Electromagnetism and Magnetic Monopoles
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Physics & Astronomy Institute for theoretical physics Electromagnetism and magnetic monopoles Author Supervisor F. Carere Dr. T. W. Grimm June 13, 2018 Image: Daniel Dominguez/ CERN Abstract Starting with the highly symmetric form of electromagnetism in tensor notation, the consideration of magnetic monopoles comes very natural. Following then the paper of J. M. Figueroa-OFarrill [1] we encounter the Dirac monopole and the 't Hooft-Polyakov monopole. The former, simpler { at the cost of a singular Dirac string { monopole already leading to the very important Dirac quantization condition, implying the quantization electric charge if magnetic monopoles exist. In particular the latter monopole, which is everywhere smooth and which has a purely topological charge, is found as a finite-energy, static solution of the dynamical equations in an SO(3) gauge invariant Yang-Mills-Higgs system using a spherically symmetric ansatz of the fields. This monopole is equivalent to the Dirac monopole from far away but locally behaves differently because of massive fields, leading to a slightly different quantization condition of the charges. Then the mass of general finite-energy solutions of the Yang-Mills-Higgs system is considered. In particular a lower bound for the mass is found and in the previous ansatz a solution saturating the bound is shown to exist: The (very heavy) BPS monopole. Meanwhile particles with both electric and magnetic charge (dyons) are considered, leading to a relation between the quantization of the magnetic and electric charges of both dyons and, when assuming CP invariance, to an explicit quantization of electric charge. Finally, the Z2 duality of Maxwell's equations is extended. When P SL(2; Z) duality is assumed for electric and magnetic monopoles satisfying the Bogomol'nyi mass bound a dyonic spectrum is found in the orbit of the electron. Contents Preface 1 I Electromagnetism and relativity 2 1 Electromagnetism and electromagnetic duality 2 1.1 Electromagnetism in vector calculus notation . .2 1.2 Electromagnetism in tensor notation . .4 2 A magnetic monopole 6 3 Relativity 7 3.1 The structure of flat spacetime: Minkowski space . .7 3.2 Lorentz transformations . .8 3.3 Proper time . .9 4 Electromagnetism and relativity 11 5 Lagrangian mechanics and actions 13 5.1 Classical mechanics in integral form . 13 5.2 The relativistic Lagrangian . 14 5.3 The classical electromagnetic Lagrangian . 14 5.4 The relativistic electromagnetic Lagrangian . 15 5.5 The Hamiltonian . 16 II Symmetries, fields, gauge theories and the Higgs mechanism 17 1 Symmetry 18 1.1 Classification of symmetries . 18 1.2 Symmetry breaking . 20 1.2.1 Explicit symmetry breaking . 21 1.2.2 Spontaneous symmetry breaking . 22 2 Some classical field theory 23 2.1 General forms of the Lagrangian density . 23 2.2 Noethers theorem . 24 2.3 The Hamiltonian density . 25 3 Gauge theory 26 3.1 Classical abelian gauge theory . 26 3.2 Gauge invariance of the relativistic electromagnetic Lagrangian . 28 3.3 Gauge theory for the Dirac Lagrangian . 29 3.4 The general strategy for U(1) . 29 3.5 Yang-Mills theory . 30 3.6 Again Yang-Mills . 32 3.7 The case of SO(3) ..................................... 33 3.8 Yang-Mills in general . 34 4 The Higgs mechanism 36 4.1 The Higgs field and its potential . 36 4.2 Massless gauge bosons from a broken global symmetry . 37 4.3 The Higgs mechanism . 38 III Magnetic monopoles 39 1 The Dirac monopole 40 1.1 The search for a global vector potential . 40 1.2 The Dirac quantization condition . 41 2 Dirac monopole: Dyons and the Zwanziger-Schwinger quantization condition 44 3 The bosonic part of the Georgi-Glashow model 48 4 Static, finite-energy solutions in the 't Hooft-Polyakov ansatz 52 4.1 The solution of H and K in the 't Hooft-Polyakov ansatz . 53 4.2 The topological origin of the magnetic charge . 56 5 The difference between the Dirac and the 't Hooft-Polyakov monopole 61 6 Mass and the BPS monopoles 62 6.1 The Bogomol'nyi bound . 62 6.2 The BPS monopole . 63 6.2.1 The BPS monopole in the 't Hooft-Polyakov ansatz . 64 7 Electromagnetic duality revisited 67 7.1 The Montonen-Olive conjecture . 67 7.2 Quantization of dyons . 68 7.3 The Witten effect . 69 7.4 P SL(2; Z) Duality . 70 7.5 The modular group . 70 7.6 Orbifolds and the dyonic spectrum . 72 IV Discussion 75 A Tensors 77 A.1 Basistransformations: Co- and contravariant objects . 77 A.1.1 Co- and contravariant indices, raising and lowering indices . 77 A.2 Transformation properties of tensors . 77 B The Hamiltonian density in the 't Hooft-Polyakov ansatz 78 C The fundamental group of maps between circles 80 C.1 The structure of loops . 81 C.2 Homotopy equivalent spaces . 82 C.3 The fundamental group of maps between circles . 82 D Lie groups and Lie algebras 85 D.1 A note on SO(3)...................................... 