Nick Land recently pointed to Robin Hanson’s 1998 essay, The Great Filter, which looks at possible explanations to Fermi’s Paradox, by looking at critical steps necessary to galactic colonization and asking where the greatest barrier to further progression is. Land sees this implying that many civilizations are exterminated Shortly thereafter, Jim posted a response that the Great Filter lies not before us, but behind us.
While Jim might be right that intelligent life of similar caliber to humans is rare in the galaxy, he starts his post off with an assumption that seems to me to be unwarranted:
There seem to be no great obstacles to intelligent life devouring the galaxy.
In fact, obstacles are legion in a project of interstellar colonization. The first of these is the sheer distance between stars. The nearest star, Proxima Centauri, is over 2,000 times further away than the Voyager 1 spacecraft, which, launched in 1977, is the furthest manmade object from Earth. To send a living crew that distance, the spacecraft would require speeds at least hundreds of times that of the voyager probes, and the amount of energy required rises with the square of the speed, so hundreds of times the speed requires myriads of times the energy
Second, not all stars are going to have habitable way stations for a prospectively interstellar civilizations to colonize and develop before sending out more colonists to the next waypoint. Proxima Centauri is again a good example. As a red dwarf flare star whose stellar flares make any potentially habitable planets that might orbit it less accommodating and possibly entirely useless for the purpose of installing a colonial civilization capable of sending future generations of colonists to worlds further out. This challenge both effectively increases both the necessary distance traveled per voyage between stars and the cost of the measures put in place to allow colonists to settle an alien world.
Earth has countless amenities that we don’t even notice by virtue of our having evolved in response to Earth’s environment. The amount of oxygen in the atmosphere closely matches what is ideal for us because we are sculpted around such parameters, not vice versa. As such, the likelihood of finding a planet with the right oxygen levels to support human life outside of a sealed environment with the gas balance controlled by humans is minimal and the probability only goes down when other parameters like temperature or surface pressure are added. This adds an additional challenge, because it limits the possible development of any potential daughter civilization, as it will likely be confined to an environment sealed off from the original atmosphere of the planet it inhabits. Since such civilizations serve as launching pads for a second wave of colonization to stars further on in any scenario by which a civilization successfully conquers an entire galaxy, this also introduces a barrier beyond the initial wave of colonization.
The third and most important problem is motivation. Humans have done a remarkable job of proliferating around the planet and we have the ability to venture into space, though we haven’t yet sent ourselves out of Earth’s gravity well. However, contrary to Robin Hanson’s suggestions, our ability to colonize and proliferate on Earth combined with our ability to reach beyond the Earth’s atmosphere does not imply that we will actively pursue a voyage to the stars. We have a fundamental difference in the magnitude of the challenges that face humans colonizing Earth versus colonizing a planet around another star, and ultimately, because it requires a capital investment that dwarfs that of any human project past or present with little to no hope of a return on investment for those who stay on Earth.
While it is true as Robin Hanson suggests, that natural selection rewards those who break with the established strategy in ways that exploit previously untapped resources, the means by which it does so tend not to involve an actual conscious desire to proliferate. Instead, strategies developed through natural selection involve the confluence of many instincts. Instincts, which have if anything, proven themselves to be thwarted by the mix of technology an intelligence, as the use of contraception shows. Yes, it’s possible that a select few individuals could muster the necessary resources to back a project to send Earthly life to a planet orbiting another star, but the chance of success combined with the cost and sheer difficulty of such a mission makes it unlikely that such individuals will arise.
Ultimately, the barriers to interstellar travel do represent a great filter preventing the colonization of the galaxy by an intelligent civilization. Whether they are the Great Factor is difficult to judge. However, it should be noted that as exterminators go, the lack of will to send colonists to other stars is one of the slowest-acting civilization killers in existence, and probably less consequential than more proximate causes.
A Wandering Learner 
This is a really good article. The ramifications of Fermi’s Paradox aren’t taken seriously enough by most thinkers. Clearly there must be major obstacles to spreading across the galaxy, and I agree with you that they lie ahead of us.
On the other hand, they may lie behind us, too. What if Babylon had kept going; is there any reason they couldn’t have developed spaceflight a thousand years ago? Civilizations may mobilize incredible resources, but they are inherently prone to dysfunction and collapse, and I think we can already see cracks forming in our own:
http://awesomescience.us/the-singularity-part-iii-or-all-is-not-well-in-the-world
I’m also not convinced that the physical distances you focus on are of exceptional importance. Yes, the distances are great. But given artificial environments of sufficient quality, many, many human lifetimes can become available for interstellar journeys of enormous distances. The problems of escaping and re-entering planetary gravity wells remain, of course, but we know that they can be dealt with, because we’ve already dealt with them fifty years ago. So I think your discussion on motivation is where the real issue lies.
What would be especially helpful would be a series of intermediate steps, like breadcrumbs to follow upwards. There may be economic incentives to developing the technology for space habitats already; if people ever want to start living at the poles or underwater, they’ll get plenty of practice with biodomes. They may even get good enough with them to survive, sealed up, for generations. But what incentive is there to get to the moon? Mars? Pluto? Alpha Centauri? Physical barriers can’t be considered by themselves; they need to be compared to the incentives for surmounting them.
William Bainbridge had some really interesting ideas on religion providing those incentives. His “Religions for a Galactic Civilization” may be dated (and time has not been kind to his hopes for Scientology), but I still find him to be thought provoking:
http://mysite.verizon.net/wsbainbridge/dl/relgal.htm
Good article RA. I read it some time ago and found it thought provoking. Was reminded of it tonight when watching the latest episode of Prof Brian Cox’s Human Universe on BBC. He discussed the Fermi Paradox (without naming it as such).
I’ve had a similar journey to you in some respects (faith!), but find it fascinating that one possible solution to the paradox is that Earth is not as mundane as has been assumed but really is more special than we have imagined.
Shared the link to your post on Twitter, in case you wonder why your stats have a sudden spike on this one!
Cheers
The question of life on other planets is pointless to even consider,the scope of human knowledge we have at our disposal when evaluating questions like this is our own tiny slice of reality in planet earth. Cognition without empirical data every step along the way leads to pure gibberish and can only rely on projections of existing knowledge. Humans not so long ago assumed other human civilizations existed on other planets they saw in the solar system and with their limited knowledge,with their tiny paradigm that does seem the most rational idea. And I have a feeling if the nature of this thing we occupy is ever understood these musings will prove equally as stupid.
So we have to come to peace with the fact that’s there’s some things we simply have no evidence or data on and that we can’t even comprehend or understand and that for now or maybe forever we cannot know.