Most people, when they think about exploring the galaxy, think about sending out human colonies. It's natural to think we would explore it just as we do the Earth, it's the only way we know. To send machines instead of humans, especially machines that can replicate, may seem frightening. But - I'd argue, humans colonies are by far the most scary way we could explore the galaxy. It might well be a case of "look out galaxy (and Earth), the monsters are coming" :). 

So what can we do? What is a responsible way to explore our galaxy, with current understanding of science, biology, and society,  and could this explain why our galaxy is not filled right to the brim already with extra terrestrials?

After explaining the problem in detail, I'll draft out a possible way to do safe self replication with robots, but it's not a method that could be used ethically with humans. Could there be other solutions, and if so, could those also explain why the galaxy is not filled with extraterrestrials?


So, first, is it possible to continue colonizing new frontiers endlessly? Could we, or an extra terrestrial, do this? Or if not, how long would it be before we have to stop?

We can get an over-estimate by asking an easier question - how long would it take before all the available matter consisted of humans, supposing we continue with unrestricted exponential growth? Obviously that's not practical, but as an upper bound, there is no way we could keep growing beyond that point.

And to simplify the calculation, since we only need an upper bound, let's forget about speed of light issues. If we can't travel faster than light, we run out of matter sooner. Will come back to that later.

Let's suppose that the population doubles every century. Then the calculations get simple, every thousand years, the population increases a thousand-fold. (210 = 1024 or a bit over a thousand). After another thousand years (total of two thousand years), it's increased by a factor of a million. After three thousand years, the original population has increased by a factor of a billion and so on.

Also, let's ignore for now the detail that most of the atoms are hydrogen or helium, and human bodies require somewhat rarer atoms such as carbon, nitrogen, oxygen, and various trace elements (maybe our descendants can convert any atom into any other atom).

Let's start with just the galaxy.

One estimate for the mass of the galaxy is 1041 kilograms.  The mass of an adult human is at least 15 kilograms. So if the exponential growth continued, all the matter in our milky way gets converted to humans within around 13,000 years. Something has to "give" here.

Chandra Bahadur Dangi, the shortest man in the world, a Nepalese farmer, weighs 15 kg. If the entire galaxy was converted to humans, there isn't quite enough matter for 1040 humans the same size as him. At some point population doubling has to stop. Could this be why our solar system is not filled with Extra Terrestrials?

The mass of the observable universe (excluding dark matter) is about 1053 kilograms. So, if you could continue, with exponential population growth, doubling every century, you convert all the matter in the observable universe into humans within 18,000 years. (Here I say observable universe because the universe probably continues well beyond the horizon of what we can see).


You can also see it in another way, starting with Earth, and supposing about 10 billion people will be a decent population for the Earth (with advances in technology). So then if we double every century, then one century later you need two Earths. A thousand years later, you need a thousand Earths. We can do that within the solar system by converting all the asteroids in the asteroid belt into Stanford Toruses. See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths.

But now, supposing that only a small percentage of our population leaves our solar system through interstellar travel - then two thousand years from now, we'd need a million Earths. By then we'd have used up every spare bit of asteroid and comet in the inner solar system, quite probably most of the Oort cloud and most likely have started dismantling the planets because there isn't enough living space unless you do that.

So, if we double every century, then by two thousand years from now, every year we'd be making a thousand Earths worth of habitats from materials we can find in our solar system. And three thousand years from now, we'd be doing that in a thousand solar systems, and four thousand years from now, we'd be doing that to a million solar systems.

By five thousand years from now, we'd have done that to every single star in the galaxy and be looking for new galaxies to fill. By seven thousand years from now, we'd be doing that to all the stars in a thousand galaxies every year, filling them to the brim with human habitats.


So, it's absolutely clear, as it would be to any intelligent extra terrestrial, that we can't possibly continue to have "new frontiers to explore" indefinitely if we continue in this way. If we colonize the galaxy, we will run out of "new frontiers" well within 13,000 years unless we can find a way to stop this exponential growth, and a way to intentionally leave some parts of our galaxy uncolonized. 

If we can't travel faster than the speed of light, then something has to "give" sooner. The galaxy is 100,000 light years in diameter. At some point well before our 13,000 years, the population would be growing so fast, your spaceships, traveling at sublight speeds, simply can't discover new matter fast enough to keep up with the population growth.

We can have new frontiers in understanding, new frontiers in many ways. Perhaps those can continue endlessly. We may be able to have new frontiers for exploration endlessly also, if we can find a way to do that without continual population growth.

But when it comes to exploring space and then converting places we find into habitats for humans - at some point this is going to stop.


