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Where Is Everybody?: Implications of the Fermi Paradox

March 20, 2021 by Benjamin Hollon
7 minute read


This week’s post is a research paper I wrote this week on aliens. That’s right, aliens. My research papers tend to have a flair of the ridiculous (I’ve heard that my old teacher still talks about my paper on Religion in Space; maybe I should publish that one soon), and this one is no exception.


For thousands of years, people have looked at the sky and wondered if they were alone in the universe. The Fermi Paradox, named after Italian-American physicist Enrico Fermi and developed by Michael Hart, famously asks why humanity has not seen extraterrestrial civilizations if life is common in the universe. When it was first published, the general answer to the theory was that life was not common, eliminating the paradox, but recent discoveries have shown that the galaxy is full of planets that could potentially support life. If humanity were to discover such life, though, it would probably mean the eventual doom of the human race, regardless of the form that life takes or its intentions toward humans.

The whole debate centers around the Drake Equation, which attempts to estimate the frequency of intelligent life in our galaxy. Grossly simplified, the Drake Equation can be written as the product of *Npq *where N is the number of planets in the galaxy that can support life, *p *is the chance that––on a planet that can support life––a technological civilization will arise, and *q *is the probability that a technological civilization will survive in a way in which it is observable to other galactic species for a significant amount of time. This product gives the number of civilizations humanity can reasonably expect to observe in our galaxy. As no extraterrestrial life has been observed, this argument assumes “prima facie” that there is none to observe. In this case, either or q is extremely low; since the choice is between life being unlikely to survive or life that has arisen being unlikely to survive to make an observable difference in the universe, humanity, as a civilization that wants to make its mark on the galaxy, should hope that p is low rather than q (Aldous). If life is so uncommon that there are no other observable civilizations in the galaxy, humanity’s own existence should be called into question as so unlikely that it should not have come to be, creating a sort of reverse Fermi Paradox: humanity no longer wonders why alien life does not exist but why human life does. As only one civilization, humanity, is known, the sample size is too low to make any reasonable predictions about the frequency of life and that avenue of exploration is, for now, a dead end. On the other hand, uncommon life being the best-case scenario does not mean that life is uncommon. To explore the possibility of high creation rates but low survival rates, Robin Hanson came up with the Great Filter theory.

The Great Filter theory is perhaps the strongest argument behind the idea that intelligent life tends to destroy itself. It assumes that life, on its journey toward intelligence, meets multiple barriers, or filters, which block most species from continuing. “The Great Silence [i.e. the Fermi Paradox] 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. 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” (Hanson). In essence, this theory purports that one step (the “Great Filter”) is so improbable to pass that practically every lifeform that meets it fails and dies out. The question then becomes this: if there is an evolutionary point past which it is almost impossible to survive, is that point ahead or behind humanity? If it is behind, humans have the best-case scenario implied by the Drake Equation; human civilization is alone in the universe and free to expand and dominate it. But given that life (namely, humanity) has reached this point, there is a non-zero chance that the Great Filter is ahead of humans. In this case, the universe would be full of life, but all of that life destroys itself before becoming observable at a galactic scale. Humanity would likely destroy itself within the next couple of centuries, as it is extremely near the delimiter of galactic communication which the Great Filter prevents reaching. This would be a worst-case scenario for humanity. If it discovers life on another planet (assuming that life is not capable of the galactic communication the Great Filter prevents), it becomes more likely that the Great Filter is ahead; the more advanced the discovered life, the higher the likelihood. Again the conclusion is that humanity should hope there is no other life in the universe.

