In "Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths" I looked at a possible future where humans could colonize space and build habitats using materials from the asteroids. Later on, once we have fusion power able to build miniature suns to warm our new habs ,it would be easy to spread to the Oort cloud. Then, since Oort clouds of stars mingle, we would be well on our way to colonization of the galaxy

So, why haven't ETs colonized the galaxy already? It would be an extraordinary coincidence for an ET to evolve even as recently as a hundred million years ago, leaving plenty of time to colonize the galaxy. At first sight it seems unlikely that we could be the first civilization in our galaxy.

So ETs should have colonized our solar system and the Earth millions of years ago. Where are they?

Fermi's paradox, made more acute by space hab colonization

This is Fermi's paradox - he was the first to think of the idea. Usually it's thought in terms of planets, colonizing the glaaxy by hopping from one planetary system to another.

But it's made more acute when you stop thinking of planets as the places a technological ET would colonize and instead think in terms of space habs. I argued that in the not so distant future we may think of space habs made from materials mined from the asteroids and comets as far more favourable places to set up home than planetary surfaces.

Once we have solar systems filled with trillions of independently evolving space habs, then it is hard to believe that many of the solutions proposed for the Fermi paradox would apply to them all.

They surely wouldn't all devolve to animals, or all become danger averse individuals who no longer travel anywhere , or all retreat to communal minds or digital worlds, or all make themselves extinct with technology. Especially once they start to spread to other star systems, then in almost any scenario at least a few would survive, to start the process of galactic colonization over again. 

The mystery deepens - planets around old stars

Another thing that makes this paradox more acute is the recent discovery by Kepler that planets can form around old stars

The first stars in our galaxy (the mysterious hypothetical "population III stars "), though purely hypothetical, not yet observed directly, must have been made of the same material that formed after the big bang - hydrogen, helium, and a little lithium and not much else, so couldn't have planets even with water or ice. As more complex elements were created in stars and supernovae, planets like Earth could form. It used to be thought that only the newest "population I" stars like our sun was one of the first stars to be able to form planets. This new discovery though suggests there could be planets around somewhat older "population II" stars, which formed maybe up to 10 billion years ago.

You could imagine perhaps they have some "prime directive" as in Star Trek to keep Earth pristine. Yet, even if they keep Earth pristine, the sky should be full of their habs. Where are they? Do they have a "super-prime" directive to keep not just Earth pristine but our entire solar system? That's one solution, yes, but would everyone, all the trillions of ETs, in all those habitats, follow this directive?

We may colonize our solar system in the near future

This might happen sooner than we think, with ideas such as Skylon, which would be able to fly directly into orbit from a runway like a jet plane, and various ingenious ideas for reducing cost of heavy lift into orbit (including JP Aerospace's orbital airship).Skylon, may enter commercial flight by 2022, able to fly to the space station and back again from a reinforced runway, like a jet airplane

This is a computer generated image of the Skylon flying into orbit - it takes off from a conventional runway and has an ingenious motor that is able to work like a jet in the atmosphere, breathing air and so massively reducing the amount of fuel needed, yet can also work like a rocket in space. To do this, it has to cool down the incoming oxygen from 2000 C to -200 C in a hundredth of a second, sounds impossible but apparently it works in the tests. It may fly to the ISS as soon as 2022.

Eventually perhaps travel into space will be as easy as travel between continents today. If so, we can expect billions eventually to fly into space. There are a billion aeroplane flights a year today on the Earth. 

Perhaps within a few decades we will build our first Stanford torus in space. By then this will surely be no more expensive to do than current mega-engineering projects on Earth such as the tallest skyscrapers and the longest bridges.

Video fly through of a Stanford Torus style space habitat by Uzi Bento

We could already build a Stanford torus well within the financial capabilities of just one of the major space faring nations such as the US, Russia, or the ESA if it was made a priority for a decade or so - at least - the researchers thought so back in the 1970s in the Stanford study. It has become easier to build with advances of technology since then, and will surely soon become similar in scale to other mega-engineering projects in the not so distant future.

