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The setting: Very distant future, Earth is long gone/forgotten/just not around anymore. Humanity, whatever it may consist of (people, AI, something in-between) inhabits a single vast generation ship that roams the stars with no fixed destination, using material from the star systems it arrives at to refuel, expand, and do whatever else. They don't stay though, once they're done in a system they just pack up and leave.

Not allowing FTL travel in this universe would be my first guess, but I feel that even then it would be possible to at least retain casual contact between systems up to a couple dozen lightyears away, on a "let's sort it out before the colonists leave" basis.

My question is why might this be the way a civilisation exists as opposed to maintaining an interstellar group of colonies?

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    $\begingroup$ How many people are on the ship? One great reason to not have people spread across many colonies is that there simply aren't enough people to run them. $\endgroup$ – HDE 226868 Jul 18 '16 at 14:21
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    $\begingroup$ "And lo, the prophecy was written: thy shall not make a home under a single star". No one even remembers when the third testament was written, but religion's a big deal aboard ship. $\endgroup$ – FraserOfSmeg Jul 18 '16 at 15:18
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    $\begingroup$ Planets are too easy to destroy. Simply push a large asteroid into a collision course with the colony, and it is toast. $\endgroup$ – doug65536 Jul 19 '16 at 10:12
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    $\begingroup$ @doug65536 +1 for Planets are too easy to destroy. Oh Worldbuilding, I love you. $\endgroup$ – ktyldev Jul 19 '16 at 10:48
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    $\begingroup$ Landing is hard $\endgroup$ – Ewan Jul 19 '16 at 22:21

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In the savannah of the universe, stars are the watering holes needed to sustain life, but full of predators and dangerous.

Colonies are fixed targets, perhaps there are some hostile alien entities around most stars that are hostile to other forms of life (or all life). Note: in the far future these entities could easily be of terrestrial origin, out of control AI nanotech or other similar hazards.

The danger in one star system can be mitigated/defended against for short visits, but will attract larger swarming attacks from nearby stars, almost like an immune response. Assuming no FTL these attack groups from other stars would take decades to get to the current star system, making it safe for the short term, but sticking around for longer extremely dangerous.

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    $\begingroup$ While the basic premise may be correct (predators exist around waterholes), the usual evolutionary response is to move in vast herds rather than as a single individual. I would expect a swarm of colonies, much like a flock of birds or school of fish (more appropriate to the 3D environment of space). xkcd.com/1377 $\endgroup$ – Thucydides Jul 18 '16 at 14:41
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    $\begingroup$ @Thucydides While the xkcd comic is much appreciated and topical, I'm not sure you can apply the herd logic to the layout and number of colonies. After all, flocks of birds and schools of fish are mobile and comprised of individuals, whereas a 'herd' of colonies would be comprised of several independent and stationary settlements with multiple people. Unless you're suggesting a swarm of smaller-than-huge ships is more likely than a single super duper big one? $\endgroup$ – ktyldev Jul 18 '16 at 14:48
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    $\begingroup$ @monodokimes I think a fleet of ships makes more sense. One huge ship is a single point of failure. If there is a critical malfunction on one of a million ships, then there's a loss, but it's not catastrophic. But a critical failure on your megaship could threaten all of humanity. $\endgroup$ – Kys Jul 18 '16 at 15:13
  • $\begingroup$ A large fleet of ships is exactly what I am suggesting. There is nothing inherent in the description of a space colony to say it is stationary, indeed space colonies will either have to be mobile to gather resources or have a large fleet of ships to bring them resources and export trade goods. $\endgroup$ – Thucydides Jul 18 '16 at 17:24
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    $\begingroup$ @Thucydides Indeed, it should be distributed throughout whatever asteroid belt(s) the system has, with a few missions to some moons that have promising amounts of ice that could be harvested without having to waste much fuel getting out of their gravity wells (we're probably talking about building catapults or space elevators there anyway). A solar system is a big place, and there will be work all over. $\endgroup$ – Monty Harder Jul 18 '16 at 21:59
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Why shouldn't they live on that vessel?

As long as it isn't more dangerous or less comfortable than living on a planet, why bother to settle on a planet? Depending on how long the journey with this ship took, the people in it where born in the ship. It's their home. That's their way of life.

And being mobile has some advantages. The sun in your system reaches the end of its life? No problem: just move on. Asteroids crashing on the planets in your system? Not your problem: you just move out of the way.

