The Fermi Paradox
Posted on May 31, 2006 Posted by John Scalzi 66 Comments
SFCrowsnest has a review up on Old Man’s War which is mostly positive, and that’s good. Yay! It also notes that the book “wantonly ignores the Fermi Paradox and offers no explanation as to why his galaxy is so densely populated with myriad alien species.” For those of you not in the know, the Fermi Paradox is the idea that is best summed up as “if there is intelligent life out there, why isn’t it here already?” In the particular case of OMW, I suspect the question might be better tuned as “if the universe is filled with hostile intelligent life, why aren’t we dead already?”
Well, that is an interesting question, isn’t it. The review is entirely correct, incidentally: I don’t really explain that aspect of the story. In terms of OMW, the reason I don’t explain this has to do primarily with me wanting to leave the question open a bit, because I think it’s fun to let readers speculate; the question begins to be answered in The Ghost Brigades and will be more so in The Last Colony, but as I’ve said elsewhere about the Old Man series, in my mind the ambiguity of some aspects of the series is a feature, not a bug. I like to read what it is that readers of the series have to say about it. It’s nice to live in a time when one can get out on the Internet and see reader speculation in that regard. Naturally I understand why people want answers from me rather than have that ambiguity there, but I hold the opinion that in science fiction, not everything is improved with a “logical answer.” Case in point: Midichlorians.
In the really real world, I like to answer the topic of the “Fermi Paradox” with two questions of my own:
1. Why don’t we have a moon base?
2. Why is there no McDonald’s in my hometown?
In both cases there’s no practical bar against either — both are able to be accomplished materially, although admittedly one’s easier than the other — and yet here we are without a moon base or a McDonald’s in Bradford, Ohio. Once you figure out why we don’t have either of these, you know why the Fermi Paradox isn’t really a paradox, and also frankly isn’t really that interesting of a question. I have my own thoughts as to the answer to both questions and what it means for the Fermi Paradox, but I’ll leave it open for you folks to discuss if you want.
That sounds like the snarky comment you’d get from your younger sister; “If your so special why aren’t you King!” Except it is all gussied up with a formal title. Fermi Paradox indeed. Just because something is possible doesn’t mean it is probable. Take my biological ability to reproduce for example…
I just gotta say that I’ve always hated the term Fermi Paradox. In my day, if your model didn’t the data, it was called a bad model, not a paradox.
Yeah, I kind of feel that way too, Stan.
kero
Just because something is possible doesn’t mean it is probable. Take my biological ability to reproduce for example…
So your biological ability to reproduce is possible but improbable?
I don’t think you want speculation in that arena, do you?
John Scalzi
Fermi’s “paradox” has many conceivable solutions in my mind, which is why I have never really considered it.
First and foremost is, of course, the “non-science fiction solution.” That’s where the speed of light really is a speed limit and everyone interesting is just too far away.
Then there is the “bad breath” solution whereby it is speculated that most intelligent life forms would rather avoid us, if possible.
Then there is the “scale” solution which was most famously postualted on the Outer Limits where the housewife crushed the tiny space ship with her foot. This could go the other way as well.
And then there is the “phase lag” solution where there were intelligent races close by but they died or otherwise went away.
I could go on, but I’ll spare you….
Why would you want either a mcdonalds or a moonbase?
(who did i have that conversation about men and going to the moon and to mars with? justine? gwenda?)
3/4 through “Ghost Brigades”.
Myself, I hate black jellybeans.
I wonder if “Ghost Brigades” will catch some flak for seeming pro-UN, the same way OMW caught some criticism for appearing to be pro-military. Not that this makes any sense, but for a certain kind of mind, green-skinned men blowing away insectoid aliens with nano-weaponry is *clearly* a commentary on contemporary politics.
There’s no moon base in Bradford, Ohio?
I’d move.
The answer to both your questions is “economics.” McDonalds has decided that Bradford, Ohio is too small a market for a McDonalds.
