Sorry, I accidentally made two copies of this article with different titles. And both have comments, so not sure what to do.

Unless you want to read the comments here, please visit the newer copy, Why ETs Won't Need to Colonize or Expand.

Science fiction stories often suggest that ETs, and our future selves also, would be expansionist, colonizing the galaxy, taking over worlds, and so forth. It's natural enough, because we are expansionist ourselves. But it's actually quite easy to see that ETs simply can't have expanding populations, at least not for very long. Not if they are anything like us. 

It's a simple calculation which I covered before. If their doubling time is once a century, say (for  ease of calculation) - then in a thousand years, their population multiples by a little over a thousand (two to the power ten). So after two thousand years it has multiplied by a million, by a billion after three thousand years and so on.

This clearly has to stop in quite a short time compared to the age of the universe or the length of time it must take for ETIs to evolve, both of order of billions of years.


The calculation for humans for instance shows that, starting with just two humans, and doubling every hundred years, you'd end up with an Earth's mass of humans well within 8,000 years. Or from our present population we'd get to the point where we have to create an Earth's mass of humans every century well within 5,000 years. (Techy details: the mass of the Earth is around  5.972*10^24 kilograms and the Guinness Book of Records smallest adult humans are more than 15 kilograms).

If our population doubles every 50 years, as it is at present, we get to this point within 2,500 years.

Assuming we don't dismantle Earth itself, we'd need to dismantle the entire asteroid belt, Mars and its moons, our Moon, Venus, Mercury and that still wouldn't be enough for a single population doubling, just for the extra mass of humans never mind food and habitats.


In that calculation I assumed humans of 15 kg, and will do so in all the calculations. That's  the mass of the smallest known adult man in the Guinness book of records.

Chandra Bahadur Dangi, the shortest man in the world, a Nepalese farmer, weighs 15 kg. In these calculations I assume that typical adult humans don't get any smaller than this in the future.
If you suppose somehow we had really tiny humans, as tiny as an Etruscan shrew, only 2 grams

Etruscan Shrew - smallest known mammal,  If you think humans can evolve to get as small as this in the next few thousand years, through genetic manipulation, add an extra millenium to the figures for doubling each century, an extra 500 years for doubling every 50 years, and an extra century for doubling every ten years.

This just puts off the inevitable for a short period, just a blink compared to the ages of stars or galaxies.

- that is a thousand fold decrease in mass. If doubling every century, that gives your population an extra millenium. If doubling every ten years, it gives it an extra century. Just add a thousand years or a century respectively to the upper bounds if you think it is possible for future humans to evolve to be as small as this - which I suppose may be possible.


If you assume humans can get even lower in mass than 2 grams - has to be a limit at some point. There are about 10^28 atoms in a human body. If you can think humans can get as low in mass as a single atom, add an extra ten thousand years to the calculations (or one thousand years if doubling time is ten years) and you've got it. This is discussed some more in the comments to this article..


At some point we have to stop this exponential growth. Indeed long before 5,000 years from now, probably. If we continue like this, then 1,000 years from now, we'd have a thousand times the present population of Earth.

We could get that far by converting the entire asteroid belt into space habitats. 

But if it is an exponential, once we've reached this stage, we have to repeat that feat every century (or whatever the doubling time is).

Then a thousand years later there are a thousand times as many humans again, so we will now need to create that many new habitats for humans ten times every year. 

Even that seems unlikely. So it doesn't seem likely we can continue like this for 2000 years. At the faster doubling rate of every 50 years, then that would happen by 1000 years from now. So one or two millennia of exponential growth would seem highly unlikely. Never mind making an Earth's mass of humans every century, 5,000 years from now (alternatively 2,500 years from now).


Colonization of other stars won't help at all, just postpone it slightly. By 13,000 years, a similar calculation, we need to convert an entire galaxy into humans every 100 years. And within 18,000 years we need all the matter in the observable universe every 100 years. Just to make humans.

That's assuming Faster than Light travel of course. Some think we may achieve it some day with the Alcubierre drive or similar technology. 