For more about all this, see "Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths" 

As the process accelerates, our solar system may be filled with habitats. We are surely not more than a century or so away from that if we keep going with the present rate of technological advance. 

Once that starts, once we have self sustaining habitats in space, it is hard to see how humans and our descendants could become extinct or anything happen to halt the colonization process. Especially once colonists start to reach other nearby stars.

At the edge of the spreading wave of colonists there would be adventurous explorers who set out in faster spaceships, perhaps a tenth of the speed of light. Even with only one in a trillion doing that, the spread would be rapid. 

Suppose for instance that the most adventurous explorers travel one light year further than everyone else (a ten year journey), and that it takes a century to build a new colony (after all they have everything there already, have brought their technology with them in their spaceships and it is a familiar place for them, the vacuum of space, using materials from comets and asteroids to build new habs, few surprises). Well the entire galaxy then would fill with colonies within 10 million years.

If they travel only a hundredth of the speed of light and travel only a tenth of a light year each time still they will colonize the galaxy within a billion years.

So if there are other ETs in our galaxy, why haven't they done the same? 

Possible solutions to Fermi's paradox

There have been many solutions proposed to the Fermi's paradox. So I won't talk about them all, but just go into a few that might be especially interesting or give a slightly different perspective from the usual treatments of the subject.

According to some of these ideas then we may be the very first ET to arise in our galaxy. According to the other ones, our galaxy may be full of ETs but they are either non technological or, the last solution is an idea of my own. I will suggest that any ETs that are technological and sensible enough not to make themselves extinct, would inevitably decide not to colonize the galaxy but would intentionally restrict themselves to their solar system or a limited area of the galaxy for colonization.

It is hard to come up with a scenario where ETs would be dangerous to us

I thought I'd just talk about this briefly as Stephen Hawking has famously said we should be careful about contacting ETs in case they are dangerous.

If Earth and our solar system was attractive for them as a place to live, then the solar system would already be filled with their habitats. We wouldn't have had a chance to evolve because they would have colonized the Earth long ago, billions of years ago most likely.

As for taking our wealth, our animals or plants or ourselves - well any ET with billions of years old technology, would surely have no problem replicating anything they find of interest, any biology etc. They could create a new Stanford torus type habitat as easily as we can build a house, one especially designed just for housing their finds. 

With space habs so easy to make, for them (surely), they wouldn't need the surface of the Earth at all. With their 3D printers, able to print at sub nano-scale they would also probably have universal replicators just as in Star Trek. This is not so far into our future, even 3D printers able to print food may not be far away (has already been done in the lab) - though other parts of Star Trek such as transporters, and force fields remain fantasy.

I enjoy stories about alien invasions of the Earth, am able to read them just as I read stories about magic and dragons etc, using suspension of disbelief. But once you start to think of them as creatures who are at home in space and find everything they need in comets and asteroids, there is only one story I can remember now that seems plausible, where the Aliens found things on Earth that they wanted to take away from them.

In this story, they are collectors who want to collect our art treasures, and just as collectors on Earth want to have an original Van Gogh rather than a replica you could imagine a really keen art collector who wants to have originals of our artifacts (whatever seems interesting to them). But their ideas of what is precious and collectible on Earth might not align with our ideas on these matters... Sorry I can't remember the name of the story or the author right now, maybe someone reading this knows?

You could count ET as well, taking away a few plants as collectors, is plausible, similarly, like the collectors of art, that they want to propagate the plants from originals. 

It seems if they exist, at least they have strong laws or morals or directives governing their behaviour or again they would be here already as colonists.

They might turn our ideas on their heads, some of them, and so indirectly be disruptive in that way, but I doubt myself that they would be dangerous to us directly. In any case we have already announced our presence via radar and TV; there is nothing we can do about that, and listening out to find out more about them seems wise whatever they are like.

Solution 1. Is it highly improbable for technological ETs to evolve, so we are the first in our galaxy?