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    $\begingroup$ Have you tried blue? It's the new red! $\endgroup$ – Casey Kuball Jul 18 '16 at 18:47
  • $\begingroup$ Sorry, but I don't get it. Could you help me? $\endgroup$ – lokimidgard Jul 18 '16 at 20:41
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    $\begingroup$ WALL-E reference, where people are living in the ship. $\endgroup$ – Martheen Jul 18 '16 at 21:46
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    $\begingroup$ It's a reference to WALL-E, mainly because someone on earth tells the colony to continue living in the ship indefinitely, because the earth is uninhabitable. I believe you're giving @monodokimes directive A-113! :) $\endgroup$ – Casey Kuball Jul 18 '16 at 23:13
  • $\begingroup$ Some crazy settlers might try to colonize some planet - but if the harvesting for the mother ship is thorough, there will be no inhabitable planet left once they are done and leave the system. - If you don't want to desolate all planets you pass, it could also be that some small groups of a few dozen people are anti-technical and settle on some planet living in nature. But they will not develop to a space-faring colony, so are irrelevant. Or there are sinister forces on board, which make accidents happen, so these settlers don't live for long... $\endgroup$ – Falco Jul 19 '16 at 12:51
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The ship started out as a generational ship to colonize some far-away planet. However, when approaching that planet, nobody wanted to be a colonist. They lived on the ship, their parents lived on the ship, their grandparents lived on the ship. Indeed, some even doubted the historical record and claimed that no one ever had lived on the surface of a planet, that it was far to dangerous to try, with all that uncontrolled weather, volcanism, and so on. And even assuming that Earth existed (nobody alive remembered any communication from Earth anyway), what could the people there do about it if the colonists simply didn't colonize, but keep on the ship?

However they did face one problem: Population growth. But they didn't have the facilities to build a new ship (after all, the ship was not intended for that; the necessary facilities would have been built on the planet after colonization), so they decided to just add some sections to the existing ship. They kept orbiting the destination sun, as it provided them with energy, and harvested asteroids as those didn't have the gravity well cost.

As the ship grew, so did their skills and abilities, and over the course of a few hundred years, they indeed reached a level where they could build new ships directly. But the existing ship had developed so much that any newly built ship would have necessarily been a step back for any inhabitant. They developed decks with soil; there was definitely not enough soil to spare for a new ship to immediately copy that achievement. Building up new soil would need centuries. Also, building a new ship would have meant duplicating all the systems of a ship, the drive, the sensors, the main computer, while extending the ship meant just adding a bit to what already was there. So they added layer over layer to the ship, and after few generations they didn't even think about it any more; extending the ship was just the way they lived.

And extending got even easier over time, as the surface of the ship grew. So they ended up with a giant ship containing all of the known mankind. If Earth and its inhabitants still existed, nobody could say for sure (and actually, most were now convinced that it never had existed, that it was a myth from those people millennia ago).

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    $\begingroup$ Population growth! This does not compute. There is nothing easier to control than fertility and demographics. This is a myth pure & simple. This only happens with extreme poverty & a lack of contraceptives. One big ship, the first thing they will fix is its population. Suggesting that it was easier to maintain one big ship than build another is good. $\endgroup$ – a4android Jul 19 '16 at 4:37
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    $\begingroup$ Population growth will not be a serious problem as a5android explained, but other than that I love this answer. +1. $\endgroup$ – ApproachingDarknessFish Jul 19 '16 at 5:29
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    $\begingroup$ @a4android - "only happens with extreme poverty & a lack of contraceptives" ... or a religion that opposes the use of contraception. $\endgroup$ – Jules Jul 19 '16 at 7:21
  • $\begingroup$ Population growth can happen due to how the economy and/or politics is set up, and that may not be corrected due to entrenched interests. $\endgroup$ – clacke Jul 20 '16 at 6:25
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    $\begingroup$ @a4android That's assuming that someone reasonable is in charge of the forces in society, and/or that the cost of adding modules to the ship is higher than the cost of going against individual rational behavior within the societal framework. $\endgroup$ – clacke Jul 20 '16 at 9:50
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Why might they want to stay on their ship and live a nomadic lifestyle rather than settle down? There could be a couple of different reasons:

1. They are being hunted

Humanity has an enemy, and can't win in a straight up fight. In fact, we've tried that, and it cost us dearly. The survivors exist in this generation ship, and are hoping to reach a remote corner of the galaxy where they might be safe.

Unfortunately, so far the enemy has proven extremely dedicated in their pursuit of the remnants of humanity. Why that might be is up to you. Maybe us humans created an AI which rebelled against us, tried to wipe us out, and is trying to finish the job. Maybe we encountered an alien species which wants complete and total dominance of the universe, and simply moves from solar system to solar system wiping every other sentient species out, and they've found us - we know that any colony will be destroyed sooner or later. .

2. A final destination

Humanity has found out that there's this perfect paradise somewhere at the end of the universe, and are seeking it out on a journey which will take thousands of generations to complete.

Maybe the source of information was some higher order being, or promised them by some much beloved religious leader; either way, they believe this information completely.

3. Cultural reasons

Humanity has embraced a culture of exploration, and we're sticking with it. (this is probably the weakest reason of all)

