Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott Cards Enders Game, where one character ages only eight years in space while 50 years pass on Earth. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox: an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster.

Time dilation goes back to Einsteins theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. The faster you move through the three dimensions that define physical space, the more slowly youre moving through the fourth dimension, timeat least relative to another object. Time is measured differently for the twin who moved through space and the twin who stayed on Earth. The clock in motion will tick more slowly than the clocks were watching on Earth. If youre able to travel near the speed of light, the effects are much more pronounced.

Unlike the Twin Paradox, time dilation isnt a thought experiment or a hypothetical conceptits real. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. The relative motion actually had a measurable impact and created a time difference between the two clocks. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay).

So in your question, an astronaut returning from a space journey at relativistic speeds (where the effects of relativity start to manifestgenerally at least one-tenth the speed of light) would, upon return, be younger than same-age friends and family who stayed on Earth. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so its not something we can readily answer. But if youre trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, youd have to be moving at close to light speed.

Theres another wrinkle here worth mentioning: time dilation as a result of gravitational effects. You might have seen Christopher Nolans movie Interstellar, where the close proximity of a black hole causes time on another planet to slow down tremendously (one hour on that planet is seven Earth years).

This form of time dilation is also real, and its because in Einsteins theory of general relativity, gravity can bend spacetime, and therefore time itself. The closer the clock is to the source of gravitation, the slower time passes; the farther away the clock is from gravity, the faster time will pass. (We can save the details of that explanation for a future Airlock.)

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How does time dilation affect aging during high-speed space travel? - MIT Technology Review