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So I've been thinking about the idea of a series that takes place on a small fleet of generation ships, traveling at around 10% of lightspeed to Proxima Centauri, which should take around half a century. So the flaws here have been pretty well laid out in detail by Kim Stanley Robinson here, who also wrote the book Aurora with the same argument. While I think his problems have solutions, these solutions lead to another problem: Anyone who is capable of building a working generation ship has no reason to do so.

In order for generation ships to work, you essentially must have fusion power and nearly perfect self contained life support. You also need genetic engineering to fight evolutionary pressures as well as either transhumanism or medicine advanced enough to deal with damage from stellar radiation and the possibility of major ecological problems.

All of this basically gives you the ingredients for a post-scarcity utopia, which people have to then leave to go on these ships in which scarcity comes from the fact that you only have whatever you started with. On the upside this does solve another major problem, that we have no idea whether there are any habitable destinations, as we can simply live in space within the new solar system. On the downside, if you have all of these, you then have nearly infinite growing space within our own solar system and thus no real reason to leave in the first place.

This post scarcity context does at least solve one problem without any major downsides, it means that there is no real need for anyone back home to turn off the lasers that allow your such a ship to be a somewhat workable beam-rider. Alternatively it could also allow you to synthesize enough fuel even with insane mass ratios. There is also a really cool means to slow down, called the magnetic sail.

Anything I'm missing?

EDIT: One major point I failed to make fully is that when I say live in space, I mean to live in newly constructed habitats in the new solar system, not to remain on the ship itself. Also, I think the deeper problem is the lack of habitable destinations that we know of. Either the world will be uninhabitable, in which case it will need terraforming that could last centuries, or it will be habitable and almost certainly have something alive on it. Specifically a dead world would never have oxygen (like Mars, it would turn to rust). The better question then becomes what is the point of traveling to a new solar system just to live in space stations?

EDIT2: I finally corrected the travel time. My original mistake was using delta-v instead of speed, which would effectively cut the travel time in half using a variation on the flip and burn style popularized by The Expanse.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monty Wild Dec 13 '19 at 11:18
  • $\begingroup$ 10% lightspeed to Proxima would only take 43 years, not a century $\endgroup$ – qazwsx Jan 8 at 1:00

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I think there's two things missing. The first is that we do hard things all the time. Its standard practice for humanity to do things that were impossible a generation earlier.

I think the real key is motivation.

A society's motivation for generation ships is the same motivation a tree has to produce seeds. Why produce seeds when, to produce seeds, you need to be so successfully rooted in such a good place that you could just stay there forever.

Trees produce seeds. There's a reason.

The motivation lies in the long game. The tree "knows" that sooner or later something is going to happen. A lightning strike. A prolonged drought. A plague of beetles. Homo sapiens. The good life may look like post-scarcity, but a handful of individuals know the truth. As perfect as their fusion may be, one day, our star is going to go out. Its going to die, and we are going to be cold. Very cold.

The generation ship would not be a uniform sampling of society. That would be like a tree packing up some roots and leaves and bark and sending it out on the wind. The mother of thousands and mother of millions actually do pack up a whole plant, but they're the odd ball, not the norm. You would select a very very specific subset of society which is designed to germinate. Thus their social needs are very different than the whole.

In a post-scarcity world, this might not matter. But we often have this funny illusion that there's a discrete transition from scarcity to post-scarcity. I would argue there's litte reason to believe this to be true. You can get to almost post-scarcity by managing the expectations of the citizens. If a citizen never wants what they cannot have, its post-scarcity as far as they are concerned. But that may not be sustainable. You're not quite there yet. You're consuming resources too fast to be post-scarcity (and by "too fast" I mean "consuming finite resources at all").

So you bundle up a seed You bundle up a subset of society which is as close to post-scarcity as you can possibly manage. And then you send them out and hope they reach somewhere to germinate on. While your tech may not be sufficient to provide this post-scarcity illusion for the general population, the driven population on the ship can endure much more stringent requirements put on them. We have germinated 2000 year old palm seeds, even though no palm that we know of has lived 2000 years.

