The Fermi paradox is neither a problem nor a paradox, so it’s always baffling to me when it’s brought up. It’s like those annoying trolley problems: they’re stupid and unrealistic and pointless, except that they make you think about your assumptions. It’s only when people focus on the minute details of the question, rather than thinking about what the answer says about yourself, that you want to yell at people to shut up, they’re missing the point.
The Fermi ‘paradox’ was only fascinating to the physicists and engineers who were sitting around wondering about how they were going to get into space and explore strange new worlds because they assumed those strange new worlds were populated with other physicists and engineers who were thinking the same thing. In a rational world, they would have simply said, “Oh, my assumption must be wrong, let’s move on.” But no, instead they started inventing excuses for the absence of aliens, instead now assuming that there must be hordes of frustrated scientists and engineers out there who are pinin’ to visit Earth, but are stymied by the speed of light or their predilection for building nuclear weapons first and exterminating themselves or that they’re using some super-duper communications technology we haven’t invented yet. All their rationalizations seem grossly anthropocentric.
As a biologist, we have a collection of assumptions, too, only our assumptions all seem to default to making the absence of aliens an entirely ordinary conclusion. Life is probably common in the universe — all it seems to require is redox chemistry (universal, obviously), proton gradients as an energy source, which can be easily generated in lots of ways, and time, which the universe has lots of. We don’t expect a multiplicity of engineers, because they’re not common even here on earth. We tend to expect bacteria-like and algae-like organisms, because those are ubiquitous here. But we’re unsurprised that they aren’t hailing us, because we similarly do not expect an algal population in Australia to launch a transcontinental probe, land it on my desk, and slither out to plant a flag and claim it in the name of their colony.
My assumptions could be wrong, but because they’re grounded in known science, I don’t expect them to be. To me, the Fermi paradox is simply confirmation of a reasonable inference.
Where this gets troublesome, though, is that some creationists use it as confirmation of what they think is a reasonable inference — that life exists nowhere else in the universe, but is the product of a unique creation event here on Earth.
In a sense, Christian presumptions and its claim of historicity for biblical miracles is more consistent with what should be happening given the premises of evolutionary science. A complex and powerful Godhead with anthropomorphic habits, dimension-jumping beings doing God’s bidding or working against it, frequent interventions in history accompanied by bizarre occurrences in nature—isn’t this what we’d expect in a universe given all the oddities of physics in the context of evolutionary randomness?
I’d grant the guy one thing: the absence of aliens is an observation compatible with the hypothesis that life only exists on one planet, ours. However, he’s wrong that we should accept the possibility that any outlandish scenario could occur in the history of the universe — there are natural laws that seem to be pretty consistent in their operation, which is going to constrain the range of possiblities — and he is even more wrong when he suggests that one particular bizarre scenario that just happens to coincide with his religious preconceptions ought to be “expected”. He really reaches to turn his mythology into a science-fiction story.
So, given the sheer magnitude of theoretical possibilities granted by known science, to say nothing of the unknown science waiting to be discovered, what is really so random and strange about, say, an alien being flooding the earth in order to destroy a genetic perversion of humanity bent on destroying the original species this same alien had crafted?
The answer, of course, is “nothing.” Yet, we suspect Dawkins et. al. would grant any alien scenario so long as it doesn’t involve a tri-conscious being making periodic manifestations among ancient Semitic peoples about 3,000 years ago, which in a rather singular case used as its avatar a first-century personage born in the days when Quirinius was governor of Syria.
I have to raise two objections to his fantasy.
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When Richard Dawkins and others suggest that they are open to the idea of aliens having intervened in the history of life, that acceptance is general — they are not inventing a convoluted, contrived series of events — and contingent on evidence for such an intervention being found. Are there phenomena we don’t understand yet? Yes. Could they have been important in the origin of life? Sure, but you have to be specific about the mechanism you are arguing for, and provide good evidence that it happened.
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Your scenario must be compatible with all of the reliable, available evidence. There was no global flood in the history of humanity, so a model that depends on a significant event that has already been falsified is garbage. We also know that humanity had a founder population much larger than 8 people, and that the young earth creationist timeline is incompatible with physics and geology and paleontology and even recorded human history.
Another revealing thing about this article: it purports to complain about science’s interpretation of the Fermi paradox, but it doesn’t cite any science — instead, the only sources the guy mentions are science fiction, and even at that he doesn’t mention any SF books, but only SF and horror movies.