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    $\begingroup$ In regards to the third point (cultural reasons), you could opt for a fanatical environmentalism that oversimplifies to "don't muck it up": humans arrive, take the minimum we need for our purposes, and move on before we can ruin the local world. Perhaps this evolves from an environmental destruction of Earth? $\endgroup$ – Ghotir Jul 18 '16 at 18:54
  • $\begingroup$ @Ghotir - that actually did cross my mind after I wrote my answer, but was too lazy to edit, lol. Frankly, I don't think you could sell so many people on living on a spaceship. Us humans love feeling the ground beneath our feet, and sooner or later a faction would demand to be allowed to start a colony, and be left to their own devices. How would you stop them short of military action? However, if aliens are chasing humanity no one would want to stay behind. Seems more realistic to me. $\endgroup$ – AndreiROM Jul 18 '16 at 18:55
  • $\begingroup$ The first reason is the premise of the Knights of Sidonia series. I'm not a fan of the plot, but I felt the premise and world-building behind the series was done exceptionally well $\endgroup$ – knowads Jul 18 '16 at 23:03
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    $\begingroup$ Being hunted? Run & scatter would be safer & smarter strategy, $\endgroup$ – a4android Jul 19 '16 at 4:40
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    $\begingroup$ @a4android maybe at one point there were more ships and they split up, or got destroyed. The OP doesn't specify that no other humans can exist in the galaxy, just that no one on this ship should want to stay behind and live on a planet. $\endgroup$ – AndreiROM Jul 19 '16 at 13:14
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Centralization

even if it is a large population it is definitely going to need to stick together if it desires to even have a chance at surviving. Also a vessel of the magnitude you assume is very large. So large that it may even need to have its own government. In this case Stability of the government is easily guaranteed if there is just a single ship and not multiple colonies. (Multiple colonies Can mutiny on their own or even declare independence from the group also having multiple colonies may split your populace into factions which I do not believe to be desirable given the set of circumstances)

Unity

When people stay together on the same ship they have a greeter chance of fighting together for greater causes. In case assuming an attack of the big vessel you speak of the loyalty of the crew towards its cause and also its co-ordination between itself may be of vital importance. Given that it is just one ship easy and effective co-ordination is very easily possible

Bigger Ship = More Firepower

If the ship is supposed to represent the carrier of the last remnants of humanity it is obviously very strong. Capable of repelling attacks and has amazing weapons and stealth capabilities. In case an attack ever happens this one large ship may be able to easily take over enemy vessels in order to salvage them or to take them as hostage. Which could give the crucial battle edge to these survivors

Also if you have a single ship retreating and regrouping is much more easy since you don't leave anyone and always go with your crew. Though it may not appear morally very appeasing retreating in some cases is the best option available

Colonies Can Mutiny Easily (Can also be read as civil wars are dangerous)

It is possible that colonies are not happy with the deal they are getting and hence may be possible that they can mutiny. Since no FTL travel is allowed the word of mutiny may reach the government too late for the government to even stand a chance. A civil war may tear the remaining human population to shreds from where it may reach a level so low that it may never be able to stand up again.

You Cant repair a Planet or a Colony but you can repair a ship

A ship is a dynamic system You can repair it easily it may even repair itself given the lever of technologies. Planets unfortunately are not the same. Once you destroy them It will be very difficult (Read Difficulty->impossible) to even repair the planet. In this case a single ship would be an advantage due to it being just more resilient.

Or may be these people are happy as they are

May be that the population of your ship is happy and comfortable with its nomadic lifestyle. It may have even completely adjusted to it so why take pains to colonize a planet if everybody happy with this nomadic way. Colonizing planets is hard indeed

You Don't want to destroy the planet the way you destroyed your home

May be humanity has some guilt left in it about the horrible stuff it did to this planet so they may for some emotional reason decide that they can't destroy another planet's well and good ecosystem and remind themselves that they are not gods and have every right to play with other planetary systems

The colony may also have some unexpected and not so friendly neighbors

If you set up a colony it is possible that some intelligent alien species which already resides in that system may get upset. In which case it is probably going to look great for humans who are by now a bunch of nomads. An all out war or covert warfare would not be favorable at all especially in the enemy's home turf. Also since your population was probably drifting about in space in search of a new home it also has terribly less experience of interstellar warfare which is another fact that plays against it

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Every sufficiently advanced civilisation will reach the stage where it starts building advanced generation ships. These are attracted to habitable planets, where they can expand the ship, refill various resources and acquire knowledge. These visits almost never end well for the populations of planets visited. (Some of the first were nearly fatal for our ship, but we have had enormous technological progress since then.) This kind of event is rare enough for sedentary civilisations simply not to prepare for it, but also frequent enough to be the key reason why, as a general rule (as far as we know), every sedentary civilisation will be destroyed sooner or later. The inhabitants of the ship, on the other hand, don't draw attention to themselves by orbiting a star, and they know well in advance when they will reach a potentially inhabited planet. Sometimes they even manage to develop biological weapons that extinguish the entire population without ever being discovered. This makes it so much easier to plunder a planet for its resources.

Theorists say that there must be similar ships that are able to detect other generation ships from far away and feed on them. They say that by leaving a trail of destroyed civilisations behind us, we may attract these meta-predators. This is because the results of some of these destruction events can be observed from very far away in much the same way that we find our victims. We don't have (yet?) what it takes to become such a meta-predator. So we'll continue with our current lifestyle, which is at least safer than staying on a single planet.

Some theorists also claim that there must be sedentary civilisations that feed on ships like ours that underestimate them. But nobody really believes them.

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Obviously, there must be many reasons to go with a large ship. People may fiddle with smaller designs, but you need reasons for big ships! Here are some:

Shielding

Shielding a population from the nasty, UV and other radiation induced-related deaths and symptoms can be quite costly. There could be a particular size of ship, which is large, that produces a strong enough magnetosphere for humans to live in.