This is the first solution I want to look at. So, scientists have looked at evolution on Earth, and nearly everything happens so quickly on the geological timescale. It's now thought that there was life on Earth already within a few hundred million years of its formation, and quite probably almost straight away. So, the way this is usually interpreted is that it must be really easy for life to evolve. Also there were many other steps along evolution to humans, and again these steps, most of them, happened quickly enough so that they seem reasonably easy too.

However, now let's turn this idea on its head. What if it is really hard for life to evolve at all and typically takes billions of years. Suppose there are ten different steps like that along the line to evolution of humans and each one typically takes billions of years. In that situation, most civilizations will not arise until tens of billions of years in the future, but there has to be a first civilization. In that first civilization, then all the steps would get completed, just by chance, within a period of say 5 billion years. Now look at the time line for that civilization. You find that the most likely thing to happen is that the steps are all evenly spaced and each one takes quite a short time. 

This is an idea suggested in a 2008 paper by Andrew Watson (see also the Odds of intelligent life in the universe)

So if we are the first technological ET in our galaxy, we would expect to see a geological past with everything happening very quickly. So, this doesn't by itself prove that life can get started really easily.

In this situation the first civilization in the galaxy is a rather lonely one as it gets underway typically billions of years before everyone else. Perhaps the reason there are no colonists here yet is because we are first?

If so then we need to take especial care, because the future of the galaxy may depend on our actions.

Other civilizations are bound to evolve even if all the five stages he identified take ten or even a thousand times as long. Orange dwarf stars have habitable zones which last for trillions of years, as compared with the billions of years of sun like stars ("yellow dwarfs") and there are many more orange dwarfs than yellow dwarfs in our galaxy. 

So surely eventually our galaxy would be teaming with many different ETs; so long as the early ETs such as ourselves leave it in a reasonably pristine state.

Perhaps life originated before Earth

This is another way that we might be the first in the galaxy. If you look at the complexity of life, and the way it evolved, it gradually got more complex, from the earliest cells without a nuclei, through cells with a nucleus (eukaryotes), worms, fish, and finally mammals. You can measure the complexity by counting the number of non redundant nucleotides - basically - the amount of essential information in the DNA. If you do that on a log plot, they all lie close to a straight line. 

This suggests that life is getting gradually more complex over the billions of years timescale of evolution. It doubles in complexity every 376 million years, so roughly every billion years or so, the information in the DNA gets about six times more complex. You can trace that trend back for nearly four billion years, to the time when the first prokaryotes evolved (the earliest primitive cells without nuclei).

Before that we have a huge gap in our knowledge, nobody knows how the first primitive cells could form. They are far too complex to just arise by chance from prebiotic chemistry, with many hundreds of complex chemicals including DNA and RNA and proteins, ribosomes to make the proteins, and enzymes interacting in complex ways. There must have been less complex precursors.

So we know our picture of the origin of life is incomplete, and the idea of Richard Gordon and Alexei Sharov was to plot this complexity and trace it back to get an idea of how long it took to evolve to the first primitive cells.

Plot of log of complexity of life by non redundant nucleotides against date of first occurrence in evolutionary record in billions of years. (Sharov, 2012).

Surprisingly, they found that their graph hit the zero line about 10 billion years ago. To put this another way, we know only about half of our evolutionary history, and the rest of our history is a blank.

If that is an accurate estimate of the time needed, it is no surprise that there are many competing ideas of abiogenesis with no obvious way to tell between them, or that our laboratory experiments have come nowhere near to creating life from pre-biotic chemistry.

However, the Earth is only 4.7 billion years old. So if life evolved 10 billion years ago, it must have started on another star. This is not too surprising actually. If a star with life on it passed through our solar system when it was forming, the life might easily transfer to our planets via meteorite impacts.

This then helps resolve the paradox, when it is combined with the evidence from Kepler that there were planets around the older population II stars. These planets could have evolved life and one of those then seeded our sun.

Then if it takes about 10 billion years to evolve to technological civilization, again, we could easily be the first to evolve in our galaxy. Perhaps there simply hasn't been enough time since the first stars with planets for life to evolve around other stars in the galaxy.

There are other civilizations but they are incapable of building radio telescopes or spaceships

Look at the beings on the Earth that are tool using and so closest to technology, outside of ourselves, and ask yourselves - how far would they be likely to develop towards our technological achievements?