I guess this should be no surprise, that someone who mangles logic and misunderstands a hypothesis doesn’t read any books (except, maybe the Bible) and definitely doesn’t read any real science. He doesn’t seem to recognize irony or projection, either.
Meanwhile, the aliens arising from the imagination of modern science fiction, because they have no affiliation whatsoever with the evidence at hand, have a little more than the whiff of blind faith associated with them. Unlike say, Christian faith, where powerful objective evidence creates an ongoing intellectual crisis calling one to abandon subjective thinking, blind faith in something lacking any objective basis leaves only the subject’s imagination as the focus of query.
If that was intentional, it’s kind of funny — “powerful objective evidence” for Christianity? Hah. I fear he’s being serious, though.
Odds are that in an enormous universe, life managed to tumble its way to sophisticated intelligence more than once. But in addition to all the arguments regarding how that life might evolve and whether it would care about things we care about, or communicate in any way we can understand, we have the additional problem of a really tiny sample size, even if we include 1000+ extrasolar planets.
Who can say how often life forms? And then how often it forms into something capable of leaving the planet, however temporarily? We only have one species on Earth that does that, and we have a fuckton of species here. Pretty low odds!
What we consider ” powerful objective evidence” is different than what they do. For example the evidence of prophecy in the Bible is compelling for many and poof the Bible is of supernatural origin. Selectively choosing passages with interpretations geared toward only one end yields conclusions they find unassailable. It’s astonishing how smart people can think so poorly.
The Fermi ‘paradox’ is one for a drunken lunch. It you look at how our planet was created its quite apparent that to make a life capable planet in a not-violent enough to kill off life every few hundred million years then we end up with a very small number of regions of space where that could happen and they are statistically sufficiently far apart not to have been able to communicate with each other at light speeds.
Odds are that in an enormous universe, life managed to tumble its way to sophisticated intelligence more than once.
To know the overall odds of at least one occurrence, you need the sample size and the probability per sample. You need both terms to get the product.
I don’t think we have any clue as to probability of human-like intelligence evolving on a planet. We have no other examples and we don’t know enough about the process.
So obviously I’m personally unpersuaded by any form of an argument that relies on the universe being big. You somehow need to grapple with whether intelligent life happens on 1 time in 10^10, 10^20 or 10^50 to decide between “frequent”, “occasional” and “unlikely to be even one other species.”
Obviously there is selective advantage to behavioral complexity — organisms that have a complex sensorium and can respond to sensory input to generate behavior to better obtain food, avoid predation, and favorably modify their environment can be more successful. Clearly evolution on earth has driven that (it’s not the only path, obviously, but it’s one that the metazoa have followed) and by now there are many species that aren’t that far from our capacities. Our technological civilization seems to me (granted this is purely intuitive, sample of 1) a not highly improbable outcome, given enough time. I think it likely there are others out there, but it is not surprising to me that we haven’t been able to detect them. We are undoubtedly very difficult to detect from distant stars. Everything else — about the typical life span of civilizations, whether any of them want to expand to other star systems and whether that is plausibly possible, whether anybody out there is interested in communicating — is speculation, but it doesn’t seem unlikely to me that yeah, they’re out there somewhere, but we may never know it, alas.
This all leaves out the single best reason you shouldn’t expect alien contact or be surprised it hasn’t happened yet – and that’s isolation. If there were thousands and thousands of space-faring civilizations out there, even within our galaxy, we’re still a needle in a haystack. Terrestrial planets like Earth are notoriously difficult to detect because of their low mass, and when you have an 8-planet system like Sol, an intelligent being would reasonably assume any gravitational gymnastics are coming from the four huge individually detectable gas giants orbiting it. Therefore the first problem is anyone visiting Earth would be doing so incidentally as part of an investigative team either purposely looking for Earth-like planets or who gets all wet thinking about the physics of gas giants.
Relatedly, any traveling spaceship in our solar system is going to have a hard time detecting terrestrial planets without setting up shop for decades, unless they get really lucky. To us Earth is the pale blue dot, but it wouldn’t even show up blue at distances as minor as Mars-orbit.
