Licentiatavhandlingar / Institutionen för fysik

Permanent URI for this collectionhttps://gupea-staging.ub.gu.se/handle/2077/28689

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    Using Lyapunov Exponents to Explain the Dynamics of Complex Systems
    (Department of Physics. University of Gothenburg, 2024) Storm, Ludvig
    Complex systems often display chaotic dynamics, characterised by being exponentially sensitive to changes in initial conditions. Such systems are in general difficult to analyse, due to the large number of nonlinearly interacting degrees of freedom. Dynamical-systems theory provides a framework for analysing such systems. One of the tools from this theory is the Lyapunov exponent, which quantifies the rate at which initially nearby trajectories converge or diverge over time. The exponent can be used to study how the stability of a complex system depends on different system parameters. The finite-time Lyapunov exponent can be used to reveal organising structures in the phase space of the system that separate it into different characteristic regions. These structures are referred to as Lagrangian coherent structures. In this thesis, the Lyapunov exponent and Lagrangian coherent structures are used to explore the properties of complex systems. In the two presented papers, artificial neural networks are analysed, which are machine-learning algorithms with a large number of interconnected nonlinear computational nodes. We show that these systems can be analysed as complex dynamical systems, and show, among other things, how this perspective helps shedding light on how the neural networks learn to perform classification tasks. Additionally, a project on how microswimmers can escape through transport barriers in flows using orientational diffusion is presented, where the transport barriers are Lagrangian coherent structures.
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    The mechanical Paul trap in the upper secondary physics laboratory
    (2023) Kilde Löfgren, Sebastian
    In physics, toy models are often used in popular science contexts. Further, toy models have also been used to communicate some important principles of abstract phenomena qualitatively. One such example can be found in the 1989 Nobel laureate Wolfgang Paul’s Nobel lecture. Previous research on models in science education has identified that using models and engaging students in model development could improve learning outcomes. However, an important issue is making model development meaningful for students in similar ways that scientists can use models and the modelling process to learn. This thesis investigates the usefulness of a toy model of the Paul trap, the mechanical Paul trap, for the upper secondary physics classroom. To do this, two studies were conducted, considered part of one design experiment. The first study investigated the mechanical Paul trap and its similarities and differences to a linear Paul trap to develop a physical model and a simulation for the design experiment. The second study used a design-based research approach, guided by variation theory, to conduct a mixed-method study developing a laboratory exercise and investigating how students interacted with and what learning was made possible using the mechanical Paul trap. The laboratory exercise was developed over three phases, consisting of three to five laboratory sessions each. Results from the first study provided suggestions for constructing an accessible and affordable physical model of the mechanical Paul trap. In addition, it resulted in the development of two similar simulations but in different online environments. The second study identified patterns of variation, highlighting critical steps where struggling and successful lab groups differed. One example is that it was critical to discern the threshold frequency as a step to discern the rotational frequency-trapping time relation. Further, findings also support the argument that the level of openness in laboratory exercises should be chosen deliberately by the teacher, with both the current groups of students and the complexity of the laboratory exercise at hand. Finally, it contributes to the discussion regarding the use of models in physics education by exemplifying how working with models can better help students understand the limitations and use cases of different models in physics.
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    Prospects for performing tests of quantum electrodynamics with high-intensity lasers
    (2023) Olofsson, Christoffer; Institutionen för fysik, Göteborgs Universitet
    Charged particles emit radiation in the presence of strong electromagnetic fields. Emission implies a recoil on the charge, altering its dynamics and is referred to as radiation reaction (RR). Current experiments are on the verge of probing the quantum regime of RR by usage of intense laser pulses colliding with high-energy electron beams. The framework for quantum RR is described by quantum electrodynamics (QED) and becomes nonperturbative in the strong-field limit (SFQED). Theoretical knowledge beyond it is limited but could potentially be detectable with upcoming laser facilities. Here, the presence of electron-positron cascades and low-energy emissions masks any signal emergent in such experiments. Hence, the appeal for strategies to extract signals of SFQED and/or the adoption of advanced statistical techniques. In this thesis, we develop strategies to retrieve information in laser-electron experiments. Firstly, we establish a selection rule based on the kinematic properties of electrons and their emissions to form a descriptive reading at the detector. Secondly, we explore the role of tight focusing of lasers to attain extreme regimes of interest of which we derive the optimal solution we call the bidipole wave. Finally, we propose an experimental framework capable of inferring parameters of models designated to capture SFQED effects using Bayesian techniques.
