I want to summarize and add some commentary to the article[1] posted by macu as a reply to another comment.
It's amazingly interesting and well argued. In short, the author says first that any sufficiently advanced civilization will at one point send self-replicating probes that will eventually colonize all of the galaxy and even universe. And it takes only one match to light a fire, and this is important--that is, for a sufficiently advanced planet, it would take only one rebel, one mad scientist, to set the colonization, by robots, of every habitable body in motion.
Assuming the above is true, then why do we not see any sign of these probes? It follows that there must be a Great Filter that prevented them from ever being created. Things that qualify as such a filter are not plenty, and include the original formation of life and the transition from procaryotic to eucaryotic (taking a couple billion years).
Thus if we find life so easily formed on Mars or anywhere else (especially eucaryotic cells), we can assume these past events are not really Great Filters. Then, the only reason we haven't found a single probe is that any sufficiently advanced civilization destroys itself before it ever gets to that point. If it's not nuclear weapons, then there must exist technologies that are sure on our path to discovery while at the same time guaranteeing our extinction--again, think in terms of things that only need one outlier to use it in a way that compromises the existence of all on Earth for good.
So he concludes that we must pray that we never find life anywhere else, because it would in turn give us hope (but not assure us) that there might exist no Great Filter ahead of us, and the Great Filter was indeed in our conception as life--thus we are the one single unfathomably lucky planet to ever have harbored life. If nothing potentially impedes our expansion, we will be the one sending probes and expanding to everywhere.
Adding my own expansion on this, I believe it's extremely unlikely that there is some irrevocably cataclysmical tech to be discovered before ever we are able to send self-replicating probes--ones who can mine the raw materials needed for unbounded expansion.
I don't think we're that far from that point, and taking from our own anecdotal existence since it's the only one there is, even if we do annihilate ourselves before sending them out by chance, other civilizations might not have done so, if they existed. Thus indeed the only tech that would assure a Great Filter is the self-replicating technology itself. And that is a strong point--the author seems to ignore the fact that these probes wouldn't really be the expansion of humanity, it would be the expansion of drones. And what might the precursor techs for such a replicating machine look like? If superior AI is needed at any rate, we could think that the tech leading to sure extinction is indeed robots who decimate us humans. And since they would do this as early as chance would afford it, the robots eliminating us would be as dumb as possible--thus we can assume they would be incapable of advancing technology on their own; or of coordinating an event like launching themselves into space, and thus would never leave Earth. In this scenario, we can assume every civilization that ever got a shot in going to space ended up extinct, their planet ridden with dumb self-replicating robots who can never launch themselves into space.
Since self-replicating robots are the hypothesis of the author's argument, then indeed it makes sense that every potential civilization either never reached this tech (the Great Filter is in life's conception, or otherwise somehow denies the existence of self-replicating robots), or it must absolutely reach it, in order to colonize space, but in doing so it declares it's extinction.
So, like the author, I also hope[2] that we never find life elsewhere, that we might put our tiny hopes in thriving as a race, the only one that ever existed.
It's an interesting thought experiment, but not too meaningful. The universe is still young; it's been around for 13 billion years, the Earth for 6. Given the immense distances involved, and the complexity and resources required for interstellar travel, it's not surprising we wouldn't have seen life.
And, for a moment, let's suppose your thought experiment is true, and there's an alien probe somewhere in our solar system. Would you be surprised we haven't found it yet? There're an awful lot of places left to look.
1. I was wondering if we could apply an anthropic type of argument here? We are having this argument at this time and place because we predate the colonization by self-replicating bots / other end-of-world scenarios. Multiverse version: any time we're having this discussion, we are on the surviving branch predating self-replicating bot colonization that renders the planet unlivable. I realize this type of argument can be extended to reach absurd conclusions, but nonetheless it's an entertaining thought.
2. What is life? What is civilization? What are their goals? Why do we assume a society of relatively independent individuals? Is it even a sustainable model for a post-human level civilization? Aren't we anthropomorphizing aliens a bit too much? What if a civilization either dies out at human-level stage or necessarily reaches a mental unification point and all its further activity is inward focused? I think we're on the first stages of self-awareness compared to rocks and jellyfish, imagine the level of self-awareness experienced by a far more advanced civilization. Would it necessarily be resource and space-greedy and try to colonize the galaxy? At least it's not immediately obvious to me why it would.