If you rely on wars and starvation to stop population growth - that means that half of your entire population of humans, world wide, die before they reach child rearing age every century, from starvation or through warfare. Over the entire galaxy.

I think we can assume that for us, and for most intelligent ETIs, this is not an ethically acceptable solution.


Even with a very slow exponential growth of say a doubling every thousand years, you just multiply all those numbers by ten, and we'd still run out of new frontiers well before 130,000 years. That's just a blink in geological time.

If we do encounter an extra terrestrial civilization, the chance they evolve to technology within the same hundred thousand year period as us, over a history of billions of years, is tiny. So surely they'd be a hundreds of thousands of years old, or most likely a millions of years old civilization.

You can get around this if all civilizations are short lived, then we could meet a short lived civilization that is only the latest of many waves of colonization of the galaxy. But if they have started to colonize space - and this pattern of exponential growth has taken off - then it is almost impossible for them to go extinct. Because if they become extinct around one star, well, another star a thousand light years away won't even know about that for a thousand years. It's hard to see a ripple of extinctions affecting them all. So long as there are a few left, anywhere in the galaxy, then they could restart the colonization process. And the colonies that do the most rapid exponential growth would be the ones that fill the galaxy soonest.

That same reasoning also suggests that such a slow exponential couldn't be sustained for long. Because even if for some cultural reason or biological reason that was true of many colonists - there'd be sure to be some group of colonists, out of contact with the rest, who have much faster exponential growth. And they would be the ones who spread most and colonize the galaxy.


So, since they aren't here yet, haven't already colonized the solar system and filled it to the brim with ETs and converted all matter in our solar system into ETs - it's clear that any extra terrestrial intelligent civilization we may find that is older than, say, a few tens or hundreds of thousand years, must have found a way to stop exponential growth. (If their lifetimes are similar to ours that is - less time if their lifetimes are shorter, longer if they have lifetimes of thousands of years before they replicate).

Actually, we probably have found a way to stop exponential growth already ourselves. We reached "peak child" a few years ago, and so long as that continues and we don't get another upturn in the number of children born, then the predicted near doubling in the future to 11 billion people arises because people are getting healthier and living longer. That then would be our last doubling of our population. See Viewpoint: Five ways the world is doing better than you think 

Whether we are able to keep this up, as the more optimistic projections suggest, or whether the number of children in the world will start to increase again at some point - at least it shows it is possible for an intelligent species to reach a situation where it no longer has exponential population growth. 

So, what could we, or they, do after that, to explore a galaxy without running into this population explosion scenario?

One solution might involve using self replicating machines, or "von neuman machines" as they are called, after the mathematician Von Neumann who first explored some of the theoretical ideas for that machines could replicate themselves.


I think we may be surprisingly close to this actually. Not the versatile, creative, evolving and highly autonomous Von Neumann probe of science fiction, able to replicate almost anywhere, and do almost anything. But with limited capabilities and exact replication, yes, maybe quite soon.

We do have 3d printers now. There's even the "RepRap" project, developed with the aim for the 3D printer to print out all its own parts. RepRap - RepRapWiki

RepRap from Adrian Bowyer on Vimeo.

The most difficult part to print is the computer chip but we have nanoscale 3D printers as well. Once you can print out a transistor, even relatively crude in structure, you can in principle print out a (very slow probably) computer chip. As this continues, I don't see why a 3D printer couldn't print out all its components in the not so distant future, even a crude computer chip (although it would probably continue to make sense to use commercial chips instead of printed chips for some time to come).

Once a 3D printer can make itself from simple stock materials - then you need to find a way to do that in space, to print out all the components for asteroid mining. But some forms of mining are very simple.

Particularly, you can extract nickel by surrounding a meteorite in a bag which you don't need to heat very far, by the Mond process. Can fill it with carbon monoxide, and heat it by solar power to 50–60 °C. And end result is nickel carbonyl which is a gas, and fills the bag, all ready for use in a high temperature 3D deposition printer which you then attach to the bag.

For details, see SHEPHERD: A Concept for Gentle Asteroid Retrieval with a Gas-Filled Enclosure. You can also listen to Dr. Bruce Damer talking about this as guest speaker on David Livingston's "The Space Show".

So, as long as you can make do with mainly nickel parts, you could get much of the way using just this technology. Of course, you would need to have other elements as well, it's just a start. But - a bit like a miner making a hammer first - your self replicator, when it arrives in a new solar system, could make the easy tools first, using this method - and then - gradually make more complicated machines for harder to extract but important elements.

In principle we could already make a start at a "clanking replicator" in this way, using very slow computers printed using existing 3D printers - or even make printed clockwork "babbage machines" to do the computing for our printers.

