Last summer, a British astronaut made the news when he referred to someone’s theory that unidentified flying objects (UFOs) in our skies may be future humans looking back at us. Tim Peake is a decorated test pilot and was the first European Space Agency (ESA) astronaut from the United Kingdom, with over half a year spent in space including on the International Space Station (ISS); he was answering the question with good humor and skepticism.

But it got me thinking.

Time travel is an iconic storytelling mechanism. It’s hard to say for sure what the very first time travel story was, but people point to H.G. Wells’ novel The Time Machine as the first description of, well, a time machine. Previously, stories like Rip Van Winkle suggested a telescoping of time, but by supernatural rather than mechanical means. In the century-plus since Wells’ influential novel, time-travel fiction has become almost its own entire genre.

See Also:

US Energy Department says Covid-19 likely resulted from lab leak

People travel through time in machines; they appear to do it without machines; they disappear into pocket dimensions or alternate realities where time is different. Sometimes, like in Stephen King’s story “The Jaunt,” someone disappears into a rift and reappears far older. In the all-time great episode “The Inner Light,” Star Trek’s Captain Picard spends a lifetime inside an anomaly. Time has passed for them normally, but their pocket dimension existed outside of regular time.

Read more: Popular Mechanics