Aug 4 2008 1:55pm

The Fermi Paradox: OK, where are they, then?

The Fermi Paradox states simply that if there are aliens, where are they? It can be seen most elegantly stated in the Drake Equation, where, simply paraphrased if there has been a certain amount of time, and there are so many stars, and so many planets capable of life, and so much life, and so much intelligent life, and a certain percentage of that gets off the planet, then why haven’t we met any aliens. You can plug in any numbers you like in most of those holes, because we just don’t know the answers. Planets capable of life might be much rarer than we think, and intelligent life might be much rarer. You can’t fiddle with time, but you can get a quite satisfying answer to the Fermi Paradox at any of those other link levels. We only have one example of an intelligent species, and we’ve only barely got off the planet and haven’t even got out of our own solar system yet.

The thing that makes the Fermi Paradox interesting for SF is that like the speed of light, you have to have an answer for it. It can be any answer you like, but it has to answer it. This has led to some lovely creative possibilities and, unlike the Singularity, seems to me to be a constraint on the genre that encourages positive things and fascinating speculation.

So, there’s Ken MacLeod’s Engines of Light solution—the aliens are right here, they’re just keeping a low profile. We don’t see any evidence because they don’t want us to. This is also the position of Roswell conspiracists and so on. They’re here but hiding.

A little further out, there’s “they’re nearby, keeping an eye on us, but leaving us alone for their own benign reasons.” That’s Heinlein’s Have Spacesuit, Will Travel—the aliens are vastly more powerful than we are, and they’re leaving us alone until we’re more ready to join them. They don’t want us until we graduate. We don’t see any evidence because they’re so much more advanced. You see this also in Carl Sagan’s Contact and in Clarke’s 2001. This allows for stories where we later join them and have lots of planets to leave alone, as in the Star Trek Prime Directive, and Lloyd Biggle’s Still Small Voice of Trumpets universe.

Then there’s “Earth is a preserve but aliens aren’t so wise.” In David Brin’s Uplift books the universe is full of aliens who have each uplifted other species to full sentience. Planets are gardened for intelligent species, and Earth is lying fallow ready for the next such, while we evolved by mistake—or did we? There’s also a short story—author forgotten—where there are planets full of mindless human-like meat animals, whose alien owners come back to harvest them now and then. There’s also Jerry Pournelle’s Janissaries where aliens are quietly recruiting groups of humans from Earth for very dubious projects.

Related to this is the fairly common idea that they’re just too alien to be interested in us unless we bother them.

These are pretty good answers because they’re inarguable. I mean if they’re there and they’re hiding, of course we see no evidence. QED. But there’s something a little unsatisfying about “we don’t see them because they’re cheating.”

There’s half a ton of SF where the aliens just happen not to have got here yet and could wander by at any moment. Larry Niven’s Known Space is like that, more power to its Outsiders. Le Guin’s Ekumen is also like that. Also, there’s no FTL travel, and crawling along takes time—it’s surprising how few SF universes there are with no FTL or slow FTL. There are generation ships, but not much  like the Ekumen where it takes ten years to get anywhere.

Then there are books that break one of the links of the Drake Equation. C.J. Cherryh plays with the “habitable planets” link in her Alliance/Universe novels—very few suns have planets that can support life, humans live in space stations around inhospitable planets. When they do find planets with life, it usually isn’t intelligent, or advanced. (Later she connected these books with the Chanur books where there are lots of aliens, with no explanation yet.) Bujold’s Miles books break the link at intelligence. Life is common, intelligence is vanishingly rare. After all, there are lots of animals as smart as a dog, there’s only one as smart as people.

Vinge’s Zones answer the Fermi Paradox without ever needing to directly address it—if the interesting part of the universe is a long way from here, of course they’re not here! And in Marooned in Realtime he uses the Singularity as an answer--if aliens all have Singularities and go away, that also fits.

In Saberhagen’s Berserker books, technological civilizations destroy each other. They aren’t here because they’re all off fighting elsewhere, and when they show up, watch out.

Perhaps my favourite answer to the Fermi Paradox is Terry Bisson’s “They’re Made Out Of Meat”.

