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  1. My civilization has global free energy, understands the physics of antigravity, etc.
  2. It was able to advance technologically through its own efforts. Six separate civilizations become a confederation that shares technology.
  3. They also locate the technologies of their predecessors (Atlanteans), which advances their society rapidly.
  4. Society is reduced to hunter-gatherers after a comet strikes the earth in 10,800 BCE.
  5. They revered the earth and used only biodegradable products; thus, the only remnants of their civilization are the megalith structures still being found on the earth.
  6. I had been thinking 6 million people, but do not want to be way off base.
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    $\begingroup$ Welcome An-Ima. I'm a bit confused about the timeline, are you saying that they're all extinct now but what were their maximum numbers? Or that they started developing tech from the Atlantean tech from a population bottleneck 10,800 Ka ago - and you're asking how many there are now? I.e. are you asking about maximum sustainable population on Earth, or ..., I'm confused, they could easily have left the planet if they wished with their tech level. Please clarify. $\endgroup$ Commented Mar 7, 2022 at 18:27
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    $\begingroup$ What is a "kya"? $\endgroup$
    – Daron
    Commented Mar 7, 2022 at 18:41
  • $\begingroup$ “Thousand years ahead”. But what do you mean by “minimum population?” Are you looking for how many survivors from the comet you needed to reach a current 6 million? $\endgroup$
    – Vogon Poet
    Commented Mar 7, 2022 at 22:09
  • $\begingroup$ @Daron: Thousands of year ago. Similarly. mya (or better, Mya) means millions of years ago. (And "ago" refers to years before 1950, which is called "present' by archeologists, paleontologists and geologists.) $\endgroup$
    – AlexP
    Commented Mar 7, 2022 at 22:17
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    $\begingroup$ @An-Ima Dude just write "thousands of years ago". $\endgroup$
    – Daron
    Commented Mar 8, 2022 at 13:40

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Technology and science are limited primarily by population. Take medicine, for instance. Medicine has throughout the ages often has only advanced by the study of rare diseases (both genetic and pathogenic). If the rarity of a disease is such that only 1 in 1 million are affected by it, and if medicine requires at least 100 examples of such people to be able to make sense of it... then nothing short of a population of (roughly) 100 million will suffice.

That population can a generation of 1 million, over a period of 100 generations possibly. But then we're still limiting the advance of medicine quite harshly (that's several millennia after all).

Medicine isn't the only science thus limited by population. Genius itself is rare. And geniuses tend to benefit from the so-called "network effect". A lonely genius might be worth X innovation. But two geniuses who can correspond and collaborate and compete? They are worth more than 2X. And ten geniuses are worth more than 10X.

Thus, the higher the population, the more minds there are that can correspond and collaborate and compete, and innovation in general is accelerated.

Since many technologies and insights build upon earlier technologies and insights, achieving those more slowly can in many cases delay the more advanced versions.

Furthermore, higher populations allow greater numbers of people to specialize in the many specialties and sub-specialties that need to be researched. You can't afford 15,000 high energy particle physicist researchers if your total population is only 1 million. But without those, likely you have no chance at all of fusion (and if you somehow do discover the secret to that without them, then it's a fluke down to nothing more than luck, and you'll miss the next innovation).

I contend that for much of the technology and science that you are used to, today, it would be impossible to develop that without a population of about 7.5 billion (+/- 500 million) that we actually have. This of course contradicts the various fictions you're familiar with where some lone scientist or tiny clan of super-geniuses comes up with technology that rivals or even surpasses the rest of humanity.

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  • $\begingroup$ +1 and suggest the OP reads Guns, Germs and Steel where this idea is explored more thoroughly and mapped against actual human development on this planet. $\endgroup$ Commented Mar 7, 2022 at 21:42
  • $\begingroup$ Though perhaps there is some critical mass of automation where a very advanced civilization can at least be maintained at a significantly lower population. (Until there's one unexpected breakdown...) $\endgroup$
    – Jedediah
    Commented Mar 7, 2022 at 22:06
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    $\begingroup$ @Jedediah Perhaps true. But even then, we're not there yet! Suggesting that this theoretical civilization would have had to pass through the 8bil level, then went down from there. $\endgroup$
    – John O
    Commented Mar 7, 2022 at 22:08
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    $\begingroup$ "Medicine has throughout the ages often has only advanced by the study of rare diseases": citation needed. Medicine has by and large progressed by understanding the anatomy and physiology of the human body and by learning to diagone and treat common illnesses. Do you have even one example of an important medical advancement brought about by the study of a rare illness? $\endgroup$
    – AlexP
    Commented Mar 8, 2022 at 23:11
  • $\begingroup$ @AlexP About 150 different named genes come to mind. But the first I was personally aware of was the story of the man who took a bullet wound to the stomach that healed improperly, allowing the physician who treated him to actually study digestion. There are numerous rare injuries that have lead to insights into how the human brain functions. All of these rare injuries and diseases are even more rare with tiny populations... with the smallest, they might not even occur at all. I don't need "citations", this isn't my college essay paper and you don't get to fail me. $\endgroup$
    – John O
    Commented Mar 9, 2022 at 5:14
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In order to maintain genetic diversity for a "long" period of time, the general view is a minimum of 10k and ideally 40k individuals.