But in practice it would be hugely expensive surely, if it did work. To make the first one anyway - that would be a kind of "Babbage machine" type project just now.

Part of Babbage's machine - a hugely ambitious undertaking in nineteenth century Britain to build a complete programmable computer entirely out of cogs and wheels. He developed many of the ideas of modern computers. Ada Lovelace then worked on ideas for ways to program it, and as a result has been described as the world's first computer programmer. But - the complete machine was never built. This was a hugely expensive undertaking, and it also stretched the technology of its day to the limit.

Augustus Ada King, Countess of Lovelace, who designed a program for Babbage's machine to calculate Bernoulli numbers, so is sometimes referred to as the first ever programmer. Her program couldn't be run as the machine was never completed, but it was correct and would have worked if the machine had been completed.

In the same way it would be a hugely complex and difficult project to attempt to build an asteroid mining self replicating machine right now. But in the future, this too might become feasible and common place.

It may not take many decades, before we can print out computer chips from simple raw materials. And once we can do that, it's likely to be not that long before we can make at least a large scale "clanking replicator" which you can just put into space, say, and send it off to convert suitable asteroids into copies of itself plus something useful, such as solar panels, or solar sails, or space mirrors. Or use it to make self replicating ice miners to bring ice to space habitats for fuel or water.

The other missing piece in this puzzle is how to get the replicators to assemble the copies of themselves from their components, once made. But this also doesn't seem that far out of reach.

This was an early 1980s NASA idea for a self replicating factory on the Moon.

But now we could make a much smaller version, surely.

We could do just about everything here, except manufacture high quality computer chips. And with nanoscale printing, those may not be far away.


The big problem is - what if this self replicating machine starts to evolve? It could turn into something quite different from what you originally intended, and that's the reason for caution about self replicating machines, with the potential dire consequences explored so often in science fiction.

However, it could be made safe. Surely any sensible ET would do that - just as we make our knives with handles so that we don't cut ourselves every time we use them.

Just as we build knives with handles and sheaths to keep ourselves safe so we don't cut ourselves - surely also we will build self replicating machines so that they can't hurt us. A Carter Cutlery damascus Bowie style blade. Image credit: Murray Carter

I'm not going to try to anticipate how an ET would do this with experience of thousands or millions of years of technology. But, it is something we could do already, or at least, in the near future, so let's see how we could do it right now with our current and near future technology.


One way to make them safe is to limit the number of generations, because that means a shorter time-line for errors to creep in.

Perhaps your machines only replicate for a dozen generations and stop. Surely even if there are errors and it evolves in some way through accumulated errors within its design tolerances - even though designed to reproduce exactly, it can't evolve that far in only a dozen generations.

Then you have mechanisms in place that immediately destroy any flawed replicators. Probably they check themselves for flaws, and if any are found that don't pass their self-tests, they automatically destroy themselves.

Indeed, before sending any replicators to another star, they could all check each other and themselves for any error in replication and then any that are flawed get recycled.

Amongst the flaws you check for would include the inability to check themselves for errors, or anything that disabled the twelve generation limitation. But - you'd do a thorough check of the entire machine, that every piece is identical, scan it, test its circuits, multiple hash checks of any code, check responses to tests etc. And you have the original as well as the replicated versions, so they check themselves against each other as well.

Whatever they do here has to be provably verifiable to a high degree of accuracy. After all, you are going to have billions of these machines eventually.


Each replicator when it arrives at a new star starts making copies of itself to send to other stars. And stops - say after it's made at most a hundred copies, the exact number here doesn't matter, just some limit to make sure it doesn't go on endlessly making everything into copies of itself. These are all sent to other stars.

Anything it makes for use in the same star is non self replicating, machines for particular tasks.

And it stops if it detects any prior instances of itself in the same solar system (have a radio signal it sends out to say "I am here" and first thing it does when it comes to a new star system is check for that signal. If it finds that another copy is already there - it switches itself off, destroys itself, or whatever).

After a million years or so, every star in the entire galaxy is visited. Your replicator has made at most a hundred times as many copies of itself as there are stars in the galaxy, so there is no chance of runaway replication. And most of those copies it made are inactive, having switched themselves off or destroyed themselves because they arrived at a star that had already been visited. This may seem very wasteful, but it's just a tiny amount of material and energy from every star and as for human effort, there is none extra involved at all.

That's how I'd design them anyway.


You also would want some method that lets you stop the whole thing if anything goes wrong.