1. WhitK
"Bordered in Black" by Larry Niven for the forgotten story?
2. jimboweb
Don't forget Alastair Reynolds' Inhibitor universe, which spins off Saberhagen's idea and imagines the ancient Inhibitor construct which plays whack-a-mole with any intelligent race that sticks its nose too far out. In Reynolds' books, the Inhibitors are keeping the galaxy "clean" of intelligence for their own unknown purpose.
3. Tom Scudder
I know you're allergic to media adaptations, jo, but for others, "They're Made Out of Meat" has been made into a lovely short film.
Clifton Royston
4. CliftonR
Charlie Stross recently had a discussion about this in his blog.

I'll trot out again my favorite answer, from Greg Bear's Forge of God: intelligent races only survive their infancy if they have learned to shut the hell up and broadcast nothing, because noise attracts predators.

Seriously, though - if you look closer at that "time" question, we are still relatively early in the history of the universe. It took roughly 1/3 of the lifetime of the universe for life to evolve to intelligence on Earth. (Earth is roughly 4.5 billion years old; the universe appears to be about 13 billion years old.) Then subtract the first generations of stars, which would have been unable to support life as we know it because the heavier elements like carbon hadn't yet formed; a generation or two of stars had to be born and die before stellar systems with a composition like ours. That leaves a much shorter window for life to have evolved anywhere than it appears at first glance.

Even if you dice the numbers such that intelligence is highly probable in the long run, it's not implausible that we might be among the earlier intelligences, and that they may be maturing as we are. Or as we hope to, should we live long enough.
Stephen Hunt
5. Stephen_Hunt
And there's always that old favorite, the Prime Directive!

Paul Weimer
6. PrinceJvstin
My solution to the Fermi Paradox is a bit of a variation on the Singularity idea by Vinge, blended in with a couple of others:

Intelligent Species do not last long at a level of development where meaningful interaction with their peers is possible. They either evolve/develop beyond our ken or blow themselves up, or go extinct for other reasons.

The Drake Equation lacks a variable for time--for the probability that civilizations potentially capable of communication will co-exist in the same time frame.

There could be a bronze age civilization on a planet circling Alpha Centauri B and we wouldn't know it without going there. Conversely, they would have no idea that *we* are here.

And if humanity were to go extinct in a millennium, they might rise to technological prowess just as alone as we seem to be.
Del C
7. del
The Drake Equation lacks a variable for time

The Drake equation lacks lots of things, but that isn't actually one of them.
Stephen Covey
8. coveysd
I fear that CliftonR / Greg Bear is right: intelligent species hide from possible predators.

In my own (July) blog post, The Fermi Paradox: Where Are They?, I proposed two other possible answers:

1) Technology is extremely rare, because planets capable of supporting it are rare. Most temperate worlds would be dry and lifeless, or water worlds without dry land. We may indeed be unique.
2) Intelligence is an anti-survival trait. We'll soon be extinct.

It is difficult to believe that we could be the first intelligent technological civilization. It's not so hard to believe that we could snuff ourselves out. So many ways, but with so little time and space to try them all out.

For an SF writer, the Fermi Paradox is a challenge. As Jo points out, it should be addressed. Either a reason why they're quiet, or a reason why we're alone.

Knowing how good people are at keepng secrets, or abiding by rules, the quiet possibility is implausible for a galactic civilization spanning millions of worlds. Something humanity might accomplish in only a few million years.

That only leaves the possibility-make that probability-that we are alone.

And leaves the question (raised by Jvstin): How long will we survive as an intelligent, tool-using civilization?

The Fermi Paradox suggests the answer: not long. Not long at all.

The good news is that all this provides great fodder for interesting SF stories. (Just not ones involving aliens.)
9. Mark Duffett
Deeply un-PC on several levels to say so, but there is this possibility: that the Judeo-Christians have it right, and there was only one Garden of Eden...
10. crotchetyoldfan
Fermi's question was tied in with the concept of self-replicating von Neuman machines having had enough time to spread throughout the galaxy, starting from a single source and assuming that FTL is not possible.

CliftonR (#4) points out an aspect of this question correctly (and anticipates a timeline I've been putting together for a blog entry): it is entirely possible that we are the first or among the first intelligent/technolgical species to arise in the galaxy.

In this particular case, it actually helps inform on the subject if we make the assumption that the human race's evolution (which encompasses our solar system) is the standard to measure everything by.
Derek Bizier
11. tke.hijacker
Meat with thought... I would argue that the samples that they have taken come from trailer parks and should not be looked at as the best example of our kind.

