The Search for Extraterrestrial Intelligence (SETI) faces persistent uncertainty due to Earth's sole example of advanced civilization. The Fermi Paradox remains unanswered 60 years after its inception. However, this uncertainty sparks numerous hypotheses about the possible whereabouts and behaviors of extraterrestrial intelligence.
The Percolation Theory Hypothesis suggests that civilizations may be constrained by physics and planetary capacity. A recent study from the University of the Philippines Los Baños explored this, indicating that intelligent life's expansion is time-limited and likely exponential.
The Percolation Theory Explained
Percolation Theory, in essence, explains how networks respond when nodes or links are removed, leading to their fragmentation into smaller connected clusters. One of the earliest applications of this theory to the Fermi Paradox can be attributed to Carl Sagan and William I. Newman in 1981.
In their paper titled "Galactic Civilizations: Population Dynamics and Interstellar Diffusion," they proposed a unique perspective on the absence of contact with extraterrestrial civilizations (ETCs). They suggested that the reason might be that interstellar exploration and colonization do not follow a linear progression.
Sagan and Newman presented the notion that interstellar exploration resembles diffusion. Geoffrey A. Landis further echoed this idea in his 1993 paper, "The Fermi Paradox: An Approach Based on Percolation Theory," in which he argued that the laws of physics impose limitations on the speed of interstellar expansion. Landis asserted that expecting a uniform motivation among extraterrestrial civilizations is not realistic.
Considering the existence of numerous extraterrestrial civilizations, it's likely that at least one would have undertaken interstellar exploration, possibly for unknown reasons. However, colonization is resource-intensive and lengthy, making it reasonable to assume that not all civilizations would invest in such an endeavor for long-term gains.
This perspective draws parallels with human societies, which include both exploratory cultures spanning vast distances and those without such inclinations.
Likewise, a 2019 paper titled "The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States" by Professor Adam Frank and his colleagues from NASA's Nexus for Exoplanetary Systems Science (NExSS) argued that interstellar colonization would likely occur in clusters, considering that not all potentially habitable planets may be suitable for an alien species.
Finding Extraterrestrial Intelligence
The research team examined the limits of cosmic settlement in various cosmological models: static, matter-dominated, and dark energy-dominated Universes. In the preprint of the paper, researchers applied a Logistic Growth Function to understand how the number of settled planets changes over time.
Surprisingly, they observed similar settlement patterns in all scenarios, despite the Universe's expansion. In dark energy-dominated universes, distant planets could become unreachable due to their high expansion rate, while matter-dominated universes allowed for potential reachability as the comoving Hubble Sphere shrank over time.
The study suggests that advanced civilizations tend to follow a three-phase pattern: slow initial settlement, followed by a rapid expansion phase, and finally a slower settlement rate again as reachable planets are exhausted.
Currently, the Milky Way galaxy may be in Phase I, with only a few civilizations engaged in interstellar settlement. However, once a critical number of such civilizations are active, we may enter Phase II, characterized by a faster settlement rate, increasing the likelihood of contact with extraterrestrial life.
The results also touch on the challenges of humanity becoming an interstellar species, especially in a universe dominated by Dark Energy. While the expansion rate limits how far we can reach, the volume of reachable space is continuously expanding, offering hope for the survival and development of our species. The study raises exciting possibilities for future cosmic interactions and the search for advanced civilizations.
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