The EM Drive is highly controversial. Many say it’s impossible that such a drive could work but testing so far says that it does. This experimental evidence is usually dismissed as error or an unintended side effect that would not produce thrust.
Why is this drive so controversial? Because it uses no propellant. This makes it seem like it violates Newton’s third law of motion. If you throw nothing out the back, there can be no momentum pushing the vehicle forward.
The very fact that it needs no mass to chuck out the back is exactly why this system is so exciting. Instead of giant fuel tanks all you need is to generate electricity. It’s usually the propellant mass that runs out long before the ability to generate electrical power is exhausted. With this drive and some solar panels you’d practically have an infinite (infinite over time that is) amount of thrust available.
There’s a new explanation of what might be happening with these experiments that involves pilot wave theory. Although discounted for a long time in favor of the Copenhagen interpretation, it has been gaining popularity in physics circles lately.
Amazingly this sounds to me exactly like the “aether drive” like the ones used in Space 1889.
There is a dark side to this. Because the drive can produce thrust for very long periods of time, they could be used to push relativistic kill vehicles. A relativistic kill vehicle is basically, a very fast moving mass that can be pointed at a planet or some other target. As a vehicle reaches something like 20% of the speed of light, it has accumulated a really large amount of energy.
How large? Planet killing large. No death star required. Just a rock with a nuclear reactor and an EM Drive attached. Give it a few light years to get it up to speed and a few dozen years to make the trip.
The really scary part is that if you’re traveling at a significant fraction of the speed of light, it might be nearly impossible to see the weapon coming.
So a working EM drive is exciting, but also terrifying.