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    Visualizations of particle-field interactions
    (2022) Johansson, Andreas
    Visualizations within physics education are critical for learning physics and can be realized in a classroom with experiments, demonstrations, digital tools, mathematical analysis, or other representations, all with different levels of abstraction. This project aimed to determine whether the concept of field (i.e. electrical, acoustic, or optical fields) can be demonstrated, visualized, and applied in various experiments. In the first Paper, an experimental setup for visualizing charge particles’ motion in an electrical field was built. Designed learning activities were performed, and the effects on Swedish upper secondary school students’ conceptual understanding were tested. This work shows that students’ understanding of the interaction of charged particles with electrical fields increases more than without if a lecture includes an experiment that visualizes the phenomenon, either live or videotaped. In Paper II a remotely operable optical trap was realized and used to levitate and investigate charged droplets remotely from a classroom. Visualizing and measuring many fundamental physical processes are described. The motion of charged particles in electric fields and the photon pressure of light is described as well as how it can be safely demonstrated for a class. In Paper III, an optical trap is used to visualize the electron´s quantization. In this work, it was shown that the effect of a single electron addition can be magnified and observed by the naked eye and measured with a ruler analogous to Millikan’s experiment. The droplet is optically trapped and uncharged in an alternate electric field by an alpha radiation source. A strong electrical field was applied and as the uncharged droplet gained charges from the ionized air it jumped a well-defined step depending on how many electrons were added. The smallest jump corresponds to the addition of one electron, i.e. one elementary charge, and longer jumps are multiples of this. Finally, in Paper IV, a new type of experimental method to determine the volume of microliter-sized droplets in acoustic fields is described. By using a simulation of the acoustic field to assist in setting the cavity length a fast and self-calibrated method is presented.
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    The versatility of optical levitation
    (University of Gothenburg / Göteborgs universitet, 2022) Marmolejo, Javier Tello; Department of Physics / Institutionen för fysik
    The breadth of applications of optical tweezers in life sciences is so large, that it merited Arthur Ashkin a Nobel prize in 2018. Although optical levitation in air was invented first, it has not managed to be so widely adopted as optical tweezers. Here, I present 4 different applications to showcase the versatility of optical levitation. First, I show the forces acting on a particle through the construction of a fully manipulable damped driven harmonic oscillator. The transition from the over- to under-damped regimes is clearly recognizable and a big harmonic resonance can be observed by eye. Second, to measure the particle’s displacement in the previous experiment, we focused the scattered light with a lens. Here, I noticed an intriguing spider pattern that we fully explain by analyzing the spherical aberration of the lens. This resulted in a clear visualization and measurement of optical aberrations. Third, once we were free of aberrations, we were able to measure minute movements of the particle. This allowed us to create a modern version of the Millikan experiment where we add individual electrons to a single levitated droplet and observe the quantization of the electron by eye. Fourth, as the droplets evaporated we found a fascinating pattern in the scattering intensity. This turned out to be a directional Mie scattering spectrum with over 100 evolving Fano resonances arranged in a comb structure. We intuitively describe this spectrum through a quantum mechanical analogy to a spherical well potential. This converts our experiment into a “toy atom” where we see quantized angular momentum, ground and excited states, and tunneling. Through these 4 different applications, I provide an overview of optical levitation and its wide applicability
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    Topological stabilizer code on a honeycomb lattice
    (2022) Srivastava, Basudha
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    Smart microswimmers in complex flows
    (2022) Mousavi, Navid
    Zooplankton are playing a pivotal role in balancing the life on Earth. By grazing on phytoplankton and then serving as food source to larger aquatic animals, they pave the way of redistributing the Sun’s energy and formthe second level of the aquatic food chain. They are also important for the global carbon cycle and, as a result, their effect upon the climate is another reason that highlights their importance. Being small, they experience the flow as viscous. Nevertheless, they are able to each day migrate long distances efficiently. Their daily vertical migration is the largest natural migration of biomass on Earth, which is not well understood. In this thesis,we used a model to analyze the optimal navigation strategies for vertical migration of planktonic microswimmers in turbulent flows. Passive strategies for vertical swimming, such as gyrotaxis, where the swimmer is bottom-heavy and hence obtains a tendency to point upwards, do not have as good performance in turbulent flows as they have in quiescent flows. We present here active mechanisms that a microswimmer, similar to a juvenile copepod, can exploit to significantly increase its vertical migration efficiency in turbulent flows. We find that the modeled swimmer utilizes different mechanisms in two and three spatial dimensions. In two dimensions, they mimic longer swimmers by actively reorienting. This results in an increase in the rate of upwelling region sampling, which leads to a significant increase in swimming speed against gravity. On the other hand, in three dimensions, it turns out that actively keeping the swimming direction aligned against gravity is more efficient. Both mechanisms are found to be robust to moderate perturbations of the flow and swimmer parameters, and they explain how swimmers that do not benefit from passive gyrotaxis can obtain notable vertical migration rates.