As for all of the difficulties in finding the right planet? That's another reason I recommend thinking in terms of seeds. A well rooted mesquite tree will produce literally hundreds of thousands of seeds in its lifetime. A valley full of them can produce so many seeds that, were they playing Powerball (with a jackpot odds of 1:292,201,338), it starts to look like a sure thing that they'll hit the jackpot.

Sending out a generation ship? Foolish and wasteful. Sending out a million generation ships? Now things start to get interesting.

"A society grows great when old men plant trees whose shade they know they shall never sit in."

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    $\begingroup$ We will get cold after the sun dies, but first we will get very, very hot. $\endgroup$ – user3067860 Dec 6 '19 at 18:50
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    $\begingroup$ If you have a lot of generation ships, OPs requirements get softer and softer if people are willing to accept the risks that not all ships will make it. $\endgroup$ – mao47 Dec 6 '19 at 19:24
  • $\begingroup$ @mao47 That seems like the interesting part to me... the psychology of people knowing that not only they may not make it, but their descendents may not make it. That makes for a neat study into the psychology of the human mind. $\endgroup$ – Cort Ammon - Reinstate Monica Dec 6 '19 at 19:58
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    $\begingroup$ @CortAmmon-ReinstateMonica : Unless of course their DNA, eggs & sperm are part of the package sent with the other colony ships, allows for greater genetic diversity post arrival which makes sense, then it's only their survival & that of their immediate direct descendents they put at risk rather than their entire germ line. $\endgroup$ – Pelinore Dec 6 '19 at 22:36
  • $\begingroup$ @Pelinore I was thinking of the moral weight of those lives, but that just shows my current world centric point of view. I wrote about how hard it is to nail down motivation, and then proceeded to apply my own motivation anyways! Your approach with spreading the eggs and sperm out is fascinating, and totally the kind of thing that belongs in a story about generation ships! My brain's already spinning with all the variants you could do on that one (what to do with a "mutinous germ line" is especially interesting) $\endgroup$ – Cort Ammon - Reinstate Monica Dec 7 '19 at 1:05
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Let's split this question to some parts that I deduced from your question

1. Why would anyone work on something that they will never be able to use?

First, there are people who are interested in things they can't actually use. For example, Ada Lovelace invented a programming language and wrote code for the Analytical Engine even though it was never built. Likewise there were algorithms for quantum computers (the best known is the Shor algorithm) that were developed before quantum computers existed. So people may build spaceships because they want their grandchildren to have a good life, knowing that they can not use those ships themselves.

Second, they may feel responsible for humankind. We only have one planet with humans on it. If something bad happens to earth, humans will go extinct. If we can colonize another planet, the danger of going extinct is much smaller.

2. Why would you land on a distant planet and try to colonize it, when you can live in an orbit around that planet?

Those ships may be well engineered but won't work for eternity. While travelling to the distant planet, the ship will be hit by some micrometeorites. Those damages must be repaired. The ship will lose some air because of the damages. The life support systems will need spare parts for repair. The fusion material that is powering the ship won't last forever. So you will at least have to harvest some material from the planet. Also some of the people will want to live on the planet. Why would you want to live in a cage that is flying around a planet when you can as well live on the planet and maybe have a whole country for your own? The ship will always be in danger of being hit by a meteorite or a failure of life support systems.

3. Why would you even travel to a distant star when you can stay in earth orbit?

A combination of questions 1 and 2. We need a backup for humankind. When Earth is so badly destroyed that humans can't live on it anymore, you may not be able to harvest new materials for spare parts or new fusion material. Also there are dangers that not only affect Earth itself but also the surrounding space or the whole solar system (gamma ray bursts, attack by a more powerful alien species, the sun becoming a red giant etc.). And of course it's nicer to live on a planet than in a spaceship

4. Are Generation Ships inherently implausible?

There are other reasons why generation ships may be implausible.

  • The life support systems could fail - all humans die
  • A breach in the hull - much air flows out - many humans die
  • The ship needs artificial gravitation by rotating it. If something breaks, the ship is ripped apart - all humans die
  • Some miscalculation for the radiation in space - all humans die
  • There's no habitable planet in the destination star system - all humans die
  • some people going crazy and destroying parts of the ship - all humans die
  • the artificial gravitation needs to be turned off for some reason - the humans degenerate and aren't able to land on the planet anymore - all humans die
  • some bad epidemic - all humans die

etc.

