Thanks to Einstein, we have learned how to accomplish a certain kind of time travel: All you have to do is accelerate to near light speed or through a powerful gravitational field, and—for you—time will slow. Einstein called this Zeitdilatation: time dilation. It’s not as handy as a time machine; for one thing, there’s no going back. Instead, you could think of time dilation as an anti-aging technique. It leads to the so-called twins paradox thought experiment: The twin who zooms off at near-light speed returns home younger than the one who stays behind. In his 1956 novel Time for the Stars, Robert Heinlein put time dilation to work as a plot device employing literal twins, Tom and Pat. Humanity urgently needs to explore the stars. (By the way, it needs to do so because Earth is grievously overpopulated: It has grown to the gargantuan level of 5 billion inhabitants.) So off Tom goes on a spaceship while Pat stays behind.
When Tom returns, a 22-year-old by his reckoning, Pat is a codger of 89, because 71 Earth years have passed. “Let me look at you,” says Pat. “I knew intellectually that you would not have changed with the years. But to see it, to realize it, is quite another thing, eh? The Picture of Dorian Gray.” Tom then marries his great-grandniece, and they live relativistically ever after.
James Gleick, in Quartz, sobre Time for the Stars, de Robert Heinlein