For another thing, humankind is producing absolutely nothing evidential of civilization that would help a spaceship pinpoint us. Contrary to what most people think, no radio or microwave transmission, from us or for us, is capable of traveling very far with the cohesion either for detection or for transmitting information. In the book/film Contact you have the first television broadcast strong enough to be produced world-wide falling on the ears of beings on Vega – this would never happen. A broadcast many times that strength would cease to be intelligible before it reaches the gas giants. You would need a series of buoys acting as repeaters to send messages from one end of the solar system to the other, and even then you will get severe quality decay.
It is practically impossible for two space-faring civilizations to detect each other unless one first finds the other by chance and together they set up an infrastructure for communication.
Addendum: which I’ll throw in here only because I’m mad at the movie industry again. It is not possible to communicate with extraterrestrials. Got that? 100% not going to happen. Our biologies would forbid it. We would be lucky to find a way of illustrating rudimentary concepts like “peace” or “threat”, and it would take many generations, regardless of level of technology. Language has a biological basis our young are primed to imprint the local version onto, and they lose this ability at puberty. Even if you did the responsible thing, and sat a truckload of babies in front of an alien speaker, they would learn nothing, because they are primed to communicate only with each other. In terms of how the biological basis for language works, you’d need to create a race of interspecies drone children in the faint hope they could act as translators – how’d you like to draft that bill?
I think it would be worth distinguishing between the Fermi equation, and the Fermi paradox. The paradox only comes in if you put values into the equation that suggest we should have contacted other civilizations by now, which (as many erstwhile commenters have pointed out) is unlikely given recent estimates about, for instance, the relative rarity about areas of the universe stable enough for life to develop without being, eg, fried by a gamma pulse every hundred million years.
It’s a thought experiment that forces us to try and estimate values for some of them. How many stars have planets? How many planets support life? How many of those can support multicellular life? They are all reasonable questions.
The Fermi paradox in itself is a really great source of story ideas, with tons of really interesting ideas.
Nitpick: I think of bacterial aliens as real aliens too.
Even finding bacteria of non-terrestrial origin would be a revolution for biology.
Or any species advanced enough to travel to or communicate with other planets probably already realized that humanity is a shit species and rightfully wants nothing to do with us.
The early terms of the Fermi equation are being nailed down with some pretty good estimates. Turns out nearly every star has planets statistically. As Myers suggests above, life itself may be inevitable given the right entropic conditions. It may be (pardon) universal. So the observed result (lack of obvious chatter on the radio spectrum) has some explanation besides the rarity of planets or life. Which is interesting. Eliminating those possibilities gives us lots of possible explanations. A few of them are obvious, like the possibility that multicellular life is vanishingly rare for whatever reason (low probability, regular gamma bursts, etc), or that intelligent life is simply extremely impossible. Some are less obvious, like the simulation hypothesis (solipsistic navel-gazing, if you ask me) or that radio just isn’t used by advanced civilizations, or that if it is, it’s so well encoded (see Shannon) or encrypted that we can’t extract information from it and it just looks like static.
Then there’s the more sinister explanations, which make for great SF. Berserkers. Intelligent life automatically incinerating itself before it becomes starfaring. Or even that the economics of starfaring make it impossible.
Like i said, it’s a great source of story ideas. :)
Rob Bos,
I think you mean the Drake Equation.
Some of my friends had a fancy dress party once, where the theme was “Aliens”. They spent ages making costumes to look like Klingons and Xenomorphs and Greys and Tyranids and the like.
I went as the Fermi Paradox by not showing up.
@cartomancer (#11) Yes, you’re right, of course. Extended brainfart, thank you.
mannnn, I always interpreted the FermiParadox as only saying the probability of life elsewhere in the universe is “no zero”.
All the numbers were thrown in just to show how big numbers of possible locations (aka ) can still give non-zero results no matter how tiny the probability of life.
I simplistically thought that was the point Fermi was trying to make, not calculate an actual number. All the extrapolated secondary questions are just glommed on to make different puzzles.
pffft, just me. [slinking away]
re myself @14:
oops, I missed @11. Drake Eqn was what @14 was actually talking about. tsk tsk
I don’t understand why this has you in a swivet. The quite complete Wikipedia article on the paradox lists as the #1 explanation, “Extraterrestrial life is rare or non-existent” and as #2, “No other intelligent species have arisen”. Number 5 is “Periodic extinction by natural events”. And that seems to cover most of the objections raised in the main post and following comments. So, problem solved?