3. Another point is, what scales are we looking for? Doesn't our scale essentially depend on the planet / star system we evolved in, and the scale at which original life started? What basic building blocks / resources does alien life use? Maybe they rely on physical resources we haven't yet explored (dark matter, etc). Essentially I'm agreeing with the people saying "we don't know what we're looking for".
I really liked point 2. It seems to be the default assumption that we as a species should seek to increase our chances of survival by looking to colonize other planets. However it is entirely possible that our mindset may change in the future and we come to the conclusion that, even with capable technology, it is better for us to live out our existence on this planet.
"Assuming the above is true, then why do we not see any sign of these probes?" Because for example these probes are as tiny as a nano-meter? Hey it is the year 2012 here on earth. In the last decades we have already shrunk computers from the size of a entire room to fit in our pockets and soon our blood. And we ourselves can already build nano-technology, despite not being able to travel around our galaxy. What tells us that intelligent life has to build "probes" so big that we are able to detect them. Maybe these probes are just as large a nanometer and are already flying towards us or were near us or are already on Pluto. Or maybe other life forms shrink themselves to fit into a trillionth of a trillionth of the diameter large spaceship to be able to travel around space faster than light. All of the theories are just expanding from current knowledge as of today, not factoring in any stunning surprises of how the universe might really work :)
This seems like a good point on the surface, but since it takes only one outlier to dominate the universe (by Bostrom's train of thought), then it would have happened anyway. It's the law of smart for one, dumb for all. Assuming most life forms would decide to live minified, any life form not deciding so would have immense advantage to consume all untapped resources. Following this logic, and since any sufficiently advanced replicators would all be competing for the same space, the most overwhelming kind would take over all others.
Suppose that we start near the edge with a self-replicating probe that can spread outwards no faster than Voyager 1. In 2 billion years, every solar system in the Milky Way would be chock full of factories trying to expand.
And there is no need to assume that it is so slow. We have already designed technologies to send probes to stars much, much faster than Voyager 1. Here is one. Put a bunch of solar panels on Mercury, and use the power to shine a laser into space. That can push on a light sail, pushing the probe outwards at something like 1% of the speed of light. Then as it nears its star, let the sail fly away, shine the laser again, and have its reflected light brake the probe.
With this level of technology, which may be feasible for us in just a few hundred years, the colonization of an entire galaxy can be reduced to tens of millions of years.
If technological life is out there in our galaxy, in the blink of cosmic time finding evidence of it will not be a search for a needle in a haystack - it will be closer to finding a piece of straw in a haystack.
I find much confused logic in Nick Bostrom's argument, and huge assumptions:
"I will return to this scenario shortly, but first I shall say a few words about another theoretical possibility: that the extraterrestrials are out there, in abundance but hidden from our view. I think this is unlikely, because if extraterrestrials do exist in any numbers, it’s reasonable to think at least one species would have already expanded throughout the galaxy, or beyond. Yet we have met no one."
Sorry mate, but it's not reasonable to think that at all. The galaxy is colossal. We have exactly zilch insight into how even a two-star civilization might work, and the kind of problems it would have - and he's saying it's reasonable to assume the existence of a civilization with dozens of thousands, or hundreds of thousands, of millions of stars?
It's possible, maybe, but it's not reasonable, and it doesn't disprove the idea that we simply haven't looked deeply enough for ET yet. As Jill Tarter of the SETI Institute once remarked [1], it's like asking if there are any fish in the ocean, filling a cup with ocean water, and upon discovering that it has no fish in it, you conclude that all the world's oceans don't, either.
In short, we haven't done the experiment yet - we may as well refrain from making conclusions. Our galaxy could house a million times the diversity of species in Star Trek, Star Wars, Battlestar Galactica and Mass Effect combined, but it may take a while to find evidence of any of that.