One way to do this, is that I'd have a continuous beacon that we have to keep sending. It would be encrypted in a non reversible way - so they can't generate their own signal. Then design it so they all stop working if you stop sending this beacon. For replicators within the solar system, you'd have a beep every day, or even, every minute for replicators on the Earth. When the beeps stop, all self replication stops, and all the machines just stop working and wait for the next order. If they get nothing more after some elapsed time, they self destruct or whatever.

For interstellar travel it could be just a beep every hundred years, enough to make sure all the self replicators stop working and become inert if the parent civilization forgets about them or becomes extinct. In this scenario, if this is what other ETs did, our solar system may well have many of these forgotten probes, but if so, they'd be inert or have destroyed themselves and if we find an active one, that would mean its parent civilization probably is still in contact with it.


You could also design it with a spatial limit. Perhaps it has to remain within a hundred light years of a beacon in our solar system, or else it switches itself off. That might be a good precaution at least for the first such machines, while you get used to how they work and what they can do. Well of course to start with you'd make prototypes limited to our solar system, or even to parts of our solar system - then can advance to ones designed to explore the nearby stars, and then it might be time, with great care, to make one that can explore the entire galaxy.


You'd also need to make sure any such mechanism can't be bypassed by fledgling civilizations that might find probes that use technology way in advance of their own, and not understand the need for a failsafe like that.

I can well imagine that if humans found self replicating machines in our solar systems right now, and learnt how to bypass its failsafe mechanisms that the ETIs built in, it's entirely possible that someone or other would do just that. And set it evolving and replicating with exponential growth to fulfill some end they have. So a responsible ET would surely take steps to make sure that this sort of thing can't be done by immature civilizations that haven't thought through the consequences of their actions.


So- yes I think we are not that far away from it, maybe a few decades, but would need to be very careful.

We can of course make self replicators right away if they are living cells, microbes. And so that's another approach, synthetic life. Which requires a very high level of care if we ever design synthetic life expected to reproduce in the wild.

So far any experiments are carefully designed so they can't possibly reproduce outside the laboratory the (main one was an experiment that added two extra bases making DNA with six bases - designed so the new bases couldn't be made in the wild, only in a laboratory).

That counts as a von Neumann machine. So long as they replicate, it doesn't matter for the definition that they can only replicate in particular environments.

However synthetic life has the problem that it can evolve, and is not very controllable at present.


I'm not suggesting that we are at all close to the science fiction scenario of a general purpose self replicator - one that can survive almost anywhere by prospecting for materials and using creative fluid ways of working with them. Never mind the idea of a self replicator capable of any degree of intelligence. That may be far away.

But we might not be far away, as in, a few decades perhaps,  from a self replicator that can make copies of itself in special situations, e.g. to make solar panels from metal rich and silicon rich near earth asteroids using comparatively simple tools designed to work in that situation, and that situation only.

And once we get that far, we may not be far away from building an interstellar self replicator. One that's capable of colonizing, one per star, nearly all the stars in the galaxy, even if it can only build copies using iron nickel asteroids (say).

Given advances in interstellar propulsion, this approach could end up with one of these machines around just about every star in our galaxy within a million years. Just for the effort involved in building one self replicator.

Something like that. Details to be worked out.

Before long all the nearest stars  have them and eventually entire the galaxy does. But this process of replication comes to a complete halt after twelve generations by which time every star in the galaxy has one of those machines in its system.

Then when they encounter an interesting system, say one with habitable planets, they make more (non self replicating) robot spaceships to explore those systems in detail, and tell us what is there. 

I'd see that as a possible first phase, done with great care and forethought about possible consequences, so this is just a very preliminary draft idea.


So, our conclusion so far, if you did send humans, and the population grew exponentially, then in a surprisingly short period of time, compared with the timescale of evolution or geology, the entire galaxy, would be crowded by humans, more crowded than any of our cities or slums. It's no solution for an expanding population, not in the very long term.  All this would do is to transfer problems of exponential growth into the galaxy, written large across the skies.

This might be the reason Extra Terrestrial Intelligent creatures aren't living in our solar system already. Perhaps they are too sensible to fill the galaxy with evolving exponentially growing colonies.

So, if they do colonize, it is by some method that gives them confidence and security that this can't happen, with all the suffering it would entail. And the ones that are not that sensible, simply don't get as far as creating such colonies, but destroy themselves or at least their space exploration capabilities, at an early stage. They might keep doing that, going into space, then by accident or in warfare destroying all their space colonies, and grounding themselves and destroying their technology, in one cycle after another, until they become more sensible.


You might well ask - but does this happen to everyone? Are there no exceptions, not a single exponentially growing technological extra terrestrial that manages to overcome the obstacles of interstellar travel?

