The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion and Steady States (1902.04450v2)

Abstract: We model the settlement of the galaxy by space-faring civilizations in order to address issues related to the Fermi Paradox. We explore the problem in a way that avoids assumptions about the intent and motivation of any exo-civilization seeking to settle other planetary systems. We first consider the speed of an advancing settlement via probes of finite velocity and range to determine if the galaxy can become inhabited with space-faring civilizations on timescales shorter than its age. We also include the effect of stellar motions on the long term behavior of the settlement front which adds a diffusive component to its advance. The results of these models demonstrate that the Milky Way can be readily 'filled-in' with settled stellar systems under conservative assumptions about interstellar spacecraft velocities and launch rates. We then consider the question of the galactic steady-state achieved in terms of the fraction of settled planets. We do this by considering the effect of finite settlement civilization lifetimes on the steady states. We find a range of parameters for which the galaxy supports a population of interstellar space-faring civilizations even though some settleable systems are uninhabited. Both results point to ways in which Earth might remain unvisited in the midst of an inhabited galaxy. Finally we consider how our results can be combined with the finite horizon for evidence of previous settlements in Earth's geologic record. Our steady-state model can constrain the probabilities for an Earth visit by a settling civilization before a given time horizon. These results break the link between Hart's famous "Fact A" (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy.

• The paper introduces a dynamic model of galactic settlement fronts using probes with finite speed to quantify colonization rates across varying stellar densities.
• It demonstrates that finite civilization lifetimes can lead to partial settlement, leaving pockets of uninhabited systems despite rapid expansion.
• Numerical simulations reveal local clustering of settleable systems, suggesting that such spatial patterns may explain Earth's lack of contact with extraterrestrial civilizations.

The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States

The paper by Carroll-Nellenback et al. explores a critical question related to the Fermi Paradox through the modeling of galactic settlement by space-faring civilizations. The authors propose and investigate a model by which the spread of these civilizations might occur across the Milky Way, focusing on mitigating assumptions about the motivations or intentions of such extraterrestrial entities, referred to as 'agency'. The primary objective of this investigation is to provide insights into why the galaxy seems uninhabited despite the potential for widespread civilization, as suggested by the Fermi Paradox.

Main Contributions

1. Modeling Settlement Fronts: The paper introduces a dynamic model for the settlement of neighboring {settleable} stellar systems using probes with finite velocity and range. The model accounts for the randomized distribution and motion of stars, introducing a diffusive component to the settlement front's advance. This allows for a more comprehensive understanding of settlement across the galaxy compared to models assuming static stellar positions.
2. Rate of Expansion: A quantitative analysis is presented to estimate the speed of a settlement front. The authors identify three critical regimes: low, intermediate, and high densities of {settleable} stars. The results indicate that the spread of civilizations can occur swiftly under conservatively assumed parameters, suggesting that the Milky Way could potentially have been settled over timescales significantly shorter than its age.
3. Steady-State and Partial Settlement: The paper introduces the potential for a steady-state model where not all {settleable} systems are colonized, even if the Milky Way has experienced a sweep by a settlement front. By allowing for finite civilization lifetimes, the authors demonstrate that the galaxy may support a population of interstellar civilizations, with pockets of settled planets, while others remain uninhabited or temporarily unsettled.
4. Implications for the Fermi Paradox: The authors address Hart's Fact A by demonstrating that the lack of settlement on Earth does not necessitate being the only advanced civilization in the galaxy. Factors such as finite settlement lifetimes, the Aurora Effect (the difficulty of finding suitable worlds), and statistical fluctuations in settlement patterns mean that the absence of evidence is not evidence of absence.
5. Numerical Simulations and Local Clustering: Computational models support the analytic findings, revealing clusters of {settleable} systems that could continually resettle each other after the original civilization's demise. This clustering can lead to regions within the galaxy that remain unsettled, which could explain the lack of contact with Earth.

Theoretical and Practical Implications

The theoretical implications are profound as they suggest a re-evaluation of existing assumptions about the galactic presence of technological civilizations. Practically, the models indicate that efforts in the Search for Extraterrestrial Intelligence (SETI) could focus on regions of higher stellar density or zones expected to have frequent encounters, as these regions could harbor civilizations even if Earth has not been contacted.

Speculatively, future research might explore the cultural and psychological factors influencing the willingness of civilizations to engage in interstellar settlement. Additionally, interdisciplinary studies incorporating astrophysics, sociology, and planetary science could further unravel the complex dynamics governing exo-civilizations and their interaction with the cosmos.

In conclusion, this paper provides a robust framework addressing the Fermi Paradox by articulating how civilizations might evolve in an already dynamically stable and settled galaxy. It challenges the notion that human civilization is unique in the Milky Way by quantitatively demonstrating viable pathways that align with the lack of observable extraterrestrial presence on Earth today.