Aspects of Quantum Gravity
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ASPECTS OF QUANTUM GRAVITY AHMED FARAG ALI Master of Science, Benha University, 2008 A Thesis Submitted to the School of Graduate Studies of the University of Lethbridge in Partial Fulfilment of the Requirements for the Degree DOCTOR OF PHILOSOPHY Department of Physics and Astronomy University of Lethbridge LETHBRIDGE, ALBERTA, CANADA © Ahmed Farag Ali, 2012 Library and Archives Bibliotheque et Canada Archives Canada Published Heritage Direction du Branch Patrimoine de I'edition 395 Wellington Street 395, rue Wellington Ottawa ON K1A0N4 Ottawa ON K1A 0N4 Canada Canada Your file Votre reference ISBN: 978-0-494-88392-1 Our file Notre reference ISBN: 978-0-494-88392-1 NOTICE: AVIS: The author has granted a non L'auteur a accorde une licence non exclusive exclusive license allowing Library and permettant a la Bibliotheque et Archives Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par telecommunication ou par I'lnternet, preter, telecommunication or on the Internet, distribuer et vendre des theses partout dans le loan, distrbute and sell theses monde, a des fins commerciales ou autres, sur worldwide, for commercial or non support microforme, papier, electronique et/ou commercial purposes, in microform, autres formats. paper, electronic and/or any other formats. The author retains copyright L'auteur conserve la propriete du droit d'auteur ownership and moral rights in this et des droits moraux qui protege cette these. Ni thesis. Neither the thesis nor la these ni des extraits substantiels de celle-ci substantial extracts from it may be ne doivent etre imprimes ou autrement printed or otherwise reproduced reproduits sans son autorisation. without the author's permission. In compliance with the Canadian Conformement a la loi canadienne sur la Privacy Act some supporting forms protection de la vie privee, quelques may have been removed from this formulaires secondaires ont ete enleves de thesis. cette these. While these forms may be included Bien que ces formulaires aient inclus dans in the document page count, their la pagination, il n'y aura aucun contenu removal does not represent any loss manquant. of content from the thesis. Canada Dedication To Ebtesam, Ibrahim, and my parents. iii Abstract We propose a Generalized Uncertainty Principle(GUP) consistent with String Theory, Black Hole Physics and Doubly Special Relativity. This modifies all quantum mechan ical Hamiltonians and predicts quantum gravity corrections. We compute corrections to the Lamb shift, simple harmonic oscillator, Landau levels, and the tunneling cur rent. When applied to an elementary particle, it suggests that the space must be quantized, and that all measurable lengths are quantized in units of a fundamental length. We investigated whether GUP can explain the violation of the equivalence principle at small length scales that was observed experimentally. We investigated the consequences of GUP on Liouville theorem. We examined GUP effect on post inflation preheating, and show that it predicts an increase or a decrease in parametric resonance and a change in particle production. The effect of GUP on the creation of black holes is investigated to justify the experimental results from the Large Hadron Collider. iv Acknowledgements First and foremost, I would like to express about my deep gratitude for my supervisor Dr. Saurya Das whose expertise and attention to both detail and the big picture, provided invaluable guidance for my research, all the while being so friendly and straightforward during our discussions. He has been and remains my principal mentor, from whom I learned about various useful techniques and some of the important research problems in quantum gravity. The material presented here could not have been developed without his assistance which I appreciate very much. I am also very grateful to his guidance through the early years of chaos and confusion. I would like to thank Dr. Arundhati Dasgupta very much for teaching me Introductory Quantum Gravity which helped me very much to write the review chapter about approaches to quantum gravity of this thesis. I learned a lot during this course. I would like to thank Dr. Mark Walton for being so supportive and for his useful comments during the PhD program. I am greatly indebted for his useful suggestions in my meetings with the advisory committee. I should be thanking Dr. Kent Peacock for his kind advice and useful comments during the PhD program. I really appreciate that he kindly paid my attention for some fundamental questions about the discreteness of the space and for suggesting some possible answers. I am greatly indebted to Sheila Matson for