Dynamical friction in slab geometries and accretion disks

The evolution of planets, stars and even galaxies is driven, to a large extent, by dynamical friction of gravitational origin. There is now a good understanding of the friction produced by extended media, either collisionless of fluid-like. However, the physics of accretion or protoplanetary disks, for instance, is described by slab-like geometries instead, compact in one spatial direction. Here, we find, for the first time, the gravitational wake due to a massive perturber moving through a slab-like medium, describing e.g. accretion disks with sharp transitions. We show that dynamical friction in such environments can be substantially reduced relatively to spatially extended profiles. Finally, we provide simple and accurate expressions for the gravitational drag force felt by the perturber, in both the subsonic and supersonic regime.Comment: 10 pages, 8 figure

Public archaeology and political dynamics in Portugal. A reply to Bednarik

info:eu-repo/semantics/publishedVersio

Are Our Brains Subcutaneous Machines of Truth-Optimization?

Strategies purporting to determine the meaning of inner states of belief-content in terms of their inferential role usually assume the inner structure of the human inferential competence to be that of first order logic plus identity. Considerations of computational complexity and cumbersomeness of representation tend to undermine the plausibility of combining such strategies with this assumption. In this paper I contend that appealing to rules of default reasoning won’t make things turn out any better for the inferential role functionalist.info:eu-repo/semantics/publishedVersio

Mathisson's helical motions demystified

The motion of spinning test particles in general relativity is described by Mathisson-Papapetrou-Dixon equations, which are undetermined up to a spin supplementary condition, the latter being today still an open question. The Mathisson-Pirani (MP) condition is known to lead to rather mysterious helical motions which have been deemed unphysical, and for this reason discarded. We show that these assessments are unfounded and originate from a subtle (but crucial) misconception. We discuss the kinematical explanation of the helical motions, and dynamically interpret them through the concept of hidden momentum, which has an electromagnetic analogue. We also show that, contrary to previous claims, the frequency of the helical motions coincides exactly with the zitterbewegung frequency of the Dirac equation for the electron.Comment: To appear in the Proceedings of the Spanish Relativity Meeting 2011 (ERE2011), "Towards new paradigms", Madrid 29 August - 2 September 201

A criança do Lapedo e as origens do homem moderno na Península Ibérica

Durante as três últimas décadas do século XX, o debate sobre as origens da humanidade actual girou em torno de duas posições fortemente polarizadas (Fig. 1). Um modelo, a Hipótese Multiregional, sustentava que, após a saída de África do Homo erectus, há mais de um milhão de anos, a nossa evolução se deu num quadro de fluxo genético permanente entre as diferentes regiões do Velho Mundo, pelo que todas as populações do género Homo, independentemente do seu maior ou menor grau de isolamento geográfico, teriam sempre constituído uma única espécie. Essa espécie única teria evoluído de forma gradual, representando os neandertalenses um ponto intermédio na passagem do “estádio” erectus ao “estádio” sapiens; na Europa, portanto, teria havido continuidade genética total entre os últimos neandertalenses e os primeiros sapiens, resultando estes últimos da transformação evolutiva dos primeiros

Cognition and Rationality: Writing straight with crooked lines?

Four different approaches dominate the modern discussion around the topic of how best to define human rationality. These are the following: 1) Unbounded Rationality (UR); 2) Optimization under Constraints (OuC); 3) Heuristics and Biases (H&B); 4) Ecological Rationality (ER). Typically, proponents of approaches 3) and 4) criticize the models put forth by the proponents of approaches 1) and 2) for their cognitive unrealism. However, many ethologists contend that it makes sense to account for data gathered in animal behavior research along the lines of precisely these models. Elaborating upon this claim, Stanovich suggested that models of the kinds 1) and 2) are more appropriate to account for the behavior of creatures endowed with simple cognitive architectures rather than to account for the behavior of humans. Moreover, according to Stanovich’s own new approach, it is the cognitive complexity of humans rather than their computational limitations that makes them (partially) irrational. Could he be right? Following a suggestion made by Kacelnik (2006), I will contend that, in order to try to find out an answer to this question, one needs to realize that the term “rationality” in use in this debate has to be understood as referring to, at least, three distinct properties; and that, once one analyzes each of them properly, most of the above-mentioned approaches, including Stanovich’s, reveal themselves to be untenable.info:eu-repo/semantics/publishedVersio

The Upper Palaeolithic of Europe

The Anatolian Middle Bronze Age (MBA) is roughly contemporary with the Isin-Larsa Period followed by the Old Babylonian Kingdom in southern Mesopotamia, the Old Assyrian Period in northern Mesopotamia, the Syrian Middle Bronze I-II Period characterised by urban centres such as Ebla, Mari and the kingdom of Yamkhad, and the Middle Kingdom to Second Intermediate Period in Egypt. Many sites with MBA levels continued into the Late Bronze Age (LBA) with little change in cultural assemblages or technologies. The figure of the ruler Anitta provides historical continuity for the MBA and LBA polities in Central Anatolia. Anatolian art of the LBA is best known for its architectural sculpture and rock reliefs. The west Anatolian kingdom of Lydia, the last independent polity of Anatolia with linguistic links to Anatolian Indo- European Bronze Age languages, succeeded the Phrygian kingdom at the beginning of the 6th century.info:eu-repo/semantics/publishedVersio

