Here’s why the trip to Tau Ceti takes 12 years but the return is only four in Project Hail Mary

If you watched Project Hail Mary and paid attention to the travel times, you might’ve noticed that the trip to Tau Ceti takes over a decade, so why does the return only take about four or five years?

It sounds like a mistake at first. But actually, there’s a proper scientific explanation behind it, and even the author Andy Weir has addressed it.

So, in the story, Tau Ceti is around 12 light-years away from Earth. That means, from Earth’s point of view, even travelling close to the speed of light, the journey should take well over a decade. And that’s exactly what happens on the way there.

But then things get a bit weird on the way back. According to Weir, if Ryan Gosling‘s character, Ryland Grace, decided to return to Earth, it wouldn’t feel nearly as long for him. “It would be between four and five years,” he told Entertainment Weekly, talking about the journey from Grace’s perspective.

So how does that make any sense?

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This is where relativity comes in. Basically, when you travel extremely fast, like close to the speed of light, time doesn’t pass the same way for you as it does for people back on Earth. From Earth’s point of view, the trip still takes something like 11 to 12 years. But for the person on the spaceship, time actually slows down. So, to Grace, the journey feels much shorter.

But that’s not all. There’s another strange effect going on too. The distance itself sort of “shrinks” from his point of view. So instead of travelling the full 12 light-years, it actually feels like a much shorter distance.

Interestingly, one person on Reddit summed it up in a really simple way. They wrote, “Relativity. It’s really freaky. From Grace’s point of view, it would only take about five years, because it would seem to him that the distance to the Sun is a lot less than 12 light-years. From Earth’s point of view, it’d take well over a decade for the Hail Mary to travel back from Tau Ceti, and if we had good enough telescopes to see Grace moving around through the window, he’d appear to be in slow motion.”

From Earth, everything is normal, long distance, long time. But from Grace’s perspective, both time and distance are kind of compressed.

So, the trip to Tau Ceti takes around 12 to 15 years for people on Earth. The trip back is still long for Earth. But for Grace, it only feels like about four or five years. In other words, both things are true at the same time.

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