Nobody has figured out how to make enough money from a lunar base to justify the astronomical (pun intended) cost of building it.
Neither factor is necesarily permanent. Bradford could grow (or the target market for McDonald’s could shrink) and the costs of going to the Moon could fall (or profits rise) to justify the base.
JonathanMoeller: I don’t know that I would characterize TGB as pro-UN – it certainly raises concerns about the actions of the CDF.
Chris Gerrib:
“The answer to both your questions is ‘economics.'”
An answer is economics.
There was a Calvin & Hobbes cartoon that stated:
“The surest sign that there is intelligent alien life is that it hasn’t tried to contact US.”
Not having a McDonald’s in Bradford is a feature, not a bug
The Fermi paradox assumes that growth is an unlimited, unconsciouis process, and that it will always happen no matter what. That given a chance, life will expand across the entire universe.
The limited data Fermi had when he came up with that fit his idea, but, just a few decades later, it already doesn’t. We’ve already stopped expanding into a lot of the available spaces here on Earth, and the acceleration of population growth is on the decline. It’s not quite a choice the species is making on a conscious level, at least to some degree, but I see no reason why it couldn’t be a completely conscious one in a few thousand years or so. The paradox assumes that we, or any other intelligent species, would chose to expand at an exponential rate. Why would we? What would we get out of it?
If the answer is “economics”, then the solution is really simple: switch the two things around.
1) It is uneconomical to build a moon base on the Moon (since it is such an expensive piece of equipment, airlocks, hydroponic plants, etc.). Plus, a moon base is clearly solely a cost center.
2) Building a MacDonalds is a lot cheaper than building a moon base.
Therefore,
3) Build the MacDonads on the moon: In addition to being cheaper to build, it is clearly a revenue center (and the sky is the limit for the price of a big mac on the moon). Clearly the place would pay for itself in just a few months.
4) On the other hand, maybe the best place for a moon base would be Bradford, Ohio: it would cost a lot less to buil the moon base in Bradford (no need for airlocks, nor hydro plants), and it could easily become a revenue center instead of a mere cost center (by renting it to the local SF author – who could then claim to be the only SF author actually writing in a moon base).
Now that I have have done the hard work and solved both problems, someone else should think about the details…
The Fermi Paradox and a n explanantion just makes for dull story telling.
John – an answer is economics. But that wasn’t your real question. Here’s my answer (cross-posted at my blog.
Fermi was no dummy. He knew that, for 99% of the history of humanity, the aliens would have to find us by landing on Earth.
Implicit in his question, however, was the assumption that aliens would go forth, if not to look for us, just to explore. Some of the pessimists who argue that man is alone in the galaxy point out that, even at relatively low rates of population growth, eventually any alien species will fill the galaxy. I think both groups are missing something, namely demographics.
Consider humanity (since it’s the only species we can consider.) For 99% of human history, the demographic pressure has been to have as many children as possible. Worried that the kids will get eaten by a saber-toothed tiger? Have more. (It works for rabbits.) Worried that somebody needs to be around to run the farm when you’re old and senile? Have more children. In agriculture, you get a bonus – even very small children can do useful work.
All this changes when man becomes industrialized. Building steam engines is a non-trivial task. And you can’t have a rocket ship if you can’t build a steam engine. Education becomes more important, and guess what – children become a short-term liability.
Carried to extremes, as in Western Europe and Japan, birth rates fall below replacement levels. The only reason the US population is growing is due to immigration. Even third-world countries, like Mexico and Kenya, are seeing the rate of population growth fall.
What does this have to do with aliens? Plenty. Most mass migrations in human history were semi-voluntary at best. My great-great-grandfather came here from Lithuania because that was the only shot he had at getting enough cash to buy a farm in the old country. As soon as the pile under his mattress was big enough, back he went. Fortunately for me, a 14-year-old boy who became my great-grandfather stayed.
