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I have a scenario where Aliens must escape their Star system because a star 5 light years away from them is going nova soon. They find humanity and learn all about them in a few years - our whole history, the biology of all animals, etc. Then they establish contact and send us gifts that improve our level of technology and seem to be very friendly. Without going into too much detail, they eventually tell our politicians about their master plan.

They plan to come to Earth and make it their second home. They are carbon-based and breathe Oxygen, but are quite different than humans - for example, our atmosphere is not breathable for them because of the different ratios of Oxygen/Nitrogen and the presence of some other gasses. So...they need to "terra" or rather "xenoform" our planet. They make a deal (and an ultimatum of sorts) with human politicians that infrastructure will be built in China, Mongolia, and a separate one around India (smaller) to give shelter and food for all of humanity, and humans will live there separated by a special barrier...and the rest of the planet will be heavily changed to fit the Alien biology.

Aliens are powerful enough to terraform other planets, but not enough to do it quickly and economically-and changing Earth will be much quicker and economical; they just need less Oxygen and tweak some other things. Since lifeforms are generally egoistic, they are not ready to Invest 100x of their time and resources when they have a beautiful planet like Earth here and now.

So now for the question: is it possible to fit all of humanity in China, India, and Mongolia? Knowing, of course, that we are talking about a society that can build city-sized buildings and skyscrapers, and that production of food is easier because of genetic engineering and certain megaprojects.

Also, feel free to comment on what kind of improvements can be made to this setup. Maybe this barrier I write about is not feasible, so building cities similar to Saudi Arabia's "Line" will be a better option.

EDIT:By "fit" I mean: could humans live "normally" like in some densely packed city and country,or would it be an instant catastrophe? Lets say that most people could have a small apartament like cheaper places in Tokyo,some groups (from the poorest countries) would live like the homeless in India and only richest of the richest would have really comfortable houses or luxury apartaments. Something like that.

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    $\begingroup$ Gases mix freely. This is a very important difference between gases and liquids. The only way for one part of Earth's atmosphere to have a different mix of gases from the rest of the atmosphere is to keep it enclosed in an vessel, such as a glasshouse. Aside from the small engineering problem of building a glasshouse covering all of China, India and Mongolida, glasshouses have lots of problems, such as the need for active cooling and the need to regulate the atmosphere inside. The question is envisioning a technology level so much in advance that it is indistinguishable from magic. $\endgroup$
    – AlexP
    May 4, 2023 at 11:16
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    $\begingroup$ See also: How many fed per sq kilometre?. $\endgroup$ May 4, 2023 at 11:25
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    $\begingroup$ Why would such an advanced civilization need do colonize Earth? Why would a nova even be a problem for them? They could build a barrier (thin mirrors would work wonders) between their solar system and that star, and that should be easier than terraforming Earth. Or they could prevent star from going nova via star-lifting. $\endgroup$
    – Negdo
    May 4, 2023 at 12:07
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    $\begingroup$ @Slarty Unwilling and uncooperative would die due to terraforming, e.g. suffocate or eaten by alien frogs. Forced relocation is also an option. Aliens are capable of interstellar travel, certainly they can deal with Chinese government. Elites will get better deal than the masses, but advanced enough civilization could provide few billions of humans with all they need. If people had to chose between moving and death, they would move. Most of them anyway. These are finer details of the story, they are far beyond the scope of the question. $\endgroup$
    – D'Monlord
    May 4, 2023 at 17:53
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    $\begingroup$ @Michael Richardson "Xenoforming something like Mars would be much, much easier" - or not. It depends on what aliens want from a planet and what technologies they have. Both of which are unknown. $\endgroup$
    – D'Monlord
    May 4, 2023 at 17:55

2 Answers 2

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Easy. The main limiting factors are food, water, energy and shelter. All easily solved by high enough technology like space hydroponic farms, water desalinization, fusion reactors, megaskysrapers/underground cities. If humans and aliens have broadly similar needs in food and water, which seems likely, these problems would be solved by aliens anyway.

I would avoid the magical border thing as per AlexP comment. Just make aliens or humans xenophobic enough to make the idea of sharing the same space impossible. It might even work better for the story.