It isn’t necessary to assume many other worlds populated by scientists and engineers; it is only necessary to contemplate the staggering number of possible life-bearing planets, a number that has only kept multiplying as we learn more, see e.g. the Kepler mission. That plus Fermi’s original contribution, a back-of-the-envelope estimate that if an interstellar civilization arose, it could have visited and even colonized much of the galaxy in a time that is really short in geological or astronomical terms. It was that which made him blurt “where is everybody?” That’s a reasonable question that is based not on parochial assumptions but on reasonable estimates based in the huge numbers.
gee let me continue to run my “mouth”.
Fermi Paradox is easy. remember , so big that the probability of finding us is infinitesimal. Why so arrogant to think that life will inevitably find us, in particular?
Even the SETI variation. Why think aliens will talk interstellar the same we talk transcontinental? Distance is an issue.
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still. I disagree with PZ, it is a paradox. [MW: (originally denoting a statement contrary to accepted opinion)] In that intuitive answer is different than actual answer. We, at first, think of things one way when the problem requires a different way. At first we think it is similar to finding the single red m&m in a barrel of random M&Ms. When it is more like finding a single molecule of alcohol in the worldwide ocean of water. That’s the paradox. I think. umm *showing myself out*
@taraskan #6:
I disagree. Park your ship 40 million miles off one of the sun’s poles and start looking towards the ecliptic for wandering stars. Unless your snazzy interstellar vehicle is equipped with naff telescopes and shoddy computers that are crap at image analysis, you should have the four terrestrial planets within a fortnight*.
*Possibly an exaggeration, if only for Mars, but it certainly wouldn’t take decades.
Detecting alien planets would be relatively easy if you’re a super-advanced alien civilization with spacefaring capabilities. Build some truly massive formations of space telescopes, and you’re golden – and that’s not even getting into weirdness like using gravitational lensing by the central star of their solar systems and so forth.
Maybe that’s why nobody’s shown up, if they’re out there. What counts as a desirable solar system for an advanced space-faring civilization might be very different than what we consider to be as such, and any system is going to get an intensive telescope search before they spend all the energy and resources necessary to send an actual probe there (and those will be substantial costs, even hand-waving away potential engineering snags). On top of that, interstellar probes are going to be as small as possible to save mass on the transit, and you don’t really need to send them to investigate solar systems constantly – once every few hundred thousand years is enough.
I don’t even know what physicists and engineers would get out of contact. There are countless solar systems out there, with plenty of interesting features to study and doubtless resources. It’s biologists who would have some reason to care about ours in particular.
A minor point, but I question whether we should expect that Christians have read much if any of their bible. Based on and admittedly small and non random sample I have found that Christians who have tried to convert me on many occasions show almost complete ignorance of their “holy book”. I have read it, cover to cover at least twice, and I find they almost universally in my experience show few signs of being actually familiar with what’s in that book.
Of course the largest sect of their religion actively discouraged their members from reading it so I suppose it isn’t surprising that Catholics should show such ignorance, but in my experience neither do most Protestants.
Rich@18:”I disagree. Park your ship 40 million miles off one of the sun’s poles and start looking towards the ecliptic for wandering stars.
Not so simple. You are either in orbit or falling into the sun. A log slow polar orbit would probably be fine, but that’s not exactly “parked”, but it would have to be a hell of a long way further out than 40 million miles.
A single orbit at 40 M.M. would go round the sun in *much* less than a year. Earth’s orbit is more than twice as far out and it’s period is one year. Anything closer orbits faster.
At 40 million miles you are only slightly further out than Mercury’s orbital distance. You could see all the minor planets in binoculars. A 60 power spotting scope would show disks for all of them.
But anyone close enough and technologically advanced enough to put a satellite in orbit around the sun would also be able to build a telescope with a mirror of, say, 1000 km diameter in a distant orbit from their sun. That would be much easier and they could examine millions of stars optically.
But failing that, why would they pick our particular sun for their probe? Why not the Alpha Centuri system or any one of a million or so similar stars within a few dozens of light years?
Probability is low, but the sample size is enormous:
~100Bn stars in the Milky Way. Planets outnumber stars (most stars have planets, and some have several).
