Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
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Item Interdisciplinary Reasoning about Energy in an Introductory Physics Course for the Life Sciences(2014) Dreyfus, Benjamin William; Redish, Edward F; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energy is a unifying concept that cuts across physics, chemistry, and biology. However, students who study all three disciplines can end up with a fragmented understanding of energy. This dissertation sits at the intersection of two active areas of current research: the teaching and learning of energy, and interdisciplinary science education (particularly the intersection of physics and biology). The context for this research is an introductory physics course for undergraduate life sciences majors that is reformed to build stronger interdisciplinary connections between physics, biology, and chemistry. An approach to energy that incorporates chemical bonds and chemical reactions is better equipped to meet the needs of life sciences students than a traditional introductory physics approach that focuses primarily on mechanical energy, and so we present a curricular thread for chemical energy in the physics course. Our first set of case studies examines student reasoning about ATP hydrolysis, a biochemically significant reaction that powers various processes in the cell. We observe students expressing both that an energy input is required to break a chemical bond (which they associate with physics) and that energy is released when the phosphate bond is broken in ATP (which they associate with biology). We use these case studies to articulate a model of interdisciplinary reconciliation: building coherent connections between concepts from different disciplines while understanding each concept in its own disciplinary context and justifying the modeling choices in deciding when to use each disciplinary model. Our second study looks at ontological metaphors for energy: metaphors about what kind of thing energy is. Two ontological metaphors for energy that have previously been documented include energy as a substance and energy as a location. We argue for the use of negative energy in modeling chemical energy in an interdisciplinary context, and for the use of a blended substance/location ontology in reasoning about negative energy. Our data show students and experts using the blended ontology productively when the two ontologies are combined in a coherent structure, as well as students getting confused when the ontologies are not coherently combined.Item Energy, stars, and black holes in Einstein-aether theory(2007-07-12) Eling, Christopher; Jacobson, Theodore A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years there have been hints of Lorentz violation in various approaches to quantum gravity. Lorentz violating physics has also been proposed as an explanation for unexpected observational anomalies such as atmospheric cosmic rays apparently observed above the GZK cutoff, the flatness of galactic rotation curves and the accelerating expansion of the universe. In this dissertation we will consider an alternative theory of gravity that exhibits Lorentz violation. This ``Einstein-aether" theory is a four parameter class of theories where a dynamical unit timelike vector field (the ``aether") is coupled to gravity. We will focus particularly on energy, stars, and black holes in the theory. First, using pseudotensor methods we find expressions for the Einstein-aether energy. These are then applied to find the energy in both linear and non-linear regimes. Enforcing the energy positivity of linearized wave modes yields an important constraint on the four parameters. An expression for the energy of an asymptotically flat spacetime is also obtained, but a complete positive energy theorem remains elusive. Next, we study in detail non-linear spherically symmetric solutions in the theory. The time independent asymptotically flat solutions fall into two classes depending on whether the aether is aligned with the timelike Killing vector. ``Static" solutions aligned with the Killing vector describe the interior and vacuum regions of fluid stars. We characterize properties such as maximum masses and surface redshifts for candidate neutron star equations of state. Only tentative observational constraints on the theory are currently possible due to uncertainties in neutron star physics. Black hole solutions, which must be non-static, are shown to exist in a class of Einstein-aether theories using numerical integration. The geometry outside the horizon is very similar to the Schwarzschild solution of General Relativity, but there are qualitative differences inside. Finally, we investigate classical two-dimensional Einstein-aether theory as a toy model that could be used to study the Hawking effect and quantization in a Lorentz violating setting. We conclude by examining directions for future research.