied scientific explanation. The question of whether the EM Drive is a huge step forward for science, or simply a refresher course in the importance of taking careful measurements, has vexed NASA engineers. Last year they announced thrust readings that could not be falsified by any means they devised, but in their own paper they went on to actively disown the results. Now, a physicist from Plymouth University may have figured out an explanation for the EM Drive’s stubborn refusal to sit still: with a whole new theory of inertia, we could explain both the EM Drive’s anomalous thrust and a long-standing mystery in physics.

Repeated testing, with multiple versions of the EM Drive built by multiple independent sources, have all failed to prove that it is not generating the thrust reported by prior tests. Against all odds, the EM Drive’s abilities are seem to be holding up to scrutiny, and thus seemingly in contravention of the law that every action must have an equal and opposite reaction. EM Drive inventor Roger Shawyer thought he could get around this by invoking a process called vacuum polarization, arguing that the system takes transient particles that appear spontaneously in space, turns them into a plasma, and expels them out the back. If true, this would mean that the EM Drive really doesn’t break the laws of physics but, unfortunately, it doesn’t seem to be the explanation.

This theory could equally fix the EM Drive’s problem with Newton’s Third Law by positing a whole new theory of inertia. Relativity predicts something called the Unruh effect, in which any accelerating body should observe an amount of extra heat relative to its acceleration. Put differently, the faster you accelerate, the hotter the universe should look; wave a thermometer in absolute zero, and in principle its movement should cause it to observe a temperature very, very, very slightly above absolute zero, especially if you can wave it at relativistic speeds.

The new study‘s argument relies on a further idea called Unruh radiation, which refers to the unconfirmed idea that the observation of this heated universe will stimulate the release of real particles — in other words, particles from the pure vacuum of space, not unlike our vacuum polarization particles. In the vast majority of cases, this theory predicts the results we’re used to seeing in the world around us, same as the classical theory of inertia. But its predictions diverge from tradition in one area: extremely small accelerations, or, about the level of acceleration (perhaps) observed in the EM Drive.

The idea is that, since the wavelength of Unruh radiation would increase as acceleration decreases, for extremely small accelerations a body should be experiencing Unruh radiation with a wavelength longer than the observable universe. With this being the case, inertia may only take on whole-wavelength units over time. Behaving in this way is to become “quantized,” to exist only in some multiple of an indivisible unit of measure (“a quanta”). So, at very low accelerations, inertia jumps from tiny magnitude to slightly less tiny magnitude without going through all the intervening values we would expect.

Evidence for this theory may predate the EM Drive. Scientists have long observed a phenomenon called the Flyby Anomaly, in which spacecraft performing a flyby of Earth will move noticeably and reliably faster than we calculate they ought to. The study’s author claims that this new theory of inertia could explain this effect, and produce more accurate inertial predictions that better reflect our observations. On the other hand, it’s not like this is the first theory that could explain the Flyby Anomaly, and most of the others don’t have to posit whole new theories of inertia to do it. Occam’s Razor would have us assume the simplest explanation — but Occam’s Razor is just a guideline, and certainly wrong from time to time.

In the context of the EM Drive, this new inertial effect would cause thrust inside the EM Drive’s truncated cone section. Different wavelengths of Unruh radiation will be allowed at either end of the cone, due to the change in diameter. This means that as particles bounce back and forth inside the cone, their inertia would have to change as well. According to our good friend the law of conservation of momentum, this means the particles will have to generate thrust — that is, thrust without the need to bring fuel.

To us, this sounds far-fetched. It’s arguing that the acceleration caused by the EM Drive is a product of…the EM Drive’s own acceleration? Inertia is associated with the release of particles which can be manipulated to produce inertia. Now, we still have to input energy in the form of electricity, so it’s not quite a perpetual motion scheme — but it’s not far off. If confirmed, this is the sort of insight that could give us hover-cars, or with a cheap source of abundant clean power, hover-castles, too. This is perpetual thrust for nothing more than the cost of electricity.

It’s important to note that the Unruh radiation theorized to cause this behavior has not been confirmed to exist, so in principle we have a hypothesis built on a hypothesis — not the strongest of footing. However, it does make some testable claims — and according to the paper, they’ve managed to predict the observed acceleration in the EM Drive to within an order of magnitude, in every case.

More exciting is the fact that they have concrete ideas for future work. One idea is that, if the cone shape is truly causing the observed thrust by providing an asymmetrical environment for the Unruh radiation, then flipping the cone also ought to flip the direction of the thrust. In other words, it should be trivially easy to produce a backwards EM Drive with only the cone direction changed, and thus should produce the same amount of thrust as before, but in the opposite direction. Another prediction is that putting a dielectric in the reaction chamber should increase the drive’s thrust output, which is both an easily testable idea and exciting, if true.

The fact is, the EM Drive is still a big mystery. Its readings are now far better supported than they were at this time last year, but that’s not to say that they have been accepted by a majority of the scientific community. Any eventual explanation for this seemingly impossible behavior will, almost by definition, be an astonishing change to basic physical theory — all we really need to do is hope that the thrust readings hold up to testing in space and elsewhere.