I suspect that there is indeed intelligent life out there. And I’m absolutely certain that we’ll never see any sign of intelligent life, nor will they see any sign of us. Space is huge, the speed limit is absolute, and civilizations don’t last forever.
it helps to have a background in astronomy. astronomers have a funny way of naming things. for example, to an astronomer, “ice” is any volatile element or simple molecule with a low boiling point, regardless of its actual phase. a “yellow dwarf” is a normal sized star. don’t get me started on “planetary” nebulas. and they have no word for a solid object. anyhoo, my point is:
an astronomical “paradox” is simply a puzzle – why are things this way? perhaps most famous is olbers’ paradox, “why is the night sky dark?” it is this question that confirmed that the universe cannot be both infinitely large and infinitely old.
similarly, fermi’s paradox seeks to determine the parameters for where not to look for intelligent life elsewhere. it’s a starting point, not a conclusion.
… “powerful objective evidence” for Christianity?
I simply have to call POE on this!
“Relatedly, any traveling spaceship in our solar system is going to have a hard time detecting terrestrial planets without setting up shop for decades, unless they get really lucky. To us Earth is the pale blue dot, but it wouldn’t even show up blue at distances as minor as Mars-orbit.”
We’re going to be able to resolve spectra of extrasolar planets in our lifetime, so it would hardly take a spacefaring race decades to detect water and free oxygen in the atmosphere of Earth.
I detect a little whiff of something else with that guy, and its powerfully objective.
@#20: “I don’t even know what physicists and engineers would get out of contact. ”
Well, its at least conceivable they might provide them with knowledge…and perhaps even provide paleontologists with a complete record of events that transpired on Earth over the last 4.6 billion years. An oft overlooked part of the ‘paradox’ is contingent on just how advanced a technology can get…and whether the term ‘organism’ should be limited to the natural variety.
I take a more primitive approach to the Fermi Paradox as I really don’t know the intricacies of it. I’ve always just heard of it as “Where is everyone? Shouldn’t someone have arrived by now?”
I can just see some wise men advising the Aztec emperor, Cuauhtemoc, that there were no people on the other side of the ocean because otherwise someone would already have visited. Oops hello Mr Cortés.
#21 Ed Seedhouse
/Of course the largest sect of their religion actively discouraged their members from reading it so I suppose it isn’t surprising that Catholics should show such ignorance, but in my experience neither do most Protestants.
Actually I don’t think largest sect ctively discouraged their members from reading [the bible] so much as they discouraged reading and interpreting it on one’s own. Th idea seems to have been let’s keep the wackier interpretations under control with by having someone around having some biblical scholarship.
Having seen some of the really weird interpretations coming out of protestants, with no biblical knowledge, have managed to concoct (snakes anyone?) this may have made sense. I once knew one nutcase who after reading the bible for years discovered that it ordained that men should wear beards. Who knows where he got this as IIRC he was reading the bible in English, German and possibly Hungarian.
My suspiciaon is that Catholic knowledge of the bible, gained by having bits and pieces read at them at Mass on a regular basis + the blasted sermons is not all that inferior and possibly wider albeit probably shallower than most protestants.
Bob Altemeyer in his book /The Authoritarians makes the point that only 20% of protestantss (may be a student sample) had read all the books in the KJV of the Bible and 19% had never read one book of the bible from end to end. So it is not all that surprising that they don’t know much about it.
I like Zach Weinersmith’s answer:
http://www.smbc-comics.com/comic/2010-09-18
I’m unclear on this statement “We also know that humanity had a founder population much larger than 8 people.” What is the evidence?
I’m probably not thinking “population” enough, but does the evidence have to do with bottlenecks and genetic drift?
If all of humanity was descended from eight people six thousand years ago, we’d have far less genetic diversity as a species or else there was a point in time when mitochondrial genetic drift was accelerated to a rate that we don’t see today (which requires an additional explanation for how that happened).
Thanks microraptor @32. It was the implied 6000 years ago that was throwing me.
@29
Leviticus 19:27: Ye shall not round the corners of your heads, neither shalt thou mar the corners of thy beard.
Fermi Paradox is easy. remember space is big, so big that the probability of finding us is infinitesimal.
The Fermi Paradox is actually the observation that space is not big, relative to the timescales of stars. There’s so much time to go back and forth across the galaxy in a few billion years that if intelligent life is common then the prospect of not finding us is infitesimal.
I can just see some wise men advising the Aztec emperor, Cuauhtemoc, that there were no people on the other side of the ocean because otherwise someone would already have visited.
