Time, Tense, & Rationality

In this thesis I try to advance our understanding of the nature of time. In particular I defend the idea that there is an objective difference between the past, the present, and the future; a metaphysical tense. This is in opposition to the idea that these distinctions merely mark an aspect of our perspective on entities in time. I argue that tensed beliefs – beliefs that go hand-in-hand with tensed language – are essential to our lives as rational animals. Firstly, they are essential to our practices of providing reasons for action and acting for reasons. Secondly, they are essential for our lives as emotional animals whose emotions are appropriately responsive to the world. Perry has argued that indexical – including tensed – beliefs are essential for actions. In order to attend my meeting, it is not enough that I know that it is at 2pm, I must also know that it is now 1:55pm. Examining Perry’s argument I show that its proper conclusion is that tensed and first-personal beliefs are necessary for rational actions. I argue that reasons are facts (not belief/desire complexes or intensional entities). Further, the rationality of an action derives solely from these reasons, so that when an agent is not mistaken their action is rational purely insofar as it is done for a reason that justifies it. This means that beliefs are required for rational actions only to the extent that they provide an awareness of reasons and thereby enable an action. A proper understanding of rational action thus enables me to say that if an action must involve one belief rather than another in order to be rational, this must be because the former belief involves an awareness of a reason, hence fact, that the latter does not. Combining this with the proper conclusion of Perry’s argument we can say that tensed beliefs are required in the place of any tenseless beliefs in rational actions, and therefore must involve an awareness of facts that the latter cannot capture. Given that our actions are by and large rational, it follows there are facts captured by tensed beliefs not captured by tenseless beliefs. There is a metaphysical tense. Prior has argued that some emotions involve tensed beliefs and Cockburn has furthered this to show that the appropriateness of some emotions depends upon these beliefs. It is inappropriate to grieve a future death or fear a past danger. I show that the appropriateness of emotions stems from the reasons they are felt for and that these reasons are revealed by the beliefs involved in these emotions. This enables me to argue that if an emotion must involve one belief rather than another to be appropriate, then this can only be because the former belief captures a reason that the latter does not. In combination with Prior/Cockburn’s conclusion I am thus able to argue, analogously to the case of rational actions, that if there are emotions which must involve tensed beliefs to be appropriate and there are examples of appropriate such emotions, then metaphysical tense is real. My thesis thus derives a conclusion about the nature of time from our nature as rational animals. These arguments also have implications for a proper understanding of first-personal indexicals, which must now be recognized to pick out facts not captured by non-first-personal language. The former of these conclusions has been famously attacked by McTaggart, and the latter by Wittgenstein, and so I will also say something to rebut these criticisms. My arguments also have implications for certain issues surrounding the cognitive significance of co-referring names/natural kind terms which I will show to be unproblematic

Spitzer and ISO Galaxy Counts in the Mid-Infrared

Galaxy source counts that simultaneously fit the deep mid-infrared surveys at 24 microns and 15 microns made by the Spitzer Space Telescope and the Infrared Space Observatory (ISO) respectively are presented for two phenomenological models. The models are based on starburst and luminous infrared galaxy dominated populations. Both models produce excellent fits to the counts in both wavebands and provide an explanation for the high redshift population seen in the longer Spitzer 24 micron band supporting the hypothesis that they are luminous-ultraluminous infrared galaxies at z=2-3, being the mid-infrared counterparts to the sub-mm galaxy population. The source counts are characterized by strong evolution to redshift unity, followed by less drastic evolution to higher redshift. The number-redshift distributions in both wavebands are well explained by the effect of the many mid-infrared features passing through the observation windows. The sharp upturn at around a milliJansky in the 15 micron counts in particular depends critically on the distribution of mid-infrared features around 12 microns, in the assumed spectral energy distribution.Comment: 6 pages, 2 figures, accepted for publication MNRA

Multiphase modelling of the effect of fluid shear stress on cell yield and distribution in a hollow fibre membrane bioreactor

We present a simplified two-dimensional model of fluid flow, nutrient transport and cell distribution in a hollow fibre membrane bioreactor, with the aim of exploring how fluid flow can be used to control the distribution and yield of a cell population which is sensitive to both fluid shear stress and nutrient concentration. The cells are seeded in a scaffold in a layer on top of the hollow fibre, only partially occupying the extracapillary space. Above this layer is a region of free-flowing fluid which we refer to as the upper fluid layer. The flow in the lumen and upper fluid layer is described by the Stokes equations, whilst the flow in the porous fibre membrane is assumed to follow Darcy’s law. Porous mixture theory is used to model the dynamics of and interactions between the cells, scaffold and fluid in the cell–scaffold construct. The concentration of a limiting nutrient (e.g. oxygen) is governed by an advection–reaction–diffusion equation in each region. Through exploitation of the small aspect ratio of each region and asymptotic analysis, we derive a coupled system of partial differential equations for the cell volume fraction and nutrient concentration. We use this model to investigate the effect of mechanotransduction on the distribution and yield of the cell population, by considering cases in which cell proliferation is either enhanced or limited by fluid shear stress and by varying experimentally controllable parameters such as flow rate and cell–scaffold construct thickness

