The law of conservation of momentum says that a rocket (or anything else) can't accelerate forward without some form of exhaust ejected backward. But in 1998, a British engineer named Roger Shawyer announced the seemingly impossible—he had built a closed system that could generate thrust.

Twenty years later and many scientists still call the Shawyer's EmDrive impossible, but that hasn't stopped DARPA, the Defense Department agency that funds potential technological breakthroughs of all kinds, from putting serious money behind it.

An Irritating Anomaly

EmDrive concept SPR Limited

Here's how the EmDrive works. Imagine you have a truncated cone—a tube wider at one end than the other—made of copper. Seal it, then fill it with microwaves. Like other electromagnetic radiation, microwaves exert a tiny amount of pressure. But because of the shape of this device, they would exert slightly more force on one end than the other. So, even though it’s a closed system, the cone would experience a net thrust and, if you had enough microwaves, it would gradually accelerate.

Build it to a large enough scale and you could revolutionize propulsion.

But all of this should be theoretically impossible, hence the skepticism hurled by respectable physicists and SGU , a skeptic website that compared the idea behind the EmDrive to someone trying to move a car forward by pushing on the dashboard.

NASA Eagleworks’ EmDrive NASA

Undeterred by the fact that it would seem to be physically impossible, independent imitators testing the EmDrive theory have nonetheless reported small but measurable thrust from their own EmDrives. These include Chinese researchers at Northwestern Polytechnic in Xi’an, NASA’s Eagleworks , and the American company Cannae, which plans to launch a commercial version into space. A German team at Dresden is evaluating the EmDrive and will report next year, though early results suggest thrust measurements could instead be stray magnetic fields .

To the physics establishment, these reports of positive thrust are an irritating anomaly, the result of experimental error and wishful thinking. But about a decade ago, before China reported its results, as the idea of a propellant-less drive began to swirl, DARPA quietly got involved, according to Shawyer.

“DARPA attended the original 2008 EmDrive meeting at the Pentagon, chaired by Joe Rouge, the then director of the National Security Space Office,” Shawyer told Popular Mechanics. “I was then invited to a meeting with DARPA at their Arlington HQ to discuss an R&D program.”

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Jess Sponable, formerly a program manager at DARPA in charge of the XS-1 Spaceplane project , says that he maintained an interest in the EmDrive’s progress well before China. Although he did not fund any EmDrive programs, Sponable believes the interest in these findings is justified.

“Given the number and diversity of claims about EmDrive and other exotic physics, my opinion then and now is that DARPA should invest modest sums to experimentally assess such claims, albeit only where credible experimental evidence exists,” Sponable told Popular Mechanics.

"We must strive to beat the other guy to the punch line and ensure there will never again be another Sputnik moment."

This applies even where the underlying science is unclear or disputed, and especially if there is a risk that someone else, like China, might get there first, Sponable says.



“The DARPA mission is to embrace and advance transformational change in the U.S. military, but…we must strive to beat the other guy to the punch line and ensure there will never again be another Sputnik moment,” says Sponable. “If DARPA does not gather this evidence and publish the results, positive or negative, then who in the U.S. government will?”

More recently, Shawyer has been in discussion with Mike Fiddy, the manager behind the latest DARPA initiative, Nascent Light Matter Interactions , or NLM. This will explore new and little-understood phenomena, such as the apparent thrust generated by the EmDrive. Fiddy confirms that DARPA has previously funded work related to the EmDrive but says this is a fresh start.

“The NLM program is new and is focused on Nascent Light Matter interactions where ‘Light’ implies electromagnetic waves and not only visible light,” Fiddy told Popular Mechanics.

The Unfounded Physics of the EmDrive

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DARPA's $1.3 million contract includes developing theories to reconcile the EmDrive with known physics, and the basis of such a theory already exists. Enter Mike McCulloch, a lecturer in geomatics (the math of positioning in space) at the University of Plymouth, U.K.

“McCulloch's research will model and test the interaction of light with strongly resonant cavities, and it relies on a prediction from quantum theory that accelerating objects experience a thermal background known as Unruh radiation,” says Fiddy.



McCulloch and has already published over 20 papers on his theory of Quantized Inertia, or QI. It’s also known as Modified inertia by a Hubble-scale Casimir effect (MiHsC). This is a radical theory with wide-ranging implications that affects everything from galactic rotation to Dark Energy. McCulloch has already indicated how QI could reconcile the EmDrive with existing physics .

“It would be a game changer because if we understand the thrust effect then we can enhance it.”

“I am approaching it with a sense of opportunity,” McCulloch says. "It would be a game changer because if we understand the thrust effect then we can enhance it."

His QI theory has already been met with some resistance, as it challenges some widely accepted but unproven beliefs such as the existence of dark matter. But in science, facts are always king.

Tiny Thrust, Big Possibilities

Michael Stillwell

Rather than the tiny forces claimed by NASA—a few micro-Newtons, or the weight of a large ant—a properly engineered EmDrive could theoretically produce hundreds of milli-Newtons ( as claimed by Chinese scientists ), similar to the weight of a smartphone. That will make it easier to demonstrate that the thrust is not a measuring error or some other random effect.

Rather than microwaves, the experiments to validate McCulloch’s theory will use light with one experiment traveling in a loop and another with a laser bouncing off asymmetrical mirrors. Nobody has built this type of EmDrive before, but the inventor thinks it has some advantages.

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“There is no reason why EmDrive should not work at optical frequencies,” says Shawyer. “This approach would result in small EmDrive thrusters, with high specific thrust output.”

If successful, the technology could be quickly applied to station-keeping for satellites, keeping them in orbit for extended periods. McCulloch says it would cut the cost of space launches by a factor of at least ten. Instead of giant rockets and inefficient rocket boosters which waste energy lifting their fuel, spacecraft could have sleek, efficient, electrical EmDrives.

“It would make interplanetary travel easier and will make interstellar travel in a human lifetime possible for the first time,” says McCulloch.

But the doubters are still going to doubt, because that’s how science works. Unruh radiation, a key part of McCulloch’s work, is still just a theory, yet to be detected conclusively in the laboratory. As Rochester Institute of Technology astrophysicist Brian Koberlein has noted, the experimental evidence for the EmDrive is currently at the level of background noise. And, as he writes in Forbes , any theory supporting the EmDrive has a lot of work to do:

“The idea not only violates Newton’s third law of motion, it violates special relativity, general relativity, and Noether’s theorem. Since these are each well-tested theories that form the basis of countless other theories, their violation would completely overturn all of modern physics.”

McCulloch’s work will likely continue under scrutiny, with the smallest details setting off all kinds of intense debate among scientists who live and breathe this stuff. But DARPA has, for the moment, anyway, deemed the potential of a working EmDrive worthy of at least some further investigation.

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