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.

## HOW LONG BEFORE THE MASS NEEDED FOR EXTRA HUMANS EVERY CENTURY IS MORE THAN THE MASS OF THE EARTH?

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*1024 kilograms and the Guinness Book of Records smallest adult humans are 15 kilograms, and 5 kilograms respectively).

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.

You get to the point where you need to convert the sun's mass into humans just two thousand years later (or a thousand years later with the faster doubling rate). Or to get from a population of two, to a population with the mass of the sun (for instance when colonizing another star system), with a doubling time of a century, takes less than ten thousand years (or five thousand years). And once you get that far, you need to convert at least a sun's mass of matter into humans every century to keep going.

(Techy note: every population with an exponential growth pattern has a doubling time. For instance our world population has been growing exponentially for many decades now, with a doubling time of about 50 years. It's a similar idea to the half life, the time taken for the population to halve, for populations that decay exponentially.)

## WHAT IF HUMANS ARE EVEN SMALLER THAN THIS?

In all these calculation I assume future humans with an average mass of 5 kg. That's the mass of the smallest known adult human, a lady, Jyoti Amge, in the Guinness book of records.

Chandra Bahadur Dangi, the shortest man in the world, a Nepalese farmer, weighs 15 kg. Jyoti Amge has a mass of only 5 kg. In these calculations I assume that typical adult humans have a mass of at least 5 kg.

The idea is that in the future if populations run out of resources, humans could get really small. That five kilogram seems a reasonable guess at a lower bound, that probably they won't get much smaller than this, on average, as there isn't much time for evolution in a few thousand years.

If you suppose somehow we could get really tiny humans, in the next few thousand years, perhaps through genetic manipulation, in the near future - well it seems a reasonable guess that they won't get much smaller than the smallest mammal in the world - the tiny Etruscan shrew, only 2 grams. That's more than a thousand fold decrease in mass.

If you think this is a possibility - then for a doubling every century, that gives your population an extra millennium or so. If doubling every ten years, it gives it an extra century plus a little bit. Just add a thousand years or a century respectively to the numbers I give here and you've got it.

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 millennium 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.

## WHAT ABOUT EVEN LOWER MASS HUMANS

If you assume humans can get even lower in mass than 2 grams - still there has to be a limit at some point, so long as we are made of atoms. There are about 1028 atoms in a human body.

If you can think the rather science fiction scenario is possible, where "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.

## POPULATION INCREASING BY A THOUSAND TIMES THE CURRENT POPULATION OF THE EARTH NEARLY EVERY MONTH

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).

## CONVERTING AN ENTIRE GALAXY'S MASS INTO EXTRA HUMANS EVERY CENTURY

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.

## WITHOUT FTL

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.

## AT SOME POINT - ONLY A BLINK IN TIME - HUMANS - AND ETIS - HAVE TO STOP EXPONENTIAL GROWTH

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.

## FACTORS THAT WOULD FAVOUR VERY RAPID POPULATION GROWTH, DOUBLING TIMES PERHAPS TEN YEARS OR LESS

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 destroy 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.

## HOW WE MIGHT STOP - IN BEST CASE - AS SOON AS 2050

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.

## CONFIDENT PREDICTION THAT OUR POPULATION WILL STOP GROWING SOME TIME IN THE NEXT FEW MILLENNIA - WILL THIS HAPPEN PEACEFULLY?

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.

## WHEN DO ETIS (OR HUMANS) TYPICALY STOP EXPANDING?

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?

## DEDUCTIONS FROM ABSENCE - BECAUSE THEY AREN'T HERE ALREADY

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.

## WHAT WE CAN SAY ABOUT THE MAXIMUM REGION FILLED BY ETI COLONIES

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.

## MIGHT THE MAXIMUM REGION BE LESS THAN THE SIZE OF A GALAXY?

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).

## WHY WOULD THIS BE - IF ETIS ALWAYS STOP BEFORE FILLING A GALAXY?

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.

## WHAT ABOUT POPULATIONS THAT PLATEAU OR CRASH

Typically in nature populations expand exponentially for a while, but they can't keep it up as they run out of resources. So various things can happen They may plateau.

Logistics curve for populations that plateau (public domain image from wikipedia)

Or you get boom and bust cycles, as happens with lemmings for instance.

Every few years the numbers increase a thousand fold, then suddenly crash. Not through swimming into the sea - that's a myth.

Just through predators and starvation.

See the Amazing Lemming. For some reason there have been no population peaks in recent years, possibly a side effect of global warming and less snow cover.