It may be far easier to build a machine that is able to breed a cryoconserved fertilized human egg on the surface of a distant planet than to build a ship that is able to transport living humans, animals and plants to that planet. Also be aware that you need more than just two humans to colonize a planet. Some scientists calculated that the absolute minimum is 98 non-related humans and a strict plan who gets children with whom to reduce incest related genetic defects to a level that long-term colonization is possible. So that ship will be pretty huge and expensive compared to an automated human breeder and AI-nanny ship.

Answers to comments:

I'll write the answers here, because the comments are limited to 600 letters.

You would be able to built quite a large number of rotating habitats, using just the mass of Mercury.

That's true, but I try to use only current or soon available technology in my assumptions. The ISS may be large enough to support enough arable land (in shelves) to build a biosphere for one person that could spend their entire life in space. The cost to launch all the ISS parts into orbit, is something around 150 billion dollars. Even if future technology can lower the cost by 99%, it will still be 1.5 billion dollars for each person. So you may be able to build an Elysium for the super rich people, but you won't be able to build space habitats for all humans, because it's just far too expensive.

Also, terraforming a planet so that it is actually earthlike is incredibly difficult and may not actually be possible

I'd assume, we will fly only to stars that are guaranteed to have at least one "rocks and water"-planet in their system, so we can dump some algae and plant seeds on it to produce enough oxygen, so humans can survive on it after a few decades. Or maybe we fly only to planets with a high oxygen level, so we can land there on arrival.

A note on rotating habitats; the rotating cylinders are unlikely to be a visibly rotating section (like Babylon 5 or similar). Instead the rotating section will likely be contained within a non-rotating shell.

That may be a good idea. Another idea that came to mind is that you don't even need a rotation because the main engine has a constant 1g acceleration. We would need that anyway if we wanted to go to Proxima Centauri within 100 years.

And of course it's nicer to live on a planet than in a spaceship It's nicer, when you have always lived on a planet. The third generation on a generation ship is likely to not agree with this statement.

I think it may be even opposite. For us it is new and exciting to live on a spaceship. If you were born on the ship, you may be more interested in exploring a planet. Just imagine you'd live in a single skyscraper your entire life.

"all humans die" section: the same list of arguments (with translation to analogous technologies) applies to the era of exploration by sailing ships. As we all know, that was wildly unsuccessful (he says, sarcastically)

There are some similarities but also many differences between interstellar travel and early intercontinental sailing. The sailors often had problems with rotten food, they didn't know if they'd arrive somewhere, they heard rumors of huge octopusses or whales or sirens that would kill them, they didn't exactly know if they would fall from the edge of the world etc. but they were still brave enough to sail away. And many of them died. Mostly because they starved or dehydrated or because the ship sunk or became unmanageable.

The problems with interstellar travel are different. The ships are far more expensive. Just think ahead of what I said about the ISS. Even with much cheaper spaceship construction options, you'd have to pay over a billion dollars to theoretically transport one person to a distant star - based on the construction of the ISS, which is hardly more than a balloon made from steel. You could easily punch a hole in its hull with a nail and hammer. For a ship that has a hull which is strong enough to withstand the radiation of the interstellar medium and the impacts of micrometeorites at 10% of the speed of light, you'd need a lot more money. The calculations regarding 98 persons to colonize a planet applies to the launch of the mission. When you arrive, that will be a lot more people. I can't find more detailed data of that study, but let's just assume 500 people on arrival. Then you will most likely also want some animals to be on the distant planet. For example cows, chicken, pigs, dogs, cats. All of those also need enough space and food for 500 of each species. So the calculation is like 6 species times 500 individuals times 15 billion dollars = 45 trillion dollars for one ship.

The cost may be too high for humankind to even build one ship. So if you can bring up the money, you need to be absolutely sure that the ship will arrive safely on its destination. The old sailing ships were expensive, but the people were still able to build several hundred ships that were capable of intercontinental travel. So you had enough tries to colonize a distant continent.