A basic nuclear rocket—the kind we could build today—can reach 10 kilometers per second. That's gets a robotic factory across the galaxy in 1.5 billion years. The oldest habitable stars are considerably older than that.
Top quality propulsion can shrink the time to 50 million years, which means somebody can colonize the galaxy faster than dinosaurs can evolve into talking monkeys.
So there has been plenty of time for robotic probes to colonize the galaxy. So where are they?
The time between dinosaurs and talking monkeys is basically irrelevant. The important figure is the time between the formation of the sun and talking monkeys - 4.5 billion years. You're also making some assumptions about how easy it is to build a ship capable of crossing interstellar distances and also reproducing itself.
In fact, using current technology, we actually could build a "self-replicating probe" loaded with algae and other photosynthetic single-celled organisms. So we really can't rule out ourselves as the evidence we're looking for. We might even be evidence that such probes usually malfunction.
Minor nit: is there evidence that dinosaurs evolved into monkeys? didn't they all get wiped out by an asteroid?
AFAIK, mammals existed during the dinosaur era. But they occupied a tiny ecological niche and were physically tiny. They survived the asteroid hit because they were tiny.
The absence of dinos enabled them to eventually occupy niches occupied by dinos.
My hunch is that it's true: that when we get better at detecting it, we'll discover bacteria/viruses/spores of various kinds all over the place (including Mars and other solar system locales).
I heard recently that if the earth was 3 times larger, the most advanced chemical rocket would not have the energy density to leave the atmosphere.
Scientists now know that with out a Jupiter, the asteroid belt might not form in such a way to deliver 'water' and protect us from large comets roaming the galaxy.
Even now it may not be possible to leave our solar system without a significantly different understanding of physics. Voyager is just no leaving our solar system, and it will take decades for it to actually leave the effect of our sun. Today if we launched a craft it probably would get just about as far as Voyager in just about the same amount of time, but maybe our power source could power a more efficient version for a decade longer, getting us just a bit further than voyager will, but still no where near Alpha Centauri.
For us to explore the depths of space, it would need to be much easier and cheaper than it is today.
For it to even be plausible a DIY Scientist as you suggest would need to create energy density so powerful, it would be hard to imagine a world where that sort of thing is not regulated.
For us to study sub atomic physics, we have a structure that is 20 miles in length, this is not the research of... a DIY community.
Today we argue about Iran getting enriched plutonium for nuclear research, when some think they want to build a bomb and others think they want to build a power plant. I really can't imagine a neighbor of mine being able to ... take that step without raising some alarms.
Your mad scientist would have to be on a far off island/ comet/ moonbase with all of the resources and man power required without interference from others, but still have access to raw materials.
I am just not sure we will find other worlds in humanity's lifetime.
We may be able to colonize mars and a few other large bodies in our solar system, but leaving it is a different story.
> Your mad scientist would have to be on a far off island/ comet/ moonbase with all of the resources and man power required without interference from others, but still have access to raw materials.
well unless he, or his supporters, are also granted dictatorial control over a significant portion of the planet.
Well let's not get ahead too much. Suppose such a civilization has existed in another galaxy, there is probably very little chance that they would be able to jump to another galaxy. The distances are huge.
And there is also the possibility of having intelligent civilizations never reaching the space age for one reason or another.
We have only ONE specie on Earth among billions which reached this point, it is not like we have much of a sample to make any rule.
All this is just assumptions based on very small data pool. Meaningless.
What if we are the replicated life sent out to colonise the universe. It would be hard to self-replicate knowledge and especially knowledge of our own origins over such distances and tiny cracks.
#justsayin
I am also not too sure the maths adds up either.
Lets assume that this "spread everywhere" civilisation does exist, and it fortuitously started evolving just after the Big Bang, and neatly at the "centre" of the universe (i.e. somewhere that is not next door to us, nor the furthest point possible from us)
Lets also assume it took them roughly 4Bn years to evolve to the Promethean level (age of the Earth) and so have been spreading for past 10 BN years.
And (yet) another assumption - they spread uniformly outwards, travelling at speed of light (!) but taking a recharge period to, I don't know, consume the resources of a planet, build a few houses, before spreading again.
