5,535 research outputs found
Breaking boundaries:Charge density waves, quantum measurement, and black holes in theoretical physics
This thesis, titled âBreaking Boundariesâ is a journey through three topics united by the theme of boundaries in physics. First, the journey begins with an investigation into charge density waves (CDWs) and their nearly commensurate phase, focusing on the materials 2H-TaSe2 and 1T-TaS2. An extensive treatment of Ginzburg-Landau theory is covered with an extension into truly two-dimensional systems. This extension is used to study spiral patches of commensurate charge density waves observed in experiment. The research leads to a novel perspective on CDW behaviour with the existence of a spiral CDW phase in a range of materials. Secondly, transitioning to the quantum realm, the thesis addresses the quantum measurement problem, emphasizing the constraints any valid theory must possess. It critiques existing models, demonstrates the non-linearity of objective collapse theories, and proposes a minimal model that bridges quantum mechanics and classical physics. Thirdly, the thesis delves into black holes and specifically the phenomena of thermal radiation due to a horizon. First, we explore analogue models that mimic the thermal spectrum near a black hole horizon, to pave the way to experimental realization. Then we focus on the region far away from a black hole horizon and challenge the notion of remnant radiation at this position. With a theoretical toy model, we study the regime and find a non-evaporating black hole. This questions the validity of standard Hawking radiation calculations.In conclusion, the thesis navigates through the boundaries of material behaviours, the quantum-classical divide, and the enigmatic nature of black holes. It highlights the blurring and breaking of boundaries in physics, offering new perspectives and promising avenues for future discoveries
Faster inference from state space models via GPU computing
Funding: C.F.-J. is funded via a doctoral scholarship from the University of St Andrews, School of Mathematics and Statistics.Inexpensive Graphics Processing Units (GPUs) offer the potential to greatly speed up computation by employing their massively parallel architecture to perform arithmetic operations more efficiently. Population dynamics models are important tools in ecology and conservation. Modern Bayesian approaches allow biologically realistic models to be constructed and fitted to multiple data sources in an integrated modelling framework based on a class of statistical models called state space models. However, model fitting is often slow, requiring hours to weeks of computation. We demonstrate the benefits of GPU computing using a model for the population dynamics of British grey seals, fitted with a particle Markov chain Monte Carlo algorithm. Speed-ups of two orders of magnitude were obtained for estimations of the log-likelihood, compared to a traditional âCPU-onlyâ implementation, allowing for an accurate method of inference to be used where this was previously too computationally expensive to be viable. GPU computing has enormous potential, but one barrier to further adoption is a steep learning curve, due to GPUs' unique hardware architecture. We provide a detailed description of hardware and software setup, and our case study provides a template for other similar applications. We also provide a detailed tutorial-style description of GPU hardware architectures, and examples of important GPU-specific programming practices.Publisher PDFPeer reviewe
Meta-critical thinking, paradox, and probabilities
There is as much lack of clarity concerning what âcritical thinkingâ involves, even among those charged with teaching it, as there is consensus that we need more emphasis on it in both academia and society. There is an apparent need to think critically about critical thinking, an exercise that might be called meta-critical thinking. It involves emphasizing a practice in terms of which âcritical thinkingâ is helpfully carried out and clarifying one or more of the concepts in terms of which âcritical thinkingâ is usually defined. The practice is distinction making and the concept that of evidence. Science advances by constructing models that explain real-world processes. Once multiple potential models have been distinguished, there remains the task of identifying which models match the real-world process better than others. Since statistical inference has in large part to do with showing how data provide support, i.e., furnish evidence, that the model/hypothesis is more or less likely while still uncertain, we turn to it to help make the concept more precise and thereby useful. In fact, two of the leading methodological paradigmsâBayesian and likelihoodâcan be taken to provide answers to the questions of the extent to which as well as how data provide evidence for conclusions. Examining these answers in some detail is a highly promising way to make progress. We do so by way of the analysis of three well-known statistical paradoxesâthe Lottery, the Old Evidence, and Humphreysââand the identification of distinctions on the basis of which their plausible resolutions depend. These distinctions, among others between belief and evidence and different concepts of probability, in turn have more general applications. They are applied here to two highly contested public policy issuesâthe efficacy of COVID vaccinations and the fossil fuel cause of climate change. Our aim is to provide some tools, they might be called âhealthy habits of mind,â with which to assess statistical arguments, in particular with respect to the nature and extent of the evidence they furnish, and to illustrate their use in well-defined ways
LIPIcs, Volume 251, ITCS 2023, Complete Volume
LIPIcs, Volume 251, ITCS 2023, Complete Volum
Proceedings of SIRM 2023 - The 15th European Conference on Rotordynamics
