Symmetry, Transactions, and the Mechanism of Wave Function Collapse
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Path Integrals in Quantum Mechanics
Path Integrals in Quantum Mechanics Dennis V. Perepelitsa MIT Department of Physics 70 Amherst Ave. Cambridge, MA 02142 Abstract We present the path integral formulation of quantum mechanics and demon- strate its equivalence to the Schr¨odinger picture. We apply the method to the free particle and quantum harmonic oscillator, investigate the Euclidean path integral, and discuss other applications. 1 Introduction A fundamental question in quantum mechanics is how does the state of a particle evolve with time? That is, the determination the time-evolution ψ(t) of some initial | i state ψ(t ) . Quantum mechanics is fully predictive [3] in the sense that initial | 0 i conditions and knowledge of the potential occupied by the particle is enough to fully specify the state of the particle for all future times.1 In the early twentieth century, Erwin Schr¨odinger derived an equation specifies how the instantaneous change in the wavefunction d ψ(t) depends on the system dt | i inhabited by the state in the form of the Hamiltonian. In this formulation, the eigenstates of the Hamiltonian play an important role, since their time-evolution is easy to calculate (i.e. they are stationary). A well-established method of solution, after the entire eigenspectrum of Hˆ is known, is to decompose the initial state into this eigenbasis, apply time evolution to each and then reassemble the eigenstates. That is, 1In the analysis below, we consider only the position of a particle, and not any other quantum property such as spin. 2 D.V. Perepelitsa n=∞ ψ(t) = exp [ iE t/~] n ψ(t ) n (1) | i − n h | 0 i| i n=0 X This (Hamiltonian) formulation works in many cases. -
Quantum Field Theory*
Quantum Field Theory y Frank Wilczek Institute for Advanced Study, School of Natural Science, Olden Lane, Princeton, NJ 08540 I discuss the general principles underlying quantum eld theory, and attempt to identify its most profound consequences. The deep est of these consequences result from the in nite number of degrees of freedom invoked to implement lo cality.Imention a few of its most striking successes, b oth achieved and prosp ective. Possible limitation s of quantum eld theory are viewed in the light of its history. I. SURVEY Quantum eld theory is the framework in which the regnant theories of the electroweak and strong interactions, which together form the Standard Mo del, are formulated. Quantum electro dynamics (QED), b esides providing a com- plete foundation for atomic physics and chemistry, has supp orted calculations of physical quantities with unparalleled precision. The exp erimentally measured value of the magnetic dip ole moment of the muon, 11 (g 2) = 233 184 600 (1680) 10 ; (1) exp: for example, should b e compared with the theoretical prediction 11 (g 2) = 233 183 478 (308) 10 : (2) theor: In quantum chromo dynamics (QCD) we cannot, for the forseeable future, aspire to to comparable accuracy.Yet QCD provides di erent, and at least equally impressive, evidence for the validity of the basic principles of quantum eld theory. Indeed, b ecause in QCD the interactions are stronger, QCD manifests a wider variety of phenomena characteristic of quantum eld theory. These include esp ecially running of the e ective coupling with distance or energy scale and the phenomenon of con nement. -
Dirac Notation Frank Rioux
Elements of Dirac Notation Frank Rioux In the early days of quantum theory, P. A. M. (Paul Adrian Maurice) Dirac created a powerful and concise formalism for it which is now referred to as Dirac notation or bra-ket (bracket ) notation. Two major mathematical traditions emerged in quantum mechanics: Heisenberg’s matrix mechanics and Schrödinger’s wave mechanics. These distinctly different computational approaches to quantum theory are formally equivalent, each with its particular strengths in certain applications. Heisenberg’s variation, as its name suggests, is based matrix and vector algebra, while Schrödinger’s approach requires integral and differential calculus. Dirac’s notation can be used in a first step in which the quantum mechanical calculation is described or set up. After this is done, one chooses either matrix or wave mechanics to complete the calculation, depending on which method is computationally the most expedient. Kets, Bras, and Bra-Ket Pairs In Dirac’s notation what is known is put in a ket, . So, for example, p expresses the fact that a particle has momentum p. It could also be more explicit: p = 2 , the particle has momentum equal to 2; x = 1.23 , the particle has position 1.23. Ψ represents a system in the state Q and is therefore called the state vector. The ket can also be interpreted as the initial state in some transition or event. The bra represents the final state or the language in which you wish to express the content of the ket . For example,x =Ψ.25 is the probability amplitude that a particle in state Q will be found at position x = .25. -
Many Physicists Believe That Entanglement Is The