So, of chimpanzees, parrots and finches, bears, badgers, octopi, otters, elephants, crows, sting rays, most of those would have a pretty hard time building a telescope. Only the chimpanzees would have nearly as easy a time as ourselves and they would have issues too because of the lack of a fully opposable thumb (their thumbs are short and so they can't achieve a pad to pad type grip).

 Alex the grey parrot, famous for his language skills. He had a vocabulary of 100 words, showed understanding of their meaning e.g. using colour words correctly, used syntax and other traits showing basic language competence.
Alex the grey parrot, famous for his language skills. He had a vocabulary of 100 words, showed understanding of their meaning e.g. using colour words correctly,  used syntax and other traits showing basic language competence. If ETs were similar in structure to a grey parrot they would find it hard to build optical and radio telescopes and might never make any advanced technology as we understand it.

Or look at the animals capable of the most complex "animal language" closest to humans. Here we have elephants, dolphins, whales, parrots again (can be taught complex methods of communication), chimpanzees, baboons, squids (using a complex colour language changing colour to communicate).

If ETs are like cuttle fish, then however intelligent they are, or however complex their culture, they would be unable to make telescopes or other technology

Again most of those would have considerable difficulties building a radio or optical telescope. Or indeed building megastructures likely to survive over geological time.

So of we are the first technological civilization in our galaxy, it doesn't rule out non technological civilizations.

If these examples from Earth are anything to go by, it seems that perhaps you get many non technological civilizations evolve for each single technological civilization. Perhaps even such civilizations arose in our past. Brain size is not a good indicator, the parrot has a tiny brain but is close in its language and tool using capabilities to a human.

Suppose, just for instance, that there were dinosaurs capable of communicating using language and capable of philosophy and mathematics etc. If unable to use or make complex tools, they might have left no trace of their civilization for us to discover. 

Also, again, by the same argument, if it is indeed much more likely for a civilization to be non technological, then it would seem that the first civilizations on any planet would be most likely to be non technological. So who knows perhaps we have had such civilizations on our planet in our past. We wouldn't be able to tell, not if they arose many millions of years ago and didn't construct durable physical artifacts. They also probably would be rare as fossils just as ancient humans are, clever and not easily trapped and tend to be helped by their friends when they get in trouble. 

Next to impossible to spot in our own geological past, they would be totally impossible to spot on other planets around other stars, without a really close look.

(This argument that it's likely we had non technological civilizations in our past on the Earth itself is my own idea though it is often suggested for other stars and planets)

Then there is a developing idea that perhaps the planet we live on, Earth, is not the most likely place for life to evolve. There seem many more chances for habitats like Europa, with its ocean below an icy crust, warmed up by tidal heating. We know several possible habitats like that even in our solar system - Encladus also has clear evidence of sub surface water as it is actually gushing into space in jets. Also Ganymede, and Titan may have sub surface water oceans amongst others (in case of Titan is in addition to its surface oceans of methane / ethane).

Europa, a moon with an ocean under the ice, natural colour. Photo by Galileo spacecraft, NASA

Europa. The most promising habitat for complex life outside of the Earth is its subsurface ocean which is believed to be oxygen rich because of the action of ionizing radiation from Jupiter on the water ice which eventually makes its way into the ocean. The ocean may be kilometers below the surface. If an ET was a fish or squid like creature living within a Europa like ocean, it would not only not be able to make telescopes - it would probably have no way of knowing that there is any more to the universe than its ocean habitat. It could continue as a civilization for millions of years with no knowledge of any of the inhabitants of the stars and galaxy outside of its ocean.

The most promising of these for higher life is Europa because its ocean is probably oxygen rich (the oxygen is created in the surface ice by the strong radiation of the Jupiter ionosphere). This means its ocean could have higher animals able to use energy in oxygen to travel quickly. If it does have life, it might have the equivalent of fish, squids, sting rays, and octopi. 

