You’ve probably heard about that crazy EM Drive by now. You know, the one that converts electrical power directly into thrust without going through any sort of intermediate chemo-physical reaction. That one.

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.

Roger Shawyer, inventor of the controversial EM Drive. The drive works by firing microwaves around inside the metal cavity. The flat plate at the big end gives them something more substantial to push against than the rest of the cavity, so thrust is produced that way. As shown, the device in the test rig should appear to get a few grams heavier when the device is switched on.

Roger Shawyer, inventor of the controversial EM Drive. The drive works by firing microwaves around inside the metal cavity. The larger end allows the microwaves to apply more thrust in the direction of the flat plate than against the walls of the rest of the cavity, so thrust is produced that way. As shown, the device in the test rig should appear to get a few grams lighter when the device is switched on. This is sort of like mounting a fan on a boat and blowing wind into your own sail. It shouldn’t work, but it does.

Shawyer’s criticss say that according to the law of conservation of momentum, his theory cannot work as in order for a thruster to gain momentum in one direction, a propellant must be expelled in the opposite direction, and the EmDrive is a closed system.  That’s the big problem – according to Newton’s third law, everything must have an equal and opposite reaction, which means that something needs to be pushed out the back of propulsion system for it to move forwards. Since the EM drive doesn’t use any fuel propellants, it doesn’t have an exhaust, so it can’t produce thrust. This argument, however, is kind of like physicists arguing over why a honeybee can’t possibly fly, even though it obviously does – because the EM Drive seems to ignore what physicists think and produces thrust anyway.

The potential solution appears to be photons. The physicists at the COMSOL company, the University of Helsinki, and the University of Jyväskylä in Finland think they know what’s really going on.

According to their new peer-reviewed study published in AIP Advances, the EM drive doesn’t actually defy Newton’s third law, because it does produce exhaust.  Apparently the exhaust is actually light, or more specifically, photons that have become paired up with another out-of-phase photon in order to shoot out of the metal cavity and produce thrust. Because they have to pair up to escape the fuel cavity, the wave forms  cancel each other out and they have no net magnetic field. So, they don’t register on a magnetometer, and they’re not visible either, yet they’re roaring with kinetic energy. The EM Drive just converts energy directly into an invisible, undetectable exhaust – undetectable except for the thrust it produces.

“The EM drive operates by the same principle, for example, as a jet engine, where the high speed exhaust gases backwards (opposite reaction) push the airplane forwards,” one of the researchers Arto Annila, told ScienceAlert over email.

“Light at microwave lengths is the fuel that’s being fed into the cavity … and the EM drive exhausts backwards paired photons,” he says. “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.”

An independent scientist has confirmed that a paper by scientists at the Nasa Eagleworks Laboratories on achieving thrust using highly controversial space propulsion technology EmDrive has passed peer review, and will soon be published by the American Institute of Aeronautics and Astronautics (AIAA). Dr José Rodal posted on the Nasa Spaceflight forum – in a now-deleted comment – that the new paper by scientists at the NASA Eagleworks Laboratories will be entitled “Measurement of Impulsive Thrust from a Closed Radio Frequency Cavity in Vacuum” and is authored by “Harold White, Paul March, Lawrence, Vera, Sylvester, Brady and Bailey”. Rodal also revealed that the paper will be published in the AIAA Journal of Propulsion and Power, a prominent journal published by the AIAA, which is one of the world’s largest technical societies dedicated to aerospace innovations.

There is also a line of text that EmDrive enthusiasts believe could be from the paper’s abstract, which reads: “Thrust data in mode shape TM212 at less than 8106Torr environment, from forward, reverse and null tests suggests that the system is consistently performing with a thrust to power ratio of 1.2 +/- 0.1 mN/Kw ()”. This doesn’t sound like a great power conversion ratio, we’re talking micro-Newtons to Kilowatts here.

And that’s the last we’re going to hear of it for a while. Shawyer is now working with a private aerospace firm in the UK to develop the next generation version of the EM Drive.

“We’re trying to achieve thrust levels that go up by many orders of magnitude, where the q values of the cavities are between 1 x 109 and 5 x 104. Once you reach the levels of thrust we anticipate we will reach, you can apply it anywhere,” he told IBTimes UK.   “The work we’re doing is difficult and expensive and the people paying for it don’t necessarily want to give it away to the rest of the world, but EmDrive will make a huge impact and a lot of people have thought of a lot of things to apply it to.

The question of whether it works or not is essentially put to rest. It does. The new open question is “What can it be used for?”  The idea of being able to launch spacecraft without having to allocate all that weight and engineering to handle material propellant systems is a transformative one. If this works the way we think it will, nothing we imagined about space travel for the next hundred years will be the same.

And of course, the other question: “When will it be ready?”