So, if populations stabilize or fall, the big pressure for immigration and colonization fall off as well. Granted, some small percentage will go no matter what. But then, you run into the problem of small numbers. Assume 1% of the alien race leaves. 1% of a billion is a million – roughly the population of Montana. Montana is a big state, but a damn small planet.
So, why haven’t we met aliens? They’re just not spreading out very fast. And I suspect neither will we.
It’s also not unreasonable to think that the Solar System has been visited, but that the alien explorers chose not to stay here for colonization, perhaps because there were not compatible planets for their life forms, or perhaps because the life then on Earth made it too difficult for them to colonize for some reason.
This is similar to the economics of a moon base argument – essentially, the cost of exploring is much cheaper than the cost of settling down when it comes to space.
I think the real point of the Fermi “Paradox” is that it follows from reasonable assumptions, and thus makes you question those assumptions. If I reason correctly, it implies that at least one of these statements is true:
intelligent species don’t last forever;
intelligent species stop expanding at a certain point;
intelligent species are not evenly distributed in time (that is, we’re among the first of them);
intelligent species are avoiding us / hiding from us;
there are no other intelligent species.
Saying “oh, it just hasn’t happened yet” discounts the third point – it’s been a long time since the beginning of the universe, so even slow or occasional growth should have reached here by now unless one of the other statements is true.
Too bad you don’t live here in TX. Every town with 1200 or more people has a Dairy Queen. No exceptions. Do you have one???
Dave
No. No franchises or chains of any sort, save gas stations.
yes there is no Mc donalds in your town but i am willing to bet there is pretty obviouse proof in your town that Mc Donalds exists….you are taking the Fermi paradox placing your own definition of it incorrectly then saying that it is untrue becouse your incorrect definition of it has a literal meaning that is easily dismissed…
not very big of you John….
My opinion is that those who think the aliens should have found us by now (or vice-versa) either-or-both underestimate just how big the galaxy/universe/what-have-you really is, or overestimate the level of technology which is either achievable at all, or achievable by a sentient species before it destroys itself.
This last is one – of many – points considered to some degree in Vernor Vinge’s new novel, Rainbows End.
Incidentally, a constrasting set-up of alient interaction to that in Old Man’s War can be found in Karl Schroeder’s Permanence. It’s worth a read.
Joshua, you’re going to have to be more coherent (and less rude) if you want me to make sense of that statement.
Denis: It may also be true that intelligent aliens existed, and may have even visited us, before we had the wherewithal to communicate with them. Our planet has been around for 4.5 billion years, with life starting around 3.5 billion years ago. By comparison, it’s been little more than 100 years since the first radio broadcasts from Earth started wending their way through the galaxy (making the relatively safe assumption that humans were the first terrestrial species to figure out how to send signals over radio waves).
John H has a point. An extraterrestrial species that had been exploring exponentially (like with von Neumann machines) could’ve stumbled into our solar system a billion years ago, mined some asteroid or hydrogen fuel from Jupiter, and left, and we’d never know it.
Folks seem to forget we are in the galaxy equivalent of the boonies. All the cool interesting stuff is in the galatic core.
Joshua, you’re going to have to be more coherent (and less rude) if you want me to make sense of that statement.
Think Ring world…or if that is to exotic for you think radio signals.
I think Denis got it right…and occums razor points to number 5.
As to me being rude I think no matter what I say you will say that about me…don’t know what it is but I rub you the wrong way…maybe you just don’t like my name. It isn’t important.
Joshua Corning:
“As to me being rude I think no matter what I say you will say that about me…”
Well, no. When you’re not being rude, I don’t say that you are being rude. But when you are, I note it. Which is to say that if you don’t want me to note that you’re being rude, stop being rude. Simple. Saying something along the line of I’m “always going to think you’re rude” sounds to me like you want to palm off on me what are your own social failings, and I’m not going to be having any of that, thank you.