In our solar system we have just the one Europa with an oxygen rich ocean, but Jupiter sized objects are common around other stars, and may well have moons like Europa. So Europa could easily represent as widespread a type of habitat as the Earth or more so. If ETs develop in Europa type habitats, they would probably find construction of a telescope really hard to do, even octopi. Hard to discover fire. They have a covering ice sheet kilometers deep to get through before they can see the night sky at all. They might have no idea at all that there is anything else outside of their ocean, anything else would just be philosophical hypotheses.

So again this may be a common form of civilization.

If these civilizations are more common than our type, then they would be likely to be the first to form. So in that case again, probably there would already be many, billions of years old, Europa type civilizations in a galaxy by the time the first technological Earth like civilization arises.

In this picture, though humans may be the first technological civilization there may be many advanced non technological civilizations in our galaxy that may be highly evolved in all other directions (maths, philosophy, literature, poetry and the "arts" whatever they are for them) and have no technology at all as we know it.

Are ETs super-careful because they are afraid they will become galaxy destroying von Neumann machines?

This is a last idea, is an idea of my own. I haven't seen it discussed anywhere else and am just presenting it as something to think over.

So, first of all need to explain what a von Neumann machine is, just in case you haven't come across the idea before. It simply means a machine able to replicate itself. It is an idea that was developed by Von Neumann the computer pioneer in a very abstract way, as a logical mathematical idea related to the way computers work, he developed it into a type of cellular automata a bit like the Game of Life. 

Nowadays however it means a physical machine able to make copies of itself from basic raw materials. We are getting quite close to that capability. If we develop 3D printers with nanoscale resolution, a computer chip might be something you could print out on your printer not needing a complex factory to make it. If that ever happens, then the printer could print all the components of more 3D printers, and you could start to think about an entire self replicating machine which you could send, e.g. to the asteroid belt, to mine the asteroids and make many copies of itself to speed up the mining process. Or to terraform a planet or other mega-engineering projects like that. It could be a big thing, an entire factory, this is called a "clanking replicator" - a big machine able to make copies of itself. Or it could be a tiny nano-scale device a bit like a living cell but is a machine. 

Picture of the ORDbot Quantum 3D printer
Example of a 3D printer. These can only print to low resolution at present. But eventually we may be able to make sub micron resolution printers, and even nanoscale 3D printers. Indeed we already have sub micron scale 3D printers. 
Nanoscale 3D printing by nanoscribe
sub micron 3D printed microstructure by nanoscribe
Using nanoscale 3D printing perhaps a computer could eventually print out all the components of a computer without need to build huge factories to make microchips. If so then a reasonably small self replicating machine might become feasible.

In discussions of self-replicating machines there is a lot of discussion of ways to make sure that nothing goes wrong. What for instance if you make a self replicating machine which makes paper clips. It seems a great idea, increases productivity of your paper-clip factory massively - until you realise that like the sorcerer's apprentice that you haven't built in an off-switch. It then proceeds to turn everything within sight into paper clips. You've also made it able to evolve to become increasingly more efficient at making paperclips, so it then evolves resistance to anything you do to try to stop it.

Goethe's Sorcerer's Apprentice, 1882, drawing by Ferdinand Barth
Goethe's Sorcerer's Apprentice, about to unleash the magic that will soon go out of control. What if a paperclip manufacturer uses a self replicating machine to make paperclips, without quite understanding the consequences?

(Adapted from Nick Bostrom's whimsical "paper clip manufacturing AI") 

For this reason, then its thought that ETs able to make self replicating machines would surely be super careful about deploying them. 

You could colonize the entire galaxy with self replicating machines, just send an interstellar probe to the nearest star with the capability to make more copies of itself when it gets there. Then tell it to make copies of itself to send to other stars, and each of those to make more copies and so on. Within a very short time you have probes around every single star in the galaxy. 

But then what if something goes wrong with the programming of one of those machines and it starts to make everything into paper clips. Then you could have a galactic scale paper clip event horizon. Even if you are really careful to make everything redundant etc, error correction and so on, can you be totally sure this will never happen? These would be highly complex machines. It would just need an occasional copying error in trillions of machines for something to go wrong like this.

