It's impossible. But science appears to say it's happening. A prototype space engine seems to be producing energy from nothing. And it's just passed peer review.
The reactionless thruster was met with a surge of excitement - and scepticism - when reports of its science-fiction sounding ability to pull fuel out of the void of space itself emerged in 2001.
If true, we could be walking on the surface of Pluto much sooner than anyone dared dream.
Despite a bevy of tests, including this one by NASA's propulsion workshop (Eagleworks), the bold claim simply hasn't gone away. Now it's another step closer to credibility.
The formal study into the controversial, physics-defying EM Drive was formally published in the Institute of Aeronautics and Astronautics' Journal of Propulsion and Power late last week. This means the processes and measurements used in testing the performance of this 'impossible engine' could not be faulted by a panel of independent scientists.
It doesn't mean it's real, though.
It just means something's happening that isn't yet fully understood.
Dr Patrick Neumann, the Australian inventor of a highly efficient space ion engine which uses metal as its fuel, says the idea is exciting.
At best, he says, it could be an extraordinarily efficient means of powering our way to the stars.
At worst, there's an unrecognised, unwanted spanner in the works that everyone will think was obvious - in hindsight.
SOMETHING FROM NOTHING
The EM Drive is essentially a chamber with a hole at one end with microwaves bouncing around inside.
There is no fuel supplied.
But, when turned on, the drive generates thrust.
This shouldn't be happening.
Worse, it violates some of the most fundamental laws of physics that our understanding of the universe - and all of our technology - is built upon.
"It's looking stronger," Dr Neumann says. "But it's still breaking the law of conservation of momentum - which is something we'd really like to keep as it's the basis of some really important bits of science and technology."
What should make the engine impossible is Newton's Third Law: "For every action, there is an equal and opposite reaction". Put simply, something doesn't move unless an outside force pushes it.
This is why engines need fuel. The fuel contains energy.
The EM Drive carries no fuel. So there is no energy to release.
"If we have to modify it (Newton's Third Law), that's fine," Dr Neumann says. "But we can't just get rid of it."
Dr Neumann's own invention replaces extremely expensive and difficult to carry Xenon gas fuel for ion engines with simple metal rods. An elecric arc produces the reaction needed to drive the engine.
So, with the 'impossible' EM drive, the only way this could really work is if an as-yet unknown mysterious reaction is happening with the very substance of space itself.
Some say particles materialise in the vacuum of space as a result of Unruh radiation - where acceleration appears to heat up and compress space itself. These particles then react with the microwaves to form thrust.
"There's an explanation that involves a little bit of a wrinkle in Einstein's law of special relativity," Dr Neumann says. "Another involves an interpretation of quantum mechanics ... where there are no pure particles, it's all about probability waves.
"It's all at the extreme edge of physics - just like we were when, in 1896, Henri Becquerel noticed his photographic plates were getting exposed while tightly wrapped and locked in a drawer. He linked it to the uranium salts stored in the cupboard beneath, and discovered radiation."
EXTRAORDINARY CLAIMS NEED EXTRAORDINARY EVIDENCE
Aerospace engineer Roger Shawyer first designed the EM drive in 2001.
What he offered was an engine that does not have to carry its own fuel supply around with it. This means a spacecraft can be much lighter. It can fly forever. And, with the gentle pressure of its thrust constantly building, it could even reach the extremely high speeds needed to propel a ship to the outer reaches of our solar system within months.
The thrust generated by the engine is less than what a piece of paper exerts on the palm of your hand. At such a small scale, it's easy for errors in measurement to produce unrealistic results.
The effect could be as simple as uneven heating across the metal thruster chamber. This would cause the air around it to be repulsed at different rates - producing a tiny, but measurable, thrust.
Of course, this would not work in the vacuum of space.
Recent tests have attempted to eliminate such influences by running the engine in a near-vacuum.
That was the nature of this science journal report: The EM drive appears to produce 1.2 millinewtons per kilowatt of power in a vacuum. That's a similar figure to what was recorded earlier, when it was simply tested in the air of a lab.
But it's still a tiny figure: The ion engine that pushed the Dawn probe to the dwarf planet Ceres operated at about 60 millinewtons per kilowatt.
Now that the study has been published in a science journal, any and all experts in the field can pour over the methodology with a microscope - looking for any artefacts (external influences) that may explain where the impossible thrust is coming from.
Dr Neumann says he suspects one influence that may not have been accounted for is interactions with the Earth's magnetic field.
"But the section on accounting for artefacts is really well written," Dr Neumann says. "At the end of the day, though, the big test will be putting it in space."
That's the only way to eliminate all possible external influences and prove such thrust exists: put the engine in space, turn it on - and watch what happens.