As for the rest of it, I’m afraid you are being no more coherent than you were the first time. I understand what Denis said, but I’m not entirely sure how it relates to what you were saying in your first post.
“All the cool interesting stuff is in the galatic core.”
Better hope not, because their planets are going to get sterilized every few hundred million years by the high energy radiation coming from the center of the galaxy. Happens too frequently for intelligent life to develop.
John – The question was a good question, and your answer was a good answer. Your goal as a storyteller isn’t to justify every plot point of your story. It’s to right the best freakin’ story you can – I’m not trying to teach you that, you seem to have grasped the concept and probably long before I did!
Joshua – I don’t mean to butt into you and John’s discussion, but I’d suggest re-reading your posts and thinking. Internet decorum isn’t a hard thing to grasp. Just imagine how people could read your words when they don’t have access to the [potentially] not so serious tone you wrote them in. Also? Listen when people suggest something and don’t throw it back in their face, often they have a point…
Abedemi –
>> “An extraterrestrial species that had been exploring exponentially (like with von Neumann machines) could’ve stumbled into our solar system a billion years ago, mined some asteroid or hydrogen fuel from Jupiter, and left, and we’d never know it.”
… hats off to that! Haha, sounds like an awesome potential start to a tacky B-movie, too (not to knock your point). It opens with a slow pan from geeky but nice archaeologist and his hot assistant who is, amazingly, only interested in geeky but nice archaeologists who are too shy to notice her. They are examining cave paintings of stick men fighting, chasing deer, dancing around fires and… OMGZZ itzz teh Almunium Falcon and Chewbaccaazzzz brother rendered in immaculate charcoal that somehow withstood the test of time! Thus an incredible and long forgotten period of Earth’s history soon to be known as Ska Wars is uncovered from it’s tomb!
Maybe the aliens don’t think we’re worth interacting yet. It’s very arrogant to assume we can just sit here and they’ll come to us.
I think Joshua’s point was that aliens and McDonalds are not comparable, because we know McDonalds exists.
However, that was the point. Take something we know exists, and observe that even such a thing is not necessarily observed everywhere it could be.
Maybe we are the aliens. The first intelligent species got the jump on everyone else and turned them (us) into them.
Maybe God doesn’t want us to meet. Yet.
Maybe we create our own universe and we don’t yet want aliens in it.
Maybe the aliens are here and we don’t know it.
Maybe the aliens are here and we aren’t.
That pretty much taps me.
A strength of the Fermi Paradox, and this applies to economics solutions, or demographic transition solutions (Abidemi), is that even if these solutions apply to all but a tiny fraction of civilisations, that tiny fraction should have filled the galaxy.
What is the difference to believing in alien intelligence and believing in a supreme being? Is one less kooky than the other because of the powers often given to that entity? We can write an equation giving a possiblity that aliens exist so does that make it science? We’ve never seen an alien so does that make it faith? Is the difference that we don’t have any fundy alien terrorists blowing things up in the name of the Greys?
I forgot to ask – has anyone mentioned typos or mistakes in “The Ghost Brigade?”
I’ve found one.
There have been a few people who have noted them. Send me an e-mail about it, Tripp. I’ll forward it.
Kero aka Kevin: One difference is the people who believe without question in extraterrestrial life are a very small minority with little or no power. Unlike, say, the Catholic Church, which has enormous wealth and political muscle throughout the world. (Not to pick on Catholics – other organized religions have power and money as well, but not nearly as much).
Tripp:
has anyone mentioned typos or mistakes in “The Ghost Brigade?”
I sort of found one on the first page, but maybe adjusting it would ruin the feel of the prose. I told John about it, and he responded graciously, of course. I’m not sure whether or not he means to change it.
[Cue visual of Whatever readers all grabbing TGB to scrutinize the first page]
I always get a kick out of how people want things defined and known…and especially can’t wait for the explaination. I once wrote a story in the story the main character is in a jail cell. He is a telepath and outside the jail cell is a switch (a reostat actually) which controls a supression field to inhibit his abilities.
