Intro-Chap VI
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Pre-metric Electromagnetism By D. H. Delphenich For every good man chopping at the roots of evil there are a thousand more hacking at the leaves. Henry David Thoreau Leaf-hacking, being more labor-intensive than root-chopping, is more likely to get funding. David Henry Delphenich CONTENTS Page Table of Figures………………………………………………………………... iv Introduction…………………………………………………………………….. 1 1. The unification of electricity and magnetism 1 2. The evolution of geometrical optics into wave optics 3 3. Geometrization of gravity 7 4. Attempts at unifying electromagnetism with gravitation 10 5. Rise of quantum electrodynamics 15 6. Spacetime topology and electromagnetism 19 7. Pre-metric electromagnetism 20 8. Summary of contents 23 Chapter I – Calculus of exterior differential forms…………………………… 29 1. Multilinear algebra 29 2. Exterior algebra 32 3. Exterior derivative 41 4. Divergence operator 43 5. Lie derivative 44 6. Integration of differential forms 47 7. Relationship to vector calculus 49 Chapter II – Topology of differentiable manifolds……………………………… 54 1. Differentiable manifolds 54 2. Differential forms on manifolds 67 3. Differentiable singular cubic chains 68 4. Integration of differential forms 74 5. De Rham’s theorem 75 6. Hodge theory 78 7. Time-space splittings of the spacetime manifold 82 Chapter III – Static electric and magnetic fields…………………………………. 86 1. Electric charge 87 2. Electric field strength and flux 90 3. Electric excitation (displacement) 93 4. Electrostatic field equations 95 5. Electrostatic potential functions 96 6. Magnetic charge and flux 97 7. Magnetic excitation (induction) 100 8. Magnetostatic field equations 102 9. Magnetostatic potential 1-forms 103 10. Field-source duality 104 ii Pre-metric electromagnetism Chapter IV – Dynamic electromagnetic fields…………………………………….. 107 1. Electromagnetic induction 107 2. Conservation of charge 110 3. Pre-metric Maxwell equations 112 4. Electromagnetic potential 1-forms 114 Chapter V – Electromagnetic constitutive laws……………………………………. 120 1. Electromagnetic fields in macroscopic matter 120 2. Linear constitutive laws 121 3. Examples of local linear media 125 4. Some physical phenomena due to magneto-electric couplings 133 5. Nonlinear constitutive laws 135 6. Effective quantum constitutive laws 138 Chapter VI – Partial differential equations on manifolds………………………… 148 1. Differential operators on vector bundles 148 2. Jet manifolds 154 3. Exterior differential systems 157 4. Boundary-value problems 159 5. Initial-value problems 161 6. Distributions on differential forms 165 7. Fundamental solutions 170 8. Fourier transforms 175 Chapter VII – The interaction of fields and sources………………………………. 180 1. Construction of fields from sources 181 2. Lorentz force 192 3. Interaction of fields 195 4. Interaction of currents 197 Chapter VIII – Electromagnetic waves ……………………………………………. 201 1. Electromagnetic waves 201 2. Characteristics 213 3. Examples of dispersion laws 217 4. Speed of wave propagation 225 Chapter IX – Geometrical optics…………………………………………………… 231 1. The generalized eikonal equation 232 2. Bicharacteristics – null geodesics 233 3. Parallel translation 239 4. Huygens’s principle 243 5. Diffraction 254 Chapter X – The calculus of variations and electromagnetism…………………… 258 1. Electromagnetic energy 259 2. The calculus of variations in terms of vector bundles 267 Contents iii 3. Variational formulation of electromagnetic problems 272 4. Motion of a charge in an electromagnetic field 275 5. Fermat’s principle 287 Chapter XI – Symmetries of electromagnetism…………………………………… 291 1. Transformation groups 292 2. Symmetries of action functionals 300 3. Examples of Noether symmetries 303 4. Symmetries of electromagnetic action functionals 308 5. Symmetries of systems of differential equations 315 Chapter XII – Projective geometry and electromagnetism………………………… 330 1. Elementary projective geometry 332 2. Projective geometry and mechanics 342 3. The Plücker-Klein embedding 353 4. Projective geometry and electromagnetism 356 Chapter XIII – Complex relativity and line geometry………………………………. 362 1. Complex structures on real vector spaces 363 2. Isomorphic representations of the Lorentz group 367 3. General relativity in terms of Lorentzian frames 381 4. General relativity in terms of complex orthonormal frames 398 5. Discussion 408 Index………………………………………………………………………………… 411 LIST OF FIGURES 1. The boundary of a boundary of a 2-cube. 70 2. The representation of a circle as a 1-chain with boundary zero. 70 3. Two-dimensional spaces that are described by 2-chains. 71 4. Magnetic hysteresis. 135 5. Initial-value problems for ordinary and partial differential equations. 161 6. The fundamental solution for d/dx . 171 7. A typical 4-cycle that intersects the hypersurface φ(x) = 0. 