So, with a observable bubble of 46 bn light years, and 10^^23 stars in it, I think the following calculation works
* there are 10^^23 stars [1], spread evenly over 46 bn light year diameter bubble
* they atarted their migration to us (to everywhere really) 10bn years ago
* between the "them" and us there should therefore be a even distribution over 23 bn light years or ~ 10^^23 - 10^^9 = 10^^14 stars.
* given a "pause" of 100 years at each star system and light speed travel in between that is still 23 bn years plus (100 x 10^^14) 10^^16 years or a ridiculous number that I have probably got wrong but is clearly older than the age of the universe many times over.
I know this can be tweaked any which way, but if we are looking at the pan-dispersal approach, even simple lower bound cases make it highly unlikely that a civilisation spreading enormously fast and comprehensicvely throughout the universe will still "never" get here for any value of "never"
"In short, the author says first that any sufficiently advanced civilization will at one point send self-replicating probes that will eventually colonize all of the galaxy and even universe."
That's the mistake. Its an extension of cultural / religious / philosophical values of the European colonial era. Not politically correct in this era or likely any point in the future.
Its the "star wars" effect applied to SETI. The "star wars" effect is we used to have emperors and knights in shining armor and mage/paladin wannabe types and castles and long choreographed sword fights so I'm sure the future will have them too. However good a hollywood movie plot, that's not realistic.
Repeating Earth's 1492 is dramatically less likely than ... a gray goo accident. Look for one of those. Or evidence of an unfortunate accident involving using black holes/singularities as a power source.
You're missing the point that it takes only one sufficiently equipped human being to set the whole colonization in motion. Given appropriate technology, it doesn't require any concerted effort or grand scheme. Just a single match lights the fire. Think of the DIY folks in 500 years. Given the size of the human population, it's a given that it will happen at some point, if the technology becomes available.
Yes, which 'rock' (point of origin) did the first human-scale single-celled organism on earth launch from? In this line of thought, the fact that we got off of that rock, grew into the ecosystem that we are today, and are now well on our way to doing it again is hopeful in itself.
Also, when thinking about this, i don't think we should discount the possibility that we have already discovered and merged with other forms of life, especially during explosive growth periods over previously uncharted territory. Maybe some forms of life we know of today were actually independent in origin.
But i definitely like this kind of intelligent design by self angle more than any other.
I agree. Like with investments in startups there could be a "spray and pray" strategy. Maybe "they" are just happy to spray very basic life forms like DNA around the universe and watch evolution happen, like how we in our labs like to watch experiments develop.
It's amazing how little we know--it might just be that an organic, living, genetically engineered being can sustain the pressure and vacuum of space (though how about propulsion?) and hostile planets, and consume raw resources out of soil and atmosphere to reproduce. That's a scary thought.
I think Nick Bostrom should have focused more on the time and randomness factor. Maybe there was life in our galaxy, but they visited us before humans have lived. Or we have been visited (think peruvian desert drawings [1]), but before we as human race were as advanced as we are today and they left our boring planet to visit another galaxy. There are probably more than 170 billion (1.7 × 1011) galaxies...
The point is, well before folks who could feel "boredom" passed by us, their self-replicating probes would have landed here, many thousand, million or billion years prior. And they would have harassed our planet to depletion, since they are dumb and drone-like. Even if a meteor somehow extinguished them, by then they would have launched themselves into all of the solar system, and by chance would land here again. Given time and randomness, these probes are like radio waves; they get everywhere once set in motion. Thus we can assume they were never set in motion.
When I'm visiting Japan I don't have to "probe" them. It es enough to take some pictures and leave. Like Star Trek, maybe they had some guidelines on their journey to us that stopped them from interfering with the history of our planet. And maybe they didn't even need our resources or "probe" the entire universe in the first place, because they invented a new way to create materials directly out of matter and anti-matter, without the hassle of mining the universe. The 'self-replicating probe' factor is just a theory by a human scientist from earth. Just because evolution may happen someplace else, that doesn't have to imply that all intelligent life ticks and reasons ultimately the same. Everything is just a big 'if this than that' logic, where every part of the theory can be trashed. Until now the human race can only reason with our own brain. More advanced life forms could reason with endlessly intelligent A.I. and could come to completely different conclusions. For example if it is a good strategy to visit or be detected by primitive animal life forms like us. Maybe they want just obverse us, like Bostrom mentioned.
