Fermi's Paradox - The Last Challenge For Copernicanism?

We review Fermi's paradox (or the "Great Silence" problem), not only arguably the oldest and crucial problem for the Search for ExtraTerrestrial Intelligence (SETI), but also a conundrum of profound scientific, philosophical and cultural importance. By a simple analysis of observation selection effects, the correct resolution of Fermi's paradox is certain to tell us something about the future of humanity. Already more than three quarters of century old puzzle -- and a quarter of century since the last major review paper in the field by G. David Brin -- has generated many ingenious discussions and hypotheses. We analyze the often tacit methodological assumptions built in various answers to this puzzle and attempt a new classification of the numerous solutions proposed in an already huge literatureon the subject. Finally, we consider the ramifications of variousclasses of hypotheses for the practical SETI projects. Somewhatparadoxically, it seems that the class of (neo)catastrophichypotheses gives, on the balance, the strongest justification tooptimism regarding our current and near-future SETI efforts

Advanced Aspects of the Galactic Habitability

Context. Astrobiological evolution of the Milky Way (or the shape of its "astrobiological landscape") has emerged as one of the key research topics in recent years. In order to build precise, quantitative models of the Galactic habitability, we need to account for two opposing tendencies of life and intelligence in the most general context: the tendency to spread to all available ecological niches (conventionally dubbed "colonization") and the tendency to succumb to various types of existential catastrophes ("catastrophism"). These evolutionary tendencies have become objects of study in fields such as ecology, macroevolution, risk analysis, and futures studies, while a serious astrobiological treatment has been lacking so far. Aims. Our aim is to numerically investigate the dynamics of opposed processes of expansion (panspermia, colonization) and extinction (catastrophic mechanisms) of life in the Galaxy. Methods. We employ a new type of numerical simulation based on 1D probabilistic cellular automaton with very high temporal resolution, in order to study astrobiological dynamics. Results. While the largest part of the examined parameter space shows very low habitability values, as expected, the remaining part has some observationally appealing features that imply, among other things, a reduction in the amount of fine-tuning necessary for resolving the Fermi paradox. Conclusions. Advanced aspects of Galactic habitability are amenable to precision studies using massive parallel computer simulations. There are regions of parameter space corresponding to a quasi-stationary state satisfying observable constraints and possessing viable SETI targets.Comment: Language corrected version, to appear in Astronomy & astrophysic

A Comment on Tectonics and the Future of Life on Terrestrial Planets

It is argued that the tight interconnection between biological, climatological, and geophysical factors in the history of the terrestrial biosphere can teach us something of wider importance regarding the general astrobiological evolution of planets in the Galactic habitable zone of the Milky Way. Motivated by a recent debate on the future of Earth's biosphere, we suggest an additional reason why the impact of plate tectonics on the biological evolution is significant on the global Galactic level.Comment: 5 pages, no figures; Precambrian Research, submitte

The Double Wreath: A Contribution to the History of Kingship in Bosnia

The fact that ban Tvrtko of Bosnia had maternal ties with Nemanjić dynasty and seized certain areas of the former Serbian Empire was used as a basis for him to be crowned king of the Serbs and Bosnia in 1377 in the monastery of Mileševa over the grave of Saint Sava. His charter issued to the Ragusans in 1378 contains the term “double wreath” which figuratively symbolized the rule of Tvrtko I over two Serb-inhabited states, Bosnia and Serbia. Tvrtko’s choice not to annex the conquered territory to his own state, Bosnia, but to be crowned king of Serbia as well required the development of a new ideology of kingship and a new form of legitimation of power. Although his royal title was recognized by his neighbours, including probably the rest of the Serbian lands, that the project was unrealistic became obvious in the aftermath of the Battle of Kosovo in 1389. What remained after his death was only the royal title, while the state ruled by his successors became exclusively related to Bosnia. Yet, echoes of his coronation in medieval Bosnia can be followed in the further development of the title and of the concept of crown and state. Interestingly, an attempt to revive the double crown concept was made in the early fifteenth century by the king Sigismund of Hungary, who requested that the Bosnians crown him the way Tvrtko had been crowned

Protivčinjenični iskazi i nefizički ceteris paribus : jedna eksplanatorna greška

I reconsider a type of counterfactual argument often used in historical sciences on a recent widely discussed example of the so-called “rare Earth” hypothesis in planetary sciences and astrobiology. The argument is based on the alleged “rarity” of some crucial ingredient for the planetary habitability, which is, in Earth’s case, provided by contingent evolutionary development. For instance, the claim that a contingent fact of history which has created planet Jupiter enables shielding of Earth from most dangerous impact catastrophes, thus increasing Earth’s habitability, leads often to the conclusion that such state-of-affairs must be rare in the Galaxy. I argue that this reasoning is deeply flawed, for several closely related reasons. In addition, the relevance of the philosophical problem of transworld identity for this kind of historical reasoning in science is put forward. This highlights many explanatory problems one faces when using historical counterfactuals in study of complex, nonlinear dynamical systems – and bolsters the relevance of philosophy for evaluation of scientific explanatory claims

Dynamical aspects of Galactic habitability in N-body simulations

Recent studies of Galactic evolution revealed that the dynamics of the stellar component might be one of the key factors when considering galactic habitability. We run an N-body simulation model of the Milky Way, which we evolve for 10 Gyr, to study the secular evolution of stellar orbits and the resulting galactic habitability-related properties, i.e., the density of the stellar component and close stellar encounters. The results indicate that radial migrations are not negligible, even in a simple axisymmetric model with mild levels of dynamical heating, and that the net outward diffusion of the stellar component can populate galactic outskirts with habitable systems. Habitable environment is also likely even at sub-Solar galactocentric radii, because the rate of close encounters should not significantly degrade habitability. Stars that evolve from non-circular to stable nearly-circular orbits typically migrate outwards, settling down in a broad Solar neighborhood. The region between $R \approx 3$ kpc and $R \approx 12$ kpc represents the zone of radial mixing, which can blur the boundaries of the Galactic Habitable Zone, as it has been conventionally understood. The present-day stable population of the stars in the Solar neighborhood originates from this radial mixing zone, with most of the stars coming from the inner regions. The Solar system can be considered as a typical Milky Way habitable system because it migrated outwards from the metal-rich inner regions of the Disk and has a circular orbit in the present epoch. We conclude that the boundaries of the Galactic Habitable Zone cannot be sharply confined for a given epoch because of the mixing caused by the stellar migrations and secular evolution of stellar orbits.Comment: Accepted for publication in PAS