Actually, this would be a compelling rejoinder If someone in the Aztec Empire had put forth the argument that civilization was common anywhere there was land. In point of fact the conditions for civilization appeared a few thousand years before the Aztecs did, and sure enough soon after that it the mast majority of people were visited.
If someone were making the argument that intelligent life in the galaxy is exceptionally rare, or necessarily extremely recent, the Fermi Paradox wouldn’t be a rejoinder.
I think one of the problems with discussions of things like the Fermi Paradox is that people are overestimating how fast and/or how far technology can go. A common discussion is about self replicating probes, and why we haven’t detected them yet. But just because we can conceive of an idea doesn’t mean it’s actually possible to do. It may simply not be possible to create a system of devices that keep creating copies of themselves until they can reach us. It gets even worse when you start talking about things like Kardashev type II and III civilisations, which use all the energy of their star and galaxy respectively. I suspect you’d need what is effectively magic to create something like that.
Well from our sample of one, we can deduce that all “intelligent” species elect Donald Trump before they get very far with space-faring. Paradox dissolved.
briquet:
The Milky Way has over 100 billion stars (and its diameter is over 100,000 light-years). If that’s the only thing you have to do with your entire civilization for a few billion years in a row, you’d have to travel to several stars every single year, if the goal is actually traveling to each of them (and potentially alerting us to their existence and rendering the paradox irrelevant). That’s not strictly impossible, if you’re sending out many different colony ships/probes at once to go their separate ways. But those would still need to have the capacity to pack up quickly (or gather resources, reproduce, build more ships, etc.) from that system once they reached it, since it’s not the end of the line.
What would be the point of racing around in every direction to all of these billions of star systems? Many of them may have habitable environments for their species or could be made suitable with some prolonged effort, a place where you could settle down and have a life. Why would billions of alien people be happy to go along with this plan for billions of years? Would none of them ever revolt from their wacky economic/political system? Would none of them ever say to themselves, “hey, looks good, I’m comfortable here, no reason to keep doing this shit, so fuck these people who say I’m not meeting my quota”?
consciousness razor@38,
You’re completely missing the point that the claim that all stars in the galaxy are visitable in the space of a few millions or tens of millions of years assumes that the probes sent out are intelligent, and build copies of themselves whenever they arrive at a system with materials that allow this. So once the first generation of probes is launched, incremental costs to the inhabitants of the home system are zero, and it doesn’t matter in the least if they decide they’re not interested any more or for that matter, elect Trump and go extinct – the process continues anyway. Note that only one civilization has to do this for every solar system in the galaxy to have a probe, or more likely a set of probes, sitting around waiting for intelligence to arise, or composing poetry, doing number theory, arguing about whether they are in a simulation, etc.
KG:
You’re talking about self-replicating von Neumann probes, which are intelligent enough but not themselves interested in anything except completing the mission they were programmed to do. I’m still missing the point. What is the point of making them? Do you think we’ll ever have a reason to make a fleet of such things?
Is the probe supposed to do anything then, after it’s done waiting for (potentially) billions of years? Or does it just “sit” there?
If the probes are intelligent, what’s stopping them from deciding they’re not interested any more? It’s a pretty strange sort of intelligence that would be capable of executing such a mission, but not capable of re-evaluating it.
Exactly, Dunc. I wouldn’t rule it out as impossible of course. But is it even remotely ethical to create numerous pathetically crippled AIs that need to last for billions of years (to do apparently nothing of any use, except consume resources that intelligent species it may be monitoring can no longer use) which can’t have some degree of autonomy because it may jeopardize the “mission” of gobbling up whatever it can and spreading all over the galaxy like a plague?
There’s a short story in here about a Von Neumann probe having an existential crisis…
The reason why alien life has never contacted earth is very well explained by the second half of your blog post.
If I were an alien, I’d stay the hell away from earth too.
consciousness razor@40
Curiosity, a job creation scheme, hope or fear that there is intelligence elsewhere, bizarre religious beliefs… whatever. The point you are still somehow managing to miss is that only one technologically capable civilization has to have decided to send out such probes for the galaxy to be full of them. If von Neumann probes are technically feasible (which you haven’t disputed), and if the galaxy is or has been full of scads of technically capable civilizations, then your motivational explanation for why such probes are (apparently – we can’t be sure) absent has to cover every such civilization.
Nothing strange about it at all. If you build an AI to carry out a specific task, of course you at least try to design it so it cannot deviate from that task.