> When I'm visiting Japan I don't have to "probe" them. It es enough to take some pictures and leave.
Using a transport infrastructure whose effects can be seen across the galaxy.
A theory of advanced races who have different sensibilities misses the point. What matters are the ones that have a growth and expansion sensibility. The first one of them to arise will conquer the galaxy.
The problem with the original argument (if I read it correctly) is that Bostrom assumes that the "growth and expansion sensibility" is a near certainty and that lack of evidence of this type of civilization, and the fact that we exist (as compared to having previously been 'harvested'), means that life does not exist elsewhere.
However, what everyone else in this thread has been saying, is that his conclusion (that no life exists elsewhere) is wrong because there are number of other possible explanations as to why we don't see evidence of this civilization. A quick summary of some of those arguments:
1. The expansion that Bostrom assumes is inevitable is actually physically impossible at the scale he describes.
2. It is possible that we are actually a by product of that expansion (DNA being the 'self-replicating' robot).
3. The expansion is happening, but hasn't reached us yet.
4. No other civilization has reached such an advanced stage yet (it is entirely possible that our 4.5 billion years was an extremely and uniquely fast time for evolution to intelligent life)
5. Super-intelligent civilizations come to conclusions that expansion is not a productive or useful undertaking.
While it is possible that life doesn't exist elsewhere (or at least intelligent life), it is by no means the natural conclusion to Bostrom's hypothesis.
6. The expansion is complete, and they are conservationists with an eye for subtlety.
I favor this possibility. Fledgling expansionists cannot know if they are wards of conservationists who have spent the past billion years developing spying technology and culling weapons. On a game theoretic basis, every sensible fledgling expanionist, even the first one in the universe, should reasonably act as if a living god is judging them.
It's also possible (more likely?) that life on any nearby planet discovered would just be a failed attempt of our own. If this is the case, then its discovery would actually be good news, it would mean we can continue in the face of catastrophe. I don't think the author touches on that.
We already know of at least one catastrophe we are rebuilding from, there's also no reason to think we weren't already rebuilding from earlier (possibly much larger) catastrophes.
Following this, there's nothing to say that we haven't already had some major effect on our surroundings (intentional or not) in some previous, possibly more complex or larger-scale growth. Our influence on our surroundings could be huge. Lack of life or anything that seems anomalous in our immediate vicinity could be our own doing. A seeming lack of evidence for this might be explained by scale, or the nature of the catastrophe, or (if it was intentional) by design.
It's been said by others HNers, but I want to stress the point further with a reformulation.
We have great difficulties to find foreign planets. We barely know what's beyond Pluto. Heck, we probably have a couple of surprises waiting for us in the solar system.
One cannot expect us to be able to detect alien probes, operating with engines and communication capabilities beyond our understanding, probably only few meters wide (if not much smaller!) and operating somewhere near our star, probes we don't even attempt to detect in the first place.
Plus there is a bias toward colonization. I agree that an advanced civilization, except if it becomes extremely xenophobic, would probably want to explore the universe, but colonize? If you have the technology, it's possible you don't need to spread physically that much.
It's a compelling point, albeit in many ways very short-sighted. Though I really do believe we're headed to a full virtual reality. Why live life--the hard, cold future life as depicted in Matrix--when you can be a millionaire in the 20s one day and a pillaging crusader on the next?
We have cultural fantasies and biases that will be hard to break free (or do we want it?), and the possibility of fully reliving those as we please will be a Great Temptation as the writer puts it.
Still I think it's short-sighted in that the people who can be as powerful in real life as everyone else can in the virtual will take advantage of it and control the masses with infinite happiness. So I don't think it's ultimately a good point to stand for the Great Filter.
> And what might the precursor techs for such a replicating machine look like?
I would wager, attempted mischief, that words themselves are the self-replicating machine, to the Universe..
