I think mine is sort of new. It combines together the ideas in a somewhat new way. And that is to look at the way populations grow. They can only grow exponentially for a short period of time. Then they will hit a limit.
You can show for instance that if a population of humans doubles every century, they have to stop some time before 18,000 years from now - as by then they would be converting all the matter in the visible universe into humans. That's stars, galaxies, black holes, everything, into nothing else just humans, every century. At some point they have to stop before that.
Long before probably. They might hit the crunch as soon as, say, four thousand years of doubling every hundred years - that means finding homes for a trillion times their original population every century. Probably well before then. At any rate well before 18,00o years they definitely have to stop.
It's a surprisingly short time period.
So it's almost impossible, if we encounter ETs, even if they expand in fits and starts, that they will be in an expansionist phase right now.
So then the main question is - how large is their population then - and what is its spatial extent? For the largest ET populations and territories?
They may have found a way to stabilize their population peacefully, as we hopefully are doing. I.e. they stop doubling naturally.
This would be a peaceful outcome where we stop our population expansion not through starvation or war but just through birth rate declining as families get wealthier and find it better to invest more in fewer children as it seems all the rich countries on Earth do at that point no matter the religion or philosophy or culture.
So that's a steady state where most of the population live long and healthy and happy lives and they have just enough children for replacement.
Or may be in a steady state where most of the progeny die very young. Same kind of curve, sigmoid curve, but it's achieved by high death rates as they run out of food.
Or could be a boom and crash like lemmings.
So - ETIs would probably be in one of those three states currently - end result of one of those - or could be other possibilities, e.g. they just go extinct or whatever, but those give an idea of their range of future possibilities.
But if they are sensible intelligent ETIs, only the first of those possibilities is going to be acceptable. And we have already shown in many ways that intelligent creatures can do things they would never do by short term instinct. E.g. simple example, conservation of whales. They'd all be gone now if we followed blind instinct like animals. Lots of other ways we are far more forward looking than animals would be if you suddenly equipped them with technology and the ability to use it.
So we can foresee things and prevent them and I suggest ETIs will be able to do so also.
So they will find a way to make sure the first outcome happens to them, not the other two.
The other thread in my reasoning here is that in the future we won't need to terraform worlds to create habitats but can just create them in the asteroid belt, and eventually the Oort cloud of any star.
So, if expanding outwards into a galaxy where anyone can set off in a spaceship and colonize any star they like - there is no way that they can ensure that all their progeny will be sensible like they are. It's a similar danger to the danger often mentioned of self replicating evolving machines. These are self replicating evolving humans, far more uncontrollable. Especially since they can also create self replicating machines, uplifted humans, cyborgs etc. And clones also to fill star systems rapidly with their kind. All of those can surely be used in good ways, and the majority will be "good colonists" hopefully.
But if expanding into the vastness of our galaxy, you'd only need a few in a quadrillion colonists to be expansionist, to favour exponential growth even at shorter periods than a century, maybe even doubling every ten years. So long as it is compatible with retaining technology - they would be the winners. Also the ones that develop superior technology to destroy others. Often gets forgotten - though the Allies did develop the atom bomb way ahead of the Nazis - and also developed radar first, and were ahead in computing - the Nazis developed missile technology that was way ahead of the Allies, and was indeed used in the early moon shots by both the US and Russia and was the basic technology behind the Saturn V etc.
And with quadrillions of colonists, unless somehow we move forward to a wiser and more compassionate future - then it will include the likes of ISIS and the Nazis - and they will be able to head off in a spaceship so far away that if we want to respond ot something we detect they are doing, we will be responding to something they did a century ago, or a thousand years ago. And for every star system colonized by people who double every century, there will be thousands, then millions, then billions colonized by the rapid ones that double every ten years.
And then the most prolific and aggressive will fill the galaxy first. And then once full, then impossible for them to become extinct. They would die off in countless quadrillions. If they have a doubling time of ten years, say, the winners of this race to colonize the galaxy - then every ten years the entire galactic population has to be halved. But even if they make themselves extinct around nearly every star - or move forward to some more advanced peaceful way of dealing with things, the same evolutionary effect will apply to the few that are left that are still aggressive population growers.
I can see this just going on and on endlessly, forever. People talking endlessly about how to stop it, and from time to time it gets quelled in local patches of the galaxy - but whatever they do, some future far worse than ISIS or the Nazis - or just an unregulated self replicating paper clip making von neumann machine emerges from beyond the light speed horizons to destroy everything again.
We avoid that on Earth because we are all in communication with each other. And there are enough of the sensible ones to keep the others in check. And even so we find it challenging, from time to time some extremist group takes over fairly large regions of our planet. But on a galactic scale, how can that work? How can we keep it in check then? With any group able to colonize beyond the light speed horizon and once we know about them, they have filled perhaps a million star systems and any response will take a thousand years to reach them?