Additionally, perhaps they've developed deflector shields, but require a huge amount of energy to operate them. Such deflector shields could also solve the issue of space junk and micrometeoroid collisions. Perhaps this huge amount of energy generation for said shields only makes sense on very large colony ships.

Heat Loss

There is also the issue of heat. Larger bodies (with small surface areas) will lose heat less quickly than small bodies with larger surface areas. Not that space is especially good at transmitting heat away (especially compared to metal), but it does still happen. Less heat loss means you spend less energy on heating, which means you can spend that energy on something else (like maneuvering).

Gravity

If your ship is large enough, you could even have people experience gravity just by virtue of being attracted to the ship. This is great because you can combat the bone/muscle deterioration and internal fluid redistribution this population will undoubtedly undergo. Most hard sci-fi spaceships seem to solve this by rotating and producing artificial gravity in that way.

I suppose this population may not have figured out the gravity thing entirely, so they could be forever doomed to live in low-g environments due to their adaptations to living in space! This is why they never settle on a planet; they consider their bodies' changes to space to be normal, and living on a planet after that would be very hard.

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    $\begingroup$ I never said my big ship couldn't spin! I haven't done any maths on just how big I want it to be, but just about everything in the Universe is spinning one way or another, so my ship might as well. $\endgroup$ – ktyldev Jul 18 '16 at 14:54
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    $\begingroup$ Although this has been out here for some time, I'd like to point out that, as a species, we aren't getting along all that well at home, where we were evolved to be...taking our self-inflicted problems on a multi generational interstellar road show - all in the same bus - maybe not a great idea. when decks 1,200 thru 1,400 declare war on decks 500 thru 900, the outcome can't be good. Better lots of smaller ships, with higher total probability of success... $\endgroup$ – Joe Jul 18 '16 at 15:06
  • $\begingroup$ @Joe Considering decks 1-1400 could be irradiated to death by a passing star if they don't join together and maintain a function system... well, MAD situations have been seen before, and we've not entered into that yet. $\endgroup$ – PipperChip Jul 18 '16 at 15:19
  • $\begingroup$ @Joe - how small? Which number of peoples U think? $\endgroup$ – MolbOrg Jul 20 '16 at 11:43
  • $\begingroup$ @MolbOrg - I have on good authority that a minimum 'safe' gene pool is around 250,000 - fairly large ship involved, but one ship, one colony. Many such ships would do better at species preservation, and eventual successful colonization. $\endgroup$ – Joe Jul 20 '16 at 13:26
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For me the difficult question is why do they expand and all head off together, in preference to building a second (and subsequent) ship and heading off in different directions.

There could be technical reasons they can't build another ship (lost knowledge, only one unobtanium crystal), or social ones (this one ship is the sole survivor of the ShipWars that resulted from the last time anyone tried it).

But the reason for not colonizing the planet can be fairly simple. First, the giant ship they arrived on is in the process of strip-mining the system for every useful resource, and won't leave behind anything worth living on. Second, the civilization is clearly accustomed to a way of living that consumes the resources of an entire system from time to time: staying behind as a "colony" in effect means limiting yourself to the resources of only that one system, so how long will you and your descendants last?

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    $\begingroup$ A reason for staying together is that if they head off in different directions, the communication lag will increase forever and wouldn't take long to get to a point that you were waiting years for replies. $\endgroup$ – ktyldev Jul 19 '16 at 9:12
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    $\begingroup$ @monodokimes: OK, if that's their thinking then you could make it also a reason for not staying put when the big ship leaves. Communication with the big ship will grow forever, and other new colonies could each be further away than the last. $\endgroup$ – Steve Jessop Jul 19 '16 at 9:55
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A simpler answer, there is no place like home, at least not yet. Generation ship is comfortable with perfect temperature, humidity, atmosferic content, etc... Worlds that can offer this luxury is very rare. Without FTL it would take thousands of years to find such planet. That said, if I was the governor of that ship, I would consider building other ships. Putting all your eggs in a single basket spells out extinction.

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  • $\begingroup$ Not bad, destiny of planet oriented religion, eternity in seeking perfect planet. $\endgroup$ – MolbOrg Jul 20 '16 at 12:15
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Selection Effects

That is, everyone who might have wanted to get off the ship has already done so, some long-forgotten number of visits ago. No one's really bothered to check back in with their descendants in centuries, for approximately the same reason no one checks back on Earth's descendants, and c'mon, there's a whole galaxy out there still to explore!

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All the fun happens at the core

Ultra-high bandwidth communication at light-speed, where sims are running on 1,000,000 X speedup relative to reality. I've described one such setting here. Admittedly, in that example it's placed on Earth, but might as well be orbiting the sun in a sub-mercurial orbit to gather power. To such people, making a trip to the moon would seem to take forever. As to other stars, who would want to be isolated for 10,000,000 years while going at 0.7c towards the dead worlds surrounding Alpha Centauri.

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A simple way of solving the no colonies situation is that in all there years they have not found a habitable planet. And that considred, why live isolated in a small domed colony on some barren rock when the ship is much more friendly to human life ( assuming they do not have overcrowding issues).

