The Multi-Generational Thingie, Continued
Alternatively, you can consider using religion as your general purpose societal adhesive. Unfortunately these sorts of stories tend to be written mostly by lazy writers with poor research skills and only a dim understanding of the workings of actual religions, who focus on the suffocating, oppressive, punitive, and claustrophobic aspects and tend to cast their heroes and heroines as the lone iconoclasts who discover that The Priests Are Lying And They Alone Know The Truth; e.g., "For The World is Hollow and I Have Touched the Sky". But even the Amish, who most people accept as having about as religiously closed a community as can be, do not have a truly closed society, as five minutes' cursory research suffices to prove.
Then there's the problem of over-adaptation. Once they've spent a few generations adapting to life on-board the ship, how do you get 'em off the bluddy thing at the end of the journey? A sufficiently clever, evil, and cynical mission designer might specify that the ship essentially self-destructs at the end of the journey, thus forcing the passengers to disembark. But given enough time, any such destruction mechanism can be disarmed, or more interestingly, diverted to other purposes. Maybe there's a story in that. "Five centuries ago, their ancestors were sent to Proximi Centauri. Now they're back — and boy, are they pissed."
What about reaction mass? Assuming the ship was accelerated up to some worthwhile fraction of c as it left Earth, you'd need nearly as much fuel to decelerate it at the end of the trip. Phil Jennings and I played intellectual hacky-sack with this one for a while but never came up with a answer we agreed on. Maybe the ship doesn't slow and the crew never disembarks? Maybe it just keeps on going, seeding every potentially habitable planet it comes across with human-like colonists sufficiently genetically modified to survive under local conditions? James Blish worked this idea over sixty years ago, and Ursula Le Guin forty years ago, but it strikes me that from the viewpoint of another species, this might constitute an act of war. "AIIEEE! There's a terrible giant mystery ship passing through our solar system and it's seeding our planet with hideous alien monsters!" Maybe there's a story in that. Or at least a script treatment...
Hmm. Hideous monsters. At any significant fraction of c, hitting pretty much any dust mote or stray sub-atomic particle would trigger a spatter of ionizing radiation. Assuming your ship has something resembling a front end, it would need some awesome shielding there to protect the inhabitants, but even so the accumulated exposure to heavy radiation over the course of several generations would produce — well, most likely a plague of cancers that exterminates the crew, but let's be kinder and imagine mutations instead. For a while I toyed with that idea: what if the multi-generation crew is expendable, and the real colonists are all in some sort of cryostasis in a heavily shielded cargo hold? There are many stories that could spring from this. What if the colonists are recognizably human children, shipped as frozen embryos and being raised on the colony world by loving but hideously deformed monsters? What if two competing colonies and cultures get established: the planned colony of "perfect" humans and the unplanned colony created by the surviving monstrous descendants of the ship's crew? What if the crew tumbles onto the fact that they are considered expendable, and start to view the frozen colonists as a source of transplantable body parts to maintain their cancerous, malformed, and increasingly cybernetically augmented bodies? Or better yet, what if they start to view the colonists as just so much frozen food?
Yeah, there are some great stories that could be spun out of those ideas.
But ultimately the idea for the story I got closest to starting to write was remarkably similar to the one that, quite independently, Henry came up with. What if someone invents a religion, for the sole purpose of getting control over lots of very affluent but otherwise very stupid people? What if someone is so convinced of the rightness of his apocalyptic vision that he uses the wealth of his followers to build an Ark in Space, to send the descendants of the Chosen Ones to another world? But the gimmick is, it's all a con, as the inner circle knows the technology to send a ship across interstellar distances doesn't really exist, and so the real plan is that the ship will just take a leisurely one- or two-century-long excursion around the solar system and then return to Earth, where it is expected that things will have settled down again and the Earthbound survivors, if any, will treat the returning ship's passengers as gods.
Except, of course, that when they return to Earth (truly believing that they are in fact arriving at an alien but strangely parallel planet in another star system — they're otnay ootay rightbay, after all), they discover that the predicted apocalypse has not happened. And so, earnestly believing themselves to be enlightened star voyagers, they plunge headlong into this "new" society, sanctimoniously determined to prevent it from repeating the same mistakes that destroyed Old Earth!
There. That is the story that I liked.
The place where I got bogged down was the religion. I didn't want to use a real religion; I have no desire to draw the attention of either litigious and affluent a-holes or the sort of people who slit the throats of infidels. So I figured this would have to be a nonsense religion, of the sort that could only possibly appeal to people with great gobs of money, enormous egos, and very tiny brains. I figured I'd make this religion one started as a joke by some 1940s musician of modest talent, in which followers gathered in "listening rooms," put on headphones and listened to recordings of Big Bill Broonzy and Bessie Smith, and meditated (at affordable hourly rates) on the profound spiritual implications of the color blue. I was thinking of calling this religion, "Cyantodigy."
And that's when the whole thing fell apart...