The Lost Generation (Ship)

Elon Musk, who may himself be from the future, says that human being must become a multi-world species to guarantee our survival. Honestly, it seems we're already in a space race between our questionable capacity to venture off world and our likelihood of collapsing technological civilization. 

If we are to establish sustainable extrasolar colonies, we'll have to deal with the immense distances involved. One reliable trope of science fiction has been the generation ship, which copes with the distance by embracing it. For the hundreds of years that the trip demands, the ship encapsulates a multi-generational society that (its founders hope) will unpack itself into a new human civilization when its descendants arrive at their destination. 

But there's a problem: the generation ship dilemma. Unless you're facing imminent social collapse, leaving a little later will probably mean gaining the advantage of more sophisticated drive technology, making for a shorter trip. Any departing ship is likely to be passed during the trip by another that left years or even decades later. Its inhabitants will arrive and find, not a pristine world, but an established society with rules and norms that may differ extravagantly from the ones that evolved in their own ship in a bottle. 

This isn't by any means a novel idea - the magnificent, time-sucking TVTropes has an entire section of sci-fi stories that have employed the idea of "lightspeed leapfrog," in which generation ships arrive and find to their immense dismay that faster-than-light travel has been discovered during their lengthy voyage, and that everyone has been sitting around waiting for them to show up.