EM Drive. Cannae Drive. Resonant-Cavity thrusters. Feed them microwaves and they produce thrust- without expelling exhaust. They’ve been called impossible. They aren’t- NASA’s first published paper of their results has passed peer-review. This means that other scientists have rigorously examined their experimental methodology and determined that they have produced valid results. Simply put it works. QED it is not impossible.
They have also been called a ‘reactionless thruster.’ They aren’t. If there were no reaction there would be no thrust. The fact that we don’t know what that reaction is doesn’t matter; we know there is a reaction because it works.
It has been said to violate the laws of physics. It doesn’t. If it violated the laws of physics it wouldn’t work. It’s just using laws of physics we are not yet familiar with or it is using known laws of physics in a novel way.
Of course they produce very, very tiny amounts of thrust. But in the last 7-8 years they have gone from producing micronewtons of thrust to millinewtons of thrust. Doesn’t sound like much, does it?
It is, though- that means performance has improved by an order of magnitude. That is a lot. Especially given that this is a brand-new technology that we don’t even understand yet. To some extent the thrust is scalable with input- give it more power, get more thrust.
I’m not a scientist, so I’m not going to get into the theories or post links to the papers. You can Google those easily enough. What I am going to talk about is the ramifications of the technology. To understand this we need to talk about spaceflight. Specifically interplanetary travel.
To get virtually anywhere interesting in the solar system takes fuel. To get there in a reasonable amount time takes a lot of fuel, and of course moving that fuel takes what? More fuel. Assuming that you are not suicidal you will need still more fuel to stop at your destination, then come home again. This means you need fuel to move the fuel to get there, stop, turn around and come back then stop again. That’s a lot of fuel. Using a chemical rocket to get to the nearest star system in mere 900 years would require a mass of fuel larger than the combined mass of everything in the universe. Going to Mars is a lot more reasonable, of course. Especially if you are not in a hurry.  With a reasonable amount of fuel you can get there in less than a year and still have enough to get home again… if you are willing to spend most of a year-and-a-half on Mars waiting for the planet to get relatively close to earth again
This is what makes the EM drive special. No fuel. Just deploy some solar-power arrays and away you go. Or carry a nuclear reactor if you can convince anyone to let you boost one into orbit. Yes, the nuclear material is fuel, but it lasts a long time for what you get from it. Mind you, you’ll still need fuel to lift off and land, but that amount is trivial compared to what you would need for the journey with a conventional rocket.
The thing about the EM drive is that while it produces relatively little thrust it produces it for a long, long time. That adds up to speed, and faster than you might think. Say you manage to get your EM drive to boost you at 1/100 of a gravity. After an hour you are only going 1152 mph. Seems kind slow. But if you accelerate at .01 G for ten days you will be going 276,480 mph. Thats somewhere around ten times faster than any human has gone before. Suddenly Mars doesn’t seem so awfully far away after all…
Of course it’s early days yet. The technology is in it’s infancy; sooner or later someone is going to figure out what is going on and how to capitalize on that to increase the efficiency. It is entirely possible that eventually these systems might produce a lot more than 1/100G. But even if they don’t, if they can even get .01G of  boost it opens the whole solar system to manned exploration and even colonization.
Yeah, I’m pretty danged excited about that.
In ‘Rage of Angels’ we encounter aliens with a fully developed version of this drive, and they use it to fly everything from fighter drones to lifting city-block size harvesters into orbit. Yeah, the tech may never get that far. Or it might. Early days, after all. Very, very exciting early days. Who knows what the next seven years will bring?