Two dimensional visualization of the Alcubierre drive. Theoretically this way of warping space could let you travel through space faster than light - the speed of light restriction in relativity is only on motion of light through space, not on the motion of space through space. The biggest obstacle to creating a drive like this is that it requires "negative energy density" - not just an absence of energy - but somehow an energy debt - and we don't have a clue yet how to create that, or if it is possible at all, except in rather transient ways such as the Casimir effect between parallel plates - seems hard to see how that could be isolated in any way and scaled up to create a drive like this. 


Many are skeptical that we ever will have such a drive. Without it, then that makes it much harder. Well before 13,000 years we have to stop because we can only explore and colonize a tiny fraction of our 100,000 light years diameter galaxy in that time.


So at some point we absolutely have to stop this exponential growth. The only question is, how will it happen?

So the same is also true of ETIs. And given that this exponential growth phase can't possibly last more than a few thousand years, unless the exponential is very slow indeed - then it is extremely unlikely that we will meet an ETI that is also in an exponential population growth phase like ourselves. It wouldn't be that surprising if we are currently unique in the entire galaxy, as an exponentially growing population of intelligent beings.

Actually - it's likely to happen much sooner than this, if we have uncontrolled expansion into the universe.  


With some not very strong assumptions:

  • That the colonization is unregulated - and the fastest growing most aggressive humans therefore colonize most planets and "win" the evolutionary race

  • That we don't have to terraform worlds to colonize the galaxy (terraforming could stop expansion for many thousands of years in each new solar system), but can just build Stanford Torus type habitats or similar in any solar system in the galaxies.
  • That we don't have faster than light communication or travel, so have no idea what is happening in a population of humans more than, say, a few hundred light years away. So there is little chance for learning from mistakes, collaboration and consensus, and we can't even try to persuade another group of humans from doing things that seem to us very foolish, because they can't hear us.

Then populations that expanded more rapidly would be favoured. If a population is able to fill a solar system every hundred years, for instance, then it will fill more solar systems than one that can fill a solar system only every thousand years.

For humans that would probably mean child pregnancy, children as young as five - or even younger, giving birth like Lina Medina.  This would be the rather horrific norm, if evolution continued in this way out of control. Or maybe a population that grows by cloning. 

Any population that did that would be the one that spreads out and occupies the galaxy. So long as it was also able to maintain space technology while doing so.

With a doubling every ten years, you could get to as many humans in a century as you could normally get to in a thousand years. Starting with a population like that of the present day Earth - instead of 2,000 years to get to the point where you need to find homes for a thousand times the population of Earth ten times every year (nearly every month), it's 200 years to get to that point, and you need to find homes for that many people a hundred times every year (twice a week).

Once you have an entire solar system filled with both types of humans - then if they are able to keep going - for every new solar system filled with humans that double every 100 years, there would be over a thousand new solar systems filled with humans that double every ten years.

As time goes on that disparity would just get larger and larger until the ones that double only every 100 years become all but extinct. Especially if the faster doubling humans are more aggressive and warlike as well and actively destory the slower doubling rate humans and take over their colonies for their expanding populations.

So - it's pretty clear that we, and all extra terrestrial intelligences likewise, have to stop exponential growth at some point, just a blink into the future in galactic timescales. So the chance of us spotting another intelligence in this same phase as ourselves seems very remote indeed.

The only out here would be if the ETIs have immensely long lives and are late developers, only able to give birth, say, at age a million years - they could continue exponential growth for billions of years before they hit this issue. More about this later.


And actually - it might well be that we stop by 2050 - green curve here

Or we might level off at ten billion around 2100.

There's plenty of hope here, because we have already reached peak child. See Viewpoint: Five ways the world is doing better than you think. There are no more children being born this year than there were a couple of years ago. The population continues to grow because of improvements in health and child mortality.

This seems a universal rule, for all religions and none, that the wealthier nations have lower birth rates, with the most wealthy ones mainly falling below replacement already. And as nations become more wealthy their average birth rate goes down, without any need for population control.

