Are only the most short-lived advanced extraterrestrial civilizations detectable?

The Fermi paradox dates back to the beginning of the search for extraterrestrial intelligence (SETI). Its asks to the question, “if there are intelligent civilizations that are traveling the galaxy, why haven’ we found them yet?” Many proposed solutions are plausible but hard to test scientifically, such as the dark forest hypothesis and the galaxy zoo hypothesis, both of which assume the existence of advanced indetectable aliens which are intentionally hiding themselves from humans.

Advances in environmental economics and the environmental humanities suggest a solution rooted in the long-term sustainability of continuous growth. In this case, advanced extraterrestrial intelligences (ETIs) have not been detected because current searches are for signs of civilizations focused on rapid economic growth from industrialism and colonialism, which may not be sustainable over timescales that allow for a civilization to likely be detected (>10,000 years).

This poses a problem for the search for ETI since a civilization that does not disrupt nature would be much harder to detect If a planet with a technologically advanced civilization starts to just look like a planet with a biosphere after a short window of time. This would significantly reduce the time during which a civilization is detectable over interstellar distances.

On the other hand, what looks like pollution from our vantage point may actually be a civilization intentionally disrupting planetary processes in a way that maintains long-term planetary habitability and the long-term survival of a civilization. In this way, long-lived civilizations that have learned to have a healthy relationship with their planet may still be detectable based on the same technosignatures used to detect civilizations still learning how to live sustainably on their home planet. This also provides concrete ideas of what a long-lived technologically advanced civilization may look like on Earth.

Calculations have been done which suggest that a technologically advanced civilization simply going from star to star, at speeds significantly less than the speed of light, could colonize the entire galaxy in just a few million years. While this is a long time from a human history perspective, it is the blink of an eye cosmologically and geologically. This leads to the Fermi paradox. If colonizing the galaxy is this easy (relatively), why haven’t we detected more, or any, ETI civilizations? More provocatively, why isn’t our planet in one of these civilization by now?

Classic Fermi paradox solutions include the alien zoo hypothesis, or the prime directive hypothesis, that the aliens are out there but hiding themselves purposefully from us because we are not ready. Another explanation is the dark forest hypothesis, that there is a hostile alien presence that intentionally destroys any civilization that becomes advanced enough to be detectable over interstellar distances, presumeably to prevent competitors on the cosmic scene. Another explanation is simply that most technologically advanced civilizations destroy themselves with nuclear war or environmental destruction before they can last long enough to be detect with current technology.

The galactic zoo hypothesis and the dark forest hypothesis are popular science fiction but almost impossible to disprove. Any challenges to the hypothesess can always be blamed on powerful aliens hiding things from us (aliens of the gaps?). The latter hypothesis, that most civilizations don’t last long enough to be detected, is more likely in my opinion.

On the other hand, we only have a sample of n =1 technologically advanced planetary civilizations. As a result, we really can’t be certain of how long the average technological advanced civilization lasts or whether the average lifespan of a civilization is long enough to make detection likely.

An explanation gaining popularity is that truly long-lived civilizations learn to use technology in such a way that it doesn’t disrupt the biosphere in the ways that are detectable over interstellar distances. As a civilization get more advanced, its technological processes begin to look more like natural processes until they merge with the biosphere from the perspective of an interstellar observer.

This solution comes from ecological economics and focuses on the proposed unsustainability of a focus on rapid and infinite economic growth and technological development, which depletes planetary resources and threatens planetary habitability. A civilization that seeks to last over geologic timescales, it is argued, will need to abandon this approach in favor of long-term sustainability. In this way, civilizations that last beyond a certain bottleneck of rapid technological advancement and economic growth may be very hard to distinguish from a biosphere lacking an advanced technological civilization, especially from interstellar distances.

This makes the concept of “technosignature” complicated. By analogy with “biosignatures,” technosignatures are supposed to be indications of technology on a planet. This could range from a crashed alien spacecraft on the Moon to industrial pollutants, like CFCs, in the atmosphere of an exoplanet.

The problem with a lot of current technosigantures is that they assume a civilization that looks like ours, one that pollutes its atmosphere, gives off high intensity electromagnetic signals, and is constantly disrupting its planetary environment through ever-growing production and consumption needs. We don’t necessarily know what a technosignature from a technologically advanced but non-colonizing, non-expansionist or non-industrial civilization in homeostasis with its planetary environment would look like.

While this is encouraging from the perspective of possible futures of civilization, from a research perspective it is troubling. It could mean that the only civilizations we can detect are immature civilizations that will either quickly evolve into non-detectable civilizations, indistinguishable from the biosphere, or simply wipe themselves out in a geologically short time period. The most easy to detect civilizations may also be the most short-lived.

Since the most common technosignatures are from disruptions of planetary processes caused by large scale economic or technological projects, it could be argued that most technosignatures searches would not helpful in detecting civilizations that have taken a more sustainable route where they exist in metabolic equilibrium with their planet. On the other hand, these civilizations may do things that look like pollution from our vantage point but are actually the biproduct of planetary stewardship.

Phenomena which we associate with pollution could also be a biproduct of a civilization effectively managing its planetary environment. For example, industrial greenhouse gases in the atmosphere of an exoplanet would also occur if a civilization was acting to stave off a coming ice age to maintain planetary habitability. A civilization may construct a Dyson sphere but fill it with habitats that support vegetation enabling artificially contained ecologies that have the potential to last much longer than those that exist on planetary surfaces in less controlled environments.

Furthermore a crashed alien probe may not be evidence of an advanced civilization that is looking for new economic resources to exploit, but one that is curious about the universe and sending robotic probes to explore, an idea explored in depth in science fiction.

If these hypothetical long-lived planetary civilizations also give off signals that appear to be distruptions to planetary processes, it means that there may be a larger time window of detection than otherwise assumed. This also has implications for the Fermi paradox because the absence of such technosignatures many not mean that civilizations are becoming non-detectable as they get more advanced.

This still does not mean that these civilizations do or don’t exist, but it does mean that it is still worth searching for technosignatures. Furthermore it gives our own civilization some concrete ideas of what a more sustainable future for a technologically advanced planetary or multi-planetary civilization might look like. This is important since ultimately we would want the future for our own civilization.

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