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The people on ship have become dependent on something only the ship can provide. Some nutrient, some drug, a specific radiation. Leaving the ship is a death sentence. The ship could represent a lost technology that provides eternal life as long as citizens return to the star chamber every few years. All the people are so old that without the chamber they all die of old age in a few weeks.

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The technological infrastructure capable of building big ships has been lost in the remote past. Worldships of this capacity are able to repair and maintain themselves indefinitely. Also, the drive-system might extremely efficient at moving big ships and less efficient the smaller the ships become. This would explain why the big ship was built originally. They have lost the capacity to make more big ships and if it's just as hard to make small versions, then that leaves them with the one big ship. This then is truly a worldship.

For example, the drive-system might incorporate exotic matter and while that doesn't need to be replaced or replenished. The worldship lacks the technological base to make more exotic matter themselves. If their main fuel is antimatter, we can safely assume the worldship can generate its own and is therefore capable of continuing to travel idefinitely.

We can assume that habitable planets are in the universe, so that by the time they find the first one the population and its culture are so well adapted to worldship life nobody wants to face the rigours of planetary colonization and everybody stays onboard.

Perhaps, if Earth is lost the worldship is on a mission to find its ancient birthplace and all they can do is search the galaxy until they find it again.

Personally I prefer the idea that the worldship has a population of explorers and scientists. In fact, this makes a lot of sense. The culture on a worldship would be highly scientifically and technologically oriented. Considering their worldship can roam the galaxy freely, with occasional stop-overs to repair, resupply and revictual, why not just go and explore and research every planetary system and any interesting parts of the cosmos you come upon.

Hey! That sounds like a great life. Where can I join?

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  • $\begingroup$ That engine building theme could be a reason, specially with FTL but might be not ftl too - as example it need lots of energy to start it working or if it's matter of luck like catching something like god particle. Explorers nice it's even more viable, spectrum of possibilities for that is more wider. $\endgroup$ – MolbOrg Jul 20 '16 at 12:27
  • $\begingroup$ @MolbOrg A worldship on this scale would be a massive vessel. Whatever propulsion system it has is going to be incredibly powerful just to provide a modest acceleration. Engines capable of handling energies on that level have to be something special. There's no reason why this shouldn't use exotic materials and systems requiring a highly specialised tech base to make. FTL wouldn't be much different. But the OP wasn't interested in a FTL worldship. $\endgroup$ – a4android Jul 21 '16 at 7:13
  • $\begingroup$ propulsion system do not have to be incredibly something. Even those microthrusters might do the job, just needs lot of them and better TWR. Usual rocket problems if we talk about engines. But energy needed itself might be limiting factor, yes. How big is your big, actually? What you think how big it have to be? By people, size, mass? $\endgroup$ – MolbOrg Jul 21 '16 at 15:47
  • $\begingroup$ @MolbOrg The OP is vague. Why not assume billions of people? The mass of the Ship will be huge! A gigaton of life support alone. Energy is the limiting factor, but the acceleration to move a huge mass requires enormous energy. Even at low acceleration. Say a minimum mass of 100 gigatons & up. You are excellent at calculating, what would the volume be? $\endgroup$ – a4android Jul 26 '16 at 7:41
  • $\begingroup$ I asked it, because I asked other peoples before, difference in perception how much is much is substantial. I calculate because I do not trust my own perception. I can be wrong some times, specially when there is no real examples to relay on, and my view point evolves over time because of models and calculations, my own and others. Some times it helps to think in therms of numbers, not any time, there are lot of examples where calculations where not useful, specially at predicting humans future. Size depends, but 45km radius sphere at 300K hull is save estimation for 1 billion of peoples. $\endgroup$ – MolbOrg Jul 26 '16 at 10:13
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There are a lot of potential reasons.

Population control

The moment you split humanity into two groups you start a chain reaction. Two groups becomes four becomes eight and eventually the universe is full of people and you have war and starvation and conflict. If you can keep everyone together then this will never happen.

Carnivores

Maybe they're running from something. This is the plot for "Sidonia" and "Battlestar Galactica". If unstoppable aliens or AI are chasing the humans then they'll want to stay mobile. If the aliens can listen in on their communications than they may want to stick together to keep from having to give up communication altogether. Thus, you end up with a single ship or fleet. There is also a subplot in "Ringworld" where the galaxy explodes (don't think about it too much) from the center outward and one species builds a generation ship to escape. They can stop by planets on the way out but cannot stay too soon or else the explosion will catch up with them.

The Universe is Already Occupied

If humans can greate a generation ship why can't anyone else? Perhaps the galaxy is already occupied. (This is one premise of Asimov's "The End of Eternity".) Thus humans become a diaspora like the Jews or Gypsies roving from star system to star system in search of a home.

Ecological/Scientific Preservation

If people have created an efficient post-scarcity society then they don't need to set up colonies and if people like being around other people then they'll all go places together. This is similar to the premise of "Diaspora" by Greg Egan.

Promoting Sentient Life

This is the premise of "2001" by Arthur C. Clarke. Aliens visit a star system, leave behind an autonomous machine to help promote the evolution of intelligence, and then move on to the next system. If they established their own colonies they would supplant the evolution of native intelligence.