Everyone who reads the story immediately after reading the story asks me, demands to know in fact what the current state of that switch is at the end of the story.
Truth is I don’t know, I haven’t the foggiest idea what state its in or why it would be in that state. Its an issue for a later time and that time hasn’t come yet.
What is the difference to believing in alien intelligence and believing in a supreme being?
One is a testable hypothesis and the other isn’t?
I had a thought about this review that just occured to me. This review and many others about OMW mentions a lot of unexplained things about the universe/world building aspects of the book.
I think these people keep missing the point, which is a point I got immediately. The book is written first person perspective. From that point of view/frame of reference the only things that are relavant are the things that the narartor/main character knows, is told, or actually has happen to him.
He can’t (and it would ruin the story if he did, because it would be a serious stylistic break) reveal those things that are not in his frame of reference. However this fact actually accomplishes one of the best things any writer can hope for at the end end of story. It leaves the reader wanting more, wantging those things explained.
Haplo Peart: You say he can’t reveal those things that are not in his frame of reference.. If you’re suggesting that he can’t know about things happening beyond his own experience I would disagree. There are ways for the character to keep informed of current events (or at least current propaganda) in the OMW universe – an obvious means would be through the BrainPal interface. But I would agree that leaving readers in the dark is a clever way to leave them wanting more.
CoolBlue: How would you test for extraterrestrial intelligence without concrete evidence for its existence?
CoolBlue:
What is your test? Wait and see? I suppose that before the heat death of the universe one will be more likely to be proven than the other. It seems difficult to get either of them out of the proving a negative stage.
The thought that aliens not only should have been here by now but should have been here since before life began on the planet brings an amusing thought to mind – if the existence of aliens is a pre-existing condition, we may have just taken any evidence as a given. “What do you mean you’ve seen no signs of aliens! The sky is blue isn’t it?”
John H
How would you test for extraterrestrial intelligence without concrete evidence for its existence?
Um, you can’t. Right now we have a hypothisis for which there is no supporting evidence.
That’s OK in science.
On the other hand, there is no one saying that there are or have been other intelligent species in the universe besides us.
Which is more than one can say for the whole “Supreme Being” thing.
Kero aka Kevin
What is your test? Wait and see?
Yep. Do you have a better idea?
It seems difficult to get either of them out of the proving a negative stage.
Science doesn’t “prove” theories; it disproves them…or not.
As a result, science can not claim that God does not exist. Nor can it claim that aliens do not exist.
However, it is clear that the possibility exists that aliens or their artifacts can actively be sought while God artifacts are difficult to discern.
John H…
I think more so what I am getting at is that some of those aspects are outside the characters frame of reference because he:
1. Flat out doesn’t care about them, or in terms of the writer putting the desire to care about them into the character’s head…they don’t matter to the story so why would the character care to know about them and then expound upon the details in the story.
2. Doesn’t have time to care about them, there are other things happening around and about the character that need his attention, and therefore need to be revealed to us as the readers through the character because they matter to the story.
Everytime the writer stops the story for exposition that break in the action needs to tell us something that is relavant to where the story is going. Otherwise why stop the story to tell us something we as readers don’t need to know about? Just for the sake of us knowing it? If the writer stopped to fill in every little details about the things that a reader might want to know suddenly you have a very long, slow and boring 1000 page book.
Chris Gerrib has the right idea, but the wrong formulation. A more encompassing answer might be phrased, “not everything which is possible will be made actual.” Note that this is still an answer which boils down to “economics,” but one which derives from the work involved rather than the monetization of that work or any analysis of its perceived utility.
There is no evidence that God exists. That’s why they call it “faith,” amigos.
CoolBlue:
“However, it is clear that the possibility exists that aliens or their artifacts can actively be sought while God artifacts are difficult to discern.”
I think you are putting the cart before the horse here. Aliens can only have artifacts if they exist, looking for their “artifacts” is the same as searching for proof they exist.
