Let's set the boundaries:

  • Same earth, same time

  • Overnight we all become immortals and totally indestructible

  • Peoples never fall ill nor need to sustain themselves

  • They actually can reproduce without feeling any pain or needing extra ressources

  • Let's put aside of the question the facts about other living species, they don't matter for this question

It's actually kinda what question we could ask if Adam and Eve had not been kicked out of Eden when they knew they could reproduce.

What do you think would happen, would the human number grow up until the end of time or would it face a decrease or even totally stop?

Edit As I was seeing this, I imagined something like the Christian God feeling bad for kicking our species out of Eden and even if it can't get us back, it would give us the immortality perk So maybe my question wasn't oriented enough and should have been: At what point will the humanity stop growing? Aka How much peoples and where would they be located (earth only?)

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    $\begingroup$ Finite space = finite population. We can neither build up nor down in an unlimited way. $\endgroup$ – StephenG Feb 6 '18 at 14:06
  • $\begingroup$ How could the population ever decrease if humans are immortal and indestructible? The population can only ever increase or stay the same. $\endgroup$ – Nuclear Wang Feb 6 '18 at 14:12
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    $\begingroup$ You absolutely have to read Death at Intervals, by José Saramago. "The book, based in an unnamed, landlocked country at a point in the unspecified past, opens with the end of death. Mysteriously, at the stroke of midnight of January 1, no one in the country can die any more. Initially, the people of this country celebrate their apparent victory over mankind's longtime foe(...) This joy is short-lived - it soon becomes apparent that the end of death presents unique demographic and financial challenges." $\endgroup$ – Renan Feb 6 '18 at 14:14
  • $\begingroup$ I don't understand the question. If humans cannot die, what mechanism could possibly reduce the human population? $\endgroup$ – kingledion Feb 6 '18 at 14:57
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    $\begingroup$ I am sorry, but this is a ridiculous question, for example: Overnight we all become immortals and totally indestructible. Peoples never fall ill nor need to sustain themselves. They actually can reproduce without feeling any pain or needing extra resources: if people have no need for sustenance or extra resources, then existence comes at effectively no cost, no one needs to eat, no one needs to worry about illness, so people can just pile up on top of each other, not worrying about eating or breathing - this is ridiculous! $\endgroup$ – Lee Leon Feb 6 '18 at 15:50

Population will never step growing. We will likely (try to) expand into space, onto the oceans, or below them. Invincibility and no need for food would make it a lot easier than now, and there will be more pressure due to growing population.

Reproduction is an instinct, both sexual and social (need to care for somebody, need to make "copies" of oneself). There is nothing in your setup that limits this instinct.

Normal limitations for unlimited population is lack of food, disease and war. You have ruled them out.

Earth will get more and more crowded, but that will not stop the reproduction instinct. We see people continue reproducing even when they have low income, live in a war zone, etc.

High-income people usually have fewer children (no good explanation, maybe they are too busy making money, maybe they prefer to invest more into each child). But perfect health and infinite lifespan will make them continue reproducing as well.

Edits, or rather additions:

  1. The premise of "indestructible" and "no sustenance" violates basic physical laws (conservation of energy). I am glad you realize it takes divine intervention to make it happen. After that, both your readers and the people in the story will expect further miracles to aid in their struggles. Here is a better way to introduce such plot devices.

  2. Unlimited population growth will lead to resource shortages. Humans will still need houses, cars, clothes, entertainment. And they will still want to have a better life than their neighbors.

  3. There will be an immediate economic crisis worse than the Great Depression of 1930's. Back then, engines and tractors eliminated some of farming jobs. Now, you eliminated all the jobs in farming, food processing, food retail and restaurants. Huge unemployment among low-skilled people, who have very little chance of re-training (unlike the doctors). To avoid revolution, we will need huge welfare programs, or massive public works (maybe US will fix its potholed roads).

  4. Developed world might eventually find a way to keep population growth limited, so technological innovation can keep up. E.g. have a limited number of reproduction licenses available either through free lottery or large fee. Violators will be force to pay the fee, kicked out of the country, or sterilized. This will work best in small, isolated and mono-cultural countries with tradition of law and order, e.g. Scandinavia or developed nations East Asia.

  5. Third world countries will descend into wars over limited resources (using excuses of religion or ethnicity). You cannot kill people, but you can bury them alive or throw them into the ocean (in shipping containers, with a few bricks for ballast).

  6. If people know that it was Christian God who did it, there will be a lot of mixed feelings, including anger over the side effects. Also note that Jews and Muslims are technically worshipping the same god; they just disagree on who's the true prophet.

Final edit. I decided to lighten up, and come up with a viable solution to some of the problems I found. All the healthcare spending of the developed world is re-routed to returning farmland back to natural state, planting trees, breeding wild animals, etc. This keeps former farm workers employed, at least long enough for society to create new jobs for them, e.g. in eco-tourism. As for population control, make welfare and public-works jobs require a birth-control implant.

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  • $\begingroup$ Lack of atoms to grow more bodies, perhaps? $\endgroup$ – NofP Feb 6 '18 at 14:10
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    $\begingroup$ High-income people having few childs and people reproducing even when low income or living in a war zone are actually the same phenomenon. When you have nearly nothing having more children is a good strategy, for when they grow a little they're extra pairs of hands who will collaborate in the fight for subsistence. For people who don't have to fight to survive, every child is a sink of money. $\endgroup$ – Rekesoft Feb 6 '18 at 14:20
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    $\begingroup$ @Rekesoft: Or, put another way, in a high risk environment, you spread your risk with multiple children in the hope that at least one will survive till reproduction. In a low risk environment, it's more efficient to make larger investments in a smaller portfolio of children. $\endgroup$ – nzaman Feb 6 '18 at 14:26
  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Feb 6 '18 at 14:31
  • $\begingroup$ Reproduction is done to replenish the species. If there is no need to replenish, there is no need for reproduction. But it also means there is no further evolution, As for immortality, what happens if you just put them on a rocket ship and send them off to space without oxygen, food, water? $\endgroup$ – Justin Thyme Feb 6 '18 at 16:22

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