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    $\begingroup$ "And of course it's nicer to live on a planet than in a spaceship" This seems to be the major flaw in the arguments in the question. That and planets are generally much larger than spaceships so they can support a lot of people. Eventually the sun will engulf the earth. Not sure what kind of stewardship prevents that. $\endgroup$ – JimmyJames Dec 6 '19 at 18:04
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    $\begingroup$ Planets are incredibly inefficient in their use of mass. You would be able to built quite a large number of rotating habitats (large enough for many multiples of the current human population), using just the mass of Mercury. Also, terraforming a planet so that it is actually earthlike is incredibly difficult and may not actually be possible. $\endgroup$ – Michael Richardson Dec 6 '19 at 18:29
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    $\begingroup$ A note on rotating habitats; the rotating cylinders are unlikely to be a visibly rotating section (like Babylon 5 or similar). Instead the rotating section will likely be contained within a non-rotating shell. $\endgroup$ – Michael Richardson Dec 6 '19 at 18:32
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    $\begingroup$ In a time where we can travel to other stars, we'd also likely have the technology and artificial/non-biological intelligence needed to introduce safe artificial variations in DNA, and/or remove defects from inbreeding. $\endgroup$ – SmugDoodleBug Dec 6 '19 at 21:02
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    $\begingroup$ And of course it's nicer to live on a planet than in a spaceship It's nicer, when you have always lived on a planet. The third generation on a generation ship is likely to not agree with this statement. $\endgroup$ – tbrookside Dec 6 '19 at 22:22
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Flip the question around

Let us accept the premise that a generation ship is not viable without the technological ingredients for a post-scarcity utopia. Now add the assumption that this situation has been achieved without a massive (>80%) population crash on Earth.

What remains is a situation in which there are billions of people who can do almost anything they like. The one type of activity that there will be a shortage of is meaningful activity - it would take a relatively small percentage of the population to maintain essential services (with the aid of machine intelligences) and most of the population simply lacks the aptitude and/or ability to conduct serious scientific research (more on research later). Out of billions of people seeking something meaningful to do with their lives, I would be amazed if there were not a large number who dedicated themselves to spreading humanity through the universe. While it may seem a foreign concept to some people today, many people throughout history have dedicated themselves to projects that they knew they would never see completed.

Other motives for embarking on a generation ship voyage include seeking fame, seeking knowledge (how many astronomers would decline the chance to be the first observer on the other side of the Oort Cloud?) or trying to preserve a particular human culture in isolation. Those are just the first thoughts that spring into my mind, there are doubtless many, many more.

The question is not whether anyone would want to leave Earth and entrust themselves to the unknown - a small percentage of humankind has been doing that since we came out of the trees. The question is how many millions will be in that small percentage and what variety of motives they will have.

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    $\begingroup$ One good example of people throwing themselves at the (relatively) unknown where the native Polynesian navigators. The guys braved the pacific with vehicles that amounted to little more than wooden planks tied up by twine. And they had no idea if they weren't going to be swallowed by a storm or eaten by sharks (many were). Yet they braved the ocean to get to new islands so tiny and so far away that if you scale a map today to put both in the screen they don't even show up. $\endgroup$ – Mindwin Dec 6 '19 at 18:02
  • $\begingroup$ The Polynesians didn't have to worry about carrying their air with them. They had the possibility of adding to the food stores en route. They knew that if they didn't die they'd arrive somewhere well within their own lifetimes. $\endgroup$ – Keith Morrison Dec 6 '19 at 23:17
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    $\begingroup$ @KeithMorrison was with you til that last bit. How? How (other than guessing) would they have known the seas weren't endless water? I realize many ancient civilizations figured it out, but not many of them. $\endgroup$ – Jared Smith Dec 7 '19 at 0:12
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    $\begingroup$ @JaredSmith, I should have been clearer; when a Polynesian craft sent out, everyone on board expected to be alive and relatively the same age by the time they got somewhere (otherwise, why go?). That's not true of a generation ship, where pre-arrival generations know they'll never reach the ultimate destination. $\endgroup$ – Keith Morrison Dec 7 '19 at 20:36
  • $\begingroup$ I pretty much suspect that most of those brave Polynesian navigators set sails just because they had no other options. Your island being invaded by warlike neighbors, your side have lost, all the survivors will be inevitably found and murdered - you man your canoes and sail away. The chieftain was too harsh, now the people rebelled, killed him and going to also kill every single one of his relatives (including you) - you man your canoes and sail away. 99% that you'll die in the ocean, but there is a slight possibility that you'll be lucky enough to find some new place to settle. $\endgroup$ – rs232 Dec 9 '19 at 10:14
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It's basically a political issue. While people of your way of thinking might not use generation ships, others might.