Well, we don't see ETIs in our solar system already, in large numbers, on every moon, planet and asteroid including our own. That may be indirect evidence that there is a self limiting process like that going on. 

Also, you can see how a self limiting process like that might work - at least, so long as we don't have faster than light travel or very easy rapid interstellar travel. 

Space colonies would be fragile, at least in the early stages, like the stage we are in now. Easily destroyed by the same technology that creates them - you don't even need nuclear weapons. A direct impact at tens of kilometers per second, even a spacecraft that makes a mistake and can't stop in time, would surely instantly destroy most space habitats. Unless you can develop the "deflector shields" of Star Trek that is :). 

Also space colonies would require high technology for their inhabitants to survive. So they would be working with the cutting edge technology in a situation where your very life depends on all the technology working perfectly. So if there are any problems inherent in the technology, it's likely to show up in the space colonies first.

So, it could well be, that this is how the process works. That it is just the way our universe is, due to the fragility of space environments, and the limitations of what you can do with technology - that you have to have learnt restraint, and be sensible, to explore space at all. 


I think we have been surprisingly sensible with our space activities so far. For instance, one sensible move was the Outer Space Treaty, which put an end to plans to build military bases on the Moon - and to have weapons of mass destruction in orbit. It was a remarkable achievement, as it's not easy to get agreement amongst so many nations with such diverse ideas and politics.

Have we ever had weapons in space? Turns out we do have guns in space right now. But not for use there.

These guns, on board every Soyuz sent to the ISS, are for defense against wild animals in case the Soyuz lands in a distant place and it takes a while to rescue the cosmonauts (this did happen once, the two cosmonauts of Voskhod 2 had to spend a night on their own in a snow covered Siberian forest, after it landed 1,250 miles off target because their automatic guidance system stopped working). (see also Voskhod 2, wikipedia

The Apollo astronauts took a knife with them in their survival kit, but no guns.

There is one example of a gun sent into space with the idea that it would actually be used there. This is the little known laser pistol which the Russians developed during the cold war.

The idea was that it could be used to damage the optics of an enemy spacecraft.
Soviet laser pistol: The secret space weapon of Russian cosmonauts (3 PHOTOS) -

It was never used, needless to say. It's not against any of our treaties to take hand weapons into space or indeed other kinds of weapons. Only weapons of mass destruction are prohibited. But the treaty stresses the need for peaceful exploration, and so far nobody has felt the need to take weapons into space intended for use there, except this one example of the Soviet laser gun. 

The outer space treaty also commits all the nations to peaceful co-operation while exploring space, and to render assistance to each other. 

The Chinese used a ground based ballistic missile to destroy one of their own old satellites. And the US explored ideas for launching an X Ray laser into space "on demand" to destroy ballistic weapons, as part of the Strategic Defense Initiative, but then abandoned that project. 

However, I think it is fair to say that our exploration of space has been peaceful so far, and that the Outer Space Treaty has been followed with no serious breaches so far. Certainly a lot more peaceful than it would be if we had US and Russian military bases on the Moon and weapons of mass destruction in orbit. Perhaps, hopefully, this is a sign that we can become one of the sensible space faring intelligences in our galaxy.

I think that humans at least have a chance of becoming the "sensible ETs" here. An advanced ETI might be astonished at how long it takes us to make decisions. We argue endlessly. Cause ourselves needless suffering in the process and take a long time to make a decision that may seem to be the only obvious solution later on when you look back with hindsight. But we do get there eventually.

Or perhaps we are just lucky? For instance, the various stories of how nuclear war was averted by individuals who made decisions not to launch a nuclear weapon (in the example of a Russian sub commander during the Cuba crisis), or not to raise an alert and so on. It may be luck, or perhaps there is more to it than that. Perhaps as a species we have an innate intelligence, that at least sometimes arises in individuals just when it is needed, and stops us from doing insanely stupid things.

Vasili Arkhipov who dissented from the other two commanders in a sub during the Cuban missile crisis, so preventing them from launching a nuclear missile which would probably have lead to all out nuclear war between the US and Russia. Is this just luck, or is it also an indication of some kind of innate sensibleness of our species that emerges at crucial moments?

This is his sub after it was forced to the surface. The US dropped depth charges as a warning to force it to the surface - the other two commanders thought this was the start of a nuclear war and wanted to launch their nuclear weapons as a desperate last response in the war. See Thank you Vasili Arkhipov.