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    TBA
    (2020) Dubey, Anshuman
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    Clustering and caustics in one-dimensional models of turbulent aerosols
    (Göteborgs universitet, 2018) Meibohm, Jan
    Heavy particles suspended in turbulent fluid flows, so-called turbulent aerosols, are common in Nature and in technological applications. A prominent example is rain droplets in turbulent clouds. Due to their inertia, ensembles of aerosol particles distribute inhomogeneously over space and can develop large relative velocities at small separations. We use statistical models that mimic turbulent flow by means of Gaussian random velocity fields to describe these systems. Compared to models that involve actual turbulence, our statistical models are simpler to study and allow for an analytical treatment in certain limits. Despite their simplic- ity, statistical models qualitatively explain the results of direct numerical simulations and experiments. In this Licentiate thesis, we focus primarily on studying one-dimensional versions of the statistical model. The results of these systems create intuition for, and give important insights into the behaviour of higher dimensional models of particles in turbulence.
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    Far-infrared conformer-specific signatures of small aromatic molecules of biological importance
    (2016) Yatsyna, Vasyl
    Our understanding of many biological processes requires knowledge about biomolecular structure and weak intra- and intermolecular interactions (e.g. hydrogen bonding). Both molecular structure and weak interactions can be directly studied by far-infrared (or THz) spectroscopy, which probes low-frequency molecular vibrations. In this thesis I present the results of experimental and theoretical investigations of far-infrared vibrations in small aromatic molecules of biological relevance. To enable a direct comparison with theory, far-infrared spectroscopy was performed in the gas phase with a conformer-selective IR-UV ion-dip technique. The far-infrared spectra of molecules containing a peptide (-CO-NH-) link revealed that the low-frequency Amide IV-VI vibrations are highly sensitive to the structure of the peptide moiety, the molecular backbone, and the neighboring intra- and intermolecular hydrogen bonds. The study of far-infrared spectra of phenol derivatives identified vibrations that allow direct probing of strength of hydrogen-bonding interaction, and a size of a ring closed by the hydrogen bond. Furthermore, benchmarking theory against the experimental data identified advantages and disadvantages of conventional frequency calculations for the far-infrared region performed with ab initio and density functional theory. For example, the conventional approaches were not able to reproduce strongly anharmonic vibrations such as aminoinversion in aminophenol. Instead, a double-minimum potential model was used for this vibration, and successfully described the experimental spectra of aminophenol. The results presented in this thesis can assist the interpretation of far-infrared spectra of more complex biomolecules, pushing forward low-frequency vibrational spectroscopy for efficient structural analysis and the studies of weak interactions.
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    Lärares formande av en yrkesidentitet relaterad till hållbar utveckling
    (2011) Bursjöö, Ingela; Göteborgs universitet / Institutionen för fysik
    Swedish schools are required to educate for sustainable development (ESD). The challenges for the teachers are demanding as the content is controversial and complex. This study focuses on exploring the different ways teachers and student teachers reflect upon their approach to ESD. Of particular interest is how they talk about the dilemmas they confront in their teaching. 1. How do teachers and student teachers describe their approach to ESD? 2. How do teachers and student teachers reflect on their personal lifestyle in relation to their professional teacher identity? For the purposes of this study a qualitative approach with interview questions were designed and conducted. This study uses open-ended questionnaires combined with focus-group as well as individual interviews. The teachers and the student teachers were asked about their reflections on their approach to ESD and how their own lifestyle affects their teaching credibility. The different phases of transformative learning were used as an analytical tool in order to better understand the variation in ways participants made meaning of ESD. Student teachers’ approaches to ESD depend on both external and internal factors. Three major external factors have been found: colleagues, time and the curriculum. The answers also describe the teaching methods in ESD as collaborative and interdisciplinary; ESD requires individual skills as well as a collective competence. The practicing teachers in the study have not received any professional development concerning education for sustainable development. However, all of them have a reflected explanation of how they teach for sustainable development. The different perceptions held by the teachers are categorized in three dimensions, described as a focus dimension, an action dimension and a reflection dimension. The focus dimension shows the direction of the teaching practice, a possible difference between environmental education and education for sustainability. The reflection dimension shows an identification of a transformative process and the action dimension could reveal a description of a dilemma. These three dimensions could be visualized in a 3D space of teachers’ approaches to education for sustainable development. All of the teachers and the student teachers describe education for sustainable development as difficult to teach since it involves conflicts and contradictions. The student teachers express more comfort in content and teaching methods, on the other hand they perceive more tensions with colleagues, time constraint and their lifestyle. Some teachers in the study even avoid education for sustainable development because of its political connotations. This implies that ESD is, indeed, difficult, and involves choices of different approaches to ESD such as normative, fact-based and pluralistic. ESD is a contested concept and involves controversial values which lead to dilemmas for the teacher. Dilemmas and critical self- reflection are important for transformative learning. These results may be important to teacher education as well as to teachers’ professional development as they provide insights for implementation of changes in the educational system. The use of the different phases of transformative learning as an analytical tool could also contribute to the understanding of how teachers form their professional identity. The findings also provides an opportunity to perform longitudinal studies as a way to identify different trajectories of teacher´s learning processes in a dynamic and changing educational system. Still, much work remains to identify, analyse and improve different aspects of teaching, in order to be able to stress that the implementation of ESD is based on both research and best practice.