Nothing can travel faster than light. And if you manage to travel close to the speed of light, things get weird.

Technically, we all experience time differently depending on how fast or slow we’re moving. But the differences are so imperceptible as to be non-existent. That’s how we can describe events as being “simultaneous”, even though according to Einstein’s theory of relativity, there’s no such thing.

It’s thanks to these small relativistic effects that GPS works. But when you approach the speed of light—or get close to something very massive—then the large-scale relativistic effects kick in.

If you travel close to the speed of light for a short time, it will seem like a much longer time to everyone you left behind. This is the twin paradox, which isn’t really a parodox at all, just time dilation in action.

There are some coincidental parallels to this kind of time dilation in old folk tales.

The Japanese tale of Urashima Tarō tells of a fisherman who rescues a sea turtle and is rewarded with a relaxing few days in an underwater kingdom, only to find that when he returns home to his village, 300 years have passed.

The Irish tale of Oisín describes the warrior’s journey to Tir na nÓg, the land of youth. He spends three years there but when he returns to Éire to see his old fighting comrades from the Fianna, 300 years have passed.

This story gives us a wonderfully poetic turn of phrase that’s still used today. The closest English equivalent is “Billy no mates”, a rather cruel term to describe someone with no friends. In Irish, we say:

Mar Oisín i ndiadh na Fianna

Like Oisín after the Fianna.