To avoid this sort of thing, I think ETIs will need to look ahead and find a way to ensure this can't happen to them.
So I think the maximum size of an ETI population will be far smaller than an entire galaxy.
It could be that we are first. Then we have tremendous repsonsibility to deal with this carefully.
Or it could be that there are mechanisms already in place that will stop us if we start to become a cancer of our galaxy, that we don't know of yet because they haven't been triggered. Some older ET civilization for instance may have seeded our galaxy with self replicators that watch out for rapidly expanding galactic colonization - and automatically and humanely sterilizes us through some future technology we can't detect, or of enough of us to keep our populations down.
Or, I think most plausible myself. That ETIs naturally stop themselves. Because if not then their space colonies are so fragile, and easily destroyed just by the kinetic effect of a spaceship accident at kilometers per second delta v never mind designed weapons to destroy them - that once you have a few billion people in space, if they have not yet achieved peaceful ways of dealing with things, and foresightedness as a culture - then they end up destroying themselves. So long as they don't destroy their home planet in the process, a few at least survive, and then they rebuild their civilization and try again, maybe learnt something from the last time, until eventually they "get it". Or the wiser more sensible ETIs may "get it" just the first time right away.
Or could be some object lesson - some very distant galaxy which we see has reached this equilibrium state of half the entire galactic population dying every 10 years, with messages from their inhabitants telling us what happened to them, how they can't seem to do anything to stop it, and warning us not to do the same.
Or messages from ETIs in our galaxy, older ones, educating us.
Whatever, I think that explains the paradox. There could be thousands of civilizations in our galaxy. But none of them are expanding exponentially, or very small chance that any are. And that they have maximum size populations that are achieved through peaceful means, without high mortality or boom and crash. And that to do that they need to have territories for their breeding populations that are small enough so that they can communicate each to the other. Which without FTL communication would mean at most a few star systems probably.
Even if we colonized every visible star, in our night sky, that's a tiny part of our galaxy. If we could make sure our colonization goes no further than that, we would have minimal impact on the galaxy. And most of our colonies would be close enough for communication to deal with problems that arise gradually over centuries. But we wouldn't be able to stop at that point, that's the problem. Not if we do it now, as we are now.
So they would need to have territories small enough so they know they can safely stop at that point. Contented. Not "hemmed in". They have everything they need in that region of space. And can explore beyond it as much as they like.
They could have explorers who make long thousands of years, millions of years long expeditions through the galaxy. But every young child would learn that to set off an uncontrolled colonization of the galaxy is as bad as letting off a fusion reaction nuclear bomb in your own home city. So for them this would be unthinkable.
Other solutions could involve ETIs with immensely long lifetimes so that they can crisscross the galaxy many times in a single lifetime - and no thought of having children before they have done that at least several times - they would expand in population so slowly it would not be an issue for them yet, since the beginning of the universe. That's about the only situation I can think of where we just possibly might find an ETI in sustained population growth.
There may be other solutions also. Maybe some of them involve ideas we won't discover for a thousand, or a million years. But one way or another they have to solve this.
But the exploration, discovery of knowledge, new frontiers also - those can be endleses. It is just in terms of numbers of individuals that they have to stop at some point. Everyone has to, the maths shows it, is impossible to sustain exponential growth.
And so our priority right now is not to set up colonies as quickly as possible. They could be the seed of our future problems indeed. The early stage colonies of billions destroying each other scenario not yet the galaxy cancer scenario.
Our priority is to learn to live together with love and compassion and peace. And of course competition - but competition that is non violent such as for instance the Olympic games, and scientific competition and such like. I think that's our challenge. And in space exploration to find out and learn, and not to think we know the answers to everything already. Play it right and we as a civilization can have millions and then billions of years to sort out things like that.
I do think we could make a start on interstellar exploration and exploration of the galaxy. But robots are probably the key there. Because self replicating robots can be controlled, ethically, in ways you simply can't do with humans.
More about all this here:
Robert Walker's answer to If we are truly the only intelligent life form in the universe, does this mean the universe is ours for the taking?
Which then links to more articles where I go into it in more depth.
The main calculations are here: Why ET Populations Can't Continue To Expand For More Than A Few Millennia
This is something I've been working on in the spirit of "Science 2.0" - just publishing these blog posts and answers, and getting comments on them. At first I wondered if someone would come up with some easy explanation or a flaw in the reasoning but nobody has to date. So I do think there's at least a chance this may be something that we don't have a solution to at present - and that if so - it may explain the Fermi paradox.
And though many of the elements I use in this argument are included in the traditional explanations of the paradox, I think this synthesis is new as far as I know. I'm very interested to hear if anyone knows of someone else who has suggested the same.
If it has been suggested before I think it is worth giving it attention.
