As we saw in the introduction, in Further into the future, if we have millions in space - can we be one of the "wise ET's"?, that leaves a major question open:

"Can we colonize our solar system, and further afield without taking our propensity for extremism and warfare with us?"

We've had a good look at many places where humans could colonize our solar system in the future. There are many of them, apart from Mars which has planetary protection issues:

If we can also make the habitats in these places low maintenance, they might even, perhaps, be easier to inhabit than Earth itself. They could be designed with an ideal climate, and free of earthquakes, volcanoes, hurricanes and so on. This may seem a somewhat far future prospect, but it is not nearly such a distant future as a terraformed planet. We could complete such a habitat in decades, rather than millennia, and it would be much easier to maintain and to fix if something goes wrong, than a planet.


When we look at our solar system, at first, it seems hostile, with nowhere in it anything like as habitable as Earth. However, in the not so very distant future, we could make our own niches in the solar system, as habitable as Earth and even, perhaps, easier to inhabit, because we can design these habitats to be optimal for us, free of earthquakes, volcanoes, hurricanes and so on. Vast kilometer scale habitats, maybe in lunar caves to start with. It would depend on making them self sustaining and low maintenance. If we can have self sustaining, low maintenance spcaecraft as well, then travel throughout our solar system should be no problem, to get to these places. For more about this see the sections:

However, I raised a question in the introduction, in the section: Further into the future, if we have millions in space - can we be one of the "wise ET's"? about whether we can colonize our solar system, and further afield without taking our propensity for extremism and warfare with us.

If the violence, terrorism and extremism of all sorts that we get on Earth propagates into space, how can humans remain in space for long? Any habitats in space will be so fragile to violent actions, that even lobbing a rock at them at a few kilometers per second would destroy it. Any group of millions of people with space technology would find that an easy thing to do. How could anyone survive a war in space like that?. With no air to breathe, there'd be no possibility of survivors hiding out in caves. If their environment control, hull integrity, or spacesuits are destroyed by the blast, they no longer have any way to survive.

Once we have hundreds of thousands, and then millions of people in space - how can we restrict space colonization to the "good guys or gals" whoever we think those would be? Longer term, if we are able to colonize other stars, the difference between positive and negative future outcomes may become even more stark.

Let's look at this more closely. Perhaps in the next few centuries we will have the ability to visit other stars in person? It might seem great at first, to think of a "civilization" like ours spreading to fill the galaxy. But - is that so desirable long term? I would like to suggest here, that like the colonization of our solar system, it depends very much on how and why we do it.

This is where i t might be wise to look into Fermi's paradox, "Where is everyone?" Why haven't extra terrestrials filled the galaxy already through exponential population growth? They should have done that in a blink of time, on geological timescales, within a million years of when they first developed the ability to set up interstellar colonies. Do they give up? Or do they destroy themselves before they get very far? Or is there some other reason? Are there pitfalls to avoid here, and executive decisions to make? Do we need to be a "Wise ET" to explore the galaxy, and if so what do we need to do?

There is a massive literature on Fermi's paradox. Here are a few sources which present some of the many solutions that have been proposed.

I'm taking a "sustainability solution" approach to Fermi's paradox here. This is an approach suggested by Seth Baum, a mathematician and electrical engineer who did a PhD in geography, and co-founder and director of the Global Catastrophic Risk Institute.

His approach is to tackle the usual assumption that an extra terrestrial would be bound to colonize a galaxy with exponential population growth, as soon as they develop that capability. What if they don't? I think it provides interesting insights into the whole debate. Most of the solutions are based on this assumption. What happens if we question it?

His idea was to look for sustainability solutions to the Fermi paradox. He writes in his conclusion to The Sustainability Solution to the Fermi Paradox

"Thus, the Paradox can only conclude that other intelligent civilizations have not sustained exponential growth patterns throughout the galaxy. It is still possible that slower-growth ETI civilizations exist but have not expanded rapidly enough to be easily detectable by the searches humans have yet made. It is also possible that faster-growth ETI civilizations previously expanded throughout the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might have left have also remained undetected. Both of these growth patterns can be observed in human civilization, suggesting that they may be possible for ETI civilizations as well."