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  • $\begingroup$ There's one other important limiting factor - sanitation. 8 billion humans crammed into an area of that size are going to produce a lot of waste, biological and otherwise. The proximity will also contribute to the rapid spread of any contagious diseases without strict enforcement of quarantines. $\endgroup$
    – RisingZan
    May 4, 2023 at 20:26
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    $\begingroup$ @RisingZan - nope. India and China are already home for ~35% of Earth population. Three times more wouldn't make much difference especially if aliens provide food - it would free up huge territory. An alien race capable of interplanetary travel would be able to process waste. "megaskysrapers/underground cities" mentioned above is the most efficient way to deal with the problem - you don't need to dig water supply and sewers to every little hut. $\endgroup$
    – D'Monlord
    May 4, 2023 at 20:48
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    $\begingroup$ This answer smells a bit too much like handwavium. Got a logistical problem? Solve it with futuristic tech. Where's the limit? Why not have energy be generated by deutero-null cold fusion reactors that are small enough to fit in a broom closet and generate enough energy to power an entire city for 1000 years with a single fuel cell? Why not have food and water be synthesized on demand and waste disposal powered by teleporters that warp everything directly to an incinerator, or even the Sun? $\endgroup$
    – Abion47
    May 5, 2023 at 3:20
  • $\begingroup$ @Abion47 Humanity could fit in China, India, and Mongolia, they just would need better technology. Guess what, aliens who capable to move their entire population among stars do have vastly better technology than us. Assuming anything else would be strange. More efficient agriculture, better energy generators and better skyscrapers. Nothing is it is unobtainium. Water desalinization is existing technology, fusion reactors are always just 20 years away and the idea that our current agricultural or construction technology is the best that could ever exist is just plain dumb. $\endgroup$
    – D'Monlord
    May 5, 2023 at 8:06
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    $\begingroup$ @Abion47 You are right,but for now I wanted to concentrate on this particular topic. I do not know if so many people would even fit in those countries,and by that I do not mean "human as sardines" kind of scenario,but more comfortable.I know that high buildings and underground housing can solve everything,but I dont want to have 73638634 Burj Khalifas in my setting,that would be kinda dull.But some percent of ultra high skyscapers are welcome. $\endgroup$
    – Mishima
    May 5, 2023 at 15:46
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"Some like to assert that everybody on Earth could be fit into the State of Texas,..."

My quotations are from this source.

... using logic as follows. The area of Texas is about 262,000 mi2. Dividing this figure by the current human population of 7 billion leaves each person with less than 100 square meters, a small plot the size of a big room about 10 m x 10 m. Sounds plausible enough, right? Without going into the fact that almost half the State is desert, notice we have not allowed for any roads, shopping malls, schools, hospitals, football stadiums, prisons, sewage plants, rivers, lakes, reservoirs, golf courses, parks, and what else? How much land does it take to support a human being?

This beginning statement makes a good point. Removing all other considerations, what's the smallest amount of space a human could stand to live in? I suspect high-density locations like Japan would point out that a 10mx10m room would be more than adequate... but that's all you'd get. Nevertheless, let's continue the story.

Let's do the math again, but this time for the entire planet. The total land surface area of Earth is about 57,308,738 square miles, of which about 24% is mountainous and about 33% is desert. Subtracting this uninhabitable 57% (32,665,981 mi2) from the total land area leaves 24,642,757 square miles or 15.77 billion acres of habitable land.

Divide this figure by the current human population of 7 billion (that's 7,000 million people!) and you get just under one hectare (2.3 acres) per person. If all the habitable land on Earth were equally distributed among all human beings present on Earth, this is the per capita share of good land per person. Again, however, we have not allowed for any nice amenities such as roads, schools, hospitals, shopping malls, rivers, lakes, reservoirs, parks, golf courses, etc. Even so, could you live on 2.3 acres?

Efforts have been made to estimate the amount of land needed to sustain an average individual human (link). A person living the lifestyle of an average American requires almost 24 acres, ten times the world per capita share.

And that's the question, although the article author is being a bit dramatic (and very simplistic). That 24 acres can include space in desert areas (roads, hospitals...) and is shared with others for complex services (roads, hospitals...). While the vast majority of U.S. citizens would fit within this context, there are some (I live near some...) who entirely subsist off the energy grid and entirely provide food and shelter for themselves with less than an acre (it's all about efficiency... and a bit of modern technology). What they can't provide for themselves consistently is water.

EDIT: I need to step in and make couple of a clarifications because some commenters seem to have lost track of the OP's question. The above article is just one of many that have been done to demonstrate — pro or con — just how much of the Earth's surface today's population really needs. One such article I read some time ago used Hong Kong's population density to prove you could crush the world's population into New Jersey. But if you're inclined to argue about the efficiency of the above article, you've forgotten why I posted it. It's nothing more than a means of demonstrating that the OP's proposed circumstance (relocating all of humanity to China, India and Mongolia) is believable. My original answer is about to say this. Arguing that less space could be used is irrelevant.

The second half of this answer is a Frame Challenge that points out that building a world has little meaning if the people reading about, using, or gaming with the world can't understand or relate to it. As alien as the aliens may want to be, leaving your audience confused isn't a good thing. Worse, it's difficult to believe that some commonalities wouldn't exist between humanity and any alien race, not matter how evolutionarily distant from us they may be. The need for food, procreation, education, and science. Competition, whether it be sport, academic, professional, or military. Etc. As was pointed out by one commenter, an alien race that's utterly incomprehensible to us would be unlikely to find any interest in us or our planet — because at the very least, their interest in our planet is something we could understand. Thus, while stashing humanity on a mega-reservation is believable, that an alien race would cross vast tracts of space to do it without a purpose... isn't. And I can't see that purpose, nor was one presented. Anyway... cheers. END EDIT

What's the point I'm making?