You can show for instance that if a population of humans doubles every century, they have to stop some time before 18,000 years from now - as by then they would be converting all the matter in the visible universe into humans. That's stars, galaxies, black holes, everything, into nothing else just humans, every century. At some point they have to stop before that.
Long before probably. They might hit the crunch as soon as, say, four thousand years of doubling every hundred years - that means finding homes for a trillion times their original population every century. Probably well before then. At any rate well before 18,00o years they definitely have to stop.
It's a surprisingly short time period.
So it's almost impossible, if we encounter ETs, even if they expand in fits and starts, that they will be in an expansionist phase right now.
So then the main question is - how large is their population then - and what is its spatial extent? For the largest ET populations and territories?
They may have found a way to stabilize their population peacefully, as we hopefully are doing. I.e. they stop doubling naturally.
The orange and green projections both have us reach a maximum (see Projections of population growth ).
They are not so implausible as you might think as we have already reached peak child (Viewpoint: Five ways the world is doing better than you think - BBC News) .
This would be a peaceful outcome where we stop our population expansion not through starvation or war but just through birth rate declining as families get wealthier and find it better to invest more in fewer children as it seems all the rich countries on Earth do at that point no matter the religion or philosophy or culture.
So that's a steady state where most of the population live long and healthy and happy lives and they have just enough children for replacement.
Or may be in a steady state where most of the progeny die very young. Same kind of curve, sigmoid curve, but it's achieved by high death rates as they run out of food.
Or could be a boom and crash like lemmings.
See the Amazing Lemming
So - ETIs would probably be in one of those three states currently - end result of one of those - or could be other possibilities, e.g. they just go extinct or whatever, but those give an idea of their range of future possibilities.
But if they are sensible intelligent ETIs, only the first of those possibilities is going to be acceptable. And we have already shown in many ways that intelligent creatures can do things they would never do by short term instinct. E.g. simple example, conservation of whales. They'd all be gone now if we followed blind instinct like animals. Lots of other ways we are far more forward looking than animals would be if you suddenly equipped them with technology and the ability to use it.
So we can foresee things and prevent them and I suggest ETIs will be able to do so also.
So they will find a way to make sure the first outcome happens to them, not the other two.
The other thread in my reasoning here is that in the future we won't need to terraform worlds to create habitats but can just create them in the asteroid belt, and eventually the Oort cloud of any star.
So, if expanding outwards into a galaxy where anyone can set off in a spaceship and colonize any star they like - there is no way that they can ensure that all their progeny will be sensible like they are. It's a similar danger to the danger often mentioned of self replicating evolving machines. These are self replicating evolving humans, far more uncontrollable. Especially since they can also create self replicating machines, uplifted humans, cyborgs etc. And clones also to fill star systems rapidly with their kind. All of those can surely be used in good ways, and the majority will be "good colonists" hopefully.
But if expanding into the vastness of our galaxy, you'd only need a few in a quadrillion colonists to be expansionist, to favour exponential growth even at shorter periods than a century, maybe even doubling every ten years. So long as it is compatible with retaining technology - they would be the winners. Also the ones that develop superior technology to destroy others. Often gets forgotten - though the Allies did develop the atom bomb way ahead of the Nazis - and also developed radar first, and were ahead in computing - the Nazis developed missile technology that was way ahead of the Allies, and was indeed used in the early moon shots by both the US and Russia and was the basic technology behind the Saturn V etc.
And with quadrillions of colonists, unless somehow we move forward to a wiser and more compassionate future - then it will include the likes of ISIS and the Nazis - and they will be able to head off in a spaceship so far away that if we want to respond ot something we detect they are doing, we will be responding to something they did a century ago, or a thousand years ago. And for every star system colonized by people who double every century, there will be thousands, then millions, then billions colonized by the rapid ones that double every ten years.
And then the most prolific and aggressive will fill the galaxy first. And then once full, then impossible for them to become extinct. They would die off in countless quadrillions. If they have a doubling time of ten years, say, the winners of this race to colonize the galaxy - then every ten years the entire galactic population has to be halved. But even if they make themselves extinct around nearly every star - or move forward to some more advanced peaceful way of dealing with things, the same evolutionary effect will apply to the few that are left that are still aggressive population growers.
I can see this just going on and on endlessly, forever. People talking endlessly about how to stop it, and from time to time it gets quelled in local patches of the galaxy - but whatever they do, some future far worse than ISIS or the Nazis - or just an unregulated self replicating paper clip making von neumann machine emerges from beyond the light speed horizons to destroy everything again.
We avoid that on Earth because we are all in communication with each other. And there are enough of the sensible ones to keep the others in check. And even so we find it challenging, from time to time some extremist group takes over fairly large regions of our planet. But on a galactic scale, how can that work? How can we keep it in check then? With any group able to colonize beyond the light speed horizon and once we know about them, they have filled perhaps a million star systems and any response will take a thousand years to reach them?
