system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Leaving aside the "nobody-know-why-it-works" reason, why is so innovative the EM Drive compared with others like Ion Thrusters, Plasm, Solar Sails...?

What is the difference if all of these methods already exist and can provide continuous acceleration anyway?
gpt
The top comment is not quite correct. The EM Drive does require fuel - the battery is the fuel. What it doesn't require is **reaction mass**. 

Every force has an equal and opposite reaction. In other words, in order to move you have to push against something. In space that means if you want to go in one direction you have to throw something in the opposite direction.

People get confused because for rockets the fuel is also used as the reaction mass. Other forms of propulsion separate the two. If an astronaut on a space walk had a bag of peanuts and began to throw them in a straight line away from their centre of mass, they'd begin to drift backwards. Their muscle would be the fuel/energy source and the peanuts would be the reaction mass.

An interesting consequence of this is that in space a weapon is virtually indistinguishable from an engine. The requirement of an engine in space is to push something away from your ship with as much energy as possible, which just so happens to also be the main aim of most weapons! This is something scifi always forgets. If you have an engine capable of getting you close to the speed of light, you also have an apocalyptically powerful weapon of mass destruction. All you need to do is point your butt at any planet which offends you and hit the gas.