NEWS FEATURE SPACE. TIME. ENTANGLEMENT. n early 2009, determined to make the most annual essay contest run by the Gravity Many physicists believe of his first sabbatical from teaching, Mark Research Foundation in Wellesley, Massachu- Van Raamsdonk decided to tackle one of setts. Not only did he win first prize, but he also that entanglement is Ithe deepest mysteries in physics: the relation- got to savour a particularly satisfying irony: the the essence of quantum ship between quantum mechanics and gravity. honour included guaranteed publication in After a year of work and consultation with col- General Relativity and Gravitation. The journal PICTURES PARAMOUNT weirdness — and some now leagues, he submitted a paper on the topic to published the shorter essay1 in June 2010. suspect that it may also be the Journal of High Energy Physics. Still, the editors had good reason to be BROS. ENTERTAINMENT/ WARNER In April 2010, the journal sent him a rejec- cautious. A successful unification of quantum the essence of space-time. tion — with a referee’s report implying that mechanics and gravity has eluded physicists Van Raamsdonk, a physicist at the University of for nearly a century. Quantum mechanics gov- British Columbia in Vancouver, was a crackpot. erns the world of the small — the weird realm His next submission, to General Relativity in which an atom or particle can be in many BY RON COWEN and Gravitation, fared little better: the referee’s places at the same time, and can simultaneously report was scathing, and the journal’s editor spin both clockwise and anticlockwise. Gravity asked for a complete rewrite. -
Dirac Equation - Wikipedia
Dirac equation - Wikipedia https://en.wikipedia.org/wiki/Dirac_equation Dirac equation From Wikipedia, the free encyclopedia In particle physics, the Dirac equation is a relativistic wave equation derived by British physicist Paul Dirac in 1928. In its free form, or including electromagnetic interactions, it 1 describes all spin-2 massive particles such as electrons and quarks for which parity is a symmetry. It is consistent with both the principles of quantum mechanics and the theory of special relativity,[1] and was the first theory to account fully for special relativity in the context of quantum mechanics. It was validated by accounting for the fine details of the hydrogen spectrum in a completely rigorous way. The equation also implied the existence of a new form of matter, antimatter, previously unsuspected and unobserved and which was experimentally confirmed several years later. It also provided a theoretical justification for the introduction of several component wave functions in Pauli's phenomenological theory of spin; the wave functions in the Dirac theory are vectors of four complex numbers (known as bispinors), two of which resemble the Pauli wavefunction in the non-relativistic limit, in contrast to the Schrödinger equation which described wave functions of only one complex value. Moreover, in the limit of zero mass, the Dirac equation reduces to the Weyl equation. Although Dirac did not at first fully appreciate the importance of his results, the entailed explanation of spin as a consequence of the union of quantum mechanics and relativity—and the eventual discovery of the positron—represents one of the great triumphs of theoretical physics. -
5 the Dirac Equation and Spinors
5 The Dirac Equation and Spinors In this section we develop the appropriate wavefunctions for fundamental fermions and bosons. 5.1 Notation Review The three dimension differential operator is : ∂ ∂ ∂ = , , (5.1) ∂x ∂y ∂z We can generalise this to four dimensions ∂µ: 1 ∂ ∂ ∂ ∂ ∂ = , , , (5.2) µ c ∂t ∂x ∂y ∂z 5.2 The Schr¨odinger Equation First consider a classical non-relativistic particle of mass m in a potential U. The energy-momentum relationship is: p2 E = + U (5.3) 2m we can substitute the differential operators: ∂ Eˆ i pˆ i (5.4) → ∂t →− to obtain the non-relativistic Schr¨odinger Equation (with = 1): ∂ψ 1 i = 2 + U ψ (5.5) ∂t −2m For U = 0, the free particle solutions are: iEt ψ(x, t) e− ψ(x) (5.6) ∝ and the probability density ρ and current j are given by: 2 i ρ = ψ(x) j = ψ∗ ψ ψ ψ∗ (5.7) | | −2m − with conservation of probability giving the continuity equation: ∂ρ + j =0, (5.8) ∂t · Or in Covariant notation: µ µ ∂µj = 0 with j =(ρ,j) (5.9) The Schr¨odinger equation is 1st order in ∂/∂t but second order in ∂/∂x. However, as we are going to be dealing with relativistic particles, space and time should be treated equally. 25 5.3 The Klein-Gordon Equation For a relativistic particle the energy-momentum relationship is: p p = p pµ = E2 p 2 = m2 (5.10) · µ − | | Substituting the equation (5.4), leads to the relativistic Klein-Gordon equation: ∂2 + 2 ψ = m2ψ (5.11) −∂t2 The free particle solutions are plane waves: ip x i(Et p x) ψ e− · = e− − · (5.12) ∝ The Klein-Gordon equation successfully describes spin 0 particles in relativistic quan- tum field theory. -
The Critical Casimir Effect in Model Physical Systems
