One of the greatest hurdles for long duration space flight is the weight cost of fuel in order to sustain maneuverability and impart acceleration over time. The Ion Drive used in the Dawn spacecraft revolutionized lightweight propulsion systems using tiny amounts of thrust compounded over time (e.g. 0-60mph in 4 days).

There’s another engine design out there that has puzzled scientists and engineers alike for nearly a decade. It’s called the EmDrive and also uses tiny, compounded forces to create thrust. The weird part about the EmDrive (and the reason it could change everything for spaceflight) is that it requires no propellant. Designed by aerospace engineer Roger Shawyer, the device uses a specially shaped cone (a resonating cavity) into which microwaves are pushed via a magnetron. Obviously this completely lacks a chemical reaction as per normal thrusters and relies on the microwaves imparting tiny forces against the cone. Physicists the world over cried foul as this seems to defy the conservation of momentum laws. However, despite the counter-textbook response, the experiment has been repeated and demonstrated to produce a thrust by notable labs to include NASA. Despite the successful lab tests, nobody could explain why it worked.

Scientists may have finally come across an answer based on a new theory of inertia leveraging quantum physics. Mike McCulloch at Plymouth University found that as accelerations grew smaller and smaller, they approach a point at which changes to inertia (momentum) do not follow a linear path but jump at discrete intervals. This effect has been observed in spacecraft before and isn’t just theoretical. McCulloch attributes the imparting of quantized momentum changes to Unruh Radiation (pdf). The explanation relies on two assumptions that he figures many theorists still won’t like, 1) that photons have inertial mass and 2) that the speed of light must change within the cavity.

Regardless of whether the explanation holds up, the EmDrive has proven itself time and time again over the past 15 years to produce a reactionless thrust.