A starting population of 40,000 people maintains 100 percent of its variation, while the 10,000-person scenario stays relatively stable too. So, Smith concludes that a number between 10,000 and 40,000 is a pretty safe bet when it comes to preserving genetic variation.

See also: How Many People Does It Take to Colonize Another Star System?

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Even if they have free energy, they need people to support the ivory tower. How many scientists do you need? How many engineers do you need to build experimental machines for the scientists? To build and maintain all the other infrastructure of a modern environment where scientists and everyone else live? How many workers do you need to work in the manufacturing plants that produce all the things the engineers want to build? (Automation helps a lot, sure, but you'd need even more engineers to design that.) How many doctors and nurses do you need, since people tend to get ill? How many salesmen do you need to sell them everything they need? How many schools and teachers do you need, since people tend to have kids? How many universities do you need to produce the same amount of high-quality researchers when the current batch dies out? How many other universities you need for all that engineers and managers? How many farmers do you need to feed them all? (Even though automation would help, again.)


To give a concrete example, the infamous town of Pripyat had a bit less than 50k inhabitants before the catastrophe. It hat 15 kindergartens and elementary schools, 5 secondary schools. It had 1 hospital and 3 clinics (whatever this means). It had 4 factories, 1 huge-ass nuclear power plant, 2 stadiums, 1 recreation park. (And much more, see the link.)

Still, basically the whole existence of that town was for the the nuclear power plant. I have not easily found, how many people were involved in actual management of the power plant (I'd guess lower hundreds), while many more might have been involved in accompanying programmes (remember, it was a power plant, so all the electrical things) and in building further blocks of the power plant. The above idea holds, though. The whole town would not exist if not the power plant; the town was actually purpose-built to construct and maintain the power plant.

Still, the now-ghost town of Pripyat was hardly self-contained in regard of building materials (incl. rare things, such as reactor construction materials, Uranium, among others), food supply, entertainment, etc.

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It's more a question of exploited landmass than population

Modern technology relies on countless scarce resources coming together from all over the planet. There is a really good scene from the book World War Z where the a character keeps a framed recipe for a pre-war bottle of rootbeer in his office as a reminder of just how hard it is to make things without a global economy.

Ingredients:

  • molasses from the United States
  • anise from Spain
  • licorice from France
  • vanilla (bourbon) from Madagascar
  • cinnamon from Sri Lanka
  • cloves from Indonesia
  • wintergreen from China
  • pimento berry oil from Jamaica
  • balsam oil from Peru

6 million people huddled together in a single city will all starve to death. Spread them out over the area of a small country, and they they can capitalize on their local resources but will have so little variety of resources that they will find themselves trapped in the iron age. Modern technology... that requires an international trade network to make sure that every possible rare resource is available for exploitation. Advanced technology facilities require things like Neodymium, Gadolinium, Lithium, etc. which are each pretty rare in their own rights, but impossible to all find in one convenient spot. This becomes even more complex when you consider all the other resources you need to mine, refine, preserve, and distribute these materials. So what you really need to ask yourself this: How many people does it take to create a global infrastructure?

Any part of the world that is still defined by hunter-gatherer economy is not somewhere you can trade with. They may live where the resource you need is, but they will not be mining it for sale to you. Early agricultural societies could exploit more resources, but were generally very self-sufficient oriented; so, they would not exploit resources they do not personally need. To really see the rise of rare resource exploitation, and the organizational infrastructure required to move that stuff where it needs to be, you need global urbanization. Cities don't need to be as huge as they are now, but you do need some form of cities covering the globe to act as hubs for the trade network to exist.

The first trade system to achieve the desired state was the Phoenician trade network. This was the first time in history human populations became both dense and spread out enough for long-distance trade of scarce or manufactured goods to be viable... so I would place the very minimum population you need at about the same population density that the Phoenicians served, but spread across the entire world. This means that the very bare minimum believable world population would be about 350 million. That said, you can't expect the whole world to be homogeneously spread out; so, I would buffer that figure by saying a more realistic number would be 500-1000 million to ensure that even more "remote" places have some sort of urban centers to exploit.

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