University of California Los Angeles The Critical Casimir Effect in Model Physical Systems A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics by Jonathan Ariel Bergknoff 2012 ⃝c Copyright by Jonathan Ariel Bergknoff 2012 Abstract of the Dissertation The Critical Casimir Effect in Model Physical Systems by Jonathan Ariel Bergknoff Doctor of Philosophy in Physics University of California, Los Angeles, 2012 Professor Joseph Rudnick, Chair The Casimir effect is an interaction between the boundaries of a finite system when fluctua- tions in that system correlate on length scales comparable to the system size. In particular, the critical Casimir effect is that which arises from the long-ranged thermal fluctuation of the order parameter in a system near criticality. Recent experiments on the Casimir force in binary liquids near critical points and 4He near the superfluid transition have redoubled theoretical interest in the topic. It is an unfortunate fact that exact models of the experi- mental systems are mathematically intractable in general. However, there is often insight to be gained by studying approximations and toy models, or doing numerical computations. In this work, we present a brief motivation and overview of the field, followed by explications of the O(2) model with twisted boundary conditions and the O(n ! 1) model with free boundary conditions. New results, both analytical and numerical, are presented. ii The dissertation of Jonathan Ariel Bergknoff is approved. Giovanni Zocchi Alex Levine Lincoln Chayes Joseph Rudnick, Committee Chair University of California, Los Angeles 2012 iii To my parents, Hugh and Esther Bergknoff iv Table of Contents 1 Introduction :::::::::::::::::::::::::::::::::::::: 1 1.1 The Casimir Effect . -
The Statistical Interpretation of Entangled States B
The Statistical Interpretation of Entangled States B. C. Sanctuary Department of Chemistry, McGill University 801 Sherbrooke Street W Montreal, PQ, H3A 2K6, Canada Abstract Entangled EPR spin pairs can be treated using the statistical ensemble interpretation of quantum mechanics. As such the singlet state results from an ensemble of spin pairs each with an arbitrary axis of quantization. This axis acts as a quantum mechanical hidden variable. If the spins lose coherence they disentangle into a mixed state. Whether or not the EPR spin pairs retain entanglement or disentangle, however, the statistical ensemble interpretation resolves the EPR paradox and gives a mechanism for quantum “teleportation” without the need for instantaneous action-at-a-distance. Keywords: Statistical ensemble, entanglement, disentanglement, quantum correlations, EPR paradox, Bell’s inequalities, quantum non-locality and locality, coincidence detection 1. Introduction The fundamental questions of quantum mechanics (QM) are rooted in the philosophical interpretation of the wave function1. At the time these were first debated, covering the fifty or so years following the formulation of QM, the arguments were based primarily on gedanken experiments2. Today the situation has changed with numerous experiments now possible that can guide us in our search for the true nature of the microscopic world, and how The Infamous Boundary3 to the macroscopic world is breached. The current view is based upon pivotal experiments, performed by Aspect4 showing that quantum mechanics is correct and Bell’s inequalities5 are violated. From this the non-local nature of QM became firmly entrenched in physics leading to other experiments, notably those demonstrating that non-locally is fundamental to quantum “teleportation”. -
Analysis of Nonlinear Dynamics in a Classical Transmon Circuit
Analysis of Nonlinear Dynamics in a Classical Transmon Circuit Sasu Tuohino B. Sc. Thesis Department of Physical Sciences Theoretical Physics University of Oulu 2017 Contents 1 Introduction2 2 Classical network theory4 2.1 From electromagnetic fields to circuit elements.........4 2.2 Generalized flux and charge....................6 2.3 Node variables as degrees of freedom...............7 3 Hamiltonians for electric circuits8 3.1 LC Circuit and DC voltage source................8 3.2 Cooper-Pair Box.......................... 10 3.2.1 Josephson junction.................... 10 3.2.2 Dynamics of the Cooper-pair box............. 11 3.3 Transmon qubit.......................... 12 3.3.1 Cavity resonator...................... 12 3.3.2 Shunt capacitance CB .................. 12 3.3.3 Transmon Lagrangian................... 13 3.3.4 Matrix notation in the Legendre transformation..... 14 3.3.5 Hamiltonian of transmon................. 15 4 Classical dynamics of transmon qubit 16 4.1 Equations of motion for transmon................ 16 4.1.1 Relations with voltages.................. 17 4.1.2 Shunt resistances..................... 17 4.1.3 Linearized Josephson inductance............. 18 4.1.4 Relation with currents................... 18 4.2 Control and read-out signals................... 18 4.2.1 Transmission line model.................. 18 4.2.2 Equations of motion for coupled transmission line.... 20 4.3 Quantum notation......................... 22 5 Numerical solutions for equations of motion 23 5.1 Design parameters of the transmon................ 23 5.2 Resonance shift at nonlinear regime............... 24 6 Conclusions 27 1 Abstract The focus of this thesis is on classical dynamics of a transmon qubit. First we introduce the basic concepts of the classical circuit analysis and use this knowledge to derive the Lagrangians and Hamiltonians of an LC circuit, a Cooper-pair box, and ultimately we derive Hamiltonian for a transmon qubit. -