Think about it. These very characters, here on this page .. motivate the pro-creation of yet more, and so much the human organism is designed exclusively for the propagation and 'existence' of these words, that eventually, to the Universe, it must look like each letter is but a sperm, off to meet some fertile substance designed to fuse another font of near endless replication, reproduction, duplication, consumption ..
If it's not nuclear weapons, then there must exist technologies that are sure on our path to discovery while at the same time guaranteeing our extinction--again, think in terms of things that only need one outlier to use it in a way that compromises the existence of all on Earth for good.
How about hydrocarbon fuels? They don't need one outlier. They seem to have chemistry and physics resulting in an economic trap for t heir use.
1. The worst-case "predictions" of the doomiest environmentalists are that the shoreline will move in a bit, and the tropics will get a bit iffy.
2. Hydrocarbons are a temporary religious dogma, not a trap. There are collosal amounts of thorium waiting to be tapped when the energy religion is reformed.
... and that large swaths of the Earth receive new weather patterns completely disrupting nearly every ecosystem, with cascading effects including no food for us
It's amazingly interesting and well argued. In short, the author says first that any sufficiently advanced civilization will at one point send self-replicating probes that will eventually colonize all of the galaxy and even universe. And it takes only one match to light a fire, and this is important--that is, for a sufficiently advanced planet, it would take only one rebel, one mad scientist, to set the colonization, by robots, of every habitable body in motion.
Assuming the above is true, then why do we not see any sign of these probes? It follows that there must be a Great Filter that prevented them from ever being created. Things that qualify as such a filter are not plenty, and include the original formation of life and the transition from procaryotic to eucaryotic (taking a couple billion years).
Thus if we find life so easily formed on Mars or anywhere else (especially eucaryotic cells), we can assume these past events are not really Great Filters. Then, the only reason we haven't found a single probe is that any sufficiently advanced civilization destroys itself before it ever gets to that point. If it's not nuclear weapons, then there must exist technologies that are sure on our path to discovery while at the same time guaranteeing our extinction--again, think in terms of things that only need one outlier to use it in a way that compromises the existence of all on Earth for good.
So he concludes that we must pray that we never find life anywhere else, because it would in turn give us hope (but not assure us) that there might exist no Great Filter ahead of us, and the Great Filter was indeed in our conception as life--thus we are the one single unfathomably lucky planet to ever have harbored life. If nothing potentially impedes our expansion, we will be the one sending probes and expanding to everywhere.
Adding my own expansion on this, I believe it's extremely unlikely that there is some irrevocably cataclysmical tech to be discovered before ever we are able to send self-replicating probes--ones who can mine the raw materials needed for unbounded expansion.
I don't think we're that far from that point, and taking from our own anecdotal existence since it's the only one there is, even if we do annihilate ourselves before sending them out by chance, other civilizations might not have done so, if they existed. Thus indeed the only tech that would assure a Great Filter is the self-replicating technology itself. And that is a strong point--the author seems to ignore the fact that these probes wouldn't really be the expansion of humanity, it would be the expansion of drones. And what might the precursor techs for such a replicating machine look like? If superior AI is needed at any rate, we could think that the tech leading to sure extinction is indeed robots who decimate us humans. And since they would do this as early as chance would afford it, the robots eliminating us would be as dumb as possible--thus we can assume they would be incapable of advancing technology on their own; or of coordinating an event like launching themselves into space, and thus would never leave Earth. In this scenario, we can assume every civilization that ever got a shot in going to space ended up extinct, their planet ridden with dumb self-replicating robots who can never launch themselves into space.
Since self-replicating robots are the hypothesis of the author's argument, then indeed it makes sense that every potential civilization either never reached this tech (the Great Filter is in life's conception, or otherwise somehow denies the existence of self-replicating robots), or it must absolutely reach it, in order to colonize space, but in doing so it declares it's extinction.
So, like the author, I also hope[2] that we never find life elsewhere, that we might put our tiny hopes in thriving as a race, the only one that ever existed.
[1]: http://www.nickbostrom.com/extraterrestrial.pdf
[2]: Just an hour ago, I would have jumped with joy if this NASA announcement was indeed confirming life.