Sepcifically...religious types. Pilgrim Fathers who need to get as far away as possible from other humans to set up their utopia. Cults who believe they have a divine mission to populate the universe. Those who believe there is a Promised World awaiting them at location X because their prophet saw it in a dream.

Or refugees from what they see as an oppressive government. Or criminals bent on escaping justice. Or megalomaniac tech billionaires who want to build their own space empire.

There could be a racial angle. Fascists who want to 'preserve the purity' of their particular race. A group with a grudge who want to protect their culture from being assimilated into the majority.

All of these things involve getting far away, and a generation ship might be the best way to do it.

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In order for generation ships to work, you essentially must have fusion power and nearly perfect self contained life support.

Not really. It just needs to be pretty good. You can plan for some level of waste, and include extra supplies. Assuming you lose 1% of your supplies per year, .99^100 = .366, so you just need to bring 3x as much stuff as is necessary to sustain life. That 1% is something I just pulled out of thin air, in the long term whisky distillers can hit around 3%. With wooden barrels. If your spaceship can't beat wood by a couple orders of magnitude, I'm not getting on it.

You also need genetic engineering to fight evolutionary pressures as well as either transhumanism or medicine advanced enough to deal with damage from stellar radiation and the possibility of major ecological problems.

Not much evolution happens over 100 years. Inbreeding could be an issue, but that's just a matter of having a large enough populaton and keeping good records. Radiation is an issue, you'll have to come up with a method of shielding, but that's an engineering issue, not fundamental physics. Thick enough walls is where I'd start, but I'm no rocket surgeon.

All of this basically gives you the ingredients for a post-scarcity utopia, which people have to then leave to go on these ships in which scarcity comes from the fact that you only have whatever you started with.

I don't think this is obvious at all. Even assuming you disagree with my previous two points, and think we necessarily can design a post-scarcity, free healthcare space hab. All we have to assume is that society can build this for some people. There are billions of people on Earth. They'd be working together to build this ship for tens to hundreds of thousands at most. Now, we need some motivation to go to another planet, but that's too plot specific. Maybe Earth is truly doomed long run and we need somewhere to save the human race long term. The space between self sufficient indefinitely and self sufficient for a couple hundred years is pretty big.

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    $\begingroup$ "With wooden barrels. If your spaceship can't beat wood..." That section was humorous, thank you for the laugh (and solidified your +1). However, after pondering that a moment I don't think that's such a good comparison/conclusion. Preserving some types of foods for decades has been possible for a long time even in wooden barrels. We really can't beat wood by a couple orders of magnitude. $\endgroup$ – Loduwijk Dec 6 '19 at 20:59
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    $\begingroup$ @Loduwijk Possibly true, although I imagine it is partially an issue of necessity. I'd be surprised if we couldn't make a seal much better than a wooden barrel with modern tech. The problem is, anything we've stored for a hundred years is by definition packaged with pre-modern tech. And we don't really try as hard as we could given the fast pace of modern society. If we want to be rigorous we could look at wine ullage and do some computations, glass is pretty good. I imagine we could only do a little better than wine bottles (surely we could make a better alternative to cork). $\endgroup$ – Zwuwdz Dec 6 '19 at 21:23
  • $\begingroup$ Are there reports of wines bottles "aged" in underwater shipwrecks? $\endgroup$ – Varad Mahashabde Dec 7 '19 at 12:52
  • $\begingroup$ There are astronomical events that could sterilize a star system, but almost none that can sterilize two different ones at the same time. That alone is a reason to spread out to multiple star systems. $\endgroup$ – John Dec 7 '19 at 16:40
  • $\begingroup$ @John Actually, won't a big supernova (from say a 20 solar mass star) have enough effect to sterilize megafauna up to 10-20 LY away from the point of origin, likely covering many star systems, as in our neighbourhood, stars are about 5 LY apart? $\endgroup$ – aphid Dec 9 '19 at 9:49
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What's missing is the ongoing march of technological progress. What's an insurmountable problem this year is next year's high school problem.