The big unknown for population projections is Nigeria. It's got a large population, is a poor country relatively, and has a high birth rate - it's projected to have a population larger than China if its population growth continues - but that seems incredible, surely it won't continue to grow exponentially for as long as that.

It's not impossible. Peak child could just be a plateau if some country or countries with a high birth rate really takes off for some reason. 

But even the "middle of the road" projection now has the population leveling off in the 2100s. And the optimistic projection sees it peaking by 2050. So I think there is reason for optimism there. 


At any rate, you can predict confidently that it's going to stop some time in the next 8,000 years because of the arguments I just gave. And I think fairly likely that it stops growing exponentially in less than 2000 years from now.

The main thing is to make sure it happens peacefully, and not through war and starvation. Right now seems it will happen due to increasing prosperity - which would be best way it can be solved.


I think this is also part of the reason why extra terrestrial intelligences haven't filled the galaxy already. They can't be expanding exponentially, apart from some extraordinary coincidence. So the big question is - at what point do they stop - how much of the volume of the galaxy or the universe do they occupy before they stop expanding? 


We might be the only intelligent creatures in the entire visible universe of course (see Rare Earth, and Lucky Planet). If so, the future is all up to us, a huge responsibility.

But if not, well, it's reasonably clear that none of the ETIs have filled our galaxy.  Even if you think they can somehow do it invisibly to us - well we don't see their tracks on Mars, some of which is explored from orbit to very high resolution. 

Curiosity's tracks photographed from orbit around Mars. We have much more capable telescopes in orbit around Mars than around the Moon which is why we can see our rovers and tracks there clearly from orbit. There are no signs of ETI tracks on Mars or the Moon or anywhere else in our solar system.

Nor on the Moon, and all the objects studied in the solar system seem unexploited by anyone. That doesn't fit with the idea of ETIs that fill our galaxy and are also expansionist. If they are here, they are not here to exploit the resources of our solar system. Or they would have done so millions of years ago. The sky would  be full of their habitats and we wouldn't be here, probably.


So surely the maximum region filled by ETI colonies at any reasonable population density is less than the volume of the entire universe. And if any colonies are galaxy filling, they haven't filled our entire galaxy yet. 

So how large is the largest region reasonably fully populated by any ETIs? Is it one solar system? Is it several solar systems? Hundreds of light years? Are there ETIs that fully occupy entire galaxies (if obviously not ours)?

Given that almost all ETIs have to have stable populations, there has to be a maximum region of space fully occupied by an ETI, one they are content to occupy and don't need any more. So what is it?

Whatever the answer is here, then - seems they must have somehow stopped their expansion before reaching our solar system. And they surely have done this millions of years ago, and most likely billions of years ago, because again it would be an astonishing coincidence to meet them immediately after they stop this expansion.

That's why I would be astonished if we found ETIs with an expanding population. And the main question then is, what is the maximum region they occupy.


We do have some experimental data here also, in the near infrared search for extraterrestrial civilizations.

This has searched many galaxies in the infrared. This is what the Andromeda galaxy looks like in the near infrared

They searched to see if they could find any galaxies that had anomalous infrared signatures. If so these perhaps could be galaxies that have been colonized pretty much in their entirety by extraterrestrials, creating Dyson spheres and so on. They searched 100,000 galaxies and found nothing obviously anomalous. So - probably ETIs, universe wide, don't go into galactic colonization in a big way (or they don't exist at all).

So the maximum region fully colonized by an ETI may, perhaps, be less than an entire galaxy. (The alternative is that they don't exist).


I think this is part of the explanation for the Fermi paradox myself. 

The other part of the explanation, as I said in my earlier article, would be that self replicating humans or ETIs are more dangerous for galactic exploration than self replicating robots (which are much more easily controlled and can be set to not evolve at all). So they'd surely send robots first.

So, you wouldn't want to set off colonies of uncontrolled self replicators, either robotic or human, at least not to any point too far away to be part of your culture. You would need to have intercommunication with them easily. 

