Spatio-temporal Constraints on the Zoo Hypothesis, and the Breakdown of Total Hegemony

The Zoo Hypothesis posits that we have not detected extraterrestrial intelligences (ETIs) because they deliberately prevent us from detecting them. While a valid solution to Fermi's Paradox, it is not particularly amenable to rigorous scientific analysis, as it implicitly assumes a great deal about the sociological structure of a plurality of civilisations. Any attempt to assess its worth must begin with its most basic assumption - that ETIs share a uniformity of motive in shielding Earth from extraterrestrial contact. This motive is often presumed to be generated by the influence of the first civilisation to arrive in the Galaxy. I show that recent work on inter-arrival time analysis, while necessary, is insufficient to assess the validity of the Zoo Hypothesis (and its related variants). The finite speed of light prevents an early civilisation from exerting immediate cultural influence over a later civilisation if they are sufficiently distant. I show that if civilisation arrival times and spatial locations are completely uncorrelated, this strictly prevents the establishment of total hegemony throughout the Galaxy. I finish by presenting similar results derived from more realistic Monte Carlo Realisation simulations (where arrival time and spatial locations are partially correlated). These also show that total hegemony is typically broken, even when the total population of civilisations remains low. In the terminology of previous studies of solutions to Fermi's Paradox, this confirms the Zoo Hypothesis as a "soft" solution. However, an important question to be resolved by future work is the extent to which many separate hegemonies are established, and to what extent this affects the Zoo Hypothesis.Comment: 14 pages, 10 figures, accepted for publication in the International Journal of Astrobiolog

Can Collimated Extraterrestrial Signals be Intercepted?

The Optical Search for Extraterrestrial Intelligence (OSETI) attempts to detect collimated, narrowband pulses of electromagnetic radiation. These pulses may either consist of signals intentionally directed at the Earth, or signals between two star systems with a vector that unintentionally intersects the Solar System, allowing Earth to intercept the communication. But should we expect to be able to intercept these unintentional signals? And what constraints can we place upon the frequency of intelligent civilisations if we do? We carry out Monte Carlo Realisation simulations of interstellar communications between civilisations in the Galactic Habitable Zone (GHZ) using collimated beams. We measure the frequency with which beams between two stars are intercepted by a third. The interception rate increases linearly with the fraction of communicating civilisations, and as the cube of the beam opening angle, which is somewhat stronger than theoretical expectations, which we argue is due to the geometry of the GHZ. We find that for an annular GHZ containing 10,000 civilisations, intersections are unlikely unless the beams are relatively uncollimated. These results indicate that optical SETI is more likely to find signals deliberately directed at the Earth than accidentally intercepting collimated communications. Equally, civilisations wishing to establish a network of communicating species may use weakly collimated beams to build up the network through interception, if they are willing to pay a cost penalty that is lower than that meted by fully isotropic beacons. Future SETI searches should consider the possibility that communicating civilisations will attempt to strike a balance between optimising costs and encouraging contact between civilisations, and look for weakly collimated pulses as well as narrow-beam pulses directed deliberately at the Earth.Comment: 12 pages, 7 figures, accepted for publication in JBI

Slingshot Dynamics for Self Replicating Probes and the Effect on Exploration Timescales

Interstellar probes can carry out slingshot manoeuvres around the stars they visit, gaining a boost in velocity by extracting energy from the star's motion around the Galactic Centre. These maneouvres carry little to no extra energy cost, and in previous work it has been shown that a single Voyager-like probe exploring the galaxy does so 100 times faster when carrying out these slingshots than when navigating purely by powered flight (Forgan et al. 2012). We expand on these results by repeating the experiment with self-replicating probes. The probes explore a box of stars representative of the local Solar neighbourhood, to investigate how self-replication affects exploration timescales when compared with a single non-replicating probe. We explore three different scenarios of probe behaviour: i) standard powered flight to the nearest unvisited star (no slingshot techniques used), ii) flight to the nearest unvisited star using slingshot techniques, and iii) flight to the next unvisited star that will give the maximum velocity boost under a slingshot trajectory. In all three scenarios we find that as expected, using self-replicating probes greatly reduces the exploration time, by up to three orders of magnitude for scenario i) and iii) and two orders of magnitude for ii). The second case (i.e. nearest-star slingshots) remains the most time effective way to explore a population of stars. As the decision-making algorithms for the fleet are simple, unanticipated "race conditions" amongst probes are set up, causing the exploration time of the final stars to become much longer than necessary. From the scaling of the probes' performance with star number, we conclude that a fleet of self-replicating probes can indeed explore the Galaxy in a sufficiently short time to warrant the existence of the Fermi Paradox.Comment: Accepted for publication in the International Journal of Astrobiology, 13 pages, 7 figure

Virulence as a Model for Interplanetary and Interstellar Colonisation - Parasitism or Mutualism

In the light of current scientific assessments of human-induced climate change, we investigate an experimental model to inform how resource-use strategies may influence interplanetary and interstellar colonisation by intelligent civilisations. In doing so, we seek to provide an additional aspect for refining the famed Fermi Paradox. The model described is necessarily simplistic, and the intent is to simply obtain some general insights to inform and inspire additional models. We model the relationship between an intelligent civilisation and its host planet as symbiotic, where the the relationship between the symbiont and the host species (the civilisation and the planets ecology, respectively) determines the fitness and ultimate survival of both organisms. We perform a series of Monte Carlo Realisation simulations, where civilisations pursue a variety of different relationships/strategies with their host planet, from mutualism to parasitism, and can consequently 'infect' other planets/hosts. We find that parasitic civilisations are generally less effective at survival than mutualist civilisations, provided that interstellar colonisation is inefficient (the maximum velocity of colonisation/infection is low). However, as the colonisation velocity is increased, the strategy of parasitism becomes more successful, until they dominate the 'population'. This is in accordance with predictions based on island biogeography and r/K selection theory. While heavily assumption dependent, we contend that this provides a fertile approach for further application of insights from theoretical ecology for extraterrestrial colonisation - while also potentially offering insights for understanding the human-Earth relationship and the potential for extraterrestrial human colonisation.Comment: 18 pages, 7 figures, published in the International Journal of Astrobiolog