There are many ideas about why these population explosions happen, not well understood. E.g. that it is due to abundance or scarcity of moss, their main food source. Or that it is due to reductions in populations of snowy owls and other predators. The predators are inversely correlated with the lemmings - that's well established - but it's hard to disentangle what is cause and what is effect.

A dead lemming on a stone in the river Revåa in Norway. After a lemming boom and bust, so many die, that drinking the water becomes a health hazard for hikers. Photo by Bjørn Christian Tørrissen

So - if we expanded without any planning, the same would happen. Either it plateaus or it booms and busts.

You might think then - so what is the problem? It's only exponential right now.

Right - yes that's true. It's obviously absurd to suppose it continues exponentially endlessly. When the exponential ends - then humans stop expanding, until they can expand again - no big deal.

But - that's the main point in this article. The question is, how does it plateau, or do the boom and bust? In nature then it does that through starvation, and through very high infant mortality. If we let it happen just as it does in nature, then we are in for a horrific future once our population plateaus or busts.

For humans, on Earth, yes, luckily it seems that the human population may well plateau naturally, or even start to decline after 2050.

So - then you could again say - "no problem - as humans fill a region of space, the birth rate will drop and everything will be fine again".

That might well be true, if humans are restricted to the Earth or our solar system.

But now look at what happens if they expand through the galaxy.

## APPLYING THIS TO THE GALAXY

If you apply it to the galaxy - yes, by the example of Earth itself, seems well possible that many individual populations will just plateau peacefully through affluence - you explore another solar system, expand, fill it, and your birth rate then goes down and you are then content until a few adventurers go on and find another star system, expand, fill it and so on.

Perhaps most human colonists would be like that, explore, expand, plateau for a few thousand or million years, explore etc.

But the thing is we would be expanding into an essentially limitless galaxy in the early stages.

And - assuming that we are still at the relatively immature social stage we are right now, or that there is a possibility of such stages occurring again in the future - there would be - probably a tiny minority - of more aggressive "ISIS" like colonists that aren't content to just do that. But expanding into our vast galaxy, a few more extremist colonists like this can then set up independent colonies, and eventually, fill entire solar systems.

They would aggressively attack other colonies and they would also start spreading to a new solar system immediately as soon as they have finished the last one.

And the most aggressive ones, with the fastest doubling times, and especially also colonies with advanced weapons technology able to overpower all the others - would be strongly favoured in this expansion. They would be the ones that colonies a thousand star systems for every single star system colonies by the less aggressive and content and peaceful colonists.

It seems an inevitable consequence of an expansion of a species that still has these tendencies in some of its members, into a much vaster galaxy.

So, how can we (or ETs) stop this from happening?

You also have the problem that with near future technology, maybe just decades, at most centuries away - that they can build self replicating machines, modify their DNA, make clones of themselves and cyborgs, etc. All of that could hugely increase their rate of population increase. And the ones that can increase their population most rapidly by these methods would be the winners of this race to fill the galaxy.

## WHAT HAPPENS IF THEY DON'T FIND A PEACEFUL SOLUTION

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, or voluntary suicide, 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.

So, unless they somehow find a peaceful solution first, their population of the entire galaxy or universe, or however far they expand, would need to be halved every 10 years through violent methods of one sort or another. That's just one ETI, continuing 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 aggressively are likely to be technologically ahead of everyone else also - how could it possibly ever stop?

I'd like also to touch on a couple of very science fiction ideas. Not at all likely, perhaps, but for completeness - not necessarily us also.

## BEINGS MADE OF PARTICLES WITH REST MASS ZERO - E.G. PHOTONS OF LIGHT

All of this argument only applies to ETIs made of ordinary matter, such as atoms, electrons etc. It doesn't apply if their bodies are made of particles with rest mass zero. Which means particles that move at the speed of light, such as photons.

This alternative is very "sci. fi." but for completeness just to mention it. No idea how such bodies could function.

For photons, then its energy depends only on the frequency. So - suppose your ET's body is made of a certain number of photons. You can double your population by just reducing the frequency of the light. If your ETs can survive halving of the frequency of their constituent photons, then they can double in population while keeping to the same total amount of energy. And that energy is equivalent to mass if they can also do total conversion of matter to energy.

If you can keep reducing the frequency of the photons endlessly, the population can increase endlessly, with the photons maybe starting as gamma rays, ending up as ultra low energy very long wave radio waves, eventually radio waves with wavelengths of light years. But it's a little hard to see that actually working in practice.

