is one where an adverse outcome would annihilate Earth‐originating intelligent life or permanently and drastically curtail its potential for future development. (Nick Bostrum).
Astronomers have used observations of local stars to estimate the number of planets these stars have, and they find that planets are remarkably common. From this you can extrapolate
how many planets there must be in our galaxy. A new study has done just that, and the number they get is stunning: they calculate there may be a hundred billion planets in the Milky Way, with 17 billion of them the size of Earth! (Phil Plait.)
Plait acclaims this because he wants to meet alien civilizations. But will we? With respect to other alien civilizations, Enrico Fermi is supposed to have asked, “Where is everyone?” Even one spacefaring civilization could colonize the entire galaxy in a few million years and the Milky Way is more than 10 billion years old. So, given that interstellar distances are short in the context of galactic time and that life has a non-zero probability of evolving it is surprising that we have no evidence of the existence of alien civilizations. It follows that the probability that a planet evolves a spacefaring civilization appears to be extraordinarily small. And so the more planets there are in the galaxy, the smaller the probability must be that life successfully evolves to spacefaring culture.
Of course, there are many plausible objections to this reasoning, including some who dismiss the Fermi paradox’s premise that the absence of evidence for extraterrestial civilizations is evidence for their absence.
But I am not willing to dismiss the Fermi problem. Others who take it seriously include Nick Bostrum and Robin Hanson, who infer that somewhere on the evolutionary path between bare rock and spacefaring, there must be one or more ‘Great Filters’.
The Great Filter can be thought of as a probability barrier. It consists of exist one of more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth‐like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology. 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. The Great Filter must therefore be powerful enough— which is to say, the critical steps must be improbable enough—that even with many billions 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.
To stop evolution, such a Filter must be “a terminal global cataclysm: an existential catastrophe.” Assuming these filters exist, are we fortunate enough to be past them, or are there still one or more in front of us?
Well, what would a Great Filter look like? It has to be something that a society on the path to a highly technological culture must encounter and that contains a serious risk that the society will destroy itself. Here is one candidate.
- Perhaps technological development sufficient for spacefaring must occur in the context of exponential economic growth.
- Exponential economic growth, while a glorious thing, will at least initially include exponential increases in the production of energy.
- And exponential increases in the production of energy are not sustainable in and pose an existential risk to a closed planetary environment. The risk occurs either because of accumulated greenhouse biproducts of energy production or simply because the heat released through energy consumption cannot be adequately dissipated.
This isn’t (quite) an argument for despair. Keep in mind that a Great Filter can be leaky, that is, it does not need to kill everything that enters it. The energy filter can be leaky and still contribute to the Fermi Paradox because several leaky filters are as dangerous as one nearly inescapable Great Filter. Perhaps a few of the civilizations that reach our level of technology find a way to guide their future development such that total energy production asymptotes while technology and well-being continues to improve.
We see these energy risks ahead of us; the Fermi paradox gives us yet more reason to give them serious heed.