85 Preface: Discussing magnetic monopoles Around 600 B.C. the Greek philosopher Thales of Miletus described the peculiar phenomena of attractive forces between two substances: Fur and amber [2]. He was likely the first to make a written description of electrical forces. Indeed it is due to the ancient Greeks (and Gilbert) that the term ηλκτρoν (meaning amber) is now used for one of the most fundamental objects in electrostatics: the electron. Furthermore, the Greeks were aware that there existed stones which seemed to attract iron when held close. The magnetic force was found, named after the Turkish city of Magnesia (now Manisa), where the stones were found. These two magical forces turned out to behave very similar. Electric poles were introduced in the theory of electrostatics. Due to a large amount of symmetry in the theories magnetic poles were introduced, in accordance with electrostatics. The first mentioning of magnetic monopoles could well have been in a letter of Pierre of Maricourt in 1269 [3]. The magnetic monopole, however, was { and to this day still is { unobserved. After the discovery of Oersted in 1820 that current deflects a compass needle, electromagnetism was born [4]. This, later on, led to Amp`eresmodel of the magnet, in which the magnetic fields of a magnet come from `little Amperian currents' [5]. This model did no good to the theory of magnetic monopoles, since in this model magnetic fields were not produced by a stationary magnetic monopole but moving electric charges. The intrinsic connection between electricity and magnetism was clearly there and Amp`ere'smodel was the first step to fathom this duality. It was Maxwell's equations, however, which were able to combine the theory of electrostatics and magnetostatics in a very elegant fashion. The four Maxwell equations, together with the equation for the Lorentz force, are able to fully describe (static and dynamic) classical electromagnetism. One of the four laws of Maxwell, however, is r~ · B~ = 0, sometimes called the `no magnetic monopole law'. This short history of classical electromagnetism might indicate that it is completely nonsensical to search for and describe magnetic monopoles. Indeed to this day they have not been seen in experiments. However one important argument in favour of the existence of magnetic monopoles is certainly the amount of symmetry in Maxwell's equations. Including the fact that over the last century gauge theories have become extremely important and very successful in the description of fundamental particles and forces (for instance in the Standard Model), together with the fact that these gauge theories rely on symmetry being an intrinsic property of nature itself, the search for monopoles might seem a bit less odd. Furthermore, in cosmology [6] and in quantum mechanics [7] reasons have come up why it might be that these monopoles have gone unseen and also are not made and observed in particle accelerators. Nevertheless, a reason why these monopoles have not yet been observed could just be the fact that they do not exist. Indeed in 1931 Dirac brought the theory of magnetic monopoles to life [7]. A magnetic monopole similar to the electric monopole was considered, which was therefore coined the Dirac monopole. In 1974 't Hooft [8] and Polyakov [9] both found inevitable monopole solutions in a Yang-Mills theory of a spontaneously broken non-abelian classical Lagrangian system. From far away, these monopoles behave in a similar manner as the Dirac monopole but close by they contain more information, including the mass of such a monopole. Another reason was found that the monopoles are hard to find: they are very heavy. In this paper properties of these types of (classical) magnetic monopoles will be described. In part I. it will start with the symmetrical formulation of Maxwell's equations in electrodynamics, first in vector calculus notation and then going to tensor notation to make it easy on readers with little experience in the latter kind. In the light of the highly symmetric formulation, magnetic monopoles will then be considered in part III, starting with the Dirac monopole and continuing on to the 't Hooft-Polyakov monopole, though a glimpse of gauge theories and symmetry breaking is given first in part II, which is needed for the discussion of the latter monopole. 1 Part I Electromagnetism and relativity Electromagnetism is a (special) relativistically invariant theory. Moreover, special relativity and electromagnetism have been two theories, which have seen development that was really intertwining [10]. For example, on completion of Maxwell's equations in classical electromagnetism, it quickly became clear that light is an electromagnetic wave. Furthermore, from special relativity it became clear that electric fields and magnetic fields are intrinsically connected via Lorentz transformations.