City on an alien planet. Art by M. Mahin.
I will now give some reasons why Fermi's Paradox is not a valid argument against the abundant existence of intelligent life in our galaxy and other galaxies.
(1) Interstellar travel is in all likelihood very, very slow. While science fiction stories talk about spaceships whizzing around quickly through the galaxy, they are indulging in wishful thinking. Einstein's Special Theory of Relativity tells us that the speed of light is an absolute speed limit to the universe. The average distance between stars in our galaxy is roughly the same as the distance to the nearest star, about five light years. This means a starship could not travel from one star to another in less than about five years. In fact, there are practical engineering reasons for suspecting that it would typically take decades to travel from one star to the nearest star. One of these reasons has to do with the difficulty of accumulating enough fuel to last for an entire interstellar voyage, and another reason pertains to the need to start decelerating long, long before reaching a star you are traveling to, to avoid overshooting it. There is currently no evidence that anything like space warps can be used for quick interstellar travel (although one cannot rule out such a possibility). The slowness of interstellar travel means that even if civilizations arose long ago in many different places in our galaxy, it is not at all evident that spaceships from such civilizations (or colonies arising from them) would have reached our planet by now.
(2) If extraterrestrials had previously reached Earth, there is a large chance they would have been unable to colonize it because it was biologically or physically unsuitable for them. The “if they existed, they would be here already” argument is undermined by the fact that there is a large chance that colonizers from another planet might come to a planet like Earth and decide not to colonize it (or fail at colonizing it) because it wasn't what they were hoping to find. The arriving extraterrestrials might have been spindly thin-legged organisms adapted to live on a planet with a gravity much lower than ours. Or they might have been organisms used to living on a planet with a gravity much greater than ours. Or they might have been organisms adapted to live on a planet much hotter or much colder than ours. Or they might have required a planet with a different composition of atmospheric gases than our planet has. Or there could have been some biochemical reason or biological reason why they couldn't live here. Perhaps their chemistry wasn't compatible with our soil, or perhaps common microbes on our planet were lethal to them. There are any number of reasons why a colonization mission from another planet to Earth might have failed without establishing a long-lasting colony, and there are also any number of reasons why colonizers from another planet might choose to head for another planet after reaching here and investigating Earth's conditions.
(3) The current non-existence of an extraterrestrial colony on Earth does not rule out that there might have been such a colony in the past, possibly existing for a very long time. We have no experience at all with any civilization lasting longer than a few thousand years. Nothing that we know about the development of advanced technology should convince us that if an extraterrestrial colony had been established it would be able to survive for millions of years. We also know that Earth is a geologically dynamic planet, and that things that existed on the planet long ago became buried or destroyed by geological processes. For all we know an extraterrestrial colony could have been created in the past, and existed for a long time. It could then have passed away and been covered up by geological processes, like those that buried so many layers under the Grand Canyon. So we cannot at all say that we have never been colonized in the past.
(4) Extraterrestrial civilizations would be more likely to concentrate on using radio and television signals to find out about other planets, rather than the infinitely more difficult route of interstellar travel. Imagine you are in charge of an advanced extraterrestrial civilization. You can probably get all kinds of information about other planets (probably including images and video) if you build yourself a really good set of radio antennae vastly better than anything we now have. Or you can build starships to try and get the same information. The first technique is 10,000 times cheaper and may give you results this year. The second technique is infinitely more expensive, and probably won't give you knowledge until many decades in the future (given the slowness of interstellar travel). Which are you more likely to concentrate on? Presumably it would be radio communication. This may help to explain why we don't get many interstellar visitors, and may not have gotten any.
(5) Super-intelligent extraterrestrials might not have any of the “lust for control” that might fuel a drive for interstellar colonization. We have no idea what type of mental attitudes extraterrestrials would have. Human beings have repeatedly shown a kind of “lust for control” that may drive imperialistic expansion. We may want to get as many countries as possible under our control, and we may get a thrill out of planting our flag on places such as the moon. But to super-intelligent extraterrestrials vastly smarter than us, such impulses may seem utterly childish. For every extraterrestrial civilization that sends out colony ships in all directions, there may be 100 that do not, because they see little benefit in creating distant outposts, given the slowness of interstellar communication. We may be projecting on our superiors a “lust for control” drive of our own they are unlikely to have (just as a 2-year old boy might try to imagine what his father does at work, and imagines him playing with stuffed animals all day, just like the 2-year old boy).