My point in all this is that I don’t see a practical difference in looking for proof of aliens and looking for proof of a diety. Except that looking for aliens is more accepted in the scientific community than looking for a diety is, and you get to use really cool gear.
Chris: 1% of a billion is 10 million. Which makes Montana a damn small moon?
Oh well, if we’re talking Fermi Problems (grin), then we’re dealing with 1 significant figure problems or order of magnitude, so being off by a factor of ten is “within the range.”
Dr. Phil
Mitch Wagner
There is no evidence that God exists.
LOL. Well no that depends on the definition of God we are using, now doesn’t it?
If, for instance, I define God as having created everything, then there is plent of evidence for it’s expistence.
Science, however deals with phenomena and because of this, such a definition is outside the domaine of Science.
Kero aka Kevin
I think you are putting the cart before the horse here. Aliens can only have artifacts if they exist, looking for their “artifacts” is the same as searching for proof they exist.
One can have a theory that speculates that some unknown thing exists which you then can go looking for. In many of the cases such as this, the theory would define the parameters of the thing which is missing.
Such situations can redily be found in subatomic physics. This is how quarks were discovered: physicists knew to look for them because of some other behavior they observed that implied their existence.
But forget subatomic physics, let’s just take the more well known atomic physics. At the beginning of the 19th Century, John Dalton speculated that elements were made up of atoms. He deduced this after experiments allowed him define what later became known as Dalton’s Law which stated
If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
And is something which every 1st year Chemistry student knows. What’s probably less well known is that there was a competing theory, or I should say a theory with which atomic theory conflicted.
Einstein proposed that Brownian Motion was consistent with Atomic theory (and created a mathematical model for it) and Thompson wrote a paper showing Cathode Rays were negatively charged particles but it wasn’t until 1911 that atoms were proved experimentally.
Now it is not unreasonable to think that it is unlikely that humans are unique in the universe giver the vastness of space and the “billions and billions” of stars out there. And so speculating what an alien artifact would look like is also a scientific endeavor. Once a model exists one can go searching for such artifacts.
The God thing, though, is much different firstly because it is so broad. If you say God created everything, well, fine, but where do you go from there?
Science seeks to understand the world and the phenomena in it. So if you say “God created the world 5000 years ago” well we can test that theory and we can prove the world is older than that.
If you say, “Well, God can do anything and he made everything look old”, science will say “OK, but we can’t test that theory so that belief is not within the domain of scientific research. And as a result, that is not a scientific theory.”
The aliens are god. They just stop by and create religions as a form of chess between themselves. “Hey, watch this one. Check mate. Your guys are all killed off now. Want to go again?”
Now this Fermi Paradox thing. First of all this is science fiction. It is allot more interesting to read when there’s something there. “We went out to the universe and no one was there” makes a boring and rather short story. This paradox reminds me of the “If a tree fell in the woods and no one heard, would there be sound?” crap. Who cares. Talk about ego. Our hearing something makes it exist? I suppose that would make us god.
The chemistry and physics examples you give aren’t quite the same thing as with aliens. They were based on other “things” in their discipline. To the best of my knowledge, we have nothing in cosmology or astronomy or anything else that leads one to believe there should be aliens, or need to be aliens for the model to be correct. There are no alien shaped holes in our view of the way things work. The only reason we have is “well, the place is so big there just gotta be aliens, there just gotta!” Which is all fine and good. However, saying the universe is so huge that there has to be aliens doesn’t seem too far away from saying the universe is so huge (and complex) that there has to be a deity.
Searching for aliens is either the most important thing humanity can do or pretty much a waste of resources. While I find it easy to believe that we are not alone in the universe, I find it difficult to justify using very many resources to search for something that has as much evidence for it’s existence as Noah’s Ark.
Kero aka Kevin
However, saying the universe is so huge that there has to be aliens doesn’t seem too far away from saying the universe is so huge (and complex) that there has to be a deity.
