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.

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...

Full story: https://www.popularmechanics.com/space/rockets/a24219132/darpa-emdrive/