21

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. ...


20

"It's the smell!" -- Smith I think the easiest answer is that there is something in Earth's biology that disagrees with them... something that can be "managed" with continuous medical intervention, but would be really annoying for them to try to settle permanently without completely terraforming the planet. Imagine, for example, that 99% of the aliens ...


19

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 ...


18

The aliens are humans. The aliens did do the calculations and they did find a habitable planet, 100,000 years ago. They are depleting their homeworld - here. On arriving on Earth and settling the colonists, the mothership immediately started working on finding the next world. These calculations take a very long time. Almost none of the humans on ...


15

The aliens evolved on a planet on which photosynthesis never developed. Oxygen is a poison to them. Some of their least susceptible adults can handle it for limited periods of time with medical support. Earth is not a healthy place for them long term and they cannot raise their children in our atmosphere. Removing the oxygen would be particularly difficult, ...


11

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 ...


11

1. They actually cannot live on Earth for extended periods of time Your premise is that Earth should be a candidate, since: "[...] some have been on Earth for 20 years. [...] Clearly they're capable of living on Earth for extended periods of time, but for the story I need them to have ruled out Earth as their home the moment they arrived. " However, you ...


9

The aliens may have decided that their time on Earth should be minimally invasive to humanity. Although they're tagging humans and using us to solve their distributed computing problems, for the most part they're allowing us to go about our day-to-day lives without undue interference. Obviously, colonizing our planet is going to be much, much more invasive ...


8

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 ...


7

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 ...


5

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 ...


4

Why go to science fiction, when there are real-world examples today, why humans don't want to go to the Moon or Mars for extended times? Bone strength. The alien's home is a 7g planet. They can live on 1g Earth for a while, but over time their bones get too weak, their blood vessels expand, and all the other ailments that humans have to recover from when ...


3

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 ...


3

Just fast forward our current real-life state of Earth a little bit: Pollution. The Earth is so full of plastics and trash and smog everywhere, and getting worse daily. Also the resources of Earth are too mostly depleted, making it not a viable target for mass colonization. (why expend all the FTL resources to get to place that is in same of even somewhat ...


3

A) The aliens are hypersensitive to gravitational changes, and find the moons slight gravitational effects (which can lift water, dont forget that) nauseating. So after the invasion, they discovered, that every half moon orbit, they conquered a planet that makes them vomit. They try to protect themselves in there ships, but if you are living in a ship anyway-...


3

"The main reason behind this question is because I think it would be interesting if there was a situation in which an individual from Alpha Centauri could literally not read a display because of the color scheme, or if conversely they could see something that someone from Sol could not." This is very very obviously possible. Here on Earth, illuminated by ...


3

Coming late to this discussion. One thing that has not been discussed is a far simpler way to accommodate a large amount of people. Instead of trying to terraform other celestial bodies, we can do something similar with close to today's technology by just going underground. Everything that humanity has accomplished has been done by exploiting just the ...


3

This was the result of First Contact For whatever reason, these aliens do not want to invade, conquer, or otherwise subjugate us under their rule. However, they need our brainpower in order to move on. So our first contract with them resulted in a trade once the language barrier was resolved: They assist us in advancing our technology along reasonable ...


2

Perhaps microbiological life is more rudimentary or non-existent on their home planet. When they arrive on Earth the microbiological biosphere becomes very apparent to them and they can’t compete with it. Perhaps the aliens who do come down to Earth can survive but there are big issues with contamination that they have great difficulty in overcoming. Maybe ...


2

No difference. Evolution does notwork on such short timescales. Jews have been cutting off their foreskins for thousands of years yet each new generation gets born with them again. Unless an extremely specific mutation randomly appears in the population, AND that mutation conveys a large benefit to the chances of breeding, AND the breeding successfully ...


2

Earth makes them sterile It's a variation on other answers, but something about Earth, its atmosphere, magnetic field, radiation, bacteria etc makes the aliens sterile. So while a sacrificial few can live down on the surface for the sake of the mission with no other noticeable ill effects long term colonisation is out of the question. These surface ...


1

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 ...


1

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 ...


1

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 ...


1

They are altruistic They recognize that life deserves a chance to thrive, and an intelligent species deserves the right to choose its own path. Humanity is still a fledgling race, so the aliens do not intervene. The computations done subconsciously don't affect our genome, so have no long-lasting negative consequences. Their calculations are for a place ...


1

When the aliens "discovered" and calculated the habitability of earth (from x light years away) - even with FTL we can assume that travelling here took them years - if not hundreds of earth years. At that time earth was in our "medieval age" - total population probably around a low hundred million - no noteworthy military danger - intact ozone layer, clean ...


1

I would think there are two reasons why the aliens have not colonised earth. While technologically superior, the aliens are biologically inferior to humans, and humans exist in much greater numbers. Meaning that there are equal chances of either losing a war, or destroying earth in the process, or winning. So they would rather not risk a confrontation. Even ...


1

Other posters here have come up with reasons not to colonize that are friendly to Earth's resident human population. I'm going to take a darker tack: Earth is doomed Why colonize a planet, if it's only going to be around for the next 1000 years? Human scientists haven't figured out that the sun will go nova shortly, but the aliens have. The aliens ...


1

Solar flares The problem is not Earth, but the neighborhood. In the search for perfect place to settle, so much attention got on the planet side of things. The alien's sun has the most perfect, calm and stable sun -- and everyone's sun is like yours, right? So it's come to a shock when they arrive and warp stop working and communications is unstable, all ...


1

The aliens evolved on a planet without a magnetic field, and Earth’s magnetic field makes them falling-down dizzy. They can manage temporarily by wearing equipment that senses and cancels Earth’s magnetic field in their immediate vicinity, but it’s bulky and imperfect and not a good solution for a long term colonist.


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