Propagation of Partially Coherent Photons in an Ultra-Intense Radiation Gas

The scattering of photons off photons at the one-loop level is investigated. We give a short review of the weak field limit, as given by the first order term in the series expansion of the Heisenberg-Euler Lagrangian. The dispersion relation for a photon in a radiation gas is presented. Based on this, a wave kinetic equation and a set of fluid equations is formulated. These equations describe the interaction between a partially coherent electromagnetic pulse and an intense radiation gas. The implications of the results are discussed.Comment: 10 pages, 1 figure, revtex

Cold hardening and dehardening in Salix

The variation in cold hardiness in Salix in the autumn was investigated using clones of different geographic origins. In late growing season, the variation was small and inversely related to a phenotypic variation in potential growth rate. When growth had stopped in response to the reduction in daylength, however, large differences in cold hardiness developed. Northern/continental clones started cold hardening up to two months earlier and showed up to three times higher inherent rates of cold hardening than the southern/maritime ones. The two components of cold hardening, the timing of onset and the inherent rate, seemed to be separately inherited traits, as judged from analyses of the prodigy of a crossing between an early-and-rapidly hardening clone and a late-and-slowly hardening one. This suggests that cold hardiness can be improved without adversely affecting growth by selecting for a late onset of cold hardening combined with a rapid rate. Also, in the early stages, cold hardening was more sensitive to low, non-freezing temperatures in the southern/maritime clones than in the northern/continental ones. Cold hardening of stems in the autumn could be monitored from the accumulation of sugars, most predominantly sucrose, raffinose and stachyose. The accumulation of sucrose started already with the cessation of growth, whilst the accumulation of raffinose and stachyose started later and was stimulated by cool temperatures. Multivariate models using sugar data could explain 76% of the variation in cold hardiness in the early stages of hardening. Changes in levels of sugars and other compounds during cold hardening could be assessed non-intrusively from the visible and infrared reflectance spectra of stems. Multivariate models using spectral data could predict up to 96% of the variation in cold hardiness. This technique is expected to greatly facilitate breeding for improved cold hardiness by allowing rapid screening of large populations. The variation in cold hardiness in spring was also investigated. Loss of cold hardiness in spring was closely related to the bursting of buds. A relatively large genetic variation in the temperature requirement for bud burst was demonstrated indicating that this might be modified in sensitive clones to improve their cold hardiness in spring

Higher-Order Defeat Without Epistemic Dilemmas

Many epistemologists have endorsed a version of the view that rational belief is sensitive to higher-order defeat. That is to say, even a fully rational belief state can be defeated by misleading higher-order evidence, which indicates that the belief state is irrational. In a recent paper, however, Maria Lasonen-Aarnio calls this view into doubt. Her argument proceeds in two stages. First, she argues that higher-order defeat calls for a two-tiered theory of epistemic rationality. Secondly, she argues that there seems to be no satisfactory way of avoiding epistemic dilemmas within a two-tiered framework. Hence, she concludes that the prospects look dim for making sense of higher-order defeat within a broader theoretical picture of epistemic rationality. Here I aim to resist both parts of Lasonen-Aarnio’s challenge. First, I outline a way of accommodating higher-order defeat within a single-tiered framework, by amending epistemic rules with appropriate provisos for different kinds of higher-order defeat. Secondly, I argue that those who nevertheless prefer to accommodate higher-order defeat within a two-tiered framework can do so without admitting to the possibility of epistemic dilemmas, since epistemic rules are not always accompanied by ‘oughts’ in a two-tiered framework. The considerations put forth thus indirectly vindicate the view that rational belief is sensitive to higher-order defeat

Mobility is the Message: Experiments with Mobile Media Sharing

This thesis explores new mobile media sharing applications by building, deploying, and studying their use. While we share media in many different ways both on the web and on mobile phones, there are few ways of sharing media with people physically near us. Studied were three designed and built systems: Push!Music, Columbus, and Portrait Catalog, as well as a fourth commercially available system – Foursquare. This thesis offers four contributions: First, it explores the design space of co-present media sharing of four test systems. Second, through user studies of these systems it reports on how these come to be used. Third, it explores new ways of conducting trials as the technical mobile landscape has changed. Last, we look at how the technical solutions demonstrate different lines of thinking from how similar solutions might look today. Through a Human-Computer Interaction methodology of design, build, and study, we look at systems through the eyes of embodied interaction and examine how the systems come to be in use. Using Goffman’s understanding of social order, we see how these mobile media sharing systems allow people to actively present themselves through these media. In turn, using McLuhan’s way of understanding media, we reflect on how these new systems enable a new type of medium distinct from the web centric media, and how this relates directly to mobility. While media sharing is something that takes place everywhere in western society, it is still tied to the way media is shared through computers. Although often mobile, they do not consider the mobile settings. The systems in this thesis treat mobility as an opportunity for design. It is still left to see how this mobile media sharing will come to present itself in people’s everyday life, and when it does, how we will come to understand it and how it will transform society as a medium distinct from those before. This thesis gives a glimpse at what this future will look like

TCP Performance in Ad Hoc Networks

Ad hoc networks are mobile wireless networks which do not have any kind of fixed infrastructure. The routing layer in an ad hoc network ties the network together into a seamless entity and provide transparent services to higher layer protocols. This thesis examines the interactions of two routing protocols, AODV and DSR and how the mobile ad hoc network environment affect TCP performance. The results presented here are as follows: the path length and the presence of competing traffic are the main factors of TCP throughput performance. The size of TCP window affects the loss rate, but the loss rate is not strongly correlated to throughput performance. Using TCP selective acknowledgement option does not improve throughput. Finally, there is hardly any difference in TCP throughput when using DSR and AODV. These conclusions are supported by extensive simulation experiments