Star Wars needs a warp drive.
Even in a galaxy a long, long time ago, time matters. More specifically, if you are planning to fight a galactic empire, you’ll need to traverse space quickly, and therefore time becomes a resource.
According to Einstein’s theory of relativity, when you move really, really fast, time slows down for you. It’s called time dilation, and it means that even if you are moving at lightspeed, a trip to Cloudcity ages the universe around you to the point where twins born at the same time have had a different number of birthdays.
According to calculations in a paper from the Journal of Interdisciplinary Science Topics — published by geeky physics students at the University of Leicester — time dilation means that the Skywalker twins cannot possibly be the same age anymore.
The paper gives an example from The Empire Strikes Back:
“Consider the separate journeys that both twins make to Cloud City. Leia travels from the neighbouring system of Anoat, while Luke travels from the much more distant planet Dagobah. Luke’s journey was ~7 days travel in his own reference frame, which was estimated to be 25 times longer than Leia’s, making her journey 0.28 days (6.72 hours) in her own reference frame.”
If Leia, in the Millennium Falcon, is traveling at 99.999% the speed of light* and Luke is traveling in an X-wing at 99.995% (the small difference matters a lot), then during Leia’s 7-hour trip the galaxy ages 62.6 days, and during Luke’s 7-day trip outside observers age 701 days (nearly 2 years). This means that when the twins first see each other on Lando’s floating palace, Luke is now 638 days younger than his twin. Oh, and that also means Darth Vader was waiting at that dinner table for a long, long time.
Relativity extends to everyone in the Star Wars universe, and the twins’ trip to Cloudcity is just one of many relativistic journeys. Each time our heroes must traverse the ‘verse time will act funny. It even gets to the point where if Han Solo did just one Kessel run, he would be chronologically older than Obi Wan Kenobi when he meets him in A New Hope — because science.
*In the Star Wars franchise we hear that ships can go “0.5 past lightspeed,” but there is only so much physics you can break (without a Star Trek-style warp drive). As far as we know, the speed of light is the universe’s speed limit.