Beyond Toulmin vs. Carnap on ‘Probability’

In Logical Foundations of Probability, Carnap put forth the view according to which there are two fundamentally different pre-scientific concepts of probability. One is best captured by its scientific explication in terms of the logical-semantic idea of the degree of confirmation of a hypothesis with respect to the set of sentences describing the evidence (probability1). The other is best captured by its scientific explication in terms of the empirical idea of the relative frequency of an event with respect to a long sequence of instances of a mass phenomenon (probability2). Both probability1 and probability2 are viewed as legitimate scientific concepts. Thus, according to Carnap, the original, pre-scientific, distinction in meaning between these uses of probability terms was preserved and furthered in the course of the development of the scientific approach to probability. Philosophical discussions about the appropriate analysis of the meaning of probability statements are therefore pointless. In The Uses of Argument, Toulmin agrees with Carnap that it is a mistake to look for a single reference in terms of which all the uses of ‘probability’ could be accounted for. However, he also maintains that it is equally senseless to try to correct this mistake by appealing to the idea of there being two different references associated with the appropriate uses of the term rather than one. According to Toulmin, both, frequencies as well as confirmation relations, are but types of evidence one is supposed to take into account in the formulation of one’s ordinary language judgments in which the term ‘probability’ occurs. But the evidence that backs a judgment should not be confused with its meaning. Toulmin therefore claims that the term ‘probability’ does not impinge on the meaning of the statement to which it is attached; it is rather a modal modifier that modulates the force with which an agent is disposed to assert that statement. The pointlessness of the philosophical discussions Carnap talks about is real; it has a different and deeper source than the one he diagnosed though. Fifty years later, on what grounds are we to adjudicate this dispute between Toulmin and Carnap concerning the right way of characterizing our pre-scientific concepts of probability? On the one hand, the distinction Toulmin draws between the force of the modal term ‘probably’ and the criteria for its use is still illuminating. On the other hand, it is hard to accept that nothing substantial is being dealt with in the debate around the question of how best to interpret our unanalysed notion of probability. Carnap’s approach to this topic is of a kind we might term ‘transcendental-teleological’. As a matter of fact, his stride into the folk-conceptual history of the notion of probability is entirely determined by his purpose of clarifying and justifying the two concepts of scientific probability he considers there to be. The strategy followed by Toulmin to deal with this issue is to bring back the study of what non-formal probability might be to an analysis of the meaning of ordinary language sentences of probability ascription. The principle underlying the use of such a strategy seems to be the principle that, whatever a pre-scientific concept of something might amount to, our only clue to determine its content is the analysis of the way normal agents use the sentences of ordinary language carrying the term allegedly referring to it in their relevant contexts. Indeed, Toulmin ascribed to his essay “Probability” the status of a “pilot investigation” introducing ideas and distinctions that throw “a general light on the categories of rational assessment”. These words indicate that he took such a principle to be of a general scope and thus that it ought to be applied to different categories used in the rational assessment of arguments. But, even if we grant Toulmin that he is right in his analysis of the semantics of many of our ordinary language sentences of probability ascription, there are other obvious sources of information concerning the lines that mark out the categories belonging to our pre-scientific thinking that he neglects. Prominent among these is empirical psychological research. One would have to be a strenuous defender of some strong version of the Sapir-Whorf thesis in order to deny this. What I propose to do in order to adjudicate this discussion is thus to evade both transcendental-teleological reconstructions of what our pre-scientific concepts had to be, given the present outline of what our scientific concepts are, and shaky intuitions about how best to interpret our ordinary language statements, and look into some recent psychological results concerning the way subjects seem to relate pre-scientifically to contexts of perceived uncertainty, randomness or chance. Preliminary as these results are, I think that an appropriate philosophical treatment of them will allow us to identify and keep the intuitions worth preserving in Carnap’s or Toulmin’s work, on the one hand, and to leave aside what is but a direct consequence of their theoretical prejudices, on the other hand. These results seem to suggest that it is indeed possible to organize the subjects’ responses to these contexts into two distinct and clearly identifiable characterizations of them: an ‘objectivist’ one and a ‘subjectivist’ one. Therefore, if we consider the way people do seem to relate to what Toulmin calls the evidence backing our utterances of probability-statements, what we get is a revealing cleavage between two ways of conceptualizing it. On the one hand, Carnap’s claim concerning the existence of two distinct pre-scientific concepts associated with our use of probability-terms might be somehow vindicated by these results. On the other hand, however, the outlines of this division do not seem to match Carnap’s characterization of the relevant explicanda. For instance, by themselves, the data do not allow us to discriminate between Carnap’s own frequentist view of empirical-objective probability (Probability2) and other approaches, such as, e.g., the classical Laplacian view of ratios of favourable cases to the total number of cases. This is, however, presumably consistent with the fuzzy nature of folk-concepts.info:eu-repo/semantics/publishedVersio