## COMPUTER SIMULATION ZERO REST MASS ENTITIES

This is also related to the idea of ETs or ourselves as entities in computer simulations. I think this is very unlikely to be possible for the reasons explained in my If a Program Can't Understand Truth -Ethics of Artificial Intelligence Babies. However it would get us rather far afield to go into those reasons here.

In any case it's not a solution, because eventually your future "human" or ETI individual's mass has to get less than the mass of an atom. Then a little later, they are less than the mass of an electron each, and then continue to get smaller.

So, eventually, within only a few millennia of continued doubling of population, you need to use zero rest mass particles to keep the population growing.

Then as with the zero rest mass ETs, then they get lower and lower in energy and if photons, longer and longer in wavelength. Which sounds fine until you realize that you eventually have a huge population, but consisting of beings consisted entirely of photons with wavelengths of many light years each.

And also every single particle of non zero rest mass in whatever substrate you have for your "computer" has to sustain exponentially increasing numbers of these photons as time goes on.

Even assuming quantum computers, and superpositions of states, it's hard to see this working indefinitely into the future for more than a few extra millennia, if that.

## UNRECONSTRUCTED "FIRST WAVE" COLONISTS

And anyway - unless the ETIs (or us) are already made of zero rest mass particles before they start colonization - you would still have the unreconstructed "first wave" colonists. Some of these would be bound to resist the suggestion that they should make way for beings made of zero rest mass particles, or that they should only have "descendants" made of zero rest mass particles. And with no communication due to light speed barrier, you won't have a society wide consensus decision to give way in this way.

They would probably destroy the substrates of the zero rest mass colonists, maybe not even knowing that they are inhabited by intelligent beings, and use them as resources. And no matter how many zero rest mass colonists you get, as time goes on you still have your rapidly exponentially increasing populations of the non zero rest mass colonists as well.

## MOST RAPID COLONIZERS - AND THE PAPERCLIP EVENT HORIZON

The ten year doubling period colonizers are by no means the most rapid, when you take into account of technology. Self replicating machines could double in an hour, like a microbe.

And amongst those quadrillions of colonists, as they disappear out of contact beyond the light speed communications barrier, some will surely eventually be stupid enough to build self replicators. Maybe just dumb machines, no intelligence or purpose as we understand it, but programmed to turn everything, say, into paperclips, or Dyson spheres, or just into themselves, and to continually self improve.

With a doubling time of an hour, the time to turn an entire solar system, including the sun itself, into the self replicators from an initial population of two (for humans) or just one for our self replicators decreases from 10,000 years (a hundred centuries) down to a hundred hours, or a little over four days. Add an extra couple of days if they are tiny, just atom sized nanobots. If our solar system was hit by a nanobot self replicators designed by some stupid distant colonist then well within a week the whole solar system would be converted, or as much of it as it has the technology to convert (including the sun itself if that is within its capability). And the initial seed of this could easily be so tiny you wouldn't spot it, landing somewhere on a distant comet in the Oort cloud. Spread through the Oort cloud until it has a huge population in stealth mode, and then strike. We wouldn't have a chance.

There might not be any purpose or intelligent goal involved here. They might be making the solar system into paperclips, if cleverly designed to evolve but not able to evolve into anything except paperclip machines - if so then perhaps 1% of the mass of the reconstructed solar system would be self replicators which immediately head off to other stars, or convert each other into paperclips - and the rest is just one huge cloud of paper clips which the original designers would have come along and scooped up in their spaceships, except that they no longer exist because of a miscalculation by a stupid colonizer. This example scenario which is sometimes used in these discussions has been called the "paperclip event horizon".

This is all science fiction right now. But the idea of self replicating machines seems to be something that is almost possible already, doesn't require many advances of technology before we could do it, either on the nanoscale or larger "clanking replicators". Mainly that they can print out computer chips with advanced 3D printers from simple raw materials. Once you can do that, then it may not be far away. As a single culture we can work out a solution to this if it doesn't happen to us too quickly, safety measures that everyone understands. But with quadrillions of colonists disappearing beyond the light speed communications horizon, then how can we control it for all future time?

I go into the dangers of uncontrolled self replicators - and on the flip side, the great benefits of controlled robotic self replicators for exploring a galaxy, in my other article Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth?

So this is another motivation to stop your population growth at some point while you still have communication with each other and consist of a single more or less cohesive culture - at least have an interplanetary or interstellar internet or similar connecting all of you, and are able to deploy trouble shooters throughout your culture to deal with issues like someone setting off an uncontrolled self replicator.