So, there are ways around that. You can for instance say that each probe produces only 10 copies and have a count down timer that starts at 12 and counts down from 1 for each generation, so when it reaches 0 then reproduction stops and it completely destroys its capability of exact replication. And also does that after it has made 10 copies of itself. 

That gets you a thousand billion probes, enough for all the stars in the galaxy three times over (300 billion stars). Design your probe carefully enough and surely you can make it fail safe down to 12 generations? Has to be extremely reliable because is not just 12 generations but also 1000 billion probes, but still, seems do-able by an advanced ET.

So, for machines you could do that, and there might perhaps be such an ET probe in our solar system already. That is the basic idea of Arthur C. Clarke''s 2001 a space odyssey, his black monoliths are Von Neumann probes.

So any ET that survives to be a space faring ET must surely pass through various technological crises. We have already passed through several ourselves. The nuclear weapons - we have had no use of them in war for some time now. Biological and chemical weapons, again had successes there. We are careful in our use of genetic modification and again in a more reckless species than ourselves perhaps all of those would have caused disasters by now.

We prevented mass starvation with the green revolution, reversed destruction of the ozone layer, eradicated smallpox, and stopped use of DDT. Yes we have had our share of oil spills and radiation leaks, but at least, we clean up after them as best we can, imagine what our Earth would be like by now if we were more reckless than we are. We have made many species extinct, but also, have saved many species from extinction. A more reckless species might have done none of these things.

So, I know that many people talk about how careless sometimes humans seem, that we don't take enough precautions with new ideas in technology. But we do have at least a modicum of sense and foresight and perhaps we have enough of that to see us through the technological challenges ahead.

So - maybe the only technological ETs that survive to become potential space colonizers are the sensible ones.

These ones then would also be the ones that take care and do not unleash unrestricted self replicating machines into the galaxy.

That would explain why our galaxy hasn't already been turned into paperclips, or worse. But cuold also explain why the ETs are not here yet. 

Are ETs, or humans, safer than self replicating machines?

The thing is, that just as machines can self replicate, so can life obviously. Now on a planet as non technological species it can't do too much harm. But in a galaxy, if you imagine that spreading wave of colonists colonizing the galaxy, they aren't all going to remain as "ur humans" just like ourselves. They will evolve. Also their ideas will develop. 

They may originate in a careful sensible civilization, and it seems they would have to or the process can't start at all. 

An anarchic war-like civilization couldn't have space colonies because they are so vulnerable. A simple weapon would easily destroy a space habitat - that is unless you have force fields and shields and so forth like Star Trek, but we have no signs of being able to do such things yet. If such things are possible, it seems unlikely that a civilization would have them when it first starts on space colonization.

So space colonization of the galaxy would surely start with a peaceful sensible ET. But after that what is to say it will remain sensible? It just needs one of those trillions of colonists to be monumentally stupid and create a self replicating machine and then you get the paperclip event horizon again, entire galaxy made into paperclips. Or worse, machines that hunt down and kill any non machine based life as in the Beserker science fiction novels of Fred Saberhagen, or Terminator.

cover art by Boris Vallejo of Berserker novel by Fred Saberhagen
A sensible ET might be wary about galactic colonization in case one of its numerous descendants in the galaxy is not so sensible and through carelessness or stupidity might unleash self replicating machines. These might turn the galaxy into paperclips in a galactic "paperclip event horizon" or evolve into Beserkers that hunt down organic life and destroy it as in the Beserker novels.

A sensible ET able to get as far as successful space colonization would need to be clear thinking and forward looking, or it probably doesn't get that far. So maybe they all look at this possible scenario and decide, the only thing to do is to not colonize the galaxy. 

Possible "Galactic Laws" to prevent unsafe replication of ETs

I wonder if there might need to be some kind of laws for this, which wouldn't be hard if everyone in the culture understood the reason for it. But, hard to see how they could be enforced if there are differences of opinion in the society about them.

This could start as local laws. Each civilization if sensible enough to reach the stage of space colonization without destroying its colonies through war, would also be far seeing enough to decide not to colonise the galaxy. They make up their own laws to prevent this.