He writes in Is Humanity Doomed? Insights from Astrobiology

"When a population expands exponentially, it typically experiences one of two fates. First, it could overshoot the carrying capacity of the ecosystem supporting it, depleting key resources. In this scenario, the population quickly suffers a dramatic crash. The damage to the population or to the ecosystem is often permanent such that the population never regrows its numbers. Perhaps extraterrestrial civilizations that continue to expand exponentially suffer similar crashes before they can expand throughout the galaxy, or before they would be observed by us or any other civilization. While civilizations sufficiently intelligent for space travel might understand the dangers of unsustainable expansion, they might not act on this understanding, just as human civilization does not always act on its own understanding of these dangers. This would explain why we do not observe other civilizations."

" The other fate of an exponentially expanding population avoids the crash. In this scenario, the population slows and possibly ceases its growth early enough that it remains within the carrying capacity of its ecosystem. This growth pattern approximates a logistic curve and is also commonly observed in populations on Earth. This growth pattern is fundamentally sustainable. We may be experiencing this growth pattern with human populations, since global human population growth has been slowing in recent decades. In this case, the reason we do not observe extraterrestrial civilizations is because they do not expand rapidly enough to fill the galaxy. These civilizations could be those that understood the dangers of unsustainable expansion and successfully acted on this understanding. They are out there, but they are hard to find."

"Note that some extraterrestrial civilizations might not expand at all. They might be highly intelligent but simply not desire to expand. Indeed, there are human populations that do not pursue expansion but instead favor other objectives. The existence of intelligent, non-expansive extraterrestrial civilizations is fully compatible with the sustainability solution to the Fermi Paradox because these civilizations do not expand rapidly—indeed, they do not expand at all. If the sustainability solution explains the absence of observation of extraterrestrial civilizations, then non-expansive civilizations and slowly-expanding civilizations could exist. Both of these civilization types would be sustainable, but they would also be hard for us to find."

So, first, what's the problem with exponential growth? Well, from where we are standing now, to start with, there's no problem. As far as we can tell, we have a vacant galaxy out there. We could expand into it, probably for centuries, no problem. But if we expand because of rapid population growth, and we need to find somewhere for all our descendants to live, then this approach will hit a crunch rather quickly on timescales of only thousands of years. Even with just a doubling of our population every century, if we did that for ten thousand years, then we would need to create a sun’s mass of humans every century to keep going with the exponential growth. For the calculation, see my Why ET Populations Can't Continue To Expand For More Than A Few Millennia.

Extra terrestrials, and ourselves, might care about such things out of concern for the fate of their distant descendants. Or for that matter, they, and we, might care about other beings too. We might care about any other extra terrestrials in our universe without technology, who might get destroyed, overwhelmed by expansionist exponentially growing populations of humans. We might also care about unique species and unusual and rare forms of life with their own biochemistry and adaptations that might be destroyed

Then there's another aspect to this too. Perhaps by the time our descendants are capable of interstellar colonization, a century or two from now, they may have the ability to extend their lifespans to thousands of year. Perhaps, who knows, eventually they might live for millions of years. If so, then those consequences, even several thousand years from now, might be ones that they actually will experience themselves. This may lead to more long term thinking and planning.

We seem to be the intelligences in our biosphere, and so, we can help direct it towards a better future. We seem to be the only components of our biosphere with the ability to foresee these future problems before they arise, as it starts to spread through our solar system and our galaxy.

Luckily we reached "Peak child" in 2005. The number of children in the world has remained steady for over a decade now. Our population will continue to increase rapidly for a while, because people world wide are living longer (on average). It seems at least possible that our population will level off naturally. Even the “middle of the road”projections now show the population trending towards 11 billion by 2100 while lower projections have it level off at ten billion or even start to decline towards the end of the century.

In the graph above, the red dotted lines show the upper and lower limits for the 95% prediction interval. The blue lines are for +- 0.5 children per couple average. You can look up the data here, projections for the whole world, or for individual countries and regions, on the graphs page for the UN population division.

If the future pans out like that, we should be okay on our planet, through to 2100, and if we can achieve a stable population by then, or soon after, as those graphs suggest, we'd be okay through into the indefinite future.

However, what about longer term? What happens if we start to set up interstellar colonies? Even if this happens slowly, if it triggers a new exponential growth process, then ugly problems soon rear their head again, as we'll see. Suppose we set up only ten colonies around nearby stars in the next thousand years, with one new colony on average each century. Let's suppose that each of those set up another ten in the next thousand years

(techy note on these calculations: in practice some of them would set up new colonies even before the first thousand years is over, but to simplify the calculation, let's ignore those - the actual numbers would be a bit larger than for our calculation).

