The Fermi paradox: what if it is not a paradox?

According to the Fermi paradox, if intelligent extraterrestrial life is common in the universe and space travel is easy, why haven’t we been visited or invaded yet? Suggested solutions include that extraterrestrial civilizations are deliberately avoiding us (the zoo hypothesis) or that most extraterrestrial civilizations wipe themselves out before reaching technological maturity where they become detectable to astronomers (brief window hypothesis). Another recent hypothesis is that civilizations do not make themselves known because they know better (the dark forest hypothesis). I would argue that most of these explanations to the Fermi paradox rely on unfounded assumptions and that the most most parsimonious (requires the least unfounded assumptions) solution to the Fermi paradox is either we have not been searching long enough or that extraterrestrial civilizations that can communicate over interstellar distances don’t exist. Since I find the idea that they don’t exist unlikely, I would argue that advanced extraterrestrial civilizations are likely to be out there but we have not found them yet.

The Fermi Paradox: introduction and classic solutions

In 1950, the Italian physicist Enrico Fermi was having a conversation with colleagues over lunch at Los Alamos National Laboratory. They had talked about aliens and UFOs and Fermi brought up the question after the conversation had moved on to other topics. If it is easy for other civilizations to travel across interstellar distances, why haven’t we been visited yet? The exact way he put it was allegedly, “then, where is everybody?“ This is the origin of the famous Fermi paradox which has framed much of the discussion about the search for extraterrestrial intelligence.

Where is Everybody? From the JWST deep field. Image credit: NASA/STScI

Classical explanations for the Fermi Paradox include the galactic zoo hypothesis, in which extraterrestrials are intentionally avoiding us. This could be because they are waiting until we reach civilizational maturity or alternatively because we are especially dangerous. Another popular classic explanation is that all civilizations reach a point in their technological development that they are likely to destroy themselves before they can communicate across interstellar distances (the brief window hypothesis). This reflects the experience of living through the 20th and early 21st centuries where human technology has created many threats which have the potential to end civilization as we know it, whether nuclear war, environmental destruction, catastrophic climate change, or more recently the threat of misaligned artificial intelligence.

Dark forests, “grabby aliens,” and great filters

More recent explanations to the Fermi paradox include The dark forest hypothesis, popularized by the novel by Cixin Liu of the same name. In this scenario, we don’t hear from other civilizations because they know better to avoid attracting the attention of civilizations that might take an interest in destroying them.

Another scenario, popularized by Robin Hanson, is the grabby aliens scenario, which argues that since an expansionist civilization could colonize the galaxy in a few million years, a fraction the lifespan of the universe, either these expansionist civilizations have not emerged yet, meaning we are likely the first, or alternatively these civilizations are very far away. Robin Hanson has also popularized the use of the term great filter as an alternative framing to Fermi paradox. The great filter of civilization is the phase or event civilizations must pass through in order to become advanced technological civilizations that survive long enough to be detectable to another civilization like ours.

The sustainability solution to the Fermi Paradox

Recent experiences with the expansion of human civilization reaching planetary limits in terms of consumption of natural resources and the capacity of the planetary system to absorb pollution and remain sustainable as led to the proposal of another solution to the Fermi paradox: the sustainability solution. The sustainability solution proposes that civilizations that last long enough to exist over geologic timescales will learn to live within planetary limits so that their presence is not disruptive to their planetary systems. This is significant from a technosignatures research perspective since it means that there may be a limited time window during which technologically advanced civilizations will be visible since the technosphere may appear to fold back into the biosphere at a certain point in terms of detection. In other words, a very advanced technosphere may disrupt the natural planetary system so little that it looks indistinguishable from just a planetary biosphere from the perspective of astronomers with our observation technology in a nearby star system.

Such civilizations in this scenario could still have a presence across the galaxy, but it may be in the form of a network of interstellar probes that exist for the sake of exploration rather than a galactic empire.

What are the implications of the possibility that the most advanced civilizations generally learn to exist in a way that doesn’t disrupt their home planet and mostly develop interstellar capabilities to explore the galaxy as implied in the sustainability solution? This implies a universe that, while possibly less exciting, is safe and benign compared to the one imagined in, say the dark forest scenario or the grabby aliens scenario.

Problems with assumptions in proposed solutions to the Fermi Paradox

Many arguments have been made for one solution to the Fermi paradox over another solution, but all these proposed scenarios appear to share the same problematic assumptions. One assumption that has been pointed out by a recent paper is that technologies and strategies employed by civilizations in Fermi paradox scenarios, such as building Dyson spheres or colonization of the whole galaxy, are technically feasible or practical. Another assumption is that sufficiently advanced civilizations will inevitably expand across the universe, forming interstellar empires. This latter assumption is called the Inevitable Expansion Fallacy by the astrobiologist David Grinspoon.

A relatively realistic Dyson swarm since a soild sphere would physically unsustainable. Image credit: Gemini.

For example, Dyson spheres may turn out to be theoretically possible but practically unfeasible due to the maintenance or energy requirements. A civilization might just decide it is not worth it. A galactic colonization program may also prove unfeasible because of long travel times between stars and the difficulty in maintaining an empire over long distances and timescales. Also the economic model of unlimited growth is already becoming obsolete as a strategy on our own sub-planetary civilization as we are hitting up against planetary limits. Does it make sense to assume that it will hold for interstellar civilizations that exist over the geological timescales required to embark on galactic colonization?

Since the brief window and sustainability scenarios do not make assumptions about a civilization being able to use technologies like astroengineering and interstellar travel, they at first seem more promising. Indeed, scenarios where most civilizations either wipe themselves out before becoming visible to SETI researchers or become indistinguishable from planetary biospheres over time is overall more consistent with historical experience. Nonetheless, such scenarios assume that the technology of hypothetical extraterrestrial civilizations is not significantly more advanced than our own.

This is a reasonable assumption, but one might push back in light of recent advances in artificial intelligence as well as the general tendency for science fiction to become science fact over time (there is an early 20th century children’s dictionary claiming that man would never go to the Moon for example). It is possible that this skepticism of colonization of the galaxy or astroengineering is ultimately short-sighted or a failure of imagination.

I believe that the solution to the Fermi paradox that requires the least amount of unfounded assumptions is either that we have not been looking long enough or that such civilizations don’t exist, at least not as described. I find the answer that these civilizations don’t exist to be unsatisfying since there isn’t a reason based on our current understanding of the history of life to believe that the emergence of life and intelligence is an extremely improbable event. Even if life only forms on one planet per a couple of galaxies, that still leaves you with billions of planets where life could form. Also, once multi-cellular life emerges, there doesn’t appear to be anything improbable about intelligent life evolving as a result.

Therefore my solution to the Fermi paradox is that they are likely to be out there but that we haven’t been looking long enough. After all, the scientific search for intelligent life beyond Earth has only been ongoing for about 70 years and our visibility bubble is only about 100 light years in radius, based on when the first radio waves left Earth. If we have not been searching long enough that means that the way to answer the question is to continue the search until we either have surveyed a significant enough fraction of the universe to safely conclude these civilizations don’t exist or until we find one. If we do find an extraterrestrial civilization, let us hope that it is more like the civilizations from the sustainability scenario than those in the dark forest scenario.

The real solution to the Fermi paradox: we were an alien kid's science experiment all along. Image credit: Gemini

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