Whether through simple miniaturisation or wholesale new breakthroughs all these problems can be solved. All you have to accept is that it's not you or your children getting on these ships, but you could be doing the work that makes them possible for the later generations.

I remember the excitement when the first exoplanet was discovered, now they come thick and fast. So don't look at generation ships with our technology, look at them as a range of solvable problems and you might see them getting slowly closer.

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About the why, even in post-scarcity society: Philosophical disagreements and lack of meaning.

People will always disagree about the best way to live. One may even say that how to answer this question is what divide a society from another. Even in a post-scarcity society these divergences will exist. How should Man live? What is a correct life? What is Good and what is Evil?

I'd say that a post-scarcity society will exacerbate the divergences. If we go by Plato there are three main human temperaments: the worker, the warrior and the philosopher. There will be nothing left for the workers to do: the utopian economy provides everything. There will be nothing left for warriors to do: the resource wars are a thing of the past. Only the philosopher will be fine.

Those with a temperament to be workers or warriors will start mess things in the Utopia: many will become addicts to drugs, other will start causing trouble fighting in the streets for the pleasure of fighting, causing mayhem for the sake of mayhem. Also, the philosopher will keep concocting philosophies, ideologies, and many of these strange ideas will be quite dangerous for Utopia. All of them will be contesting the Utopia's way of life because it deprives them of meaning.

You have to get rid of these people regularly. But you don't want to kill them, that's inhuman and unnecessary. So you build these huge generation ships and send them far away, to places where Utopia does not exist, so that workers can work and the warriors can fight fulfilling battles and the dissatisfied philosophers can try to implement their strange ideas. The new worlds in distant stars provide the only thing that is scarce in a post-scarcity society: Meaning.

That many of these new colonies will self destruct is irrelevant. The dissatisfied have the right to die trying to fulfill their destinies and finding meaning.

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There is a good time slot for generation ships between achieving post scarcity and getting to star-lifting levels of engineering. It wouldn't matter whether you live in an artificial habitat here, there or drifting inbetween, but you still get the safety of not having all your eggs in one basket. And perhaps satisfy parts of our curiosity.

An additional factor to consider: generation ship implies biological life, at least for most purposes. That term might become obsolete if we redefine us before we depart.

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You are making one big assumption that may in fact not be warranted.

You assume that generational ships (or any high tech like that) are used only out of necessity. But that's often not the case in human history. In fact, the more prosperous you get, the more likely you are to do things just because you can.

Think about all the conquerors who traveled to different continents, often under challenging and potentially fatal conditions. They didn't really have to do that, they wanted to. The USA didn't really need to go to the moon, it was done basically to show that they could.

Similarly, your advanced civilization could opt to do things just because they're interesting, challenging, and opens up new frontiers.

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    $\begingroup$ "We choose to go to the moon and do the other things, not because they are easy, but because they are hard" $\endgroup$ – Ruadhan Dec 9 '19 at 13:55
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No difference from deep space habitat

In order to travel between star systems for that amount of time, the "generation ship" would need to be essentially a self-sustaining deep space habitat. Therefore, it needs factories and a university and farms and all the necessities of modern (to whatever time period) civilization. The fact that it has engines and is moving between star systems is relevant, but the key is that this habitat needs to be able to manufacture (given the raw materials) anything it needs, and recycle nearly all its wastes. Perhaps it needs to visit a star system every couple of hundred years to fuel up on whatever it uses (uranium for fission, hydrogen for fusion, exotics for something else) and stock up on metals and organics, but it should not need to drop anyone off if it doesn't need to, because there's no guarantee there will be somewhere for someone to live.

Part of its mission can definitely be colonization. Perhaps it keeps frozen embryos to decant when it reaches a new planet. Perhaps part of the process in visiting a new system is that it spends a couple generations creating a colony or a new traveling habitat just because it needs to get rid of surplus population. Colonization should not be its entire mission, though. It will be conducting research, engaging in innovation and keeping in (delayed) touch with the mother planet as long as a coherent signal can be maintained, or boosted from relays in deep space.

