If there’s one thing guaranteed to ruffle a few feathers, it’s surely the EM Drive. Billed in some corners as the holy grail of deep space travel, others (including us) have been quick to calm expectations about what is still very much a theoretical concept at the moment.

A quick recap for those out of the loop: The EM Drive is a thruster that seems to be able to create momentum seemingly from nowhere. By bouncing microwaves inside a truncated cone, the thrusters appear to produce momentum, but no one seems to know where it’s coming from.

But now, in a new study, a researcher claims to have solved a partial puzzle of the EM Drive, reports MIT Technology Review. Mike McCulloch, from Plymouth University in the U.K., says it may be related to something called the flyby anomaly. This is a mysterious speed boost that some spacecraft have received when they fly past Earth. His paper is available on arXiv. 

McCulloch’s explanation stems from something called Unruh radiation. As predicted by general relativity, this is the idea that the universe around you warms up (obviously a very tiny amount) as you accelerate. This heat, known as black body radiation, then pushes back against the accelerating object, producing tiny shifts in momentum. McCulloch suggests this can explain inertia – the resistance objects experience when they change velocity.

So, what does this have to do with the EM Drive? Well, at very low accelerations, it’s thought that inertia becomes "quantized" – and it may be responsible for small jumps in momentum. This could partially explain the flyby anomaly, when spacecraft like NASA’s NEAR and ESA’s Rosetta experienced tiny speed boosts – a few millimeters a second – from an unknown source when they flew past Earth.

Of course, this is very much just a theory for now. But McCulloch says it can account for both the flyby anomaly and the momentum experienced by the EM Drive. Further experiments could bring us closer to an answer.

^{Some spacecraft experience a weird speed boost when they fly past Earth. NASA/JPL-Caltech}

"This proposal predicts the observations quite well, but makes two controversial assumptions," McCulloch notes in his paper. "For example that the inertial mass of photons is finite... and that the speed of the light is changing in the cavity. So it is important to suggest a definite test."

If the EM Drive is producing thrust without fuel, this would obviously be hugely important for spaceflight. It would mean we could far more easily travel to distant locations (although certainly not at "warp speeds," as quoted by some). As you might remember from your physics lessons, though, this breaks the law of conservation of momentum – hence the controversy.

But, interestingly, this propellantless momentum effect does actually seem to occur, even in repeated experiments around the world. This may just be an error in measurements, or something else unexpected. A few studies have ruled out what could be causing the momentum, but no one is quite sure yet.

So, whether this new theory is correct or not remains to be seen. Certainly, the EM Drive deserves some further attention. But don’t go holding out hope for some futuristic form of propulsion any time soon – there’s a long way to go before we even get close to understanding what’s going on.