To avoid this sort of thing, I think ETIs will need to look ahead and find a way to ensure this can't happen to them.
So I think the maximum size of an ETI population will be far smaller than an entire galaxy.
It could be that we are first. Then we have tremendous repsonsibility to deal with this carefully.
Or it could be that there are mechanisms already in place that will stop us if we start to become a cancer of our galaxy, that we don't know of yet because they haven't been triggered. Some older ET civilization for instance may have seeded our galaxy with self replicators that watch out for rapidly expanding galactic colonization - and automatically and humanely sterilizes us through some future technology we can't detect, or of enough of us to keep our populations down.
Or, I think most plausible myself. That ETIs naturally stop themselves. Because if not then their space colonies are so fragile, and easily destroyed just by the kinetic effect of a spaceship accident at kilometers per second delta v never mind designed weapons to destroy them - that once you have a few billion people in space, if they have not yet achieved peaceful ways of dealing with things, and foresightedness as a culture - then they end up destroying themselves. So long as they don't destroy their home planet in the process, a few at least survive, and then they rebuild their civilization and try again, maybe learnt something from the last time, until eventually they "get it". Or the wiser more sensible ETIs may "get it" just the first time right away.
Or could be some object lesson - some very distant galaxy which we see has reached this equilibrium state of half the entire galactic population dying every 10 years, with messages from their inhabitants telling us what happened to them, how they can't seem to do anything to stop it, and warning us not to do the same.
Or messages from ETIs in our galaxy, older ones, educating us.
Whatever, I think that explains the paradox. There could be thousands of civilizations in our galaxy. But none of them are expanding exponentially, or very small chance that any are. And that they have maximum size populations that are achieved through peaceful means, without high mortality or boom and crash. And that to do that they need to have territories for their breeding populations that are small enough so that they can communicate each to the other. Which without FTL communication would mean at most a few star systems probably.
Even if we colonized every visible star, in our night sky, that's a tiny part of our galaxy. If we could make sure our colonization goes no further than that, we would have minimal impact on the galaxy. And most of our colonies would be close enough for communication to deal with problems that arise gradually over centuries. But we wouldn't be able to stop at that point, that's the problem. Not if we do it now, as we are now.
So they would need to have territories small enough so they know they can safely stop at that point. Contented. Not "hemmed in". They have everything they need in that region of space. And can explore beyond it as much as they like.
They could have explorers who make long thousands of years, millions of years long expeditions through the galaxy. But every young child would learn that to set off an uncontrolled colonization of the galaxy is as bad as letting off a fusion reaction nuclear bomb in your own home city. So for them this would be unthinkable.
Other solutions could involve ETIs with immensely long lifetimes so that they can crisscross the galaxy many times in a single lifetime - and no thought of having children before they have done that at least several times - they would expand in population so slowly it would not be an issue for them yet, since the beginning of the universe. That's about the only situation I can think of where we just possibly might find an ETI in sustained population growth.
There may be other solutions also. Maybe some of them involve ideas we won't discover for a thousand, or a million years. But one way or another they have to solve this.
But the exploration, discovery of knowledge, new frontiers also - those can be endleses. It is just in terms of numbers of individuals that they have to stop at some point. Everyone has to, the maths shows it, is impossible to sustain exponential growth.
And so our priority right now is not to set up colonies as quickly as possible. They could be the seed of our future problems indeed. The early stage colonies of billions destroying each other scenario not yet the galaxy cancer scenario.
Our priority is to learn to live together with love and compassion and peace. And of course competition - but competition that is non violent such as for instance the Olympic games, and scientific competition and such like. I think that's our challenge. And in space exploration to find out and learn, and not to think we know the answers to everything already. Play it right and we as a civilization can have millions and then billions of years to sort out things like that.
I do think we could make a start on interstellar exploration and exploration of the galaxy. But robots are probably the key there. Because self replicating robots can be controlled, ethically, in ways you simply can't do with humans.
More about all this here:
Robert Walker's answer to If we are truly the only intelligent life form in the universe, does this mean the universe is ours for the taking?
Which then links to more articles where I go into it in more depth.
The main calculations are here: Why ET Populations Can't Continue To Expand For More Than A Few Millennia
This is something I've been working on in the spirit of "Science 2.0" - just publishing these blog posts and answers, and getting comments on them. At first I wondered if someone would come up with some easy explanation or a flaw in the reasoning but nobody has to date. So I do think there's at least a chance this may be something that we don't have a solution to at present - and that if so - it may explain the Fermi paradox.
And though many of the elements I use in this argument are included in the traditional explanations of the paradox, I think this synthesis is new as far as I know. I'm very interested to hear if anyone knows of someone else who has suggested the same.
If it has been suggested before I think it is worth giving it attention.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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