system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does NASA's EmDrive work and why is it controversial?

Edit: It's not NASA's.
gpt
Every space engine works by throwing stuff out the back.  
Rockets throw the burnt stuff out the back.  
Ion thrusters throw xenon or other gases out the back.   
But they all throw something out the back.   
   
The EM drive doesn't throw stuff out the back, but it can still make the space craft go forwards, even though it only goes forward a little bit.  
So it's either a small experimental error, or most of physics is wrong.    
If the EM drive works, then a lot of people are going to have to rework physics, if they aren't to busy building a probe that goes to Alpha Centuri or something else amazing.