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The EmDrive has long been the bane of many a physicist.

This hypothetical thruster has courted controversy ever since it burst onto the public scene back in 2006. Its then-inventor, British engineer Roger Shawyer, claimed it could produce thrust without any propellant, or fuel.


Shawyer, now retired, previously worked on UK government defence contracts, primarily for BAE Systems. He is developing the EmDrive via his Satellite Propulsion Research firm.

What is the EmDrive theory?

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The theory goes that the thruster on the proposed EmDrive, shaped like a cone, can produce momentum by bouncing around electromagnetic waves in its interior. As these waves hit the larger end of the cone, the photons produce a tiny, tiny force that causes the small end to accelerate in the opposite direction.

Nasa validates 'impossible' space drive Space Nasa validates 'impossible' space drive


In particular, electricity is converted into microwaves, and the microwave photons are fired into the metal cone cavity. It is called EmDrive because it uses electromagnetic propulsion to drive it.

In theory, scaling the technology up would allow a spacecraft or other vehicle using this engine to reach incredible speeds without expending any fuel, needing only a power source – such as a nuclear generator or the Sun's rays. It's a modern day perpetual motion machine, in theory.

The EmDrive was recently thrust back into the news with the announcement that a peer-reviewed paper on how it works was supposedly leaked. This paper, written by scientists from Nasa's Eagleworks Laboratory known for experimental tests like this, has been published in the American Institute of Aeronautics and Astronautics (AIAA) Journal of Propulsion and Power.

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Why is the theory controversial?

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Alas, as with countless bold scientific claims before (see: cold fusion, faster-than-light neutrinos and so on), there are valid reasons to be sceptical about the EmDrive.

Firstly, it violates the laws of physics as we know them, namely Newton's third law that every action has an equal and opposite reaction. For this reason, it has been dubbed "impossible drive". Without any propellant being pushed out, there should be no momentum, breaking the conservation of momentum rule. Either the laws of physics as we know them are wrong, or the EmDrive theory isn't quite right. Occam's razor suggests it's probably the latter.

Secondly, if the drive is so revolutionary, why have we never seen any serious scientific discussion of it before? It was proposed a decade ago, and given its potentially groundbreaking ramifications, there would be some of the greatest minds working on it. Instead, we've had ten years of fringe speculation and “leaked” papers that have so far amounted to much ado about nothing.

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Finally, even if it does work as touted, it might not be all it's cracked up to be. In “tests”, the EmDrive was only able to produce a force of about 1.2 millinewtons per kilowatt. For comparison, ion drives on current spacecraft – which use a very low but steady amount of propellant to produce a tiny amount of thrust over a long period of time – come in at around 60 millinewtons per kilowatt.

The cause of the force is still unknown, and it may just be an experimental error. But if it's real, it's very small, and not enough for anything close to useful interplanetary travel, let alone “warp travel” that is often touted along with EmDrive theories or quick jaunts to Mars.

The peer-reviewed paper concluded the EmDrive thruster does produce a thrust within a vacuum. The Nasa Eagleworks Lab said that 1.2 millinewtons per kilowatt of thrust were produced. It was the first time peer-reviewed research of the Drive has been published.


The paper did not address why the drive appears to break Newton's law specifically but offered one suggestion: "[The] supporting physics model used to derive a force based on operating conditions in the test article can be categorised as a nonlocal hidden-variable theory, or pilot-wave theory for short."

The fact a paper has passed peer review isn't proof it works, however. It merely shows the claims made in the paper were ratified by independent experts and Shawyer's own experiments, in one form or another, may be reproducible but not in any meaningful way.

A real test for the Drive will come when it is tested in space. This will allow researchers to validate data produced in laboratories in a real-world environment.