Without FTL, that probably means that any colonies are not much further than a few light years and probably it is safest if you don't travel much beyond your solar system for colonization purposes. 

Though for exploration without colonization you could go anywhere.

Either that or some other solution, but some way or another they have to find a way to stop this exponential growth peacefully. 


If they can't do it peacefully, then it would have to happen through violent means.

If this happens violently, they will have to be involved in constant warfare, and starvation, which has to halve their population every doubling time, whatever it is. Which, if expanding into the galaxy does evolutionarily favour the rapid reproducers -that doubling time could easily be as little as ten years. The population of the entire galaxy or universe, or however far they expand, would need to be halved every 10 years through violence, if they haven't found a peaceful solution.

And - this would all be just one ETI, contining on and on, destroying half of their entire civilization every ten years, throughout the galaxy, for all future time. Once started, once they spread out to 1000 light years, for instance, is very hard to see it stopping. And given that the ones that expand most agressively are likely to be technologically ahead of everyone else also - how could it possibly ever stop?


As a solution to the Fermi paradox, you can imagine that cultures become bored, intellectual, that they get just tired of living, and so on. In a situation like that, our intelligence has a chance to lift us above the forces of evolution in many ways.

But that assumes you are dealing with a single culture. But if it's an exponentially growing one with light speed barriers to communication -then there will surely always be at least some "remnants" that have the original vitality of the original colonizers, and they will keep fighting back, and expanding. 

One way or another, if nothing was done about it, no foresight into consequences, then the galaxy would probably fill with humans who double in population every five years or less. That then reduces the maximum of 13,000 years for total disaster down to a twentieth of that, less than a thousand years. 

Within a millennium of the first unrestricted galactic colonization, even with 'FTL, you'd end up with that situation where there is no longer any practical way to create more humans from the matter in the universe. So half your entire population of humans has to die of starvation, wars, etc, every doubling time.

Nobody could look at that possibility and see it as a desirable outcome, seems to me - either for us or for any other beings in the galaxy. So civilized beings would surely foresee that and somehow prevent it. The question is how.


Space colonies would be so fragile and easily damaged, that you'd need to have a peaceful, well ordered, and most importantly, also forward seeing civilization. At present astronauts absolutely have to be rule following,and authority respecting, like people in a submarine, or space divers, or they have no chance of surviving, because conditions there are so dangerous. It only seems easy because all our astronauts are highly trained experts, and even the "space tourists" also have to fit in with this and be trained extensively.

If you do something wrong like make a mistake while donning your spacesuit or while docking the Soyuz TMA, you endanger yourself and everyone else. 

Space is currently a place for people who are happy living in highly regulated conditions.

How that develops in the future is another matter. How will we become peaceful enough in space to have maybe eventually millions or billions of people in space without them destroying each other? Or will we stay on Earth instead, protect the Earth and keep it in good shape?

And how will exponential growth stop, and how will we avoid the traps of totally unrestricted galactic colonization?

There may be many possible ways to achieve such a civilization. It doesn't have to be be based on top down authority for instance. But one way or another it would need to be like that. 

But it would seem that one thinG it just can't be is a "free for all, most rapidly replicating and most aggressive gets the galaxy" - that's not going to work. I think any ETI will realize this, and do so some time before they get to the point where it is practical to actually colonize a galaxy.


That's my answer to the "monoculture objection" that whatever answer you come up with here, it's impossible to see it continuing to operate as cultures change, over thousands of years. If the inhabitants of Earth are reasonable for the next thousand years - what about a million years from now? What about the inhabitants of Alpha Centauri? Or any other human colonies? How can they all be so disciplined for all future time? That's the big question here. See the infrared search for extraterrestrial civilizations for a recent overview of this objection. 

So, the idea is, if it was absolutely clear and plain to ETIs that they can't safely colonize the galaxy - then that is a realization that could be so clear and obvious that it becomes unthinkable not just for one civilization but through all future civilizations - just as for self replicating machines. It would be, for them, like setting off a nuclear weapon in a city, and indeed in your own city. Especially if they also have immensely long lives so that they, or their children, would see the consequences just a thousand years into the future, or less.