So now, after two thousand years, we have a hundred space colonies. So far, fine. Now each of those set up another ten colonies, and after three thousand years we have a thousand colonies, and so on. Still there seems no problem. Exponentials are like that. For a long time nothing seems to be happening much. But then, rather suddenly, it gets you.

After only twelve thousand years, we have a trillion colonies. Our galaxy has around 100 billion stars for them to colonize. But it's worse than that. Our galaxy is around 100,000 light years in diameter. Unless the colonists have warp drive, they can't travel further than 12,000 light years (or 1,200 light years if they travel at a tenth of the speed of light). Clearly our galaxy would get crowded within a few thousand years at exponential growth, even at this slow rate of a ten fold growth per thousand years.

When you hit an exponential there isn't much you can do except to delay the effects slightly. Even tiny two gram colonists as small as the Etruscan shrews, are no help.

Etruscan Shrew - the smallest known mammal, only 2 grams, much less than a ten thousandth of the mass of an average adult human.

If you think humans can evolve to get as small as this, in as short a time period as a few thousand years, through genetic manipulation, you can add an extra four or five thousand years to the time it would take to run out of matter to make colonists, for this example situation of a ten fold increase every thousand years

Even if we go all the way to a science fiction future of. massless colonists, and have total conversion of matter into energy, and faster than light travel (e.g. warp drive), it only delays the inevitable by thousands of years. Even zero rest mass colonists need energy. Even photons, though they have zero rest mass, still have energy, which has to come from somewhere, for instance, from nuclear fusion or some other form of conversion of matter to energy.

To take this to the limit, suppose we have minimal energy "colonist photons" as vast feeble pulses of light with wavelengths equal to the diameter of the observable universe. I'm not saying that this is possible, but anything else that is possible will surely have at least as much energy as this. Still, none of that does more than to delay the inevitable. Soon, your exponential growth spurt has to stop. As usual, I'll indent the calculations so that they are easy to skip:

Even if you can reduce individual humans to tiny creatures of a few grams like a pigmy shrew, you'll soon run out of matter for all the colonists, The mass of the universe is about 3 x 1055 grams according to one estimate. If we can reduce the mass of a colonist to one gram, starting with one billion humans (say), and increasing the population ten fold every thousand years, you'd run out of matter in the observable universe to make colonists within 46,000 years.

You can try a science fiction scenario of massless colonists - could we have "colonists" in the future that have similar mass to a photon somehow? If that was possible, you are still limited because the energy of a photon depends on its wavelength.

The energy of a photon in electron volts is 1.2398/λ where λ is its wavelength in microns. There are about 1036 electron volts to a kilogram (if you can directly convert matter to energy as an advanced ETI might be able to do). So, now suppose a photon has a wavelength of 93 billion light years (the diameter of the visible universe of 93 billion light years according to one estimate) or around 8.8x1032 microns. Then it's energy will be around 1.4x1033 electron volts (1.2398/(8.8x1032)).

If we can have massless colonists, each consisting of just one photon, and use the feeblest possible low energy photons, so that each one has a wavelength so vast it spans the entire observable galaxy, then the total number of colonists we could make from the available matter in the observable universe is at most 3 x 1055/ (1.4x10-33) or about 2*1088 . So, starting with a billion colonists, and a slow exponential, multiplying population by ten every thousand years, even with warp drive, total conversion of mass to energy, and massless colonists with wavelengths that span the entire observable universe, we run out of matter in our observable universe to make these massless single photon colonists within 76,000 years.

So, even if the population increases only ten fold every thousand years, you will run out of matter to make new colonists in the entire observable universe, well before 46,000 years if the colonists have a mass of only one gram each. Or well even if the population increases only ten fold every thousand years, before 76,000 years even if each colonist consists of only a single photon with the least amount of energy compatible with fitting that photon physically into the size of the observable universe.

If the exponential growth is very slow, say, a ten fold increase every million years, the calculation is the same. It's now 76 million years to the single photon colonist end point, even with warp drive and total conversion of matter to energy. Whatever the timescale is for a ten fold increase, just multiply that by 76 and that's the absolute limit of exponential growth within our observable universe. That's going to happen no matter how those colonists evolve and what technology they have, at least if they remain within the laws of physics as we understand them today.