In science fiction, we often see empires of hundreds of worlds with populations of billions to trillions of people. That got me wondering, how many humans might ever live? To solve that problem, we need to break it down into more simple parts. The first of these is ‘how many humans can be supported by our environment’ and the second one being ‘how long will humanity survive’.
How long will humanity survive?
This is a difficult question to answer because we cannot see into the future and there are no real estimates due to that fact. Another issue with this question is the definition of a ‘human’ as it is likely that a significant part of modern-day human activities could be replaced with machines and – as I will explain soon – humans themselves could diverge over a long time spans due to genetic drift and artificial modifications like genetics. For this exercise, we will use ‘humanity’ very loosely and count all human offspring species and artificial creatures as ‘human’.
Now to answer the question we will have multiple scenarios to consider.
Scenario 1: Near Extinction
The most pessimistic of these being the “near extinction” scenario where humans die out in the very near future (say 2030) and virtually all humans that will ever live have already been born. To solve this scenario, we can simply take an estimate of how many humans have ever lived. This gives us a total population of 117 billion (or 117,000,000,000) humans.
Scenario 2: As long as most mammalian species live
The average mammalian species lives for around 1 million years before going extinct. Now we will assume that humans will survive for that period of time. To solve this scenario, we need to figure out how many humans Earth can support, as we will assume a lower bound here, and how long it takes for the population to replace itself. The UN projects that by 2100 population growth will have flattened out at 10.4 billion concurrent humans with an average life span of around 80 years. Dividing these two values gives us 130 million births per year.
Multiplying this value by the 1 million years from earlier and adding the humans already born in the past we get a total population of 130 trillion (or 130,000,000,000,000) humans that might ever live, around 1000 times more than our previous estimate.
Scenario 3: As long as the solar system exists
The sun will exist for another 5 billion years before it will die, with that we can simply redo our calculation from above and get a much larger value of 650 quadrillion (or 650,000,000,000,000,000), yet another 1000 times larger than what we had before.
How many humans can be supported by our environment?
Up until now, we have always assumed that we will not leave Earth and just inhabit all of it. However, as science and science fiction often show, this will likely not be the case. This massively complicates our calculations as there will not be an easy-to-calculate upper carrying capacity. To solve this issue here, we will need to look at how much energy a human need to survive. Some estimates have the landed needed per person at around 2 acres  which is equivalent to 8,100 m². One square meter receives around 1,350 W of energy, this will put the energy needed to support one human at about 10.9 MW of power, which we will round down to 10 MW as better farming methods are invented and because it makes the calculations a bit easier.
Scenario 1: Humans can harness all the Energy of the Sun
Now this is where it gets interesting, our sun produces 3.86 * 10^26 W of power. Dividing this by our need of energy per person we get 39 quintillion concurrent humans. This is already orders of magnitudes greater than even our highest estimate for remaining on Earth and it is the estimate of humans alive at any given point. If we use the 80-year lifespan of before and the duration of humanity altogether until the sun dies we can calculate the total population for remaining in the solar system to be around 2.4 octillion humans (or 2,400,000,000,000,000,000,000,000,000). I cannot express how many that is. But we are still not done.
Scenario 2: Humans can harness all the Energy of the Milky Way
If we figure out how to travel between stars, things get even more outrageous. Now instead of having an energy supply of ‘only’ 3.86 * 10^26 W we have access to 5.0 × 10^36W. However, we also get that output until all stars die out which will be in an even more distant future. Estimates for this post-stellar era of the Universe begin at a quadrillion years in the future. Doing the math this time gets us 6.25 tredecillion (or 6,250,000,000,000,000,000,000,000,000,000,000,000,000,000) humans. And we are still not done.
Scenario 3: Humans can harness all the Energy in the ‘Hubble Volume’
The Hubble Volume is defined as the volume of space that is receding slower than light away from us and as such everything we can ever interfere with under known physics. There are no good estimates on how much energy it produces, but it takes up roughly 3% of the observable universe and it is fair to assume that it would produce roughly the same amount of energy in linear correlation. This gets us a total energy production of 6.0 * 10^48 W of power and with that the massive population of 7.5
septendecillion (or 7,500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000) humans. This would put all humans that have lived so far into the first billionth of a billionth of a billionth of a billionth of all humans that will ever have lived.
And even here there are ways to optimize it with post-biological life, as each brain only consumes around 20W of power to run continuously, energy gain past the stellar era of the universe and computing at the Bremermann’s limit all of which could massively decrease the power needed to run a human or dramatically increase the time humanity could survive for. These will be left as an exercise for the reader.
Now what can we take away from this? The main takeaway is that we are incredibly early in human history unless we mess up in the near future and that so incredible lives are yet to be lived by humans that is impossible to comprehend just how many there could ever be.
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