Sharif Digital Repository

The goal of this thesis is to evaluate the strengths and weaknesses of the two existing formulations of Bohmian mechanics. The first one was proposed by Bohm, and emphasizes on the dynamics being second-order, and in which, the key tool for explaining the quantum phenomena is a non-classical potential, called "the quantum potential". In this approach, the postulates of de Broglie's "pilot-wave" theory, which now is known as Bohmian mechanics, are non-fundamental and are special to the atomic level. The possibility is proposed that these postulates become subjected to some modifications at the nuclear or lower levels, but no particular way for performing such modifications is suggested. The... 

In the special theory of relativity, it is not necessary to propose that there is no privileged frame of reference but because this formalism is of the epistemological level, we can not deduce this ontological conclusion. On the other hand, there are different physical motivations for a privileged frame of reference and also there is an alternative physical model for special relativity called inertial transformations which is similar to the special theory of relativity at the observational level, but there is a privileged frame of reference in that theory. In this paper, I have tried to collect motivations for a privileged frame of reference in physics, in order to prepare a source for those... 

Inflationary Cosmology is the prevailing paradigm of Early Universe Cosmology. In these models, by considering that in the first moments after the Big Bang, there has been a scalar field which has dominated the energy of the universe, we can create a an era called inflation in which the scale factor has a positive acceleration. By such an era, we can solve some of the problems of the Standard Big Bang (SBB) theory such as the horizon problem and the flatness problem, which are in relation with the initial conditions. In addition, this paradigm creates a suitable context for explaining the creation of the inhomogeneities in the universe. Therefore, these models have a predictability feature... 

One of the most simple questions of particle physics, which is still unanswered, is whether neutrino and antineutrino are the same particles or not. The answer to this question has many implications for the physics beyond the Standard Model, supersymmetric theories and unified theories.
In this thesis we first introduce Majorana fermion, the best candidate for which is neutrino. Then its implications for going beyond the standard Model have been discussed. At present time, the most important experiment which can prove this guess is neutrinoless double beta decay which has been studied in details in this thesis. The study of the theoretical and experimental aspects of neutrinoless double... 

In addition to the successes of modern science and technology in helping humanity, science has brought harmful effects ,which has led to criticisms from different domains such ethics, religion, politics, philosophy , etc. Nicholas Maxwell as an Emeritus Reader in the Philosophy of Science has shown , in several of his works, the harmful byproducts of science, and has proposed some ideas for bring about a revolution in the philosophy of science. His main thesis is to change the academic and non-academic aim of works, from knowledge to wisdom. In summary, his concerns are: inappropriate metaphysical presuppositions concerning the comprehensibility of our universe, attention to rational... 

The influences of metaphysics in physical science in contemporary philosophy of science have the main place. In this article we consideration the scientific experiment and model for understanding of metaphysic influence .we answer the epistemological and metaphysical question about scientific experiment. We notice in the approximation in the scientific experiment and modeling. In this line we inference the exponential radiation law and consider approximation in this inference and conclude that from approximation, metaphysics of individual scientist influence in the future of physical sciences. We consider the role of model in scientific explanation and with this study we understand that... 

Nucleosynthesis and CMB data clearly indicate that most of the matter
in the universe is dark and non-baryonic. There’s no shortage of particle
physics candidates for this dark matter. This candidates can be classified
according to whether the dark matter particles originated via decoupling
from thermal bath or were created in some non thermal process. Thermal relics can be categorized further as to whether they were relatvistic or nonrelativistic at the moment of decoupling. The relics which were relatvistic at this time constitute hot dark matter, while those which where nonrelativistic constitute cold dark matter. The goal of this thesis is reviewing hot dark matter... 

The advent of Quantum Mechanics in the beginning of twentieth century created a fundamental evolution on physics. The ability and performance of Quantum Mechanics in perception of natural phenomena have spread its penetration farther than physics and affected different realms of science, and especially Philosophy. As a result, several philosophic and logical principles defined according to the Quantum Mechanics, and also several of them were rejected because of their inconsistency with Quantum Mechanics. Since Quantum Mechanics was not flawless theory, some of these evolutions have created profound deviations in interpretation and perception of nature. One of these changes was denial of... 

Nucleosynthesis and CMB data clearly indicate that most of the matter in the universe is dark and of course, nonbaryonic. In the particle physics, many particles have been proposed to explain relic density of Dark Matter. These candidates can be classified according to whether they have decoupled from the thermal bath or were created in a nonthermal process. In the first class, thermal relics can be categorized into relativistic and nonrelativistic particles, at the moment of decoupling. The relics which were relativistic are called Hot Dark Matter, while those relics which were none relativistic constitute Cold Dark Matter. The goal of this thesis is investigation of cold dark matter... 