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I'm going to try an answer I don't think anyone else has given.

They tried founding colonies but they kept failing until the residents of the ship gave up

The shipborn have been traveling for a long time. The origional intent for their mission was that they would seed colonies.... but that was long long in the past.

The initial journey was a long one, while on the trip over many many generations the shipborn gradually became more integrated with the ship itself. Often in ways they're no longer even aware of. Every Shipborn carries in their blood and every cell of their body nanotech, their bodies are supported and regulated by implants that interface with countless ships systems. They even have biological technology integrated at every level that was designed on the ship, tested on the ship and implicitly relied on many conditions within the ship. Some of it they don't even think of as technology since it passes from mother to child in the womb.

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Biotech motes are even carried through the air from Shipborn to Shipborn carrying updates related to new pathogens sourced from ancient distributed ships systems.

Indeed the full understanding of some of the technology integrated into their bodies has been lost with time.

First colony

The founding of the first colony appeared to go well, while the ship remained in orbit and colonists were still regularly traveling between the ship and the surface. Indeed nothing started going terribly wrong while the ship was still close enough for ubiquitous networking to connect the 2.

But when the ship reached the edge of com rage they started receiving frantic signals from the colonists. They were succumbing to local pathogens that had previously posed no threat, their organs had started failing in unpredictable ways and colonists had started developing deadly autoimmune disorders.

By the time the ship had slowed it's course, turned around and re-entered orbit everyone on the surface was dead.

The third colony

This time the ship remained in orbit for several years to keep watch and to provide rescue in case of disaster. In all the time they waited the colonists remained fit and healthy.

But again shortly after final departure the distress beacon lights up again....

The 10th colony.

A few brave volunteers tried a final bid to survive on a planets surface away from the ship. They had countermeasures to try to protect themselves from many of the countless hypothesized causes of the previous failures. They sealed themselves inside pathogen-proof domes, they had satelites searching for possible hostile alien bioweapon incursions. They had supplies of safe food and weapons... but they didn't have duplicates of 7 of the 23 separate software and nanotech systems that the Shipborn had unknowingly come to rely on to maintain life.

And so they died like all the others.

Now it is accepted that god and a hostile universe does not intend for the Shipborn to live away from the ship. The ship is life

Sometimes a young rebel or skeptic will try, will ignore the warnings of their elders and insist on remaining behind perhaps with a half dozen of those like them. They always die.

Over-dependence on technology they no longer fully understand.

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They have the destination, they heading to Supermassive black hole.

  • reasons are TLTR, if short it allows to overcome some growth problems for a more extended period of time and have a bigger population as result.

At the moment, this is the only place I know, where is possible to extract energy more efficiently than with any thermonuclear reactions. Efficiency is around 50% of $\small mc^2$

They do not make pitstops at star systems, as it seems, they do not stop at all. The main ship does not stop. They launching probes ahead to research systems and to set up heavy elements extraction systems, like in How can I move a planet?.

The average density of stars is 0.004 per ly3, so average distance between stars on their way is 6.2 ly. (average density here, local stuff around the sun, closer to center of galaxy is denser up to 10 times, Stellar density)

Distance to SMBH is 26000ly, so they will have 4120 possibilities to get materials which they will need to build SMBH energy extraction system, and to accelerate ship for higher time dilation.

Average star, or better to say like our sun as an example, can be used to accelerate only small asteroid-size of materials with radius 25km to 0.99c speed in time of 6 years. (2-tonne density per cubic meter)

  • it can be done better than that, but still each star provides not so much star lifted material(energy flow from a stars isn't so great for the purpose of starlifting and accelerating matter to the 0.99c), but the ship system can make continuous flow of the matter for a long time by leaving systems for that in each star system where the ship is passing by. Those systems can form one system with the ship, long strands of matter from stars to the ship - super starship spider - with 100-1000 years long strands if needed. They can get all needed materials. But even they potentially be able to refine entry star system they do not do that because of star energy limitations.

Distance to SMBH is 26000ly, and 0.99c speed is 50 times dilation which makes subjective time travel 500 years, not bad actually.

This time dilation and speed 0.99c explains why it is one ship, and not many of them. Accelerating big(0.01% of Venus by mass) ship needs 181 years of the energy of 1000 stars like our sun.

So potentially we talking about 10 stars, 100years to launch, 2e18 kg ship, 1e12 humans(1e6 kg mass of ship per a human). If they prepare their departure in 500 years starting from now, it looks like good solution.

This very exciting moment

Of inserting into SMBH orbit at 0.99c speed

  • Actually isn't so dangerous, if done properly, AND if ship made from what it supposed to be made, not a tin can from 196x, but the active material(like the tool from moving planets, link above). But author might make a great scene of the fight with a great problem, where life of entry civilization is a hair from disappearing in BH.

Notes

  • energy extraction from the black hole, I'm talking about gravitational potential energy.