And then the idea is, that ETIs that are not stable enough to be able to maintain this level of understanding long term are not yet mature enough to have space colonies long term either.

So, in my view, that's why they aren't here, if there are ETIs in our galaxy. Maybe they are explorers, but if so, the galaxy - since not totally crammed to the brim with ETIs - will still be a big place for them, so then the nearest one could easily be thousands of light years away.

Either that - or we are the first. If we are first, we have a huge responsibility. Maybe we can find other ETIs in other galaxies, and if so, perhaps we can learn something from in that case.


One way to deal with the lightspeed barrier doesn't require FTL, but just ETIs with very long lifespans. if you have ETIs that live for millions, or billions of years, then some of the explorers would be able to criss-cross the galaxy many times in their lifetimes. That might then give enough stability to permit a galactic civilization. 

And if they typically gave birth, say, at age one million or one billion, then again that would hugely reduce any problems of exponential population growth. That's about the only scenario i can think of where we might possibly encounter ETIs with an exponential population growth right now - though the chances are they would not have any need for exponential growth at that stage.


If there were beings like that in our galaxy - they might help to provide the stability - and it might be they that educate the younger species and make sure they don't go off the rails and start intergalactic colonization. 

So - either it happens automatically, any intelligent species eventually once it reaches that point, will decide for itself not to colonize the galaxy. Or else, might be that they have to be educated. Either by other species -or by some object lesson - e.g. observe some distant galaxy where it has gone horribly wrong.

If so - well as only a hundred years old civilization, we might easily not yet have encountered whatever it is that either gives us the object lesson we need - or else, tapped into whatever source of knowledge or civilization is out there that will in the future educate us in this topic area. We have only just begun to build space telescopes. Many radio frequencies are not accessible to us until we build radio telescopes on the far side of the Moon because of interference from Earth. We have hardly started on laser light detection. And there may well be other ways of signaling to each other that we don't even know about - just as a couple of centuries ago nobody would have the slightest idea that you could detect radio waves, or that they even exist. There may well be other forms of energy or whatever, easily detected by some not too complicated apparatus, which to ETIs seem very rudimentary but we haven't invented it yet.


Apart from radio and laser searches, there's also the search for extraterrestrial artefacts. Most recently,  KIC 8462852. That's the one with the strange asymmetrical dips in its light curve. Down to a surprising 22%. One possibility is a combination of comet like huge clouds of material, maybe also with gravity darkening of the star.

What we've seen so far there is surely not yet evidence or even suggestive of extra terrestrials. On the other hand though, if you think there could be ET mega structures out there - well - if you are searching for these megastructures, this would be one of the top candidates on your list. Which doesn't mean you think it is a megastructure. Just that it is an obvious place to check out to see if it is.

With this idea that ETIs will always find peaceful ways to stop exponential growth - then whether there are any ETI megastructures will then depend on the point at which their population expansion stops. And on how many expand that far.

It seems highly unlikely we could figure this out a priori so will just have to search for megastructures to see if they exist. But if we don't find any - I think that just means that ETIs don't build megastructures, for whatever reason, or none close enough to detect easily. I don't see it at all as evidence that the ETs themselves don't exist.


We are also of course looking for technological ETIs. Many of the most intelligent non human beings on Earth would not be able to develop technology like us, even if they had the same level of intelligence as us. E.g. elephants?? Grey Parrots?? Dolphins - very unlikely. Octopuses - how would they develp fire?

So the galaxy could easily be filled by maybe half a dozen non technological - but very advanced long lived billions of years old civilizations - for each technological civilization. The nearest place such a civilization could exist is in our own solar system - a non technological civilization in Europa's oceans for instance would be impossible for us to detect, or for them to detect us - at present anyway.


A maximum future population of course doesn't mean at all a limit to exploration of new frontiers or limits to understanding or quality of life or insights.