## SOLUTIONS OF BOREDOM, MOVE INTO INTELLECTUAL PURSUITS, VIRTUAL REALITY ETC - DON'T WORK ONCE YOU HAVE GALACTIC COLONIZATION (WITHOUT FTL)

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 ten years or less. That then reduces the maximum of 18,000 years for total disaster even with FTL travel to other galaxies, down to a tenth of that, a little less than two thousand years.

Within a couple of millennia of the first unrestricted galactic colonization, even with intergalactic 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, suicide etc, every doubling time.

In a way it's not a big deal you might think. Living beings often have very short lives, and few die of old age. We couldn't rebuild Earth to give all the living beings here lifetimes the same as humans.

But nobody could look at that possibility and see it as a desirable outcome for a population of intelligent beings (rather than microbes), seems to me. Not only for us. Also for other beings in the galaxy - especially non technological ETs, or ones that haven't yet developed technology that would have their cultures destroyed and driven to extinction by the humans filling the galaxy, and their planets destroyed or repurposed for Earth life - in many cases before they evolve indeed.

One estimate suggests that 90% of Earth life planets have yet to form.

So that means that an ETI that filled a galaxy with its own species now, as well as destroying all present day non technological ETs in the galaxy, would also be preventing future evolution of at least 90% of all future Earth type planet originated intelligences.

Also, if there are any ETs out there that are technologically way beyond us, this is just the sort of behaviour that they would have to prevent, in one way or another, hopefully peacefully, or violently if necessary. They'd see us as a cancer of the galaxy. They would have to stop us in one way or another. Hopefully peacefully by educating us. Or enforced birth control perhaps so we can't reproduce any more, or violently if necessary.

So civilized beings would surely foresee that possibility that their colonists and their creations could turn into a cancer of the galaxy - and somehow prevent it. The question is how.

## IDEA THAT IT MIGHT JUST HAPPEN AUTOMATICALLY - SELF SELECTING, THAT ONLY THE CIVILIZATIONS THAT ARE ABLE TO FIND A SOLUTION CAN REMAIN SPACE CAPABLE FOR LONG ENOUGH

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.

And then they will put their intelligence and foresight, and their brightest minds, into making sure this doesn't happen, in one way or another.

## MY ANSWER TO THE MONOCULTURE OBJECTION

That's my answer to the "monoculture objection". If you haven't come across this objection to ETs with limited population growth - it's the idea that whatever answer you come up with here, it's impossible to see it continuing to operate as cultures change, over thousands of years.

For instance, this objection goes - if the inhabitants of Earth are reasonable for the next thousand years and don't try to colonize the galaxy - 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? See the infrared search for extraterrestrial civilizations for a recent overview of this objection.

### FIRST - THAT IT BECOMES UNTHINKABLE

So, my first idea here to answer this 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.

### SECOND - THAT THOSE WHO ARE NOT STABLE ENOUGH TO HAVE THIS LEVEL OF UNDERSTANDING AND FORESIGHT CAN'T MAINTAIN SPACE COLONIES IN THE EARLIER STAGES

And then the other 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.

### THIRD - OR WE ARE THE FIRST

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.

(By the way, if you know of anyone else who has presented this argument before, do let me know in the comments!)

### HOW COULD IT WORK ANYWAY - WOULD YOU NOT HAVE SOME PEOPLE WHO DON'T GO ALONG WITH IT

How it works in detail would depend on the culture. But we have already got many situations that required 100% compliance or close to it, on Earth. For instance eradication of smallpox. Or preventing destruction of the ozone layer. Or elimination of the use of DDT to protect the eggs of wild birds as highlighted in Silent Spring. Many things like that.

To colonize another world, you'd need star ships, advanced technology, co-operation of many people. If it became literally unthinkable, only crazy people would still want to colonize.

For sure, chances are there would be people who still want to do it, at least in a human like culture in the near future.

But - if no way has been found to do it safely - and if it was seen as something as dangerous and more so than setting off a nuclear weapon in a city - it would be regarded by everyone as a criminal act to start a wave of unrestricted colonization into the galaxy. Unless we had a solution to this, then all their peers would stop them, as if they were about to let off a nuclear weapon in a city. It would be built into the technology, safeguards to prevent this happening. Built into their justice system and so on.

I have a few other ideas to share however about how it might work, if this automatic understanding by technological civilizations is not sufficient by itself. First of all, the idea of a galactic culture that is there already. For that though, we need some way for this pre-existing galactic civilization to overcome all the hurdles that I just described.

## DEALING WITH LIGHT SPEED BARRIERS WITHOUT FASTER THAN LIGHT TRAVEL

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.