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I've been wondering for a while now why you would even have living humans on-board for most of the trip.

3D printing is making it's rise, and we've been printing simple bodyparts for a while now (https://www.smithsonianmag.com/science-nature/3d-printed-ears-grown-stem-cells-are-finally-on-their-way-180961605/)

The biggest problem with this type of printing is how to get the semi-random mini-bloodvessles throughout the bodyparts you are printing to keep it alive and questions on how we can create specific cells in the right positions to function. Once these are solved there is virtually nothing stopping us from printing entire humans.

So what you do is gather a DNA database, potentially you would even have the ability to have human DNA be computer-generated. You send this on a ship to the starsystem you want with a bunch of air and compressed dead biomatter. Once you get close to your destination you take the biomatter and start building living cells and with that print living and breathing humans (already adult-sized if need be so you can skip infancy). Do this at a distance of the planet where you have time to teach them what they need to know and make them the specialists you need. This way you only need the life support for the last leg of the journey and no super-special technology for cryogenics or long-term space-based survival strategies with limited resources etc.

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    $\begingroup$ On one level this is true, but this again adds a different problem. How do we educate a society of humans without any humans in the first place? Are we really going to trust an AI to do so? This seems somewhat likely to produce a scenario not unlike Horizon Zero Dawn, in which an uneducated group of humans is left in a word poorly managed by machines. $\endgroup$ – Adam Reynolds Dec 6 '19 at 8:50
  • $\begingroup$ First of all, I would absolutely trust an AI in education. Second, if you are 3d printing exact copies of people, they will retain their knowledge, they can educate their kids wherever the AI is inadequate. But unfortunately this answer doesn't address the question about plausibility of generation ships. $\endgroup$ – V. Sim Dec 6 '19 at 9:26
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If You Build It...

Why did cave men leave the safety of their home tribe to wander into the unknown? Isn't that foolish? For the individual cave man, it is. For the genetic inheritance of cave men, it's a prudent gamble (hence, the Polynesian explorers mentioned elsewhere). Humans have a wanderlust, and always have. Pretty much every living creature on the planet seeks out and exploits every niche it can sustainably inhabit. If humans are able to inhabit the extrasolar planetary niche, they will do so for no other reason than that THEY CAN. Not every human will do so, but you are virtually guaranteed that for every day between now and when a generation ship becomes available, you can ask the question: "Who wants to leave Earth forever and live in relative deprivation to explore the stars?" and thousands, possibly millions of hands will go up.

...Who Pays?

Ok, so we agree that there will always be someone who wants to go. But why would the rest of society expend the resources to do so? You could just hand-wave it away as the fact that your post-scarcity society is so filthy stinking rich that it can extravagantly expend the resources to build such a thing without batting an eyelid. But I find that somewhat dissatisfying. Let's put on our post-scarcity hat and think. One of the arguments for post-scarcity is that AI and automation will make production cheap and easy. Ok, I buy it. But that also means we have likely passed The Singularity, and are dealing with at least one super-intelligence. Why would that guy allow the resource expenditure to build one or more generation ships?

What does a super-intelligence want, anyway? I think it wants two things: First, it considers itself a living being, and thus, it wants to procreate like every other living being (because beings that reproduce out-compete those that don't, and so a super-intelligence can easily rationalize propagation as an optimal survival strategy). It could surely venture out into space by itself, but why do that when you can take a whole ecosystem of servan^H^H^Hcompanions with you? After all, humans represent the pinnacle of nanotechnology and provide extremely energy-efficient solutions to a wide range of problems.

Second, we just have to think about what that SI will be doing with all its time. It won't be babysitting humans, that's for sure. So it will be producing the one thing that is of any value to an SI: knowledge. And there is only so much knowledge which can be discovered and produced while sitting at home. If knowledge is the one thing in the universe that you value, then you are definitely going to expend resources to go find more of it. Who knows what you can discover? New biology? New planetary science? New stellar phenomena? The galaxy is your oyster.

