With 200 billion trillion stars (ish) stars in the universe and the 13.7 billion years which have elapsed since it all began, you might be wondering where all the alien civilizations are at.
This is the basic question of the Fermi paradox, the tension between our suspicions of the potential for life in the universe (given planets found in habitable zones, etc) and the fact that we have only found one planet with an intelligent (ish) species inhabiting it.
One solution, or at least a way of thinking about the problem, is known as the Great Filter. Proposed by Robin Hanson of the Future of Humanity Institute at Oxford University, the argument goes that given the lack of observed technologically advanced alien civilizations, there must be a great barrier to the development of life or civilization that prevents them from getting to a stage where they are making big, detectable impacts on their environment, that we can witness from Earth.
"You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe," Nick Bostrom, also of the Future of Humanity Institute, explains. "The Great Filter must therefore be powerful enough – which is to say, the critical steps must be improbable enough – that even with many billions of rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods."
Hanson proposed stages that life may have to get through (or conditions that need to be satisfied for life to occur and continue) in order to get to where we are now and beyond. These were:
- The right star system (including organics)
- Reproductive something (e.g. RNA)
- Simple (prokaryotic) single-cell life
- Complex (archaeatic & eukaryotic) single-cell life
- Sexual reproduction
- Multi-cell life
- Tool-using animals with big brains
- Where we are now
- Colonization explosion
"The Great Silence implies that one or more of these steps are very improbable; there is a 'Great Filter' along the path between simple dead stuff and explosive life," Hanson wrote in the original paper. "The vast vast majority of stuff that starts along this path never makes it. In fact, so far nothing among the billion trillion stars in our whole past universe has made it all the way along this path."
Searching the galaxy for star systems amenable to life, stars for planets, and planets for biospheres and technosignatures could tell us more about where the Great Filter lies. Could it be that the conditions for even simple life are rare (which doesn't seem likely given organic compounds found in asteroids, for example) or that civilizations hit different barriers later on, such as an oxygen bottleneck preventing intelligent species from ever leaving the Stone Age? Or is the Great Filter somewhere between where we are now, and colonizing civilizations? If that is the case, it means the filter (most likely our own extinction, unless there is another reason why colonizing civilizations remain quiet to us) lies in our future.
The good news is that finding life on other planets could tell us about where we are in relation to the Great Filter, or our own extinction.
"Searching for technosignatures alongside biosignatures would provide important knowledge about the future of our civilization. If planets with technosignatures are abundant, then we can increase our confidence that the hardest step in planetary evolution – the Great Filter – is probably in our past," as one paper explains. "But if we find that life is commonplace while technosignatures are absent, then this would increase the likelihood that the Great Filter awaits to challenge us in the future."
It could be that common threats, such as asteroids, wipe out civilizations before they have a chance to begin colonizing their galaxies, or that at some point in a technological species' development, they inevitably learn about some technology (such as nuclear weapons, or another concept we have not discovered yet), or that some other threat lurks out there for species that are technologically advanced and unwise enough to broadcast that fact. One way or the other, we guess we'll find out.
The original paper on the Great Filter "The Great Filter - Are We Almost Past It?" is available on the George Mason University website.