Stop taking our Trailer Trash Meat and take the Smart Meat!!!
12. Kate Nepveu
There was a panel on this at Readercon, but my link roundup only found one report, a general discussion of the problem.
13. Damien21
Intelligent life may be and probably is, common throughout the universe. The probable reason we have not seen them is that we are not intelligent life (despite our arrogance) and they don't want anything to do with us
14. Spherical Time
Mark @ 9: Deeply un-PC on several levels to say so, but there is this possibility: that the Judeo-Christians have it right, and there was only one Garden of Eden...

Assuming the existence of a god, that being is simply a version of Heinlein, Sagan, and Clarke's more powerful alien race.

Which presents the question: If intelligence require uplifting, who uplifted this god?
15. David Ellis
My best guess as to the solution of the Fermi Paradox:

Intelligent life is sufficiently rare that it hardly ever occurs twice in the same galaxy. Example: if it occurs, on average, in only one out of 10,000 galaxies then its hardly a surprise we haven't heard from our next door neighbors.

If this is true, then it follows that SETI's best chance of finding a signal is not to point their radio telescopes at other stars in our galaxy but at other galaxies (how prohibitively strong a signal would have to be to be detectable at intergalactic distances I don't know).

Does anyone know if SETI has done much of this?
CE Petit
16. Jaws
The Drake Equation (and Fermi's Paradox) do include time per se as (assumed) variables, but not either timescale or simultaneity. Further, there's an assumption that "intelligent life" means "civilization" buried in there.

* The time variables incorporated in the equation are geological/astronomical time; the result of the equation, though, is supposed to be in civilization time. At least in our experience, the units figures for those two types of time are off by at least four orders of magnitude, making them largely incomparable. Here's an example: Would the presumed scalar variables in the Drake Equation even notice the approximately three thousand years of irrefutable evidence of intelligent life on Earth that is theoretically observable from orbit? Nope; that would make no difference on a geological scale, but all the difference on a civilizational scale.

* The real problem, though, is the assumption that simultaneous existence — which is all the Drake Equation/Fermi Paradox attempt to discern — says nothing about simultaneous notice and communication. This is a variation on the "tree falling in the woods" question: If nobody in Civilization X ever notes the existence of Civilizations Y1...Yn, do the class Y civilizations "exist" to X? Perhaps the class Y civilizations are so far away that even though they satisfy the Drake Equation's parameters, no evidence of that existence ever reaches X during X's lifetime. Perhaps the class Y civilizations take active efforts to avoid notice. Perhaps X suffers an inward turn economically or ideologically at the technological inflection point of ability to notice and loses interest in looking.

Drake's Equation is merely a special case of Maxwell's Daemon. The math to demonstrate that isn't all that hard (well, it's hard to put into HTML, let alone bbCode), but that also points out the underlying assumption in Drake's Equation: Once civilization rises to the notice point, it stays there. That is, there is no extinction term (either through speciation or another means)...
17. David Ellis
Jaws, the Drake equation includes, as its final factor (factor L):

"the length of time such civilizations release detectable signals into space."

So your claim that the Drake equation assumes that "Once civilization rises to the notice point, it stays there" is, in fact, mistaken.
18. Emmet
In my grumpier moods, I suspect the answer to the Fermi Paradox may yet turn out to be that the universe is full of Earthlike planets containing life including roughly human-equivalent sentiences, but they're all full of libertarians who are too focused on the pre-eminent importance of their individual rights to work together on the scale needed to assemble the technological infrastructure for getting into space.
Patrick Shepherd
19. hyperpat
And then, perhaps, we are simply not looking in the right place or via the right medium. Why are we so sure that other civilizations would broadcast on the electromagnetic spectrum, and even if they do, at near certain frequencies? I remember Piers Anthony's Macroscope, which posited a different kind of particle/spectrum (not currently supported by any of our current physics models, but who says we really know everything?), where real civilizations did all their broadcasting.

