12,518 research outputs found
String Theory, Loop Quantum Gravity and Eternalism
Eternalism, the view that what we regard locally as being located in the past, the present and the future equally exists, is the best ontological account of temporal existence in line with special and general relativity. However, special and general relativity are not fundamental theories and several research programs aim at finding a more fundamental theory of quantum gravity weaving together all we know from relativistic physics and quantum physics. Interestingly, some of these approaches assert that time is not fundamental. If time is not fundamental, what does it entail for eternalism and the standard debate over existence in time? First, I will argue that the non-fundamentality of time to be found in string theory entails standard eternalism. Second, I will argue that the non-fundamentality of time to be found in loop quantum gravity entails atemporal eternalism, namely a novel position in the spirit of standard eternalism
Priority Monism Beyond Spacetime
I will defend two claims. First, Schaffer's priority monism is in tension with many research programs in quantum gravity. Second, priority monism can be modified into a view more amenable to this physics. The first claim is grounded in the fact that promising approaches to quantum gravity such as loop quantum gravity or string theory deny the fundamental reality of spacetime. Since fundamental spacetime plays an important role in Schaffer's priority monism by being identified with the fundamental structure, namely the cosmos, the disappearance of spacetime in these views might undermine classical priority monism. My second claim is that priority monism can avoid this issue with two moves: first, in dropping one of its core assumptions, namely that the fundamental structure is spatio-temporal, second, by identifying the connection between the non-spatio-temporal structure and the derivative spatio-temporal structure with mereological composition
The geometry of proper quaternion random variables
Second order circularity, also called properness, for complex random
variables is a well known and studied concept. In the case of quaternion random
variables, some extensions have been proposed, leading to applications in
quaternion signal processing (detection, filtering, estimation). Just like in
the complex case, circularity for a quaternion-valued random variable is
related to the symmetries of its probability density function. As a
consequence, properness of quaternion random variables should be defined with
respect to the most general isometries in , i.e. rotations from .
Based on this idea, we propose a new definition of properness, namely the
-properness, for quaternion random variables using invariance
property under the action of the rotation group . This new definition
generalizes previously introduced properness concepts for quaternion random
variables. A second order study is conducted and symmetry properties of the
covariance matrix of -proper quaternion random variables are
presented. Comparisons with previous definitions are given and simulations
illustrate in a geometric manner the newly introduced concept.Comment: 14 pages, 3 figure
The No Self View and the Meaning of Life
Several philosophers, both in Buddhist and Western philosophy, claim that the self does not exist. The no-self view may, at first glance, appear to be a reason to believe that life is meaningless. In the present article, I argue indirectly in favor of the no-self view by showing that it does not entail that life is meaningless. I then examine Buddhism and argue, further, that the no-self view may even be construed as partially grounding an account of the meaning of life
Spacetime Emergence in Quantum Gravity: Functionalism and the Hard Problem
Spacetime functionalism is the view that spacetime is a functional structure implemented by a more fundamental ontology. Lam and Wüthrich have recently argued that spacetime functionalism helps to solve the epistemological problem of empirical coherence in quantum gravity and suggested that it also (dis)solves the hard problem of spacetime, namely the problem of offering a picture consistent with the emergence of spacetime from a non-spatio-temporal structure. First, I will deny that spacetime functionalism solves the hard problem by showing that it comes in various species, each entailing a different attitude towards, or answer to, the hard problem. Second, I will argue that the existence of an explanatory gap, which grounds the hard problem, has not been correctly taken into account in the literature
Nonparametric estimation of the heterogeneity of a random medium using Compound Poisson Process modeling of wave multiple scattering
In this paper, we present a nonparametric method to estimate the
heterogeneity of a random medium from the angular distribution of intensity
transmitted through a slab of random material. Our approach is based on the
modeling of forward multiple scattering using Compound Poisson Processes on
compact Lie groups. The estimation technique is validated through numerical
simulations based on radiative transfer theory.Comment: 23 pages, 8 figures, 21 reference
Higher Order Statistsics of Stokes Parameters in a Random Birefringent Medium
We present a new model for the propagation of polarized light in a random
birefringent medium. This model is based on a decomposition of the higher order
statistics of the reduced Stokes parameters along the irreducible
representations of the rotation group. We show how this model allows a detailed
description of the propagation, giving analytical expressions for the
probability densities of the Mueller matrix and the Stokes vector throughout
the propagation. It also allows an exact description of the evolution of
averaged quantities, such as the degree of polarization. We will also discuss
how this model allows a generalization of the concepts of reduced Stokes
parameters and degree of polarization to higher order statistics. We give some
notes on how it can be extended to more general random media
Pulsation Frequencies and Modes of Giant Exoplanets
We calculate the eigenfrequencies and eigenfunctions of the acoustic
oscillations of giant exoplanets and explore the dependence of the
characteristic frequency and the eigenfrequencies on several parameters: the
planet mass, the planet radius, the core mass, and the heavy element mass
fraction in the envelope. We provide the eigenvalues for degree up to 8 and
radial order n up to 12. For the selected values of l and n, we find that the
pulsation eigenfrequencies depend strongly on the planet mass and radius,
especially at high frequency. We quantify this dependence through the
calculation of the characteristic frequency which gives us an estimate of the
scale of the eigenvalue spectrum at high frequency. For the mass range 0.5 <
M_P < 15 M_J, and fixing the planet radius to the Jovian value, we find that
the characteristic frequency is ~164.0 * (M_P/M_J)^(0.48) microHz, where M_P is
the planet mass and M_J is Jupiter's mass. For the radius range from 0.9 to 2.0
R_J, and fixing the planet's mass to the Jovian value, we find that the
characteristic frequency is ~164.0 * (R_P/R_J)^(-2.09) microHz, where R_P is
the planet radius and R_J is Jupiter's radius. We explore the influence of the
presence of a dense core on the pulsation frequencies and on the characteristic
frequency of giant exoplanets. We find that the presence of heavy elements in
the envelope affects the eigenvalue distribution in ways similar to the
presence of a dense core. Additionally, we apply our formalism to Jupiter and
Saturn and find results consistent with both the observationnal data of Gaulme
et al. (2011) and previous theoretical work.Comment: Accepted for publication in the Astrophysical Journal; 15 Figures and
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Evaluating Monetary Policy Rules in Estimated Forward-Looking Models: A Comparison of US and German Monetary Policies.
In this paper, we estimate two small, forward-looking, macroeconomic models for the US and Germany and we compare the implied optimal monetary policy rules. Both models have a standard structure: an I-S curve, a Phillips curve, a short term interest-rate rule and a long term interest rate determined by the Expectations Hypothesis. They are intended to fit the data while allowing for some forward-looking behavior. They are estimated from 1968 to 1998, using the full-information maximum-likelihood procedure, so that forward-looking expectations are fully model-consistent. In order to evaluate monetary policy, we compute optimal policy frontiers and we perform some simulations of the model. German optimal monetary policy is found to require a more persistent and slightly stronger response to inflation and output than the US optimal policy.Forward-looking model ; monetary policy rules
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