Spinning this off from the what to do with the dead bodies thread …
Potentially hundreds of thousands of years of a relatively small population with a hard top limit in a confined space.
Progeny have to be tightly regulated and selected. Eugenics is required. Compliance is mandatory. Creativity less so. (A recent sci fi show comes to mind not named for possible spoilers.)
Guided evolution will occur perhaps with unintended impacts. On a shorter time scale culture changes. Think of how humanity has changed culturally over even a thousand years let alone 100,000.
The few experiments done on earth to see how even a very small, completely isolated community can manage have failed spectacularly within a few years.
I would also suggest that even if a generation ship managed a 100,000 year voyage whatever came out the other end would be alien to what went in. It’d be something very different. Also, I doubt they’d be interested in disembarking at the destination. 100,000 years on their ship makes that their home and I doubt they’d have any interest in leaving it.
I’ve said before on these boards that the only reason humanity was able to thrive on Earth starting from their humble origins, is because of the large surplus of land relative to the population at the time. If two or more groups of people in a settlement get entangled in conflict, those groups could just up and leave, and settle somewhere else with arable land (for agriculturalists) or plenty of natural food sources (for hunter-gatherers).
On a generation ship, there will be no such luxury. Cooped up in a confined space with no escape from other people, I predict that the crew will rapidly start feuding and segregating themselves into factions within 20 years, ultimately destroying the community through civil war / sabotage of the generation ship.
If a drug that suppresses emotion like Prozium from the movie Equilibrium existed, then that could change the whole equation and make a confined community more viable. However, it should still require AI overlords to force the humans to take the drug at regular intervals, because as soon as one person misses their dose, the whole setup falls apart. And if AI overlords were feasible, then I’d assert that seed ships are a much better idea than generation ships because the embryos can make it to their destination without interpersonal conflict, and then be raised by AI with no innate human prejudices taught to them.
It will have been selected well before we ever get to the generation ship stage of development.
I made the point in the other thread that this project, almost of necessity, will require us to have a large and robust space infrastructure to even begin to build a ship with a decent chance of success. That means an actual space-based civilization, which will itself take hundred of years to develop.
By the time we start choosing colonists for the ship, we will have had centuries of experience living in enclosed habitats, and have developed personalities and cultures adapted to that. We’ll have seen what kinds of people make things work, and which kinds blow things up. We’ll have seen more than a few groups of Utopians, Libertarians, religious types, ascetics, hedonists, introverts and extroverts all trying their hand at developing The Perfect Society, and seen exactly how they failed, or maybe even succeeded.
Without that, we’ll never have a successful enough space civilization to build the generation ship in the first place.
A more interesting question to me is: What do we do if they decide to turn around and come back?
The OP is postulating a travel time of 100,000 years, which means they’re going much much less than even 1% of the speed of light. Even at 1% of c, it takes a full generation (30 years) to travel 0.3 light years. That’s well within reasonable communication range of the Solar System. Assuming they have fuel to slow down with when the arrive at their destination, and a decent reserve, what happens when the new generation says, “Screw this, I want to visit Paris!”?
I guess it depends on the propulsion system. If fuel is finite then, presumably, they have enough to accelerate and then decelerate at their destination (with some extra for maneuvering and some cushion for whatever). That means, once they are at cruising speed, there is no coming back. Best they can do is stop.
There is a science fiction story where the robot grows what is thought to be all the human eggs and sperm aboard. A younger guy is made, red, created, whatever about ten years later.
So all these folks begin killing each other and breaking things. They all die, and the robot tells the young kid his experience with human nature will hopefully guide him in stopping the next group from killing each other.
Dean Koontz has a novel where the spacers have been changed from people living on planets; hand like feet and stuff.
Anyway, one way to stop unhelpful evolution would be to store eggs and sperm, and not breed the crew. You’d have to change this some what for 100,000 years, of course.
As you said, it depends on the nature of the propulsion system. Fundamentally, all you really need is mass, and energy to throw the mass hard enough. Given they have a ship and resources sufficient to last 100K years, even with inevitable losses, they could decide to use the reserve masses as reaction mass.
Heck, I’d probably want the engines designed with this in mind, so as to allow maximum flexibility for when they arrive at the originally planned destination.
If you haven’t already check out the sci-fi TV series “The Expanse”. Humans have expanded through the solar system. Some humans are born and live in space. While they do not have hands for feet they have developed unique physiologies to better cope with living in space. It also makes them close to unable to survive on earth due to it’s comparably high gravity (being on earth is tortuous for them…can barely stand, hard to breathe, etc.).
I think they’ll likely evolve into something we can barely recognize, let alone categorize according to our current understanding of cultures.
One the one hand, this culture will be very small. Even a large ship with 100,000 living colonists is no more than a small city here on Earth. So in one sense, every individual will be that much more valuable. Maintaining genetic diversity as well as continuity of critical skills across generations make it likely that any society too violent or oppressive will shoot itself in the foot sooner or later.
But on the other hand, everyone is far more dependent upon everyone else. “The needs of the many outweigh the needs of the few”, and all that. Someone who is too individualized, too selfish, or too greedy can mess things up for everybody with comparatively little effort.
Have we even had a society like that? Every individual being uniquely valuable and cherished in their own right, but also expected to consider the needs of the community on at least as important a basis as their own needs?
Store a bunch of human genomes on a computer, and re-assemble the human DNA at the end of the trip. We’re very close to being able to do this already, if we haven’t already.
We are still needing novelty detection, danger response, seeing movement, and perhaps more in all directions than we do now, thinking of space less predominantly as scanning a horizon and not being stuck in arbitrary orientations or vertical and horizontal.
This comes up with having prehensile tails too, or extra fingers. You’ve got to take away from something else to power its sensation and control.