And the earlier point about civilization being so rare the only expectable place to look would be other galaxies should certainly be considered, right alongside the time value you plug into Drake's equation that specifies the length of time a civilization remains viable and emitting detectable signals before blowing themselves up or being subject to some other disaster.
20. David Ellis
Another possible solution (one I wish was true but am not holding my breath for):

We do not detect radio signals because it is possible to build FTL communication devices (think Le Guin's ansible), most civilizations develop the ansible soon after becoming seriously technological and when we do we'll join the conversation on the interstellar, inter-species internet.
21. Jeff Brandenburg
Here's another possibility: long-range communication becomes more efficient as you target it more precisely, as you use a broader spectrum, and as your encoding becomes statistically similar to noise. We may well be overhearing communication all the time, but misinterpreting it as cosmic background (a universe full of low-bandwidth messages), brown dwarves (large planet-bound civilizations leaking a lot of IR), or gamma-ray bursts (Very Important Annnouncements occasionally directed near us).
22. Gregg Cooke
Riffing on David's suggestion, consider Richard K. Morgan's "Altered Carbon" world, where FTL communications is used to transmit consciousness itself into a "sleeve" body (that may bear no semblance to the body the consciousness left). Maybe we're surrounded by aliens beaming their consciousness to us but on a frequency and medium we can't understand. So even when we do develop the ansible, what we'll here at first will just be lots and lots of incomprehensible data.

My point is, the technology part of Drake's equation may be much more complicated than originally imagined -- it may require not just one, but multiple technologies (ansible, sleeving, etc.) which have to converge in the right way at the right time. The side-effect is that we may actually be living in a crowded neighborhood but end up advancing to a very high level of technology and intelligence before we finally make contact (or may never make contact at all).

Oh, and FWIW, I think the Fermi Paradox is pointed in the wrong direction. The more I learn about evolution and memetics, the more I'm coming to the opinion that mankind needs to jump from genes to memes as its primary existential substrata. I think that all "advanced" civilizations are probably memetic, but I really can't describe what that entails (I'll leave that up to the sci-fi writers!).
23. Jeremy Yatvin
"Bordered In Black" by Larry Niven is the story you are looking for
Clifton Royston
24. CliftonR
BTW, the "period of time emitting detectable signals" also allows for one to factor in the possibility that all civilizations eventually switch to cable TV and fiber optic communications and abandon broadcast of radio and TV signals. (Or, if you like, to postulate that at that point their scientific development becomes strangled by technological monopolies and they lapse into passivity.)
Jamie Grove
25. jamiegrove
Perhaps the universe is so full of life and so much of it is exactly the same that advanced cultures have installed spam filters to keep our primitive messages out of their cosmic inboxes and proxy servers to filter our inbound packets.

I mean, how else would you treat a n00b?
26. MDK
Something I have always wondered was just how far would we have progressed as a civilization without access to cheap (and dirty) fossil fuels? This accumulated energy drove the engine of industry, allowing humans to develop higher technologies that would have been unreachable otherwise.*

We could assume a planet exactly like ours with a sepcies exactly like ours, but without coal or oil they might still be at an 18th century technology level and never attain the level of communication that could travel beyond their own planet.

So it isn't a question of life, intelligence or technology, but a question of natural resources.

Just my $0.02

*Hopefully we will be able to transition away from these fuels to other, sustainable energy source if only because fossil fuels are finite.
27. Jackie
Don't forget Frederik Pohl's Gateway series! In the later books he explored the possibility of a range of semi-intelligent to intelligent life forms that exist throughout the universe, although are found few and far between.
28. gwwiz
As a theologian and a geek I prefer to believe that we are only "effectively" alone at the moment because that's what the big guy wants.

I like to think that maybe he doesn't want to contaminate what he has in his petri dish. Kind of like a Jedi Mind trick on a cosmic scale; Effortless (for him) to cause, impossible to resist.

We ignore all signs of the others, they ignore us, etc. It keeps us free from contaminants and allows the experiment to run it's natural course.
Jeffrey Richard
29. neutronjockey
Anthropic bias.

We're still arguing that the Earth is the center of the Universe and that we are somehow unique and special through the Drake Equation and the Fermi-Paradox.

I'm going to have to agree with Terry Bisson's short story.

Call me a Bissonist.

Both Drake and Fermi fall flat because of their anthropocentric assumptions:

R* is the average rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets carbon based life as we know and understand it
f? is the fraction of the above that actually go on to develop life at some point - again, assuming similar chemical structure and biological make-up
fi is the fraction of the above that actually go on to develop intelligent life Define: intelligent life. Scientists just recently awarded the elephant with the title of "Self-aware" what makes us think we can look beyond out puny planet?
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space Assumes a similar technology structure and base. Assumes detectable by out technology.
L is the length of time such civilizations release detectable signals into space.

