Why FTL is Time Travel

FTL - View of Stars

Science Fiction is rife with FTL (faster-than-light) travel and communication, But what that often overlooks is how that automatically allows time travel. Let’s have a look.

The speed of light

I wrote about the speed of light before. That’s because the speed of light is both fascinating science, and central to science fiction.

To understand the rest of this post, there are two important things to know: the speed of light is the maximum speed of our universe , and it is constant, regardless of your frame of reference.

To summarize, nothing can go faster than light, because that would stop time and cost infinite energy. This is a consequence of the same rule of physics that says that regardless of how fast you travel, light will always travel at the speed of light for you.

Phew, that was a mouthful. Let me explain that again with an example. Imagine you travel at half the speed of light away from the sun. If you measured the speed of the light coming from the sun, it would still be the same as if you were stationary. You’d think the light from the sun would take more time to reach you, but it doesn’t, because time flows differently for you.

Keep that in mind for what comes next.

An FTL communication example

So, imagine we manage to set up a colony on Alpha Centauri, four light years away. We’d have ships that can travel at a significant fraction of the speed of light. Unfortunately, that means they still take years to cross the distance. Then, one day, we also invent an FTL radio. Whoop, whoop, we can talk to our colony in real-time!

That doesn’t sound so problematic, does it?

Now let’s add the spaceship. Talking is nice and all, but we still need to send goods and services from Earth to our colony. Well, at least we can install one of our fancy new FTL radios and talk to each other on the way.

And that’s where causality starts to break down.

Let’s do a thought experiment: the ship is well under way at near light speed, and we set up a little FTL message chain. Earth says a word to Alpha Centauri over the FTL radio. Alpha Centauri tells it to our spaceship, and our spaceship relays it back to Earth:

Earth -> Alpha Centauri: “Word”

Alpha Centauri -> Spaceship: “Word”

Spaceship -> Earth: “Word”

Cool, right. Stupidly simple experiment. Unfortunately, that’s not what would happen…

Frame of Reference

Since Earth and Alpha Centauri move at roughly the same speed, they are more or less in the same frame of reference light-speed wise. However, our spaceship is traveling from one to the other at near light speed. That means time is contracting according to the Lorentz factor.

From the points of view of our spaceship, time on earth appears to be going slower than on Alpha Centauri. And that means from their point of view the following happens:

Alpha Centauri -> Spaceship: “Word”

Earth -> Alpha Centauri: “Word”

Wait, what? From their point of view, they receive the message before Earth sends it? Weird. But… but… okay, yeah, they see it later, but of course it actually happened the other way around. You can see lightning before you hear it too. Doesn’t matter, right?

You have to realize though, this is not just some trick of the light. For the ship, the messages are actually sent out of chronological order because time itself flows differently on the ship as opposed to Earth and Alpha Centauri. And one frame of reference is no more valid than any other. And, this out-of-order stuff gets worse, because the ship sends the message to earth too….

You see, when the ship sends the message to Earth, it does so when it receives it, so before it’s sent (see above). It actually arrives back on Earth before Earth sends it out. Oh shit, time travel. And we just created a grandfather paradox!


I won’t bore you with the math about this problem, also because I’d probably mess it up. If you want, you can calculate it yourself using the correct formulas and the theory of relativity.

Regardless, this is a problem in science fiction most writers gloss over. In part because it would complicate many scifi stories, but also because it’s hard to get it right. Unless you’re an astrophysicist, and then you might still mess it up.

An interesting question remains, though: does this mean faster-than-light travel just isn’t possible in our universe? Or does it mean that time travel is possible?

The word’s still out on this, as far as I know. Maybe we’ll find out some day. But hopefully not before it actually happens.

Martin Stellinga Written by:

I'm a science fiction and fantasy writer from the Netherlands