(6) As interstellar colonies could not really be controlled from another star (and can only be communicated with after a delay of many years), other planets would have little incentive to create colonies outside of their solar system. According to the laws of physics as we currently understand them, traveling faster than the speed of light is impossible. So the idea of a galactic emperor issuing commands that are quickly obeyed over many light years is not realistic. Once a colonization mission left for some other star, it would be the master of its destiny, and it would be beyond the practical control of the planet that launched it. So what incentive would that planet have for creating such colonies? Would it be worthwhile for the sake of getting transmissions from a colony established on a distant world? But it would be much cheaper just to build radio antennae and listen to radio signals from all over the galaxy.
(7) Colonization missions by extraterrestrial civilizations could commonly be wiped out when they encounter defensive technology. Those who use Fermi's Paradox to argue that man is unique commonly speak as if some extraterrestrial civilization could send colony ships out and colonize countless other planets without difficulty, like a mailman dumping letters in a hundred mail boxes. But it could be that many extraterrestrial civilizations employ defensive technology in their solar systems, designed to destroy any incoming visiting spacecraft capable of colonization. If that happens, then any attempt by one planet to colonize many other planets would be a risky undertaking, facing a much larger chance of eventual failure before it gets very far.
(8) We might currently be receiving visitors from other planets. I refer here to the possibility that one or more UFO sightings here on Earth may actually be caused by extraterrestrial spacecraft. Now, of course, one can argue that there is no conclusive proof for that; and I would agree. But the mere fact that there are frequent UFO sightings (many of them strange and hard to explain) weakens any argument (based on Fermi's Paradox) that we are alone in the galaxy. It means the first premise of such an argument (the premise that “they are not here”) is not even beyond dispute.
(9) We may already have received one radio signal from an extraterrestrial civilization. I refer here to the famous “Wow signal” of 1977. See http://news.discovery.com/space/alien-life-exoplanets/the-wow-signal-130524.htm for a good recent explanation. The signal was exactly the type of signal astronomers expected to get from an extraterrestrial transmission. The fact that we are not sure the signal was artificial does not change the fact that it weakens the first premise of any argument (based on Fermi's Paradox) saying “since we have no sign of them, they don't exist.”
(10) Extraterrestrials may communicate through some advanced technology more advanced than radio waves. Other than the possible success listed above (the “Wow signal”), our attempts to pick up radio signals from extraterrestrials have not succeeded. But that may be because extraterrestrials commonly communicate through some technique more advanced than radio transmissions. There are all kinds of possibilities including laser communication and “subspace” communication. So our failure to pick up signals from them may simply mean we're not trying the right technique.
(11) Since intergalactic travel is presumably impossible, we have no reason for not assuming that most spiral galaxies have at least one civilization. Our galaxy of more than 100 billion stars is a spiral galaxy, and there are billions of spiral galaxies. But the distance between spiral galaxies is about 100,000 times greater than the incredibly vast distance between stars. The distance between galaxies is so great that we should assume intergalactic travel is basically impossible. Traveling between two spiral galaxies would require a journey on the order of a million years, making it unlikely that any civilization (no matter how advanced) could achieve it. Therefore any type of “if they existed, they would be here already” reasoning totally falls apart when we consider civilizations in other galaxies. No, no matter how old they might be, if they existed in another galaxy they would not at all be here, no matter how eager they might be to get here. There is no reason to think that any civilization, no matter how old, could achieve a million year journey between galaxies.
(12) The extraterrestrials may not be here because they have so many other places to go (real or virtual), and so many other things to do. Let us imagine a colony of ants in a forest using reasoning similar to arguments based on Fermi's Paradox. They might argue like this: “If there existed any beings in the world greater than we ants, they would be here already. They would be here at our anthill, trying to control or investigate us. But we see no such creatures. Therefore we ants are the most advanced creatures in the world.” The argument is a fallacy because we humans exist, but we don't go into the forest and peer into the anthill of these ants, because we have a vast number of other more important places to go to, and have a vast number of other more important activities to busy ourselves with. The same thing may be true of extraterrestrials: they may have limitless virtual computer-generated worlds to play in, and a billion places in our galaxy to travel to (not to mention billions of unimaginable and unfathomable activities far more worthy of their time than colonizing our planet). Their non-presence here now proves nothing.
These reasons are enough to rebut the most general form of an argument for man's uniqueness based on Fermi's Paradox. There is a very specific (and more elaborate) version of such an argument, built around the possibility of self-reproducing robotic von Neumann probes spreading exponentially throughout the galaxy. I will rebut that argument in a later post to this blog. My rebuttal will be based on reasons such as these: (1) self-replicating von Neumann probes are an example of runaway, cancerous technology that we would not expect extraterrestrials more intelligent than us to build; (2) if such technology had entered the solar system in the past, it is as likely as not that it would not have left any signs we could now detect; (3) rather than letting such planet-altering probes run amok uninhibited throughout the galaxy, civilizations on other planet would be expected to release counter-technology designed to find and destroy such self-reproducing probes.