Ok. But the fact remains, one difference between the two is that one is a testable theory and one isn’t.
Granted, given the lack of evidence for aliens if an individual is convinced without a doubt that they exist regardless, then such a belief is indistinguishable from religion.
I think Joshua’s point was that aliens and McDonalds are not comparable, because we know McDonalds exists.
However, that was the point. Take something we know exists, and observe that even such a thing is not necessarily observed everywhere it could be.
the problem is that there is evidence of McDonalds in John’s town…we may not have aliens but there should be evidence of them none the less. The two examples i supplied were ringworlds and radio signals…others I leave to your own imaginings
“Others I leave to your own imaginings”
Well that is certainly easier than making a coherent, specific argument.
“First and foremost is, of course, the “non-science fiction solution.” That’s where the speed of light really is a speed limit and everyone interesting is just too far away.”
The problem with that is radio or more exactly the fact that with 1960s radio telescope technology, you can beam a message across the galaxy and with an economy the size of Canada’s in 2005, TV shows on specialty channels can pay to beam messages to the stars (Specifically, to 37 Geminorum). Unlike interstellar travel, attempting deliberate radio communication is something that can be done as a hobby by a small group.
Just to be clear here: I am not talking leakage from communications but deliberate braodcasts designed to be spotted.
Now, we’ve only been looking for a few decades but so far, there’s no compelling evidence of a civilization with radios out there.
Local results for mcdonalds near Bradford, OH 45308
McDonalds – 13 miles SE – 127 N Miami St, West Milton, 45383 – (937) 698-3665
McDonald’s Restaurants – 10 miles W – 1301 Wagner Ave, Greenville, 45331 – (937) 548-1744
McDonald’s Restaurant – 11 miles E – 995 E Ash St, Piqua, 45356 – (937) 773-3100
You are in the hole in the center of the donut they will fill next.
Do aliens regard the Solar System the same way?
Now, we’ve only been looking for a few decades but so far, there’s no compelling evidence of a civilization with radios out there.
Or at least none that cares to broadcast.
They had radios. Now they’re all about MP3s and podcasting.
And bittorrent.
“intelligent species don’t last forever”
Although given the fact that C really does look like a universal speed limit, any species that managed to spread between stars would have settlements that are effectively causally isolated, which should help limit extinction events to single systems.
“intelligent species stop expanding at a certain point”
Or they spread in a patchy manner: see Landis’s percolation theory.
http://www.sff.net/people/Geoffrey.Landis/percolation.htp
“intelligent species are not evenly distributed in time (that is, we’re among the first of them)”
Alternatively, there was some kind of sychronization event. Known Space had at least one and I suspect many, thanks to the malevolent Outsiders and their brutal rule of the galaxy (Or did you think it was a coincidence that just as the Pak got out of their cage, the galaxy blew up?).
“intelligent species are avoiding us / hiding from us”
Or the ravening beams of the Nicoll-Dyson Sphere just have not got here yet…
“there are no other intelligent species.”
Which fits the available data.
Why arn’t we dead yet?
That’s because they have been watching us and we are SOOOOO BAAAAAD that we scare the bejesus out of them and who would want to release US on the rest of the Universe.
Who says we don’t have a moonbase already????
No McDonalds?
You live to far away from a town with more than 500 people.
You don’t have a high school in your town.
(no work force)
You live in a town of vegetarians.
Don’t have a main highway thru town.
any of the above.
I like the percolation theory idea. But it has this problem: it assumes cultural invariance over time, i.e. a non-colonising culture will always be a non-colonising culture, and so the “shell” of non-c worlds blocks expansion. I’m not sure that this assumption is justified over the long haul. Human history contains many, many examples of cultures changing rapidly. The Germans of 1910, to pick an obvious example, are at a rather different point on the colonising scale than the Germans of 2000.
astronomers information and correct details about galaxies