  • SMBH is pretty big 41 light sec, and the closest star is at the distance of 6.5 light hours, and it can be converted into pure energy to warm up further consumption of stars around SMBH

  • amount of energy extracted from one kg of mass dropped into BH, do not depend on mass of BH, because of event horizon radius proportional to mass, and gravitational potential $\small \sim \frac{BHmass}{BHradius} \sim 2.95 \frac{km}{Sun mass}$ so it's kinda constant

  • not sure about 50% efficiency, but something close enough, and at least order or two orders of magnitude more then all possible thermonuclear reactions from H to Fe.
  • any mass is a good mass for energy extraction with SMBH.
  • anyway, all stars from galaxy will end here, so it's just a natural direction of nature, also there will be BH near for researchers.
  • one star with mass like our Sun dropped to BH provides 8 times more energy then Hypernova, energy output, 1046J.
  • there are some reasons to fly to BH for energy, some of them in this A to Q What would come first, the colonization of the solar system or interstellar colonization?
  • to stop thermonuclear reaction of star like our sun, by converting star to cloud, needs energy like 19 millions years of work of our sun or 0.000023 of hypernova blast (1046J), or each 1kg dropped to BH, another 500000 kg might be lifted from a star(with energy produced by this 1kg).
  • Having BH nearby means ability (if there is energy for that) send stuff with almost(to some extent) arbitrary time dilation across the galaxy, using energy extracted with help of BH. Or to wait with entry civilization up some event, being at some proximity to the event horizon.
  • I tried to be correct with numbers, in a reasonable way, but errors are possible, so feel free to calculate by yourself, the calculation is funny, better then wow.
  • If someone wishes to correct my English, I welcome that.

BH is definitely good opportunity which might be exploited in interesting ways, and offer possibilities as nothing else as my knowledge we may imagine now.

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  • $\begingroup$ Instead of constantly speeding up, what if they sped up then slowed down as they approached the hole. In fact, they might not stop the first pass of the hole -- use it to slow down somewhat, then swing out and decellerate using more stars, then come back at a slower clip and stop? $\endgroup$ – Yakk Jul 20 '16 at 18:27
  • $\begingroup$ @Yakk first of all not needed, BH is good exactly with fact you might store huge amount of staff at probably any speed close to speed of light. Second, launching ship is't fast deal it takes lot of time, and will convert 500y travel to indefinite time, but they need energy to grow etc. Most important, this mass, with speed close as possible to speed of light, is needed for building system which will extract energy - kinda like Jupiter rings around BH. I suggest to read moving planets, it's long read and a bit obsolete now, but it might help to understand how such system might look like. $\endgroup$ – MolbOrg Jul 20 '16 at 19:29
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Despotism.

The captain of crewed by seven billion isn't just the officer in charge or the guy with the big hat, he's not even a feudal lord or a king, he's an emperor of unprecedented power. On a ship this size different sections will have their own command structure, their own culture, quite possibly their own food, clothing and language. The "bridge" may well be a forbidden kingdom which only those of a certain status may enter, the "engine room" a rival superpower, there will be water storage tanks containing entire oceans and the air pressure, temperature and humidity may vary by location.

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  • $\begingroup$ That sounds more like a reason to leave as early as possible than to stay to some people. $\endgroup$ – Mad Physicist Jul 20 '16 at 18:18
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The obvious answer? No other inhabitable planets. Even in the rare case that one is in the temperature range to sustain life, you'd have to put a lot of risk and basically sacrifice people each time to try the food, see if it's poison. And many may only bring death after 5-10 years. That's a lot of risk, and how do you pick who will try the food? That's a problem even on Earth, if you're in the jungle you may find nothing to eat for a long time, especially if you have no knowledge of what is edible and what is not. Maybe there are no seeds left from Earth, everything is synthesized from elemental matter. Maybe nothing will grow, or it will mutate into something inedible.

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  • $\begingroup$ I'm not sure it would even be meaningful to reason about alien foodstuffs being "edible", but I'm pretty sure we haven't looked into that, so I decided to ask a question about that exactly. Would humans be able to derive nutrition from foodstuffs found on alien planets? $\endgroup$ – a CVn Jul 19 '16 at 18:17
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    $\begingroup$ grow your food, be adult do not eat strange things, use knowledge. $\endgroup$ – MolbOrg Jul 20 '16 at 12:34
  • $\begingroup$ So, they have the ability to sustain life in the cold interstellar void. How does standing on a large, spherical rock negate that? Case in point, I'm eating a lunch I made at home inside an air-conditioned (life supported, basically) building. Just because I happen to be on a planet doesn't mean I'm going to step outside on this sweltering day and try to eat the tree outside my window. If they can make air and food in space, they can make it in a base, too. Making planetfall doesn't necessarily entail opening all the windows and going native. $\endgroup$ – UIDAlexD May 15 '18 at 17:23
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Let history be your guide. Take the essence of the thing, and make it your own. During the past, why did cities form in the first place? There are many reasons. Safety is one of the big ones. You can choose to answer this question 'to the reader/player' in a number of ways. You can allude to it during normal conversations or encounters, or directly answer it. If this is a far future setting, then there can be many ruins scattered about the universe, some of so old that they have no way to date their origins or any record of having been established. The reason that any one in particular is now a ruin instead of a thriving planet/star system can be virtually anything. A universe so mind boggling old that all known 'outposts' of man fell to ruin for as many reasons as there are stars... or, one main reason. If there is a 'one main reason' then that would almost certainly have to be tied into your main story line/culture. If you don't want to be tied down like that, then opt for diversity. Some have been mentioned already, but include