The alternate possibility is that life is, in fact, common but that it does not choose to communicate. This would bypass the Great Filter theory, as the limitation is choice to communicate, not ability. If intelligent life is common, though, why does it choose to remain silent? Should humanity cease its efforts to communicate? According to the Dark Forest theory, popularized by Liu Cixin’s novel, The Dark Forest, the situation is grim:

[The] Universe is a dark forest. Every civilization is a hunter with gun in hand and he sneaks in the forest. He must be careful enough as there are other hunters in the forest. If he discovered other lives, he can only do one thing: shoot it. In this forest, other lives are hell and constant threats. Any life that will expose his existence will be killed soon. This is the picture of universe civilizations. (qtd. in Yu)

Per the Dark Forest theory, rather than intelligent life tending to destroy itself, intelligent life tends to destroy other life. It is a sort of natural selection on a galactic scale: any civilization that is not willing to destroy competition will itself be destroyed. Even if most galactic civilizations are kind-hearted, they have no way of knowing that other civilizations are also benevolent, and they cannot communicate to ask without risking giving away their position. Even if the other civilization is at a far lower technological level, it could have a technological explosion at any time and jump ahead (potentially triggered by the contact) making it a liability. As Chao Yu points out, however, a low-level civilization would likely be monitored but not molested, as destroying it could give away the existence of the destroyer to a higher-level civilization. If this theory is true, the galaxy is full of species that are hiding from each other, terrified of giving their positions away to higher-level civilizations. And then there is Earth, oblivious to it all, sending out messages into the dark forest full of hunters asking if anyone will be friends.

Perhaps, though, the situation is not so grim. Most modern SETI (Search for Extraterrestrial Intelligence) efforts are searching for radio waves, which would likely be the first and most obvious long-distance communication method available to any extraterrestrial civilization. But advanced civilizations may outgrow radio transmissions. Carl Sagan describes this as placing humans in a situation analogous to “that of isolated societies in the Amazon basin … who lack the tools to detect the powerful international radio and television traffic which is all around them” (Sagan). Radio leakage from a civilization is also likely to go down over time, only leaving intentional transmissions, which are highly unlikely to be aimed at Earth. “Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions” (Shostak). If aliens exist and are transmitting, but using a method humanity is oblivious to, that would solve the problems posed by the Drake equation, Great Filter, and Dark Forest theory. If civilizations simply tend to stop transmitting after a short period of time, though, that would be high evidence that the Dark Forest theory might be true, as going radio silent could be a sign of discovering the Dark Forest nature of the universe and going into hiding.

The fundamental problem with solving the Fermi Paradox is that solutions have to extrapolate a line (the behavior of all civilizations) from a single data point (human behavior). Extreme amounts of speculation are involved and most theories are as good as any others. But the value can be measured by how little information is left to chance, as theories that work for an entire class of civilization are more likely to be accurate than ones that are correct only in an edge case. The Great Filter and Dark Forest fit this criterion, as each one can be applied to any civilization in a broad range; the Great Filter theory applies to all forms of life that evolve (not necessarily through Darwinian evolution) from basic to complex organisms and the Dark Forest theory purports to cover all civilizations able to communicate with others. The theory that civilizations just stop using radio, while backed up by humanity’s history, is largely speculation and cannot be universally applied, even if it is correct in some cases. But the important thing is to recognize that however strong theories are qualified, they are still only theories. Whether aliens mean the doom of humanity or not, it can still be fun to look up and wonder, like Fermi did, “Where is everybody?”

Works Cited

Aldous, DJ. “The Great Filter, Branching Histories and Unlikely Events.” UC Berkeley, 01 June 2012, https://escholarship.org/content/qt74g525bp/qt74g525bp.pdf. Accessed 18 March 2021.

Hanson, Robin. “The Great Filter - Are We Almost Past It?.” George Mason University, 15 September 1998, https://mason.gmu.edu/~rhanson/greatfilter.html. Accessed 18 March 2021.

Sagan, Carl. “The Quest for Extraterrestrial Intelligence.” Cosmic Search Magazine, May 1978, http://www.bigear.org/vol1no2/sagan.htm. Accessed 18 March 2021.

Shostak, Seth. “The Future of SETI.” Sky and Telescope, 19 July 2006, https://skyandtelescope.org/astronomy-news/the-future-of-seti/. Accessed 18 March 2021.

Yu, Chao. “The Dark Forest Rule: One Solution to the Fermi Paradox.” Journal of the British Interplanetary Society, May 2015, https://www.researchgate.net/publication/283986931_The_Dark_Forest_Rule_One_Solution_to_the_Fermi_Paradox. Accessed 18 March 2021.


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