## FEDERATIONS OF ETS

The other possibility is a network of many different "elder ets" throughout the galaxy, in touch with each other through light speed communication. (Or FTL). Where over millions, and billions of years, they've evolved a system where each elder ET is responsible for dealing with and preventing any uncontrolled waves of colonization in their sector.

## EDUCATORS - EITHER OTHER ETIS OR THROUGH OBJECT LESSON EXAMPLES

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.

So ways we could stop at that point, short of uncontrolled expansion include:

• Education or object lesson - it becomes unthinkable. It's regarded as a criminal act and is policed by the vast majority of the population of the ETs. Is built into their technology that it can't be abused in this way, can't be used for colonization. Especially if they remain within a single star system, or just a handful of star systems.
• Forcibly by other ETIs (or themselves) e.g. through birth control or sterilization. This might be the last resort for instance in that scenario of an intergalactic federation of billions of years old ETs tasked with preventing uncontrolled colonization of the galaxy, for instance. It could be done using advanced nanobes that are impossible for young ETs to detect and circumvent, or some other very advanced technology,

• Perimeter "fences" . This is an idea often explored in science fiction. That other more advanced ETs might establish a "perimeter" that they don't permit the expanding ET to go beyond - again this could be a last resort if educating them doesn't work and for some reason birth control isn't possible either. It could also be done by the expansionist ETs themselves to prevent their cousins with expansionist aims.

May be other ways - do suggest in the comments!

## WAYS POPULATIONS COULD STOP INCREASING

Having made the decision, by whatever method, that they have to stop their population growth at some point - they could do so in many ways other than just running out of resources, starvation, suicide and warfare.

• Only colonize a single star system - and send self replicating robots - and exploration parties throughout the galaxy if they want, but no colonists. Never colonize anywhere else unless their home star system is seriously threatened.
• Nomadic population, like the Puppeteers planet in Ring World (Larry Niven) for Sci. Fi. fans. The Puppeteers 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.
• 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.
• 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 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.
• A very slow exponential, say an exponent of 0.0000000001. That's just fine except - that in an expansion into a galaxy with many habitable stars and plenty of resources, the humans or ETIs that colonize a thousand star systems per millennium, and then a million in the next and so on will fill many more stars than the ones that fill one star system in the first millennium and just finished their second star system in the second millennium. So slow exponentials are unlikely unless somehow planned out or some very unusual biology. It could be possible for instance, if an adult gives birth, say, a million years after their own birth.
• Any other way that ensures a sensible maximum population. Depends on the ET probably. There may be solutions we can't even imagine until we come across them in practice, if we do. We may nay not yet have made the observations or developed the science or the concepts needed to understand them.

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.

## THE SEARCH FOR ETI MEGASTRUCTURES AND THE STAR KTC8462852

I actually started to write this in response to the recent discovery of a star KIC 8462852 with the strange asymmetrical dips in its light curve, cutting out up to 22% of the light from the star - 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 this was an ETI megastructure, we know nothing about what types of bodies they have or what their desires and limitations are. But there are some things we can assume. That they have a minimum body mass. And even if they are ultra low mass beings, with the mass of a single atom, still their exponential growth phase is all over in a blink of time compared to the age of a star.

And that then leads to the thought that 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, suicide 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?

Not saying at all that this is a megastructure. Most likely it is due to comets. Not the small relatively tame comets of our solar system, but large planet sized icy bodies in close flybys of the star evaporating in huge comet tails able to block out much of its light, maybe also with gravity darkening of the star to help explain the asymmetrical curves.

There's no way you look at this evidence and conclude "this is a megastructure". But in the other direction, SETI researchers and others are already searching for mega structures. And for someone searching for these things, it is one of the most obvious candidates for a megastructure to investigate more closely in our galaxy. They would be daft not to look at it more closely.

So this all depends on your perspective.

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. Except that they haven't totally filled our galaxy because where we are right now is not totally filled with ETIs.

The only way to find out how large their largest occupied regions of the galaxy are is to to search for them, and one way to do this is 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.

## NON TECHNOLOGICAL ETIS

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 develop 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.

## ENDLESS FRONTIERS

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.

I have a section 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.

## MY OTHER ARTICLE ON THIS TOPIC

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.

You might like my other posts on Quora

And on Science20

Robert Walker's posts on Science20

### KINDLE BOOKSHELF ON MY AUTHOR'S PAGE

And I have many other booklets on my kindle bookshelf

Just to draw your attention to the comments section. Great to hear any of your thoughts and ideas on all this. And please don't hesitate to point out mistakes, minor or major. I am always happy to be corrected!

This is intended as an "opinion piece" to stimulate discussion.