Fermi’s Paradox is often used to argue against the existence of intelligent life elsewhere in the galaxy by stating “If they’re out there, why can’t we detect any sign of them?” But how easy would it be to detect “them”? If a civilization existed on a planet 20 light-years away from Earth with exactly the same technological level as Earth today, would they be able to detect radio signals indicating intelligent life on Earth?
http://www.phys.unsw.edu.au/hsc/cosmic_contact.html
So a planet 60 light-years from earth would pick up our first radio broadcasts. The nearest star, Proxima Centauri, is 4.3 light-years away, for comparison.
Nope. We don’t have the technology to detect radio signals at the level of our current incidental broadcast radio and television emissions. However, we could detect an intentional transmission from something like an Arecibo-class radiotelescope, if it were aimed at us and we were listening at the right moment.
Actually, Fermi’s paradox doesn’t have much to do with SETI. Fermi’s paradox says that if there were advanced life out there, they should have colonized the galaxy by now, including our very habitable planet.
It’s based on the observation that civilization and technology advance so quickly, compared to the billions of years of evolution it took to get intelligent life capable of advanced technology, that the odds of us and someone else developing in parallel, before one of us could colonize the whole galaxy, is basically nil.
Along the lines of what bup said, there was an interesting article in SciAm a while ago, in which the authors attampted to show how a hypothetical advanced civilization, using only sub-light-speed propulsion, could colonize the Milky Way in a couple million years. This is approx. 0.02% the life-span of the galaxy itself. So, to put it in perspective, if the age of the Milky Way was one day, this advanced civilization would have colonized all inhabitable worlds in about 15-20 seconds. Obviously, there are a lot of assumptions built into this scenerio, but it gets at the crux of the Fermi Paradox: Unless we’re the first advanced civilization to evolve in the Galazy, it’s hard to explain why we wouldn’t have met up with the descendants of another advanced civilization by now, since in terms of galactic lifespans, it takes but a moment to spread throughout the entire galactic disk.
Actually, that’s only one of the solutions (we’re the first advanced civilization) to the paradox. There could also be a galactic quarantine on us underdeveloped critters (a la the prime directive), or it may be impossible for a civilization to survive its own coming of age (we tend to blow ourselves up).
[I think there are others, but I don’t recall them.]
I think your first alternative falls prey to the human analogy: Would we not make contact, if given the opportunity? I guess some folks think we’ll never have the wisdom to follow a Prime Directive. I wonder why.
As for your second alternative: Depressing, but all too likely.
Off the top of my head, it could be that we’re the first with our particular sort of biochemistry, or the first with an urge to exploration, or some combination, so the rest are uninterested in our planet. It could be that they have visited us, and we just don’t know it yet (shades of Chariots of the Gods, here, but it can’t be ruled out). It could be that they do visit here on a regular basis, but that regular basis is once every ten thousand years or so. It could be that once a civilization develops technologically enough that they can colonize the Galaxy, technology has also made them essentially independent of planets. It could be that an entire galactic empire did rise, but then fell a few billion years before we arrived on the scene (this could even serve to synchronize the development of post-Empire civilizations). There are a lot of possible explanations, but it does seem a bit odd.
Actually I think Fermi was speculating: if there’s other intelligent life in our galaxy, why aren’t they already here? As in physically, right here, in some form or another.
His argument was that a technological species should be able to populate, or at least explore with probes, the entire Milky Way galaxy within a “short” length of time, once they had gone a little past our level of technology. In this context, “short” means maybe a million years give or take — an enormous time-span in ordinary human experience of course, but hardly anything at all in astronomical time. If you ignore issues about the aliens’ economics, psychology, and possible motives for such a project, which are hard to know of course, it’s at least physically possible to visit every star system in the galaxy in that time frame. And the aliens could do it even assuming they’re constrained by the speed of light, which as far as we can tell, we all are.
Since there has been plenty of time for the Milky Way to churn out species of our level of intelligence and technology, and to have done so a billion years before our ancestors could even walk on two legs, Fermi asked the question, “Where are they?” As in, shouldn’t the aliens already be here by now? At least one species, if not a hundred of them, many times over? But since they aren’t here, and haven’t been here, that bodes ill for the prospect of aliens. They probably don’t exist. We are either alone in this galaxy, or at least functionally alone for all practical purposes.
(On preview I see that bup and Loopydude have beaten me to my points, and that Chronos has supplied most of the retorts I was going to offer. Ah well, better leave off here. But let me recommend Achenbach’s Captured by Aliens as a good book to read for anyone interested in this subject.)
They could even be here right now, living among us, in disguise! 
Oh well. I guess that would have occurred to a mind as brilliant as Fermi’s, and I don’t doubt he’d have a clever retort, along the lines of “Why bother?” When I first encountered Fermi’s paradox, I must admit my heart sank a little.
Or a civilization becomes more and more technological to the point where life basically no longers exists in our universe but in a sort-of online universe. That’s a bad description of the idea, but I’ve seen it in several science fiction short stories.