So for instance we might decide, no colonies in our solar system outside of Pluto, to prevent spread of space colonies to the Oort cloud - can use materials from outside of it, can explore outside of it, but can't set up colonies there.

Perhaps in a galactic civilization of many cultures, you could have some rule such as that each ET is permitted to inhabit say 16 star systems anywhere in the galaxy (I chose 16 there as a base 2 numbers because of course most of them would not have 10 fingers like ourselves). They can move from one star system to another (e.g. because it becomes uninhabitable or just for a change of scene) but if so must leave no colonists behind when they leave.

This might be some kind of federation where members all can see for themselves the need for such a rule, so evolve their own rules independently and then collaborate. Is a bit hard to see how this can work without faster than light communication, but one thing that might help is if ETs tend to become amazingly long lived with individuals with lives of millions of years. For such long lived ETs then a wait of millennia between messages might not be such a drawback.

If we are in a galaxy like that, then we might be about to join that club, some time in the next few centuries when we make contact with them, and the nearest ETs might be hundreds of light years away from us.

So that might be another reason why they are not here. This is my own idea and I welcome suggestions and discussion of it. 

There are many suggestions of reasons why ETs might be non colonizing. What I haven't seen before though is this suggestion that ETs might choose not to colonize for this parituclar reasion, because they recognize themselves as self replicators and can't see any safe way to colonize the galaxy with self replicators capable of evolution. If you know of anyone who puts this as a reason do let me know in the comments.

Do you think this is what a sensible ET would do? And presumably ourselves too? Or are there other ways around it, where galactic colonization can be safe without need for laws like that?

Do we need a super-prime directive, to explore the galaxy but not to colonize it?

If we are the first technological civilization then we may be the ones who have the responsibility of exploring the galaxy and possibly bringing technology to it, or colonizing it. We might need some equivalent of the "prime directive" of Star Trek - not to interfere with what is already there.

 USS Voyager (Star Trek)
Perhaps future humans will need some version of a Prime Directive like the Star Trek series, to not interfere with the galaxy or colonize it beyond our solar system or some limited region, only explore.

Perhaps ETs already have this. Not just for our protection but also (my own idea here) for their own protection from the possible stupidity or intentional changes their descendants could cause to the galaxy with self replicating machines and other advanced technology.

We certainly need laws for space exploration and colonization, after all we already have the Outer Space Treaty which prohibits putting weapons of mass destruction into orbit and prohibits harmful contamination of other planets and of Earth. So have made a start. There will surely be other laws.

I suggest that possibly some of those laws may involve making sure that humans don't unleash either self replicating machines on the galaxy or set up a scenario where their descendants might do that eventually after evolution and culture changes.

This might mean that we have to prevent colonization of the galaxy in case our descendants accidentally or deliberately unleash self replicating machines. Or indeed, in case things go wrong in their own evolution turning them into creatures we would not like to see in our galaxy.

I hope we can colonize at least our solar system, orbital habitats, and this could lead to many trillions of colonists in our solar system. Whether it is wise though to permit colonization of other stars I don't know. Perhaps we should visit, look, but not set up a permanent home around other stars? At least until we have a really clear idea of the risks involved.

What do you think?

I know this has no immediate practical relevance, but is interesting philosophically and is probably not too soon to start to think about it. Interested to hear what anyone else thinks about this!

Are humans or ETs in some ways safer than self replicating machines when it comes to colonization of a galaxy? If so why, what is it that makes us safer than machines? 

With self replicating technology surely not far away in the future, is there any way to be reasonably sure that none of our descendants throughout the galaxy would start an unstoppable galactic paperclip event horizon or worse? 

If so, what could we do to be sure enough of this to start colonizing the galaxy? Is some sort of law to prevent disaster possible or workable? Or some other way to stop it? Or just accept it as inevitable?

Or should we remain explorers of our galaxy rather than colonists in the near future at least? 

See also That's just a short selection. There is a vast literature on this subject, an internet search for "Fermi's paradox" will turn up pages of search results.