Roger Shawyer, inventor of the controversial EM Drive. The drive works by firing microwaves around inside the metal cavity until they find their way out, due to tighter construction on the narrow end. As shown, the device in the test rig should appear to get a few grams heavier when the device is switched on.

We could go to Mars in just ten weeks if it worked. We could revolutionize space travel in our lifetimes. All we need is for the controversial EM Drive to actually work.

The EmDrive is the invention of British scientist Roger Shawyer, who in 1999 proposed the idea that thanks to the theory of special relativity, electricity converted into microwaves and fired within a closed cone-shaped cavity causes the microwave particles to exert more force in the direction of the flat surface at the large end of the cone (i.e. there is less combined particle momentum at the narrow end due to a reduction in group particle velocity), thereby generating thrust.

The problem? According to physics, EM Drive is impossible – and scientists around the world are having trouble explaining what they’re seeing, and what they’re seeing is thrust. Not a lot of thrust, mind you – in fact, the thrust various experimenters around the world have been able to produce.  A new paper has just been released, though, that tries to explain it.

How It’s Supposed to Work

The EM Drive is, in essence, a microwave oven in a can.  The claim is, you bounce microwaves back and forth inside a metal cone in order to create motion, which generates thrust towards the narrow end of the cone.

That seems simple enough, but there’s a wrinkle: the total momentum increases as the device begins to move – the thrust is nonlinear, and it shouldn’t be like that.  First, it’s a violation of the basic law of conservation of momentum,  because it doesn’t create exhaust. There are also a number of other possible causes for the thrust being measured, mostly involving either flaws in the measuring instruments, or physical or energy effects from the testing environment that nobody can be sure are being completely filtered out. The thrust is coming from somewhere, but nobody is completely sure where.

Isn’t Light Technically a Form of Exhaust?

This paper, published in AIP Advances,c laims that EM drive doesn’t defy Newton’s third law, because it does produce exhaust. Specifically, the researchers claim that exhaust is emitted in the form of light—photons that have become paired up with another out-of-phase photon, which then shoot out of the metal cone and produce thrust.

In the paper, physicist Patrick Grahn, Arto Annila, and Erkki Kolehmainen  think they’ve figured it out.

They start off noting that when two waves of light are out of phase, they can cancel out. This is how noise canceling headphones work: microphones pick up sound, a circuit inverts the wave form and produces out of phase sound, and the two sounds cancel each other out, making things quieter. You’re actually adding sound to remove sound.  The authors, in this case, are proposing the same idea: waves of light cancel each other out, and we know this works because recording these intersections of waves on film is how holograms work. Shining laser light through the photographic record reproduces the waveforms in space and we see them as though they were coming from the original object.

Arto Annila, clarified to ScienceAlert: “Light at microwave lengths is the fuel that’s being fed into the cavity … and the EM drive exhausts backwards paired photons. When two photons travel together, but having opposite phases, then the pair has no net electromagnetic field, and hence it will not reflect back from the metal walls, but goes through.” Ultimately, those escaping photons are supposed to be the equal and opposite reaction that’s producing the EM drive’s thrust.

Brian Koberlein, in his write-up for Forbes,  adds that such a technology rests upon the  vacuum: “A cavity designed like the EM Drive could then produce a reactive thrust by basically drawing momentum from the vacuum….It’s somewhat similar to the argument that the Unruh effect could account for the EM drive, but instead of relativistic virtual particles it’s a hidden sea of zero point photons.”

Wait, Zero Point Energy?

As is often the case, if it sounds too good to be true, it probably is, and there are some catches to this. Zero point energy is a mathematical concept, stating that while it is infinitely improbable for energy to spring out of a vacuum, coming from nothing at all, it’s not actually impossible – but the jury is still out on this too.

In the end, the idea is a hypothesis that is trying to explain something that, in all probability, doesn’t even exist.  As Koberlein notes, “It’s really just an idea that, if we reinterpret the whole notion of what a vacuum is, we can have EM Drive thrust without technically violating Newton’s third law.” The AIP paper is mostly just speculative, doesn’t actually prove anything concrete, and makes no testable predictions. Still, it does point us in the direction we might need to be looking in order to explain how the EM Drive works. Just because we don’t know something yet, doesn’t mean it isn’t knowable.

Where Does That Leave Us?

It leaves us with a conundrum. The thrust from the EM Drive is a repeatable phenomenon, and the original paper has passed peer review – moreover, there is a company founded in England made to exploit the abilities of the EM Drive, and they claim that they have a new version that produces stronger thrust. Until they produce a working model, though, we’re left with debates on not only the how, but the why of it. There are more than half a dozen individual factors that could be responsible for the thrust, or errors in measurement that create that illusion.

They’re obviously still testing it, but one thing is certain: if the EM Drive works as some believe, it’s going to transform the way we think about the universe around us.

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