Why is it than in a Universe whose estimated volume is some 4.2 x 10^69 cubic miles that someone would care to broadcast their radio waves (?!) on our EXACT location.


Because the Sun revolves around us, and we're the center of the Universe.

That's why.

Brian Eisley
31. brianeisley
I agree with MDK @26. I have serious doubts about whether industrial civilization is possible without fossil fuels or some other readily-available, high-energy-density source to help boost it to that level.

Not to mention that the technology required for interstellar communication (by any method we're aware of) is largely dependent on fossil fuels and their byproducts. Imagine trying to build a PC without plastic!

And finally, it's not at all clear that we'll be able to maintain our current technological level once the oil runs out, which it's already starting to do. So I believe the factor of limited energy sources puts a severe restriction on the time that a civilization has to make contact with another--if it ever becomes capable of it at all.

I think that the time factor in the Drake Equation, normally read as "the lifetime of a technical civilization", should instead be interpreted as "the length of time that a civilization is capable of interstellar communication". That time may be very much smaller than we've ever considered.
Jeffrey Richard
32. neutronjockey
gwwiz---what is the point of an omniscient being running an experiment if you already know all of the conditions, variables...and outcomes?

As a theologian I'm sure you're well prepared with a rational discourse to answer...
33. VancouverDave
"Danger: Human", by Gordon R. Dickson is another fine explanation of our solitude.
34. Winchell Chung
There was a particularly nasty form of the Fermi Paradox: Bezerker Hypothesis in Charles Pellegrino and George Zebrowski's novel The Killing Star
35. Koowan
Maybe we really, genuinely ARE alone? In other words, flipping a coin 1 million times and coming up heads every time is just as statistically likely as any other combination. It would be a terrible shame, but it's still a distinct possibility.
Bruce Cohen
36. SpeakerToManagers
As far as I can see answers to the Fermi Paradox can be divided into 3 categories:
It's a jungle out there: if you stick your head up, it gets whacked off.
It's a desert out there: there's no one to talk to, or if there is, they're too far away to be heard.
Nobody wants to talk to us (maybe we need a better deodorant?)

In the first case, continuing to look is dangerous. In the second case, continuing to look is futile. In the third case, continuing to look might annoy someone or something. Seems to me that success in contacting alien life is likely to be successful in only the one case where technical civilizations we can talk, and who want to talk to us to are plentiful, and it's merely bad luck we haven't found them yet.
Brady Allen
37. akabrady
One possibility seems to be the obvious one to me.

As CliftonR @4 said, it took a while for the conditions to be right for us to exist.

So, if all creatures are subject to roughly the same timescale, then all civilizations would be at roughly the same level of technology. Which means none of us have yet figured out how to talk to each other or get much beyond our own backyard.

Let's even go so far as to say another civilization exists on the other side of the milky way. The milky way is 100,000 light years across (more or less). Which means this other civilization would have to be at least 100,000 years more advanced than us for us to even start to detect their earliest, weakest, most easily corrupted signals.

So, unless you assume life started on other planets a lot sooner, and that, that civilization was strong enough to last for a significant amount of time, we're not even in a situation where we can even begin to detect other civilizations.

And for gwwiz @ 28 and orchard at 30,

Assuming the light is the speed limit of the universe, God wouldn't have to perform any sort of Jedi Mind Trick. He simply has to separate the children far enough so that Tommy can't hit Suzie. That means they can still have personal agency. (If you believe in an afterlife there's nothing 'free' about free will. There's always consequences.)
eric orchard
38. orchard
Good call Akabrady! I hadn't though of that.
39. Tom Benson
Akabrady is right. There actually is a fairly straightforward answer to the paradox. People just don't spend much time on it because, well, it's not as much fun as speculating about aliens living on our planet in disguise, or hiding themselves!

But why haven't we seen colonizers?