Actually a smaller population makes the frontiers much more endless if anything. You could explore the galaxy for millions of years and continue to find places and phenomena - and maybe new creatures and beings and other intelligent species (especially non technological) that no human had ever encountered before.

It's much more of a "Star Trek" type future than a galaxy that gets crammed to the brim with humans and their creations within a few thousand years.

I have a section ENDLESS FRONTIERS about this in my other article Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth? - see the section on ENDLESS FRONTIERS. 


And - a population that stops growing, as all ETIs will have to do (apart from some very "out there" science fiction ideas) could do so in many ways other than just running out of resources, starvation and warfare.

  • Nomadic population, like the Puppetteers planet in Ring World (Larry Niven) for Sci. Fi. fans. The Puppetteers don't like to leave their home planet but take it with them when they decide to leave our galaxy. They send explorers through the galaxy, but don't have any off world colonies. They would not have any problems of this sort.
  • Could fill a few dozen star systems. So long as they are all in communication and they are confident in each other that they won't compete and break away and try to fill the galaxy - if it is clear this is totally unthinkable to everyone for all future time - then this would be safe also
  • If their planet becomes uninhabitable - as our Earth will probably be a half billion years into the future - well moving to another planet even to another solar system is just another form of nomadic population. That's not going to risk the issues of unlimited population growth through the galaxy.
  • If their lives become very long, say hundreds of millions of years, then they can keep in touch with each other over an entire galaxy no problem - you'd be able not just to send messages but actually visit your long distant cousins the other side of the galaxy several times in your lifetime. Even with a lifetime of a million years, the galaxy wouldn't seem so huge to them. If their culture also develops slowly, and say, they have their first children at age a million, or whatever, age a hundred thousand - and there is no prospect at all of that getting reduced - they would be comfortable colonizing a galaxy.
  • Any other way that ensures a sensible maximum population. Depends on the ET probably.

Those are just a few ideas. There may be many others and if we meet real ETIs the answer may be something we never thought of.


This is related to the recent discovery of a star KIC 8462852 which some think could be a partly constructed "Dyson sphere" - a megastructure of mirrors or other structures to collect all or most of the light from the star to use to power their civilization.

If sensible they will decide for themselves when to stop growing, rather than just expand until they run out of resources and have it forced on them through starvation or warfare (depending on their temperament).

So why not stop at a Dyson sphere? Or build a Dyson sphere then stop half way after they realize they don't need anything as big as this for their planned maximum population?

No way do you look at this evidence and conclude "this is a megastructure". The other hypotheses of gravity darkening, and large comets make a lot of sense. By large comets there, they don't mean the small things we call comets here in our solar system. They mean huge, world size comets made of ice all evaporating to make enormous comet tails.

So, chances are it is something like that. No need to invoke aliens to explain it. 

But in the other direction, if you are already searching for mega structures, as some researchers are doing - they surely should also look at this star, just in case. It is one of the most obvious candidates for a megastructure to investigate more closely in our galaxy.

So that leads naturally to the other ideas and questions in this article, which I felt were worth making into an article. Especially since it gives another point of view on the usual assumption of SETI researchers that if you build one Dyson sphere, then within a few million years you'd have Dyson spheres around every star in the galaxy. The idea of this article is that perhaps that is not as inevitable as some have assumed. And that perhaps no ETIs ever want to be galaxy filling, for the reasons given here, once they have time to think it through in some detail, but prefer to decide for themselves what their maximum population will be rather than just keep going until they run out of resources to grow any more. Or may be object lessons or other ETIs that educate them.


See also my Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth?

You can also get it for kindle or to read on other computers in book format also, as Replicating Robots - Safer Than Humans for Earth&Galaxy: Is This Why ETs Haven't Colonized Earth Already, Millions of Years Ago?

I haven't yet converted this article into a kindle book but may do so. I'm thinking about possibly doing a kindle book of all the articles here on extra terrestrials - from somewhat serious ones like this to light hearted fun ones such as what food you can share with an ETI host.