We have solved many other problems. For instance, some early agreements include the green revolution from the 1940s to the 1960s which prevented widespread starvation world wide. Action to prevent the scenario of Rachel Carson's "Silent Spring". Stopped destruction of the ozone layer, which is now stabilized and expected to heal. International agreements that have largely worked to prevent chemical and biological warfare (with a more foolhardy ET, perhaps biological warfare would make them extinct as soon as they discover it). Partial nuclear test ban treaty to protect the atmosphere. Protection of the sea, and of forests. Many marine and national parks to protect particular sensitive regions. News stories tend to focus on the things that go wrong but - still we may be well ahead of the curve compared with the most reckless ETs.

We are now working on dealing with the issues of global warming. As has happened so many times before it is taking what many may feel an extraordinary length of time to come to agreement about what to do about it. But - I'm optimistic that as before we will get there eventually. Take far longer than it needs to, but basically we do seem to be a sensible species and collectively do make the right decisions in the end.

So if it turns out that interstellar colonization is self destructive, I think there is a decent chance that, collectively - after probably much discussion and dissension - that eventually we'll make the right decision there also. And other ETIs would also in our situation, if they are at least as sensible, or more so, than ourselves.


I think we should explore, not colonize. And the best emissaries may be robots to start with. Interstellar probes and then, self replicating probes. I think it can be done safely. But with great care.

Instead of a "general purpose" machine of science fiction able to make almost anything, the safest thing is to, e.g., design it so it is a reasonably simple machine, and just makes exact copies of itself, no evolution, and with reasonably tightly conditioned responses to situations - with a fair amount of autonomy but also reasonably predictable.

After experience with those you might be able to tackle the much more difficult problem of a self replicator able to behave differently in different situations in a more fluid way.


And as for evolving self replicators, evolving to get better at what they do - I think that would be a total No No at present. Unless you really had a very clear idea of what you are doing, some advanced understanding we don't have yet.

But - self replicating human explorers of the galaxy are equally scary I think :). Not often said. But as well as the population explosion problem already mentioned - by the time they are a thousand light years away - we have no idea what they are doing. They also could evolve, just as the self replicating machines would do - and more unpredictably.

Our colonist descendants around distant stars in our galaxy could themselves be

  • making replicating machines
  • cyborgs
  • creating new intelligent species from all the creatures they encounter and from the animals they bring with them
  • or just invent new synthetic intelligent lifeforms for fun.

They could be doing things that are unbelievably stupid according to us but that make sense to them.

  • They could be motivated by some bizarre philosophy or religion which is more strange to us than any of the extremist sects on Earth.
  • Or could be that they are "de-evolving" to creatures with intelligence of a child but vast powers. So they don't even know what they have done.
  • They could build replicating technology or beings that destroy them, and then us

We wouldn't know until a thousand years later - by which time there could be almost anything headed our way from the colony. Say, a cloud of self replicating machines designed to turn all star systems into Dyson spheres (a spherical cloud or solid sphere surrounding a sun to capture all its sunlight to use as power) - demolishing any planets in the process to make these spheres.

Perhaps someone thought that was a cool idea and did it just because they could do it. Maybe just as a school project that got out of hand :). Or perhaps it arose out of self replicating machines programmed to evolve and solve a problem such as to create as much living space for humans as possible - and that was its solution - programmer forgot to add to the list of requirements that the solution shouldn't destroy inhabited worlds.

In that example, there's no thought behind it at all, chances are. No purpose as we understand it. Just machines, mindlessly doing what they are designed to do, and doing it a bit too well.

Could be nanobe sized machines, trillions of them, traveling at close to the speed of light, so gets to us before we even know about them, dig into the Oort cloud and make copies of themselves ready for the next step whatever it is. Cryptic machines based on technology way advanced ahead of us by many centuries (because the technological development would be bound to be uneven, some parts of the galaxy thousands of years ahead than others, and no reason why Earth should always be at the head of technology). Perhaps designed to hide and destroy themselves imperceptibly if discovered so that even when you examine the comets they have burrowed into they just seem like meteoritic dust (say), until suddenly when there are enough of them, they all come to life and do whatever they do next.

Just an example.

Or could be an army of cyborgs. Or could be any of the "Alien monsters" of science fiction - but created by our descendants, so only slightly evolved beyond us.

We haven't encountered these problems in such an extreme way on the Earth because the whole Earth as soon as it got highly technological was already interconnected. We are so connected, there is no real chance that one of the countries is more than a few years ahead of any of the others. Certainly not a thousand years ahead, or even a century.

If some strange philosophy develops, or viewpoint or approach, the whole world learns about it soon enough. 

Once we do develop mechanical self replicators, I think we will also develop ways of dealing with them. 

So long as all humans are on the same planet, or in the same solar system - we've got a chance of dealing with these things, even self replicating machines. But once we are spread out, as colonies of humans, to every star in the galaxy - what chance do we have then?