Here is a simple answer:

There exists an interstellar time-of-flight value Tau, beyond which it is impossible for any complex system, mechanical or biological, no matter how sophisticated, to survive the entropy of deep space. Faster-than-light travel doesn’t exist. Tau is a small enough value that it is 100% certain, in every time and every place in the universe, that every space-going species that ever lived or now lives, mechanical or biological, has been blocked by it.
40. glossaria
(You know, for this entire conversation, I keep hearing Hamlet's "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.")

But anyway... I'm with neutronjockey @ 29. As far as "signs of life" go, scientists are still discovering new life forms in extreme environments *on our own planet.* I think it's far too early to be saying there's nothing else out there, just because we haven't noticed it yet. But then, is humanity prepared to answer an alien contact? Would we even *recognize* one?

Consider, we've barely begun to process the languages of our own neighbors-- and I don't mean the human ones. We've lived cheek by jowl with a vast number of nonhuman lifeforms for a couple million years (give or take)... but when is the last time any of you had a deep, meaningful conversation with a dolphin? How about your cat or dog? Thousands of years of domestication and close companionship, and they *still* don't have us reliably trained on the whole "I would like to go out now, please" issue. (At least, the cats don't. Further proof of superior canine intellect. ^_~ )

In all seriousness, we're still working on the question of what, exactly, other species on Earth say to one another, and all the ways in which they say it. Naturally, we tend to start with the forms of communication humans can understand and create-- picture, sound, gesture, touch, radio signal, blog. But then, consider the olfactory equipment of a dog, or the vision of a bumblebee.

What if bumblebee society (far more advanced than we ever guessed, of course) were to develop long-distance communication by reflecting flashes of ultraviolet off their wings in high-speed semaphore? How long until humans even figured out that they were doing it, much less were able to translate? Yes, our technology could help us process the signals, but it's still a very long way from data crunching to comprehension, from comprehension to communication-- and again, we'd have to take notice of flashes of invisible light modulating at speeds faster than we can see, first. Now, how are humans to recognize an overture from another planet, when we can barely communicate with the sentient species on this one?

One more side note... neutronjockey also made a remark about "carbon-based life forms"-- my high school chem teacher once commented upon the structural similarities of carbon and silicon. I spent the rest of the class daydreaming about planets inhabited by silicon-based sextupeds.
41. David Ellis
We're still arguing that the Earth is the center of the Universe and that we are somehow unique and special through the Drake Equation and the Fermi-Paradox.

Nothing in Drake or Fermi assumes that.

The things that you call anthropomorphisms like:

"carbon based life as we know and understand it"

are just examples of them being conservative in their estimates by sticking to what we already know is possible. Additional varieties of life would be a bonus that improves the odds of ET civilizations in the universe.

And saying that factor fc---the fraction of civilizations that develop a technology that releases detectable signs of their existence into space---assumes a similar technology structure and base is not true. The could be sending their radio waves into space using biological radio emission organs for all we know (as in Benford's ACROSS THE SEA OF SUNS). That they use signals we know about and can detect does not in any way assume their technologies are even remotely similar in nature to our own.
Chris Cyr
42. Zeek
What about adding another variable?

Ap: Level of apathy. The aliens just don't care about space travel, little pink Earthmen, radio waves...what have you. I spend their days fat and happy eating the local bacon variant and drinking "bourbon," never thinking to even look at the sky.

Why assume that any alien civilization cares to develop to the point where they can be detected or detect others?
Rich McAllister
43. k6rfm
Ken MacLeod's Learning the World has a very neat Fermi Paradox resolution, based on the ideas in Lee Smolin's The Life of the Cosmos.

(One of the things I like about the book is that it's not very clear that's where he's going, so describing it might be a spoiler; I've made the text white, highlight it in your browser to read it.)


Universes have mutated progeny (black holes), so are subject to natural selection and evolution. The most common universes are the ones which create the most progeny. The star drive Ken has creates descendant universes as a side effect. Thus universes evolve to maximize the amount of star travel. That's best done by having all technological civilizations develop star travel and communication at about the same time so the first one doesn't wipe the rest out.

44. MMBB
Assume that the forces of evolution operate on a cosmic scale as well, just as they operate on Earth. Assume that some species are pacific and do not wish to interfere, but a small number of species or civilizations have no such qualms; assume some of them have low fertility, but a small number have high fertility. Assume some intelligent species are conservative or have a philosophy that cannot lead to interstellar travel, while others are expansionist and see no problem with spreading on every planet. Furthermore, assume that there is nothing special about us, so some of these aliens might have appeared, say, a billion years ago, others five million years ago, etc.