Suppose that as intelligence evolves, these speices become less and less interested in colonizing space…would these civilizations ever be detected? Take our own planet…we may well stop using AM radio very soon, and most of our TV now is delivered by fiber-optic cable. So our own emissions of RF information may well cease (to the point where they would be undetectable outside the solar system). We may also find that genetic research and improving our lives is more interestng than exploring outer space. All this talk about a “galactic civilization”…unless Einstein was wrong…I don’t see it happeneing. It would take centuries to get to the nearest inhabitable star-planet systems, and humans don’t live long enough to attempt this.
Of course, if we could perfect “coldsleep”…we might have something!
Perhaps the first we will know of alien civilisations is when they build a hyper-space bypass, blowing up Earth in the process…
Perhaps another is that advanced civilizations exist, and have the ability to spread themselves throughout the galaxy using sub-light speed methods, but don’t have the incentive to?
I know that even if one existed, I wouldn’t make any plans to step on an interstellar colony ship just so my great-great-great grandkids could land and terraform some distant planet. I like it here too much.
Other possibilities:
Individual planets do not typically contain sufficient resources to jump-start a galactic civilisation (or they do, but technological societies always deplete the resources too far before they attain the (doubtful)technologies for insterstellar travel)
SETI tells us that nobody within 5 light years (see map) is broadcasting TV or radio, nobody within 50 light years (map) is using military radar, and nobody in this galaxy ever swept the skies occasionally with a high frequency gigawatt beam (eg. Arecibo) just to see if anyone sent anything back. So, at 20 light years I believe they could detect military radar (but not TV or radio) having “structure”, and could certainly detect the deliberate high powered emissions (which we haven’t bothered with for some years, I believe).
Now, this does not necessarily mean that there is nobody there, but the alternatives are all unlikely for one reason or another:[ul][li]There might be a ruthlessly enforced galactic silence policy, and nobody uses electromagnetic signals any more. However, each such civilisation could still not prevent the leaks from their technological infancy, and it is difficult to believe that nobody, not even a malfunctioning piece of equipment or a mischievous joker, ever lets slip an electromagnetic signal detectable by the incredibly sensitive receptors on Earth.[]The aliens might have encased this planet in a “shield” impermeable to transmissions. Unfortunately, this would require such enormous resources and manipulation of spacetime that we might as well suppose that they are keeping us in the Matrix.[]Perfect compression is indistinguishable from noise (ie. a signal which has been coded for optimum efficiency simply sounds like a detuned radio). This is a genuine possibility. However, again, there must surely be some leaks from way back when the coding was imperfect, and it would seem imprudent to code eg. emergency beacons so that they couldn’t be identified.[/ul][/li]
So, it is still possible they are out there in this galaxy. However, there are two definite facts:[ul][]The technology required to emit a galaxy-wide audible signal is only as advanced as that required to make a nuclear weapon. [] We appear to be the only ones ever to have done so.[/ul] One wonders whether the two facts are related.
I think it’s a bit much to expect any civilization (including our own) to embark on an exploration project that would take millions of years to complete, given a lifespan of only about a hundred years. Who’s to say there’d be anyone waiting for the explorers when they came back? And if they’re not planning on coming back, then there’s all the less reason to send them in the first place. Does it really satisfy our desire to explore just to know that there’s some human who’s seen the other side of the galaxy up close – even if they never come back and tell us about it? When in human history has anyone ever financed a one-way mission of exploration? (Meaning one that was deliberately one-way, not just one that happened to work out like that.)
And as far as galactic civilizations, is there really that much incentive to expand beyond the point where we can even communicate with each other? If we extend ourselves more than 100 lightyears out, it would take a lifetime just to call home.
The most compelling reason to colonize is that a star won’t last forever. Would a civilization rather die violently than try to reach the next star?
However, that is one resolution of the Fermi paradox - interstellar travel is too close to impossible, or the emotional barriers make it just too sucky.
Another way around colonization, which I’ve mentioned before on these boards, is the idea of building our own star out of local brown dwarves rather than trekking off many light years in finding a pre-fab.
My guess is that if you’re 100 light-years out, Earth is only “home” in the theoretical sense. Heck, within two generations, tops, many children of immigrants may view “the old country” wistfully, but with not a huge amount of desire to visit or communicate with it.
Actually, picture a generational colony sent out at slower-than-light, with no expectation of ever returning to Earth or communicating regularly with it. Technological progress continues on Earth, faster-than-light travel is developed and new colony ships are built and sent forth. When the STL colonists arrive and emerge from their hibernation chambers, they find their utopia already overrun by McDonald’s and Wal-Mart, built by people for whom Earth is just a few hours away and thus with no desire to make anything radically new or different. That’d suck.
A couple points. Firstly, building interstellar craft may prove to be impossible in practise, even if you have the technology to do it. Not many aliens would want to board one if it only had a 0.001% chance of success. Secondly, once you get to another world, you might have to spend hundreds of millions of years terraforming it before you could live there. Even if you found a planet which had evolved complex life, you probably wouldn’t be able to eat any of the food there. We evolved on this planet, and our body chemistry is very sensitive to the presence of a whole range of chemicals, such as arsenic or lead. Basically, we can tolerate them in the abundance that we find them naturally on earth. A very similar planet with a slightly different composition would probably be poisionous to us.
Fermi’s paradox seems to assume that technology can overcome any of these barriers, and that an alien civilization would have access to effectively unlimited resources.