Scientists try to explain why the trip home always seems shorter

Whether you're traveling by plane, train or automobile, you've probably experienced the sensation that getting back from your destination seemed like it took less time than going there. And that feeling exists regardless of where you've been or why you went in the first place — it doesn't really matter whether it was a business trip, a vacation or even an Apollo mission. After all, Astronaut Alan Bean, the lunar module pilot on Apollo 12, said the trip back to Earth "seemed much shorter" too.

Previous studies have suggested that "the return trip effect" causes us to perceive the trip home to be 17% to 22% shorter than traveling in the other direction. In a recent study published in the journal PLOS ONE, researchers at Japan's Kyoto University learned that people can experience the return trip effect even when they're just watching a video of someone else's travels.

Ryosuke Ozawa and his colleagues recruited 20 men between the ages of 20 and 30, separated them into two groups and "sent" them on a trip that just involved sitting in a dark room watching one of two 26-minute movies (the run-time was not important, but it was exactly the same for both films). One group watched the cameraman walk round-trip from Point S to Point E and back, while the other saw two unconnected one-way trips. Both walks that were shown covered a distance of 1.7 kilometers.

Both groups did more or less an equal job of perceiving how time passed during the films (they were asked to make a note every time they believed three minutes had passed) but only the round-trip group "estimated that the second trip took less time than the first trip."

So what causes that? Um, they're still not sure. The researchers noted that the participants only perceived the return trip as shorter when they were asked about it after viewing the movies, not while they watched them. Ozawa and his colleagues suggested that the participants "experienced" – and that would be in air quotes if they explained it out loud – the round trip effect because they were told that they were taking a round trip beforehand. As Joseph Stromberg explained on Vox, "the explicit awareness that it was a round trip may have altered their retrospective judgment of the passage of time."

Ozawa noted that if the same study were conducted in the future, he would like to do it without tipping the participants off that they were seeing a round trip, because "this semantic labeling" might induce a bias that we already have – even if we're unaware of it – that a round trip is shorter. Even if you're on your way back from the moon.