Path Integrals in Quantum Mechanics
Path Integrals in Quantum Mechanics Emma Wikberg Project work, 4p Department of Physics Stockholm University 23rd March 2006 Abstract The method of Path Integrals (PI’s) was developed by Richard Feynman in the 1940’s. It offers an alternate way to look at quantum mechanics (QM), which is equivalent to the Schrödinger formulation. As will be seen in this project work, many "elementary" problems are much more difficult to solve using path integrals than ordinary quantum mechanics. The benefits of path integrals tend to appear more clearly while using quantum field theory (QFT) and perturbation theory. However, one big advantage of Feynman’s formulation is a more intuitive way to interpret the basic equations than in ordinary quantum mechanics. Here we give a basic introduction to the path integral formulation, start- ing from the well known quantum mechanics as formulated by Schrödinger. We show that the two formulations are equivalent and discuss the quantum mechanical interpretations of the theory, as well as the classical limit. We also perform some explicit calculations by solving the free particle and the harmonic oscillator problems using path integrals. The energy eigenvalues of the harmonic oscillator is found by exploiting the connection between path integrals, statistical mechanics and imaginary time. Contents 1 Introduction and Outline 2 1.1 Introduction . 2 1.2 Outline . 2 2 Path Integrals from ordinary Quantum Mechanics 4 2.1 The Schrödinger equation and time evolution . 4 2.2 The propagator . 6 3 Equivalence to the Schrödinger Equation 8 3.1 From the Schrödinger equation to PI’s . 8 3.2 From PI’s to the Schrödinger equation . -
A Live Alternative to Quantum Spooks
International Journal of Quantum Foundations 6 (2020) 1-8 Original Paper A live alternative to quantum spooks Huw Price1;∗ and Ken Wharton2 1. Trinity College, Cambridge CB2 1TQ, UK 2. Department of Physics and Astronomy, San José State University, San José, CA 95192-0106, USA * Author to whom correspondence should be addressed; E-mail:[email protected] Received: 14 December 2019 / Accepted: 21 December 2019 / Published: 31 December 2019 Abstract: Quantum weirdness has been in the news recently, thanks to an ingenious new experiment by a team led by Roland Hanson, at the Delft University of Technology. Much of the coverage presents the experiment as good (even conclusive) news for spooky action-at-a-distance, and bad news for local realism. We point out that this interpretation ignores an alternative, namely that the quantum world is retrocausal. We conjecture that this loophole is missed because it is confused for superdeterminism on one side, or action-at-a-distance itself on the other. We explain why it is different from these options, and why it has clear advantages, in both cases. Keywords: Quantum Entanglement; Retrocausality; Superdeterminism Quantum weirdness has been getting a lot of attention recently, thanks to a clever new experiment by a team led by Roland Hanson, at the Delft University of Technology.[1] Much of the coverage has presented the experiment as good news for spooky action-at-a-distance, and bad news for Einstein: “The most rigorous test of quantum theory ever carried out has confirmed that the ‘spooky action-at-a-distance’ that [Einstein] famously hated . -
SIMULATED INTERPRETATION of QUANTUM MECHANICS Miroslav Súkeník & Jozef Šima
SIMULATED INTERPRETATION OF QUANTUM MECHANICS Miroslav Súkeník & Jozef Šima Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia Abstract: The paper deals with simulated interpretation of quantum mechanics. This interpretation is based on possibilities of computer simulation of our Universe. 1: INTRODUCTION Quantum theory and theory of relativity are two fundamental theories elaborated in the 20th century. In spite of the stunning precision of many predictions of quantum mechanics, its interpretation remains still unclear. This ambiguity has not only serious physical but mainly philosophical consequences. The commonest interpretations include the Copenhagen probability interpretation [1], many-words interpretation [2], and de Broglie-Bohm interpretation (theory of pilot wave) [3]. The last mentioned theory takes place in a single space-time, is non - local, and is deterministic. Moreover, Born’s ensemble and Watanabe’s time-symmetric theory being an analogy of Wheeler – Feynman theory should be mentioned. The time-symmetric interpretation was later, in the 60s re- elaborated by Aharonov and it became in the 80s the starting point for so called transactional interpretation of quantum mechanics. More modern interpretations cover a spontaneous collapse of wave function (here, a new non-linear component, causing this collapse is added to Schrödinger equation), decoherence interpretation (wave function is reduced due to an interaction of a quantum- mechanical system with its surroundings) and relational interpretation [4] elaborated by C. Rovelli in 1996. This interpretation treats the state of a quantum system as being observer-dependent, i.e. the state is the relation between the observer and the system. Relational interpretation is able to solve the EPR paradox.