3 ms·

Populations can not grow exponentially for a long time, the volume of space you can colonize only grows with the third power of time given that you are limited

Populations can not grow exponentially for a long time, the volume of space you can colonize only grows with the third power of time given that you are limited to the speed of light. And within galaxies it's more like the second power due to the flat shape of galaxies. With our current rate of population growth, I think about one or two percent per year if I remember correctly, we would only make it a few percent across the Milkyway before running out of resources even if we could travel at the speed of light. You can search my comment history, there is somewhere a more detailed comment with all the numbers and assumptions. Bottom line is, if you want to colonialize the universe, you either need a population growth rate very close to zero and a lot of patience or you have to beat Einstein. EDIT: Here [1] is the mentioned comment. [1] https://news.ycombinator.com/item?id=13638012 https://news.ycombinator.com/item?id=13638012

njarboe 9y ago
Or growth and then collapses all over the place. If humans colonize the galaxy (universe is really hard), it will likely look like a growing amoeba than an expanding sphere.
- Tepix 9y ago
  Even the solar system is hard. The galaxy will be extremely challenging.
xelxebar 9y ago
This is an amazing observation. It's not terribly difficult to work out the numbers once it's been pointed out, but I'm not sure if I'd have come up with it on my own. For others who like this sort of thing, I highly recommend Isaac Arthur's YouTube channel: http://m.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g http://m.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g
vidarh 9y ago
Our current rate of population growth is declining, and projected to become negative (for a while) towards the end of this century, before resuming growth at a much slower pace. Not disagreeing with your conclusion, but assuming the current rate of population growth is relatively meaningless seeing as no reasonable model assumes current growth rates will continue (as it's been steadily changing).
- danbruc 9y ago
  I only used our current population growth rate not because I expect it to remain unchanged but because it is on the order of one percent which is towards the lower end of growth rates one will encounter when looking at all kinds of different things and systems. The important point is that this relatively small growth rate is already way too large. I don't know what you were exactly thinking of when you wrote »[...] before resuming growth at a much slower pace [...]« but maybe one-tenth of a percent? Unfortunately that is still way too large, to colonialize the Milky Way before hitting resource limits you need it down at one one-thousandth of a percent or so, for the entire universe you need to decrease it several orders of magnitude more. At least on average, if you could use something like cryostasis while travelling through the vast emptiness between stars you could afford times with higher growth rates.
  - vidarh 9y ago
    We'll resume growth, at least for some time, largely because it will take longer for everyone to get down below reproductive levels, and for life expectancy to level off. The first drop basically comes because we have a massive population bulge in the post-war years. The growth the resumes because the drop in population growth worldwide is not yet enough to offset the steady growth in life expectancy in the third world. But longer term that is likely to end up as another bulge. In terms of developed countries, most have birth rates below replacement. Once life expectancy growth worldwide has flattened out enough, and birth rates have declined to e.g. European levels, we can expect the bigger problem to be preventing terminal decline. This map [1] is quite interesting. The blue/light blue parts represents birth rates below replacement. In 2015 France too was below replacement. About half of the worlds population lived in countries with sub-replacement birth rates as of 2010, and despite the fact that the population growth tilts that towards high population growth countries on a continuous basis, the birth rates of those countries is dropping pretty much across the board, meaning it is likely that the proportion of the worlds population living in sub-replacement countries will actually increase. E.g. India is expected to drop below replacement this century too, and actually experience population decline (decline lags birth rates, of course, because you need to wait for the generation that's dying off to be larger than the one being born). So I don't think population growth is a realistic concern when it comes to interstellar exploration. On the contrary. [1] https://en.wikipedia.org/wiki/Sub-replacement_fertility#/media/File:Countriesbyfertilityrate.svg https://en.wikipedia.org/wiki/Sub-replacement_fertility#/med...

njarboe 9y ago

Or growth and then collapses all over the place. If humans colonize the galaxy (universe is really hard), it will likely look like a growing amoeba than an expanding sphere.

Tepix 9y ago
Even the solar system is hard. The galaxy will be extremely challenging.

xelxebar 9y ago

This is an amazing observation. It's not terribly difficult to work out the numbers once it's been pointed out, but I'm not sure if I'd have come up with it on my own. For others who like this sort of thing, I highly recommend Isaac Arthur's YouTube channel: http://m.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g http://m.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g

vidarh 9y ago

Our current rate of population growth is declining, and projected to become negative (for a while) towards the end of this century, before resuming growth at a much slower pace. Not disagreeing with your conclusion, but assuming the current rate of population growth is relatively meaningless seeing as no reasonable model assumes current growth rates will continue (as it's been steadily changing).

danbruc 9y ago
I only used our current population growth rate not because I expect it to remain unchanged but because it is on the order of one percent which is towards the lower end of growth rates one will encounter when looking at all kinds of different things and systems. The important point is that this relatively small growth rate is already way too large. I don't know what you were exactly thinking of when you wrote »[...] before resuming growth at a much slower pace [...]« but maybe one-tenth of a percent? Unfortunately that is still way too large, to colonialize the Milky Way before hitting resource limits you need it down at one one-thousandth of a percent or so, for the entire universe you need to decrease it several orders of magnitude more. At least on average, if you could use something like cryostasis while travelling through the vast emptiness between stars you could afford times with higher growth rates.
- vidarh 9y ago
  We'll resume growth, at least for some time, largely because it will take longer for everyone to get down below reproductive levels, and for life expectancy to level off. The first drop basically comes because we have a massive population bulge in the post-war years. The growth the resumes because the drop in population growth worldwide is not yet enough to offset the steady growth in life expectancy in the third world. But longer term that is likely to end up as another bulge. In terms of developed countries, most have birth rates below replacement. Once life expectancy growth worldwide has flattened out enough, and birth rates have declined to e.g. European levels, we can expect the bigger problem to be preventing terminal decline. This map [1] is quite interesting. The blue/light blue parts represents birth rates below replacement. In 2015 France too was below replacement. About half of the worlds population lived in countries with sub-replacement birth rates as of 2010, and despite the fact that the population growth tilts that towards high population growth countries on a continuous basis, the birth rates of those countries is dropping pretty much across the board, meaning it is likely that the proportion of the worlds population living in sub-replacement countries will actually increase. E.g. India is expected to drop below replacement this century too, and actually experience population decline (decline lags birth rates, of course, because you need to wait for the generation that's dying off to be larger than the one being born). So I don't think population growth is a realistic concern when it comes to interstellar exploration. On the contrary. [1] https://en.wikipedia.org/wiki/Sub-replacement_fertility#/media/File:Countriesbyfertilityrate.svg https://en.wikipedia.org/wiki/Sub-replacement_fertility#/med...