Then inevitably the expansionist, aggressive aliens would have been here a long time ago and colonized Earth and the signs of their presence would be obvious. There is no reason why, in fifty million years, they couldn't be both here and everywhere. There is no reason why mindless expansionism necessarily leads to extinction. The rabbits and dingos in Australia seem to be doing just fine. Ants have colonized the Earth and there seems to be no downside to it. If truly this were an untenable tactic, then all living life wouldn't be exhibiting it -- but it does. True, some species may throw the ecosystem off-balance, but there's no reason why it couldn't adjust.

Say some aliens knock themselves out or become uninterested in life because they are infinitely wise. However, the experience of life and of the human species is that once a place gets colonized (a continent, an island, the South Pole) we never abandon it for an extended period of time. If some group of humans decides to live in a sustainable manner, another group comes along and wipes them out, because there's nothing to be gained from moderation. There may be some transient trend to the contrary (a nuclear experiment that depopulates an island of all life or laws forbidding humans to go into certain areas). These things never last -- more than 500 years. Consider the hunting preserves of the nobles in the middle ages. In the end, revolutions came, nobles vanished, and all the forests in Europe were cut down and all the big game was hunted down. I cannot think of any noninterference rule that fared better than that.

I am inherently suspicious of various arguments that try to circumscribe this problem, because they sound too much like religion to me:
Aliens only reveal themselves to a few people, somewhere on the margins of civilization or two millenia ago, but for some reason are loath to show themselves on the White House lawn.
Aliens are signaling to us, but we are ignoring them / don't have the right approach to listening to them. We should be more open to their message.
The wisdom of aliens is so vast / their purpose is so foreign that we cannot comprehend them.
Aliens are so far superior to us that we are beneath their notice.
There is an invisible battle fought between the good and bad aliens hiding among us; our governments take part in it.
Try substituting "aliens" with "gods" in the preceding explanations and see my objection -- aliens shouldn't serve as a substitute to God.

Besides, maybe some aliens are like that, but what if some are like us? Noisy, fractious, polluting, not always bothering to hide ourselves? From what we know of other life here on Earth, we are by no means exceptional, so why shouldn't there have been, fifty million years ago, some aliens like us who colonized the Galaxy?
45. Drusilla
Hadn't read the Bisson story before. I think it's spot on - who wants to meet meat? (Well God does but He's unique and became meat himself.)
Torie Atkinson
46. Torie
@ gwwiz

And what does this have to do with the Fermi Paradox? On-topic, please.
47. Winchell Chung
Xenology: The Science of Asking Who's Out There, by David Brin

The 'Great Silence': the Controversy Concerning Extraterrestrial Intelligent Life, by David Brin

And there is a published book called Where Is Everybody?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life by Stephen Webb (ISBN-13: 9780387955018)
Torie Atkinson
48. Torie
@ Winchell Chung

Okay, but do you have anything to say about these books? These threads are meant to be conversations, so participate in the dialogue!
49. Winchell Chung
What I have to say is that David Brin's essays list about seven possible solutions to the Fermi Paradox, Jo Walton's original Wikipedia link lists about ten catagories, and Stephen Webb's book lists fifty possible solutions.

But so far all the solutions in the comments seem to revolve around the same one or two ideas.
Torie Atkinson
50. Torie
@ Chris Cyr

Why assume that any alien civilization cares to develop to the point where they can be detected or detect others?

Because we are assuming that aliens would be like us. Is insatiable curiosity a purely human trait? It surely could be, but I hope not! Makes for a much less interesting universe.

@ Winchell Chung

But so far all the solutions in the comments seem to revolve around the same one or two ideas.

Well sure, they're the ideas the commenters happened to find most interesting. It's not an academic study.

The Brin essay and the Wikipedia article mention something that always sort of killed my enthusiasm for first contact--the complete inability to hold "conversations" based on the sheer distances involved. Let's say with all those variables, all that improbably, and through the slimmest of chances, we DO receive a perfectly detectable signal from another civilization. What would we do about that fact? It could take generations or eons to confirm that discovery (as we send back a reply and wait for a response), or our reply might reach the other species long after it had disappeared or moved on... It makes one feel rather lonely.
51. Winchell Chung
For a signal to be that depressing, it would have to be extragalactic.