  1. War - bio weapons
  2. War - think Terminator/Matrix/Saberhagen Berserkers
  3. War - Peace called too late and ecosystem too far destroyed
  4. War - etc, etc, and what caused each war can be unique
  5. The Singularity - which could go wrong in many ways.
  6. Environmental collapse
  7. Socioeconomic collapse (huh... that is one word)
  8. Religious zealously (mass suicide, fanatical celibacy, etc)
  9. Aliens (ongoing issue, or they are gone now too)
  10. Astronomical event - Meteor storm, rogue planet collision, supernova, etc.
  11. Roanoke colony

You can literally go on for thousands of scenarios using only those seeds above. The question is how deep do you want the reasons to be.

The final conclusion, by those on the ship, is 'screw that, lets keep moving'. The ship could fail for any of those same reasons... but for whatever reason it hasn't. Lessons learned, better leadership, luck, a common guiding goal of 'lets make it to the next safe harbor together or none of us might make it', the collective fear gained by passing by countless skeletal ruins keeping everybody in good behavior for the common good, etc.

Another thing they have going for them. If no faster than light communications exist... then they could have collectively BEEN to many of these places and be a massive wealth of technologies from countless star systems who all developed technology and the sciences from countless different perspectives.

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Because of the Economy of Scale

The larger the scale your economy works on, the more effective it becomes. Smaller industries can be optimized to maximum efficiency, and larger industries become cost effective. Miraculous feats like performing stellar engineering or running an unobtanium manufacturing plant may only even be possible on the scale of an entire civilization.

The reason to build colonies is logistics — e.g. you would colonize a star system because you're mining it for raw materials and its too expensive to bring the civilization-ship to the star. But if it's not too expensive, then you don't need to bother.

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Solar FLares

In the Dr Who episode The Beast Below, the Doctor is on a similar vessel:

In the distant future, the Doctor and Amy arrive on the Starship UK, a colony spaceship containing the population of the United Kingdom who have left the planet to escape deadly solar flares.

Source: wikipedia

In this scenario, colonies are unfeasible because of solar flares (if they are within the solar system), or because there are no suitable places to colonise outside of the solar system.

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The most obvious reason to have just one ship instead of hundreds is there is something you need that there is only one of that you cannot make another.

This could be a fusion generator / force field generator / gravity generator or even a wormhole gateway back to the home planet.

It could have been built by mad scientist and nobody understands how it works. It might be some alien artifact discovered on a planet which allowed interstellar travel or perhaps it's built out of a rare material and they can't find enough to build a second one.

Perhaps even the ship itself is the alien artifact and whilst human might be living on it, still don't understand how it works beyond the basics.

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Lack of terraforming capacity.

Even though a planet is in the goldilocks zone it may still be inhospitable to humans, or life at all (see Mars). And while your humans have the capacity to build a huge ship (probably a number of self containing large ships attached to each other and ready to split up or shut off injured hulls in an emergency) they do not have the ability to actually create a liveable biosphere on a planet. So, it makes more sense to mine the asteroid belt (and subsequently other solar systems which failed to provide a liveable world) for resources to maintain their swarm.

Ethics

I've thought about this a lot for story reasons of my own, and it does make sense. It's likely that planets with the capacity to sustain life also already does that. From a moral perspective it's better to live aboard a colony ship and suck up resources from planets who don't have the capacity to develop intelligent life than to be the murderous locust scourge of the universe. Essentially it's the difference between passing the great filter and being the great filter.

Evolution

Also, lacking FTL travel in the generations it would take to travel to another star system (and discover that while the goldilocks planet is rich in iron and carbon it has neither an breathable atmosphere nor an protective magnetic field and all the liquid water has boiled off into space) your crew will probably have adapted to life aboard the ship.

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One reason for only one colony ship is that it may be the only one left.

The presupposes that there is only one known good destination.

  • If some disaster caused the need to evacuate the Earth, you would have several factions and different reactions.
  • Some people won't go and will remain behind.
  • Some people who want to go but aren't allowed to will fight to get on the ships. This might damage the ships or the Earth to Orbit shuttles.
  • Some people who can't go will try to make cure that no one else can go either.
  • Different nations or power groups may sabotage the ships of other factions to rule what is left.
  • This kind of environment will likely lead to the ships being armed to defend themselves.
  • Then after the ships have launched, the negative feelings from the previous actions will likely cause at least some of the crews to think that they can't survive in the long run if "some other group" is out there growing into a big threat.
  • You may end up with a small number if allied ships. However, since people are in a thought mode of "different is a threat," they start finding reasons that their allies are different and create a threat scenario in their own minds. Eventually, they pick each other off until there's only one left.
  • No new ships are created due to the threat that the people on the other ship will begin to differ and "become a threat."

Once they reach the destination, you have people who have spent generations living on the ship. That ship is home. There might be a few who want to try their hand at living in a gravity well that has that weird and annoying phenomena called weather but most will look at the planet and think of it as a step backward.

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