The idea of leaving a colony of humans somewhere with interstellar travel and colonization capabilities, without forethought about what the implications would be, might be considered as similar to launching an unregulated self replicating machine, or setting off a nuclear explosion in a crowded city. Something that sensible ETIs just would never do. Unthinkable. Because they would all be able to see what the potential consequences would be. Something that to them is as clear as the basics of addition and multiplication are to us, something that every child learns at an early stage.

I think in the future that this may well become unthinkable to almost any advanced ETI. And those that don't think that way, are so reckless, they destroy themselves long before they can be a nuisance to the galaxy - at least - destroy their space faring capabilities, and keep destroying them every time they try to colonize in space - until they become more forward looking. Because the first colonies are so delicate and easily destroyed. And deliberate warfare with space technology and the fragile space colonies would end up with everyone dead.

So - we need to move forward in some way to a future where we don't do things like that. Otherwise we will never explore the galaxy. And if we are as far seeing as that, able to explore space without destroying our space faring capabilities in the process - then I think we will also choose to explore the galaxy robotically. Or if we send humans at all - as explorers that wouldn't think of setting up new colonies until we have thoroughly worked out a way to do that that avoids these issues of unrestricted replication of humans and whatever entities they might create.

Either we are the first ETIs in our galaxy to have the potential to colonize it - if so it is up to us to find out what to do and to set a good example for the future. And that would suggest we take extreme care because we have no-one else to learn from and tell us what the pitfalls are (except possibly in other galaxies).

Or else - there are other ETIs in our galaxy already, if so it would be an extraordinary coincidence if they evolved the same time as us, so they would be likely to be a millions, or more likely billions of years old civilization. If so hopefully we detect them eventually and learn from them what to do and what works and what doesn't work, or is dangerous or safe to do.


If unsure how to explore, robots are safest. I mean simple ones, not able to evolve especially, and with limited autonomy.

  • They can be programmed to self destruct if anything goes wrong. Not a big deal, another robot will take its place.
  • They can go into dormancy for millions of years - if they find an uninteresting solar system say, just leave a few sentinels and go to sleep.
  • They can be programmed safely to visit every star in the galaxy.
  • They can be sterilized so that they don't bring Earth microbes to ET planets.
  • They can be designed so that you have, say, only one copy around every star (as already explained)

Many of these are impossible for human colonies, or totally unethical and impractical.


That's my preferred explanation of the Fermi paradox (the paradox, why don't we have ETIs here already if they have technology able to colonize the galaxy) - that sensible ETIs are just as careful about self replicating colonies and evolving copies of themselves as they are about self replicating machines. And that we will be too when the time comes.

But the precautions we can take with machines - exact replication, destroy them if they don't replicate exactly, limit to 100 copies and twelve generations and stop etc - these are totally unacceptable if they are humans rather than machines.

So then - maybe eventually they do colonize the galaxy but if so in other, subtle careful ways to avoid these problems. How exactly, I don't know, but perhaps we find a way to do this safely and reliably, as safely and reliably as we can do with machines. Some time, hopefully, we'll be able to do this.

Whatever their solution is, if they did it, it clearly is an approach that leads to minimal modification of the galaxy and a hard to detect presence, and without exponential growth. Because when we look at the galaxy in our telescopes - as far as we can see it seems totally unmodified and in its natural state. It's surely not all turned into habitats for extra terrestrials. And we may make the same decision also when the time comes

Perhaps they explore rather than colonize. It is easy to come up with safe scenarios for explorers.

If there were ETI individuals or families or small settlements that tour the galaxy - even if, say

  • they have immensely long lives of millions of years and touring the galaxy is something every ETI does as a youngster of one or two million years
  • and there are millions of these ETIs exploring the galaxy right now

- so long as they don't colonize and replicate anywhere but return home when finished - or so long as they are not involved in exponential population growth by whatever other method they make sure it doesn't happen - then they would leave almost no trace of their presence and the nearest ETI would be likely to be thousands of light years away.

We may be due a visit some time in the next few thousand years.


Whether humans colonize or settle or explore - that's a decision for much later once we have a much better understanding of our galaxy and also, hopefully, have managed to develop a little beyond those potentially self destructive approaches to exploration of the galaxy. How exactly that happens remains to be discovered.

And in our solar system - if the first space colonies are based around settlement and exploration - like the ones in Antarctica - I think that is the way ahead. Not push for colonization no matter what. But let it happen naturally where colonies are needed for some purpose.

Because we don't need to do a backup of the Earth - that's in one of my other posts here. There are no dangers that would not be better solved by having your backups on Earth not for the next few tens or probably hundreds of million years anyway.

