Observational Signatures of Self-Destructive Civilisations (1507.08530v1)

Abstract: We address the possibility that intelligent civilisations that destroy themselves could present signatures observable by humanity. Placing limits on the number of self-destroyed civilisations in the Milky Way has strong implications for the final three terms in Drake's Equation, and would allow us to identify which classes of solution to Fermi's Paradox fit with the evidence (or lack thereof). Using the Earth as an example, we consider a variety of scenarios in which humans could extinguish their own technological civilisation. Each scenario presents some form of observable signature that could be probed by astronomical campaigns to detect and characterise extrasolar planetary systems. Some observables are unlikely to be detected at interstellar distances, but some scenarios are likely to produce significant changes in atmospheric composition that could be detected serendipitously with next-generation telescopes. In some cases, the timing of the observation would prove crucial to detection, as the decay of signatures is rapid compared to humanity's communication lifetime. In others, the signatures persist on far longer timescales.

• The paper identifies distinct atmospheric signatures from catastrophic events like nuclear warfare, biological assaults, and industrial pollution.
• The study employs Earth as a paradigm to model how self-destruction could alter planetary chemistry, revealing potential temporal and spectral markers.
• The research underscores astroforensics as a pivotal tool in enhancing SETI strategies by addressing elements of the Drake Equation and the Great Filter.

Observational Signatures of Self-Destructive Civilisations

The paper "Observational Signatures of Self-Destructive Civilisations" explores the potential astronomical signals that might indicate the presence of extraterrestrial civilizations that have destroyed themselves. The authors, Stevens, Forgan, and O'Malley-James, approach the investigation by analyzing various hypothetical scenarios of civilizational self-annihilation using Earth as a paradigm. The implications of such findings are significant for the final terms of the Drake Equation and the various solutions to Fermi’s Paradox.

Context and Motivation

The Search for Extraterrestrial Intelligence (SETI) has long been guided by the Drake Equation, a probabilistic framework introduced to estimate the number of detectable civilizations in our galaxy. Concomitantly, the Fermi Paradox questions the absence of evidence for extraterrestrial civilizations despite the high likelihood of their existence suggested by various factors. This paper emphasizes the third class of solutions to the paradox, known as the Catastrophist hypothesis, which posits that extraterrestrial civilizations may have short lifetimes due to inherent vulnerabilities or catastrophic self-destructive events. The research proposes the idea that such civilizations could leave observable atmospheric signatures on their home planets.

Self-Destructive Scenarios and Observational Signatures

The authors categorize potential self-destructive scenarios that could occur within these civilizations:

1. Nuclear Annihilation: Globally catastrophic nuclear warfare might produce detectable gamma-ray flashes and alterations in atmospheric chemistry due to radioactive fallout. However, the comparability of these signatures to naturally occurring cosmic phenomena presents a challenge in remote detection.
2. Biological Warfare: A pandemic engineered through biological means could lead to atmospheric changes due to decaying biomass. The resulting microbial decay products, such as methanethiol and ethane, offer a short-lived spectral biosignature, although the detection window would be very narrow, demanding precise timing of observation.
3. Nanotechnology ('Grey Goo'): The self-replication of nanoscale machines could lead to the consumption of biospheric resources, producing a dusty atmospheric signature. Compared to the previous scenarios, these dust signatures might be detectable over a longer timescale if the resultant 'nano-sand' persists.
4. Pollution: Both planetary and stellar pollution could provide indications of an industrial past. Artificial compounds, like CFCs, could be detected in exoplanet atmospheres, remaining observable long after active production ceases. On stellar surfaces, unusual elemental abundances might hint at industrial activity if they exceed natural pollution levels due to planet formation.
5. Orbital Debris: Civilizations that heavily exploit orbital space might leave detectable debris rings following a Kessler syndrome, observable through transit photometry as potential artifacts caused by the loss of access to space.
6. Complete Planetary Destruction: The ultimate scenario involves the dismantling or total obliteration of a planet, suggested as a capability of advanced civilizations desiring to build megastructures. Such events could leave debris signatures akin to those of natural celestial impacts.

Implications and Future Prospects

This paper makes a case for considering self-destructive civilizational signatures as a serious component of SETI. Practically, the detection of these signatures would not only contribute to our understanding of the frequency and nature of extraterrestrial life but would also provide insights into the possible fates awaiting advanced civilizations, including humanity. The prospect of detecting such signatures provides a new dimension to the SETI initiative, allowing the utilization of existing astronomical survey data to complement traditional techniques focused on active communication.

The authors suggest that advancements in observational technologies, such as the James Webb Space Telescope and future planet characterization missions, will enhance the capability to detect these signatures. Consequently, the discipline of "astroforensics" may soon play a critical role in addressing both the probability of self-annihilation in intelligent civilizations and the broader implications for our understanding of the "Great Filter" concept.

Conclusion

While challenging, the search for observational signatures of self-destructive civilizations offers a plausible means to address outstanding questions in astrobiology and SETI. The research presented in this paper underscores the importance of considering the remains of extinct civilizations as potential evidence of extraterrestrial life, aligning well with the contemporary resources and research directions in astronomy. The confluence of theoretical exploration and observational advancements retains the potential for a significant leap forward in our search for extraterrestrial intelligence, offering both knowledge and cautionary insights.