What you're trying to do is believable, especially if the aliens are forthcoming with a bit of tech to improve the efficiency, but it's not practical.

  • Assuming humanity survived the inevitable "you're standing on my toes!" internal wars, they'd be looking for a way to oust the aliens from day #1.
  • Maybe... maybe... we could learn to overcome greed, bigotry, etc. fast enough not to tear ourselves apart (there is precedent, consider Turkey before the Bosnia and Herzegovina independence movement in the 90s), but I suspect the bedlam would be breathtaking.
  • If the aliens apply enough outside force to compel humanity into a small area, then your question is kinda irrelevant. You know... aliens with the tech to get to Earth, the tech to move all the humans, the tech to keep them there... can do pretty much anything.

But I would like to point out that your aliens are exhibiting a very odd psychology for conquerors who gobble up most of the planet for themselves.

  • Considering the suffering they'd cause among humanity and irritation they'd suffer themselves for as long as humans continued to breathe, such creatures (from a human perspective) would be far more likely to simply exterminate the humans. After all, if you have the power to force this condition upon humanity (8 billion people)... why share?

So, while I find your assertion to be believable from an implementation standpoint, I don't find it believable from a conceptual standpoint. You might think your aliens are trying to show some kind of mercy or something.... In reality, it would be much closer to wanting us to provide gladiatorial sport. Almost anything would be less cruel than to cloister humanity into a small area of their own planet with no hope of overcoming the technological advantage of the conquerors.

And we wouldn't have a hope. Allowing the hope would be a technology dichotomy. A space-faring species that could force us all into a limited area with enough efficiency to realistically stay there and survive would also have the technology to monitor almost every aspect of our society. They'd know if we were getting ahead of ourselves and stamp that out. And we're clever little monkeys... the aliens could be constantly putting out fires, so to speak.

Yeah... I'm having trouble swallowing this idea. Unless humanity was being stored away for slave labor or cannon fodder or some such, I don't understand why aliens would do this. I can't imagine a mix of desperation and compassion that would bring this situation to pass.

But, I'm unlikely to be creative/imaginative enough to justify that as a global assertion.

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    $\begingroup$ "your aliens are exhibiting a very odd psychology" - as they should, that's a huge part of what makes them aliens. Expecting human psychology from them is absurd. You are questioning the OP's premise based on idea that aliens are humans in disguise as it is often on TV. Aliens don't have to be that primitive. They are very likely to be a complete enigma with incomprehensible motivation. Full freedom for a writer. Even from humanity point of view their solution is far from being unbelievable. If humanity faced a choice between extinction and cruelty it would be cruel. $\endgroup$
    – D'Monlord
    May 4, 2023 at 15:21
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    $\begingroup$ @D'Monlord Yes and no. If the goal of this worldbuilding effort is to write a story, the alien psychology must be presented in terms that the reader can comprehend for the purposes of the story. Simply writing something sensless to humans off as "well... it's alien!" is a mark of a very poor writer. Worse, we humans can understand the motivations of cats, dogs, fish, insects... all of which are alien to humanity, just not alien to our world. The assumption that the ecology of an alien world would be incomprehensible is insensible. Even aliens need a good meal. So, no, I don't agree. $\endgroup$
    – JBH
    May 4, 2023 at 15:49
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    $\begingroup$ @D'Monlord The ecology argument is a valid one. For an alien species to have psychology that is incomprehensible to us, life itself would have to have evolved radically differently on their planet. Otherwise, things like survival of the fittest would be shared between us, which means on a certain level, we would be able to understand their reasoning if not their motivation or priorities. And that raises the question of if the aliens want to strip mine the planet or whatever, why not just be rid of us and be done with it if the alternative involves going through a lot of unnecessary trouble? $\endgroup$
    – Abion47
    May 5, 2023 at 3:01
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    $\begingroup$ And if they are so vastly different from us as to be incomprehensible, that would also mean that we were incomprehensible to them. If that were the case, why would they value our existence at all? We would be something so utterly foreign to them as to be completely beyond any hope of understanding, so why go through the trouble of containing us as a collective species in a region on our own planet, gifting us the infrastructure and technology required to house billions of people in a megacity? Why not take a handful of us for zoos, pets, slaves, or scientific study, and then burn the rest? $\endgroup$
    – Abion47
    May 5, 2023 at 3:05
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    $\begingroup$ How is invading a place and forcing its inhabitants into small areas of their former territory at all alien to even human psychology or history? $\endgroup$
    – Obie 2.0
    May 5, 2023 at 3:24

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