her kindly help from the first day I came to Canada. I should be thanking Laurie Scott for her kindly help during the final stages in my PhD program. Above all, I am very grateful to my wife Ebtesam for her unwavering love and her patience. Without them this work would never have come into existence (literally). This dissertation is dedicated to her. v Table of Contents Approval/Signature Page ii Dedication iii Abstract iv Acknowledgements v Table of Contents vi 1 Introduction 1 1.1 General theory of relativity 1 1.2 Quantum mechanics 3 1.3 Outline of thesis 6 2 Approaches to quantum gravity 9 2.1 The canonical approach 10 2.2 Loop quantum gravity 15 2.3 Graviton approach 19 2.4 String theory 20 2.4.1 Important results 22 2.4.2 The Maldacena conjecture 23 2.5 Quantum gravity approaches and generalized uncertainty principle 25 3 The generalized uncertainty principle 29 3.1 Minimal-length uncertainty relations 31 3.2 Other forms of generalized uncertainty principles 34 3.2.1 Snyder's form 34 3.2.2 Modified de Broglie relation 35 3.2.3 Alternative approaches to the GUP 36 3.3 Minimal length-maximal energy uncertainty relation 37 vi 4 Testing quantum gravity effects 41 4.1 The Landau levels 42 4.2 Simple harmonic oscillator 44 4.3 The Lamb shift 47 4.4 Potential step 49 4.5 Potential barrier 52 5 Is space fundamentally discrete? 57 5.1 Discreteness of space in the non-relativistic case 57 5.1.1 Solution to order a 58 5.1.2 Solution to order a2 60 5.2 Discreteness of space in the relativistic case 63 5.2.1 Klein-Gordon equation in one dimension 63 5.2.2 Dirac equation in one dimension 64 5.2.3 Dirac equation in two and three dimensions 68 5.2.4 Spherical cavity: Dirac equation in polar coordinates 72 6 GUP effects on equivalence and holographic principles 78 6.1 The equivalence principle at short distance 79 6.2 The GUP and Liouville theorem 81 6.2.1 The holographic entropy bound and local quantum field theory . 84 6.2.2 The effect of GUP on the holographic entropy bound and LQFT . 88 7 Quantum gravity effects on the preheating phase of the universe 91 7.1 Particle production: Parametric resonance 92 7.2 Parametric resonance with non-linear terms 94 7.3 Parametric resonance with GUP 95 7.3.1 Instability region 98 7.4 Parametric resonance in an expanding universe with nonlinear terms .... 99 7.4.1 Case when H ^ 0 and A = 0 100 7.4.2 Case when H ^ 0 and A ^ 0 103 7.4.3 Parametric resonance in an expanding background with GUP .... 106 7.4.4 Instability region 108 8 Are black holes created at LHC? 110 8.1 Hawking temperature-uncertainty relation connection Ill 8.2 GUP quadratic in Ap and BH thermodynamics 115 8.3 GUP linear and quadratic in Ap and BH thermodynamics 118 8.4 NO black holes at LHC current energy scales due to GUP 120 vii 9 Summary 123 Bibliography 130 A Proof for Eq. 3.3.1 146 B Proof for Eq. (3.3.5) 148 C Minimal coupling to electromagnetism 149 D Number of states and maximum energy 152 viii Chapter 1 Introduction Quantum gravity (QG) is the field of theoretical physics which attempts to unify quantum mechanics with the general theory of relativity. It is considered to be the holy grail which may enable physicists to complete the unification of all fundamental laws of physics. The primary approach to quantum gravity leads to a theory with unsolvable di vergences [1]. However, there are different approaches which cured these divergences such as string theory [2] and loop quantum gravity [3]. In this chapter, we will first outline the nature of general relativity and quantum mechanics. 1.1 General theory of relativity The general theory of relativity is Einstein's theory of gravity. It is based on two fundamental principles: • The principle of relativity; which states that all systems of reference are equiv alent with respect to the formulation of the fundamental laws of physics. • The principle of equivalence; the weak one states that the local effects of motion in a curved space (gravitation) are indistinguishable from those of an accelerated 1 1.1. General theory of relativity observer in flat space, without exception. The strong one states that the out come of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime. Einstein thought about the consequences of these principles for many years using many thought experiments. He then realized the importance of Riemannian geometry to construct a new theory in which the gravitational force was a result of the curvature of space-time. In Newtonian gravity, the source of gravity is the mass. In general theory of relativity, the mass turns out to be part of a more general quantity called the energy-momentum tensor which includes both energy and momentum densities as well as stress (that is, pressure and shear).