ASTRO-F - The next generation of mid-infrared surveys

We present basic observational strategies for ASTRO-F (also known as the Imaging Infra Red Surveyor (IRIS)) to be launched in 2004 by the Japanese Institute of Space and Astronautical Science (ISAS). We examine 2 survey scenarios, a deep ~1sq.deg. survey reaching sensitivities an order of magnitude below than the deepest surveys performed by ISO in the mid-IR, and a shallow \~18sq.deg mid-infrared (7-25um in 6 bands) covering an area greater than the entire area covered by all ISO mid-IR surveys. Using 2 cosmological models the number of galaxies predicted for each survey is calculated. The first model uses an enhancement of the pure luminosity evolution model of Pearson & Rowan-Robinson while the new models incorporate a strongly evolving ULIG component. For the deep survey, between 20,000-30,000 galaxies should be detected in the shortest wavebands and ~5000 in the longest (25um) band. The shallow survey would be expected to detect of the order of 100,000 - 150,000 sources.Comment: 17 pages, 13 figures, accepted by MNRA

De-blending Deep Herschel Surveys: A Multi-wavelength Approach

Cosmological surveys in the far infrared are known to suffer from confusion. The Bayesian de-blending tool, XID+, currently provides one of the best ways to de-confuse deep Herschel SPIRE images, using a flat flux density prior. This work is to demonstrate that existing multi-wavelength data sets can be exploited to improve XID+ by providing an informed prior, resulting in more accurate and precise extracted flux densities. Photometric data for galaxies in the COSMOS field were used to constrain spectral energy distributions (SEDs) using the fitting tool CIGALE. These SEDs were used to create Gaussian prior estimates in the SPIRE bands for XID+. The multi-wavelength photometry and the extracted SPIRE flux densities were run through CIGALE again to allow us to compare the performance of the two priors. Inferred ALMA flux densities (F$^i$), at 870$\mu$m and 1250$\mu$m, from the best fitting SEDs from the second CIGALE run were compared with measured ALMA flux densities (F$^m$) as an independent performance validation. Similar validations were conducted with the SED modelling and fitting tool MAGPHYS and modified black body functions to test for model dependency. We demonstrate a clear improvement in agreement between the flux densities extracted with XID+ and existing data at other wavelengths when using the new informed Gaussian prior over the original uninformed prior. The residuals between F$^m$ and F$^i$ were calculated. For the Gaussian prior, these residuals, expressed as a multiple of the ALMA error ($\sigma$), have a smaller standard deviation, 7.95$\sigma$ for the Gaussian prior compared to 12.21$\sigma$ for the flat prior, reduced mean, 1.83$\sigma$ compared to 3.44$\sigma$, and have reduced skew to positive values, 7.97 compared to 11.50. These results were determined to not be significantly model dependent. This results in statistically more reliable SPIRE flux densities.Comment: 8 pages, 7 figures, 3 tables. Accepted for publication in A&

A Study of the Microbial Community at the Interface between Granite Bedrock and Soil Using a Culture-Independent and Culture-Dependent Approach

The dissolution of minerals plays an important role in the formation of soils and sediments. In nutrient limiting soils, minerals constitute a major reservoir of bio-essential cations. Of particular interest is granite as it is the major rock type of the continental land mass. Although certain bacteria have been shown to enhance weathering of granite-forming minerals, little is known about the dissolution of granite, at the whole rock scale, and the microbial community involved. In this study, both culture-independent and culture-dependent approaches were used to study the bacterial community at the interface between granite bedrock and nutrient limiting soil in Dartmoor National Park, United Kingdom. High throughput sequencing demonstrated that over 70% of the bacterial population consisted of the bacterial classes Bacilli, Beta-proteobacteria and Gamma-proteobacteria. Bacteria belonging to the genera Serratia, Pseudomonas, Bacillus, Paenibacillus, Chromobacterium and Burkholderia were isolated from the sample site. All of the isolates were able to grow in a minimal growth medium, which contained glucose and ammonium chloride, with granite as the sole source of bio-essential elements. Sixty six percent of the isolates significantly enhanced basalt dissolution (p < 0.05). Dissolution of Si, K, Ca and Mg correlated with production of oxalic acid and acidification. The results of this study suggest that microorganisms in nutrient limiting soils can enhance the rate of granite dissolution, which is an important part of the biogeochemical cycle