Somehow, we all know how a warp drive works. You’re in your spaceship and you need to get to another star. So you press a button or flip a switch or pull a lever and your ship just goes fast. Like really fast. Faster than the speed of light. Fast enough that you can get to your next destination by the end of the next commercial break.

Warp drives are staples of science fiction. And in 1994, they became a part of science fact. That’s when Mexican physicist Miguel Alcubierre, who was inspired by Star Trek, decided to see if it was possible to build a warp drive. Not like actually build one with wrenches and pipes, but to see if it was even possible to be allowed to build a warp drive given our current knowledge of physics.

Physics is just a mathematical exploration of the natural universe, and the natural universe appears to play by certain rules. Certain actions are allowed, and other actions are not allowed. And the actions that are allowed have to proceed in a certain orderly fashion. Physics tries to capture all of those rules and express them in mathematical form. So Alcubierre wondered: does our knowledge of how nature works permit a warp drive or not?

Now, of course, our knowledge of nature is constantly changing and being updated, but what matters is right now, at this very moment, with this snapshot of our state of knowledge, can we do it? And the answer surprised him. It was yes. It’s not a firm resounding yes. It’s more like a soft hesitant yes, but a yes is still a yes, which opens up some intriguing possibilities.

And at first glance, it seems like it should be a hard no. I mean, we are talking about faster than light travel and we all know that you can’t travel faster than the speed of light. That’s baked into special relativity. And it isn’t just some speed limit posted on the highway. It’s a real hard and fast rule of the universe.

Don’t even try getting around it. We’ve been testing it for over a century, and it seems to be a result of the way that space and time weave into each other. It’s not just the speed of light. It’s the speed of causality. It’s how the universe orders itself from past to future and how causes lead to effects.

So just going really fast isn’t going to do the trick. But special relativity isn’t the end of the story. There’s a broader, more general version of relativity, which is general relativity, and general relativity says that you can never exceed the speed of light locally. That means that you can never measure yourself going faster than light, and nobody right next to you can ever see you whiz by going faster than light. If I look at the stuff that surrounds me, then by comparing myself to nearby objects I can never go faster than light.

But faraway things can get faster than light. This is how we are able to understand things like the expansion of the universe. Distant galaxies appear to recede away from us faster than light. In fact, any galaxy that’s further away than roughly 13.8 billion light-years is going to be moving away from us faster than the speed of light. And that’s no big deal.

Here we are in the Milky Way. We look around and say no, we’re not going faster than the speed of light. In fact, we’re barely moving at all.

Then hop over to one of those distant galaxies, and everyone living in that distant galaxy, they look around. Same thing. They’re not moving around at all.

We’re fine. We check our local environment, and we confirm we are not going faster than light. That far off galaxy checks its local environment and confirms that it’s not going faster than light, and we’re all good. But the space between us is expanding. And so even though neither of us is moving, we’re still getting separated faster than the speed of light.

This doesn’t break causality because that galaxy that’s really far away and receding away from us faster than the speed of light is just too far away. We can never affect it. What this means is that galaxy is beyond our influence, that we’re seeing the light that it emitted a long time ago, and we’re never going to see the light that it is emitting right now. So causality is preserved even though distant galaxies are receding faster than light.

And warp drives exploit this same loophole—never locally going faster than light.