Although the origin of matter-antimatter asymmetry remains unknown, continuing advances in theory and improved experimental limits have ruled out some scenarios for baryogenesis: for example, the sphaleron baryogenesis at the electroweak phase transition in the standard model. In this thesis, we give an introduction to some of the ideas and attempts to understand the origin of the matte -antimatter asymmetry of the universe. After the discussion of some basic issues of cosmology and particle theory, the scenarios of electroweak baryogenesis, GUT baryogenesis, and leptogenesis are outlined  

For more than a century, physicists have recognized the contrast between the asymmetry of thermodynamic phenomena and T-symmetry of the fundamental laws, and they have referred to it as the arrow of time. In this thesis we tried to give an overview of the past and present status of this problem, using various theories. It is shown that the problem becomes a question about the boundary conditions of thermodynamic systems, and then a question about cosmology, and finally it becomes the problem of initial conditions of universe. An analysis of initial condition theories is presented for knowing how each of them tries to solve the problem of arrow of time. Different views about the relationship... 

In this thesis, a general survey and reviewis done about the models that go beyond the standard model. Even it contains the great subjects like quantum gravity and alternative Quantum field theories. In addition to them, we concentrate on the supersymmetry that is one of the most important subjects in the beyond the standard model. Recently, the validityof supersymmetry has confronted a lot of controversy. This motivates reconsideration of many supersymmetric models. So we survey the main motivations and problems of the supersymmetry and finally we discuss some possible solutions to the problems  

Causality and locality are two of the fundamental problems in physics and physicists have different interpretations of them. In new physics theories like Quantum mechanics and Quantum field theory, the main point of discussion is that whether these two theories are local and causal or not? In first chapter, we introduce the principle of causality. The second and third chapters cover causality in Quantum mechanics and Quantum field theory.The last chapter is about locality  

In the early time of the discovery of the quantum mechanics and some decades after that, various interpretations of quantum mechanics were proposed. However, many years after that, the “indetermenistic” perspective well-known for Copenhagen perspective which was proposed mostly by Bohr and his colleagues became the dominant attitude among physicists. But nearly two decades after the fixation point of the Copenhagen perspective, a new interpretation of quantum mechanics was proposed by Bohm which was nonlocal but deterministic and it could reproduce all the predictions of quantum mechanics.Bohm’s theory responded to the many theoretical and philosophical objections assumed to the Copenhagen... 

Isn’t our universe ,which includes everything that is causally connected to us, a tiny and negligible part of wider and diverse “multiple universes”? Although the multiverse idea has been considered by scientists recently, it had been discussed by philosophers and theologians in the past.
Many of our physical theories,whether being accepted like General Relativity, or speculative like string theory, lead naturally to the multiverse idea .The idea of multiverse is also brought forth to take care of the fine tuning observed in cosmology, in connection with the so – called anthropic principle.
This idea appeared in order to answer to some problems of quantum mechanics, string theory... 

Bohmian mechanics (or de Broglie–Bohm theory) is a causal version of the quan-tum mechanics which its main idea was expressed for the first time by Louis de Broglie in 1927, and then rediscovered by David Bohm in 1952 with a different approach.In this theory, the state of a system of particles, is described by the position of the particles, and a wavefunction evolving according to the Schrödinger equation. John Bell in 1987, considering the double-slit experiment and emphasizing the importance of the Bohm’s wave-particle theory said: ”Is it not clear from the smallness of the scintillation on the screen that we have to do with a particle? And is it not clear,from the diffraction and... 

Many physicists believed that all four forces were once unified at high energy levels but as the universe reduced into a lower-energy states the inherent symmetry between the forces began to break down.this broken symmetry caused the creation of four distinct forces of nature. The goal of the unified theory is an attempt to look back in time to when these forces where unified as one. In this thesis I tried to give an overview of the challenges which is againsted theorists in the way of unification. In the first chapter explores the theory that try to unify the electromagnetic and weak forces are investigated.it is done by considering guage principle and spontaneous symmetry breaking.in the... 

Non-locality was taken into account, in the first place by Bell’s inequalities in Quantum Mechanics. With the emergence of non-locality, it seems that there is a fundamental conflict between the two basic theories of the 20th century, Quantum Mechanics and Relativity. Relativity is constructed upon local descriptions, but there are some hidden non-local features in Quantum Mechanics .Thus, superluminal signaling, which is in conflict with Relativity, has attracted a lot of attention and some ideas were proposed , using this non-local feature, to send superluminal signal. But, the no-signaling theorems claim that none of these features can be used to send superluminal signal. On the other... 

The measurement problem is one of the issues that is still under debate about the Quantum theory. A significant part of literature during the recent years includes the standard point of view about this problem (including the separation of microscopic and macroscopic domains) and the criticisms of some major physicists in this context. Among these, alternative theories and interpretations have been getting more attention. Experimental tests of Quantum Mechanics in the macroscopic domain and their comparison with alternative theories have always encountered various difficulties (in particular the decoherence phenomenon). Anyway, towards the end of the last century, with the advances in... 

The problem of energy is one of the most important and challenging subjects in both science and engineering. However The problem is deeper and more vital in science. In order to prob the concept of energy, it seems nessesary to know the correct meaning of that. In addition to considering and investigating different definitions of energy and exploring its relation with other quantities – specially mass, the thesis plans to search and survey about its meaning. After vast study and complete analyze around this subject, we finish the text presenting a particular conclusion concerning concept of energy and its relation with matter that shows a some what clear image of it. The subject of energy,...