The Wikipedia article (and others) mention that even with the crude technology available to us, it would be possible to colonize the entire galaxy in about 5 million to 50 million years. Given that our galaxy is about 13.6 billion years old, this means a naive calculation shows our galaxy is old enough to have been entirely colonized and re-colonized again about 270 to 2,700 times.

The question becomes: how come we don't see any alien beer cans or other artifacts littering our solar system.

More to the point, why does the human race even exist, instead of Earth being an alien colony and Homo Sapiens a species that never got a chance to evolve?
52. Damien R. S.
"In Reynolds' books, the Inhibitors are keeping the galaxy "clean" of intelligence for their own unknown purpose."

Actually they do have a clear purpose (spoiler highlight):

Andromeda and the Milky Way will run into each other in a few billion years. Supposedly the wave of star formation will massively disrupt things, and a diverse range of species wouldn't be organized enough to handle the problem. The Inhibitors were made to take over, keep people's heads down, manage the collision, and then make way for the new golden age of life.

What Fermi comes down to for me is that our observed universe is compatible with we being the only species, and expansion/exploration being possible, or with there being many species, and expansion not possible, but not with there being many species and expansion being possible. We can have the stars, or aliens, but not both. If there are aliens, we won't meet them.

(for 'alien' read "technologically advanced alien", who could colonize the stars if colonizing the stars is at all possible. Bacteria, slime, or intelligent beings trapped by lack of dry land or fossil fuels can exist in large numbers.)
Julian Hall
53. Jules
Perhaps there's a really powerful entity out there who watches over his collection of galaxies and destroys them if they become infected with life ("it really messes them up... best to start again if they get too covered in the icky stuff"). We've only survived this far because a single planet is too small an infection for him to notice.
some guy
54. NateTheGreat
We only have one example of an intelligent species

I'm surprised this slipped by everyone. Arguably, we are the only intelligent species. If we were, then there would be no need for the decades of testing of Alex the Grey Parrot, nor would we have been able to teach gorillas, chimpanzees, and orangutans sign language.

I brought this up for two reasons. The first is that while we are not the only intelligent species, we are the only one that has built a technological civilization. Given how many millions of years it took us to get to this point, this might be an indication of how rare civilizations are.

Two, when and if we do encounter an extra solar intelligence, we most likely won't realize it. We are already sharing a planet with other intelligent species, and no one here seems to have noticed. There is a good chance we will make that mistake again.
55. Pixley
I love this, it’s a good version of the sophomore arguments I’ve had.

The real reason that they aren’t dropping out of the sky is threefold. First off there is no reason to develop technology. If you’ve read James Burke’s Connections or done any reading of history you’d know that advanced technology is not a given. Christianity was necessary to the development of technology. It allowed you to look at segments of the real world for answers rather than looking at the world in a holistic manner. Religions like Buddhism require you to take the whole world into consideration before you start to come up with answers. And you have to have something like Latin, a language that is understood in many countries. That allows books to be printed and read in countries that the author does not even knows exists. All in all the development of technology is unlikely, so that might add to the lack of Gort and Klattu.

Secondly evolutionary records show that environments reach a static plateaus. Some of the dinosaurs existed for tens of MILLIONS of years with no basic change to the organism. We’ve been bombarded several times and that caused mass extinctions. Those extinctions were the only reason that mankind evolved in the first place. If the immediate solar neighborhood is cleaner, those extinctions might not have happened.

But I’d put my money on good old fashioned war. If you’ve ever seen the old classic “Things to Come” you can see how a population might be killed off without nuclear or biological agents. Since we’ve had nuclear weapons we’ve had three occasions that came close to Nuclear War. And now Iran is saying that it is going to develop a bomb. If you look at wars in the past you’ll see that often they stopped with no resolution but a plague. That is things get so crappy and unsanitary that a plague starts. In “Things to Come” the first thing that the Chief does is to start another war and try to get the means that destroyed the world in the first war. It figures.
56. Rainforest Dad
Why no love for Greg Bear? The man was seriously persuasive in "The Forge of God". Another answer is that we are some kind of sim game for 'God'. If that's the answer, then there's no need to look further. Aliens show up when we look at them and not before.

P.S. If you answered the question about Bear in an earlier post my apologies.

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