216 8. The general Fresnel quartic (biaxial case). 219 9. Fresnel quartic (uniaxial case). 220 10. The construction of the evolved momentary wave surfaces by means of Huygens’s principle. 250 11. Geodesics in the geometrical optics approximation and in the diffracted case 257 12. Bringing a current loop in from infinity in an external magnetic field. 264 13. Klein’s hierarchy of geometries. 331 14. The projective line as an affine line plus a point at infinity. 336 15. An example of a perspectivity in the projective plane. 338 16. The effect of a converging lens on parallel lines. 343 17. The projection of a helix from homogeneous to inhomogeneous coordinates. 344 18. The planes defined by an isotropic F. 360 Introduction The term “pre-metric electromagnetism” refers to the formulation of the mathematical theory of electromagnetism in a manner that not only does not assume the existence of a Lorentzian metric on the spacetime manifold to begin with, but also exhibits the appearance of such a geometrical structure as a natural consequence of investigating the manner in which electromagnetic waves propagate through that medium. Since the Lorentzian metric that appears – at least, under restricted conditions – is commonly taken to account for the presence of gravitation in the spacetime medium in the viewpoint of general relativity, one sees that in order to properly define the context of pre-metric electromagnetism one must really discuss not only electricity and magnetism, but also gravity, as well. Hence, before we embark upon the detailed discussion of the mathematical and physical bases for the theory of pre-metric electromagnetism, we shall briefly recall the conceptual evolution of the three relevant physical phenomena of electricity, magnetism, and gravity. 1. The unification of electricity and magnetism 1. In ancient times, it is unlikely that man ever suspected that the natural phenomena associated with electricity, such as lightning and static electricity on animal furs, could possibly be associated with magnetic phenomena, which were discovered somewhat later in history, and most likely in the Iron Age in the context of lodestones, which are magnetite deposits that have become magnetized from long-term exposure to the Earth’s magnetic field. However, when Europe emerged from the Dark Ages into the Renaissance the science of electricity gradually gave way to the development of batteries 2, wires, and currents, on the one hand, with the development of compasses for marine navigation on the other. It was only a matter of time before the early experimenters, primarily Hans Christian Oersted (1777-1851) and Michael Faraday (1791-1867), noticed that electrical currents could produce measurable magnetic fields around conductors, while time- varying magnetic fields could conversely induce electrical currents in current loops. Actually, this is not a precise converse, since a time-constant electrical current can induce a time-constant magnetic field, while a time-constant magnetic field will not induce an electrical current of any sort. This reciprocal set of phenomena was called electromagnetic induction . One of Faraday’s other innovations in the name of electromagnetism was the introduction of the concept of invisible force fields distributed in space that accounted to the forces of attraction or repulsion that “test” electric charges and magnetic dipoles experienced when placed at each point. Although nowadays the concept of vector fields 1 For a comprehensive historical timeline of the theories of electricity and the ether, one should confer the tomely two-volume treatise of E. T. Whittaker [ 1]. 2 Apparently, the concept of a battery had been developed in a rudimentary form by ancient cultures, as pottery that seemed to involve weak acids and metal electrodes has been unearthed by archeologists. 2 Pre-metric electromagnetism seems rather commonplace and above dispute, nevertheless, in its day the ideas of invisible lines of force apparently seemed rather mystical and dubious. It is important to note that the key to defining such fields is the association of a purely dynamical notion – namely, force – with more esoteric ones, in the form of electrical and magnetic fields. In addition to this key development, one must also observe the evolution of the theory of electrostatic forces from the early measurements by Charles Augustin de Coulomb (1736-1806) and the formulation of the empirical law that bears his name to its formulation in terms of potential theory by Laplace, Poisson, and the host of contributors to the theory of boundary-value problems in the Laplace or Poisson equation, such as Green, Cauchy, Dirichlet, Neumann, Robin, and many more. It was also found that although the static magnetic field was