It was our great honor and pleasure to host the SIRM Conference after 2003 and 2011 for the third time in Darmstadt. Rotordynamics covers a huge variety of different applications and challenges which are all in the scope of this conference. The conference was opened with a keynote lecture given by Rainer Nordmann, one of the three founders of SIRM âSchwingungen in rotierenden Maschinenâ. In total 53 papers passed our strict review process and were presented. This impressively shows that rotordynamics is relevant as ever. These contributions cover a very wide spectrum of session topics: fluid bearings and seals; air foil bearings; magnetic bearings; rotor blade interaction; rotor fluid interactions; unbalance and balancing; vibrations in turbomachines; vibration control; instability; electrical machines; monitoring, identification and diagnosis; advanced numerical tools and nonlinearities as well as general rotordynamics. The international character of the conference has been significantly enhanced by the Scientific Board since the 14th SIRM resulting on one hand in an expanded Scientific Committee which meanwhile consists of 31 members from 13 different European countries and on the other hand in the new name âEuropean Conference on Rotordynamicsâ. This new international profile has also been
emphasized by participants of the 15th SIRM coming from 17 different countries out of three continents. We experienced a vital discussion and dialogue between industry and academia at the conference where roughly one third of the papers were presented by industry and two thirds by academia being an excellent basis to follow a bidirectional transfer what we call xchange at Technical University of Darmstadt. At this point we also want to give our special thanks to the eleven industry sponsors for their great support of the conference. On behalf of the Darmstadt Local Committee I welcome you to read the papers of the 15th SIRM giving you further insight into the topics and presentations
Coherence and avoidance of sure loss for standardized functions and semicopulas
We discuss avoidance of sure loss and coherence results for semicopulas and
standardized functions, i.e., for grounded, 1-increasing functions with value
at . We characterize the existence of a -increasing
-variate function fulfilling for standardized
-variate functions and discuss the method for constructing this
function. Our proofs also include procedures for extending functions on some
countably infinite mesh to functions on the unit box. We provide a
characterization when respectively coincides with the pointwise infimum
respectively supremum of the set of all -increasing -variate functions
fulfilling .Comment: 31 pages, 2 figure
Peering into the Dark: Investigating dark matter and neutrinos with cosmology and astrophysics
The LCDM model of modern cosmology provides a highly accurate description of our universe.
However, it relies on two mysterious components, dark matter and dark energy. The cold dark matter
paradigm does not provide a satisfying description of its particle nature, nor any link to the Standard
Model of particle physics.
I investigate the consequences for cosmological structure formation in models with a coupling
between dark matter and Standard Model neutrinos, as well as probes of primordial black holes as
dark matter.
I examine the impact that such an interaction would have through both linear perturbation theory and
nonlinear N-body simulations. I present limits on the possible interaction strength from cosmic
microwave background, large scale structure, and galaxy population data, as well as forecasts on the
future sensitivity. I provide an analysis of what is necessary to distinguish the cosmological impact of
interacting dark matter from similar effects. Intensity mapping of the 21 cm line of neutral hydrogen at
high redshift using next generation observatories, such as the SKA, would provide the strongest
constraints yet on such interactions, and may be able to distinguish between different scenarios
causing suppressed small scale structure. I also present a novel type of probe of structure formation,
using the cosmological gravitational wave signal of high redshift compact binary mergers to provide
information about structure formation, and thus the behaviour of dark matter. Such observations
would also provide competitive constraints.
Finally, I investigate primordial black holes as an alternative dark matter candidate, presenting an
analysis and framework for the evolution of extended mass populations over cosmological time and
computing the present day gamma ray signal, as well as the allowed local evaporation rate. This is
used to set constraints on the allowed population of low mass primordial black holes, and the
likelihood of witnessing an evaporation
Fano-like resonance from disorder correlation in vacancy-doped photonic crystals
By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defectsPID2021-124814NB-C21, PGC2018-095777-B-C22, PID2019-109905GA-C22, CEX2018-000805-M, UAM-CAM project (SI1/PJI/2019-00052
Complex systems methods characterizing nonlinear processes in the near-Earth electromagnetic environment: recent advances and open challenges
Learning from successful applications of methods originating in statistical mechanics, complex systems science, or information theory in one scientific field (e.g., atmospheric physics or climatology) can provide important insights or conceptual ideas for other areas (e.g., space sciences) or even stimulate new research questions and approaches. For instance, quantification and attribution of dynamical complexity in output time series of nonlinear dynamical systems is a key challenge across scientific disciplines. Especially in the field of space physics, an early and accurate detection of characteristic dissimilarity between normal and abnormal states (e.g., pre-storm activity vs. magnetic storms) has the potential to vastly improve space weather diagnosis and, consequently, the mitigation of space weather hazards.
This review provides a systematic overview on existing nonlinear dynamical systems-based methodologies along with key results of their previous applications in a space physics context, which particularly illustrates how complementary modern complex systems approaches have recently shaped our understanding of nonlinear magnetospheric variability. The rising number of corresponding studies demonstrates that the multiplicity of nonlinear time series analysis methods developed during the last decades offers great potentials for uncovering relevant yet complex processes interlinking different geospace subsystems, variables and spatiotemporal scales
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