So there is no great rush. Our Earth, no matter what happens in the next few million years (timescale over which new mammal species evolve - so that is a huge future timescale), surely it will remain the most habitable place in the solar system for humans.

As we explore the rest of our solar system, our galaxy also, this remains the place where humans are able to breath the air, and feel at home. So we have to look after our Earth. And as we explore, keep that as a priority, to look after Earth and protect it from disasters self induced or things like asteroids, protect our beautiful blue planet.

Our beautiful blue Earth photographed from lunar orbit by the Apollo 11 astronauts. The moon is much more typical of the surfaces of planets in our solar system and probably in the galaxy. Our priority, as we explore the solar system, and then the Earth, must be to look after our Earth.

Then we will have a stable basis and confidence, and no need to be rushed. We can then afford to make sensible decisions as we explore the galaxy.

While protecting ourselves in this way, we also protect the galaxy too. If some form of self replicator gets out of hand, either mechanical or biological, it would probably be seen by all ETIs as a problem for the entire galaxy. We need to learn how to make not just knives for galactic exploration, which can indeed be very useful - but also handles and sheaths to protect ourselves and others in the galaxy.


This article has focused on what to do if you want to explore the entire galaxy, leaving not a single star unexplored. 

As for exploring in a more limited way, well so long as you don't set up colonies anywhere, there's no problem at all. Once you start to set up colonies, well, it needs some thought about what those might turn into, and it obviously becomes a more involved question, whether this is a safe thing to do.

I look at this whole question a little more, from another angle, in my Why Didn't ETs, Or Self Replicating Machines, Colonize Our Solar System Millions Of Years Ago? There I also look at other questions such as whether it's possible that we are the first ever technological intelligence in our galaxy - and briefly touch on a few of the other solutions to Fermi's paradox "Why isn't ET here already?".

 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.


It doesn't seem that we can have endless frontiers for population growth and colonization. But we can have frontiers in other ways.

  • Frontiers of understanding of science
  • Frontiers in development of society
  • Frontiers in understanding of ourselves
  • Frontiers in art, technology, music
  • Endless new personal discoveries.

We can also explore the universe endlessly, if we follow a Star Trek like "Prime Directive" - and I mean of course really follow it, not break it whenever it is inconvenient. If we don't colonize, or if we find a way to colonize without exponential growth, some way of limiting our population expansion as easily as we can limit the expansion of self replicating machines - then we can explore space endlessly too.

Even with trillions of explorers the galaxy would be boundless for them. Eventually we'd end up revisiting the same place that a previous explorer discovered a hundred thousand years ago. If we have good records, maybe we can read the ancient logs they left on their last visit. But it would have changed and evolved since then, and we'd have endless frontiers in that sense, even physically.

In this article I looked at a way we can explore the galaxy with machines, safely, with present day technology. Of course, such limited self replicators, deliberately made unable to evolve or to be fluid or creative in their approach, could not explore the galaxy with the inventiveness and understanding of human colonies. But it would be a safe thing to do and with minimal investment of human time we'd get a first close up look at all the stars nearby, and eventually if we wish, of the entire galaxy.

It might be possible to do it safely with more intelligent self replicating machines. And maybe even with humans also. But that would involve some future development of understanding and technology that we don't have yet. And I think we shouldn't launch out on such a path until we can see how to do it safely. And think any intelligent far seeing ETI would come to the same conclusion, eventually.


Here it might seem as if I'm being pessimistic and only looking at the worst side. But the thing is, that if we have unrestricted exponential population growth into the galaxy like this, I think it would bring out the worst side of us.

Think of whatever ideology you find most appalling and scary in the world. Now imagine that there are whole solar systems of beings with those ideologies - and much more scary ones than those. These beings are cyborgs, or they are creatures created artificially, or they have deliberately modified their own DNA, or, could be anything. These might well be the ones that are most aggressive and prolific and fill the galaxy as a result of our first colonization expeditions.

Maybe every star system for a thousand light years is filled with reasonable, sensible, careful civilizations of beings that respect each other, engage in peaceful trade, and treat other ETIs, including ones without technology, with consideration.

But you only need one "bad apple" perhaps a thousand light years away, to spoil this for everyone.

We need to find an approach that brings out the best in us, not the worst in us. I suggest that using carefully designed self replicating machines, and a "prime directive" based exploration without colonization, might be what our descendants decide to do as their first steps. And then whatever they do next would also be some form of exploration or colonization that is hazard free for them and for us and has minimal impact on the galaxy. And that might explain why the ETIs aren't here yet also.


This originated as my answer to a couple of questions on quora:

See also my Science 20 articles