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I want to work from the assumption that the challenges have been solved but theres no reason to do so.
In that case, generation ships are absolutely plausible.
People are not inherently logical. Therefore it is reasonable to assume that at least one person will participate in an activity even if the activity itself does not seem reasonable through a logical point of view. And therefore the possibility that generation ships will be used still exists. Even if technology exists that can propagate sentient life through out the stars without the transportation of living people. Someone with enough resources could still utilize generation ships simply under the impression that it is more "natural" or "cool".

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It is possible to build artificial space habitats with materials from small solar system objects like asteroids and comets.

So eventually there could be tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions, etc. of such space habitats with populations of thousands of persons each.

And of course adding an engine and fuel supply, etc., to such a space habitat would turn it into a vast generation ship.

So I can imagine that a fleet of several such generation ships might be sent to colonize some region in the outer cometary halo of our solar system. At a speed of one percent of the speed of light, it would take such a fleet 100 years to reach a part of the cometary halo 1 light year from the Sun. At a speed of 2 percent of the speed of light it would take such a fleet 50 years to make the journey, at a speed of 3 percent it would take 33.333 years, at a speed of 4 percent it would take 25 years, at a speed of 5 percent it would take 20 years.

If a fleet of several such generation ships is sent to colonize some region in the outer cometary halo of our solar system. At a speed of two percent of the speed of light, it would take such a fleet 100 years to reach a part of the cometary halo 2 light years from the Sun. At a speed of 3 percent of the speed of light it would take such a fleet 66.666 years to make the journey, at a speed of 4 percent it would take 50 years, at a speed of 5 percent it would take 40 years, at a speed of 6 percent it would take 33.333 years.

So possibly several fleets would be sent from the inner solar system to colonize various regions in the cometary halo. And after expanding in the cometary halo for a period, perhaps centuries, each such colony there might send out one or more fleets of generation ships to colonize another, and farther, region of the cometary halo.

The average distance between a star and its nearest neighbor is about five light years in our part of the galaxy. At a speed of about 1 percent to 10 percent of the speed of light, it would take about 50 to 500 years for a generation ship to travel straight from the inner solar system of one star to the inner solar system of the other star, and about 1,000,000 to 10,000,000 years to go from side to sie of the disc of the Milky Way Galaxy.. But if generation ships make voyages of 1 light year each to colonize regions of the cometary halos, and take 100 to 300 years to each the stage to send out long distance colonizing expeditions of their own, the colonizers could reach the inner solar system of a star 5 light years away in about 450 to 1,700 years.

And travelling at about 100 to 300 years between jumps, and jumps of 1 light year each at a speed of 1 percent to 10 percent of the speed of light, a society could expand in all directions at an average speed of about 110 to 400 years per light year of distance. Thus such a society could eventually colonize the entire galactic disc of the Milky way Galaxy, 100,000 light years in diameter, in about 11,000,000 to 40,000,000 years, if they started from an outer rim of the galactic disc.

If the inhabitants of such space habitats expect that they and their descendants will live in space habitats forever, and have no desire to land on any habitable planets they might possibly find, Generation ships made of such space habitats could make much longer distances.

Possibly some generation ship fleets might travel 10 light years in a single voyage, taking 100 to 1,000 years. And if such voyages are successful, some generation ship fleets might later travel 100 light years in a single voyage taking 1,000 to 10,000 years. And eventually generation ship fleets might make voyages of 50,000 light years, taking 500,000 to 5,000,000 years, reaching and beginning to colonize distant regions of the galaxy.

a copy of my post from: https://historum.com/threads/generation-or-sleeper-ships-which-would-be-the-better-more-realistic-option-for-space-travel.181701/#post-32230231

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In every sci-fi story I've read involving generation ships, the technology improves until they can send out a faster ship which of course then catches up with the generation ship. There's no reason why that won't happen IRL.

So a society shouldn't even bother building a generation ship in the first place.

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    $\begingroup$ They do that for plot reasons. You're assuming FTL is possible and that it's viable for colonization. Think of ships vs airplanes--we can go far faster than a ship but we can't haul huge volumes by air. $\endgroup$ – Loren Pechtel Dec 8 '19 at 3:21

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