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Understanding the Quantum World PDF

234 Pages·2019·18.97 MB·english
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Topic Subtopic An acclaimed physicist uses analogies to everyday life to teach the rules that govern the microscopic world of particles in this accessible introduction to quantum theory. Science & Mathematics Physics Understanding the U “Pure intellectual stimulation that can be popped into n d the [audio or video player] anytime.” er s t —Harvard Magazine a n Quantum World d i n “Passionate, erudite, living legend lecturers. Academia’s g best lecturers are being captured on tape.” th e —The Los Angeles Times Q Course Guidebook u a “A serious force in American education.” n t u —The Wall Street Journal m Professor Erica W. Carlson W Purdue University o r ld Erica W. Carlson is a Professor of Physics and Astronomy at Purdue University. She holds a PhD in Physics from the University of California, Los Angeles. Professor Carlson has published dozens of research articles in peer-reviewed journals, and her TEDx talk on emergence was well received among both scientists and nonscientists. She was elected a fellow of the American Physical Society and has received numerous awards, including Purdue’s highest teaching award, the Charles B. Murphy Outstanding Undergraduate Teaching Award. THE GREAT COURSES® Corporate Headquarters 4840 Westfields Boulevard, Suite 500 Chantilly, VA 20151-2299 USA G Phone: 1-800-832-2412 u www.thegreatcourses.com id e Professor Photo: © Jeff Mauritzen - inPhotograph.com. b Cover Image: © Pasieka/Science Source. o o Course No. 9750 © 2019 The Teaching Company. PB9750A k Published by THE GREAT COURSES Corporate Headquarters 4840 Westfields Boulevard | Suite 500 | Chantilly, Virginia | 20151‑2299 [phone] 1.800.832.2412 | [fax] 703.378.3819 | [web] www.thegreatcourses.com Copyright © The Teaching Company, 2019 Printed in the United States of America This book is in copyright. All rights reserved. Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of The Teaching Company. Erica W. Carlson, PhD Professor of Physics and Astronomy Purdue University E rica W. Carlson is a 150th Anniversary Professor and Professor of Physics and Astronomy at Purdue University. She holds a BS in Physics from the California Institute of Technology as well as a PhD in Physics from the University of California, Los Angeles (UCLA). A theoretical physicist, she researches electronic phase transitions in novel materials. Prior to joining the faculty at Purdue, Professor Carlson did research in theoretical condensed matter physics at Boston University. Her major contributions have included spin excitations of electronic liquid crystal phases, the prediction of a new phase of matter called a vortex smectic-A, and the discovery of electronic fractals in strongly correlated electronic materials. An award-winning teacher and researcher, Professor Carlson received Research Corporation for Science Advancement’s Cottrell Scholar Award and was elected a fellow of the American Physical Society “for theoretical insights into the critical role of electron nematicity, disorder, and noise in novel phases of strongly correlated electron systems and predicting unique characteristics.” At UCLA, she won the teaching associate award from the i Understanding the Quantum World PROFESSOR BIOGRAPHY Department of Physics and Astronomy. Professor Carlson has also received numerous teaching awards as a faculty member at Purdue, including the Ruth and Joel Spira Award for Excellence in Teaching (3 times); the College of Science Award for Outstanding Contributions to Undergraduate Teaching by an Assistant Professor; and Purdue’s highest teaching award, the Charles B. Murphy Outstanding Undergraduate Teaching Award. Professor Carlson has published dozens of research articles in peer-reviewed journals and has presented her scientific work at numerous conferences and invited talks on 4 continents. Her early experiments with podcasting college science courses were featured on the front page of the Chicago Tribune. She is active in outreach, having given science presentations to the public as well as to students at various educational levels, ranging from preschool through high school. Her TEDx talk on emergence was well received among both scientists and nonscientists. ii Understanding the Quantum World TABLE OF CONTENTS TABLE OF CONTENTS INTRODUCTION Professor Biography . . . . . . . . . . . . . . . . . . . . . . . . .i Course Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 GUIDES 1 Particle-Wave Duality . . . . . . . . . . . . . . . . . 3 2 Particles, Waves, and Interference Patterns . . . . . 10 3 Observers Disturb What They Measure. . . . . . . 17 4 Bell’s Theorem and Schrödinger’s Cat. . . . . . . . 25 5 Quantum Paradoxes and Interpretations . . . . . . 32 6 The Position-Momentum Uncertainty Relation. . . . 40 7 Wave Quantization . . . . . . . . . . . . . . . . . 48 8 Quantum Wave Shapes and the Periodic Table . . 57 9 Interference of Waves and Sloshing States . . . . . 70 10 Wave Shapes in Diamond and Graphene . . . . . 80 11 Harmonic Oscillators . . . . . . . . . . . . . . . . 91 12 The Energy-Time Uncertainty Relation. . . . . . . .100 iii Understanding the Quantum World TABLE OF CONTENTS 13 Quantum Angular Momentum and Electron Spin . .108 14 Quantum Orbital Angular Momentum. . . . . . . . 115 15 Quantum Properties of Light . . . . . . . . . . . . . 124 16 Atomic Transitions and Photons . . . . . . . . . . . 133 17 Atomic Clocks and GPS. . . . . . . . . . . . . . . 141 18 Quantum Mechanics and Color Vision . . . . . . .148 19 A Quantum Explanation of Color . . . . . . . . . . 159 20 Quantum Tunneling . . . . . . . . . . . . . . . . .168 21 Fermions and Bosons . . . . . . . . . . . . . . . .180 22 Spin Singlets and the EPR Paradox . . . . . . . . . 187 23 Quantum Mechanics and Metals. . . . . . . . . .198 24 Superconductivity . . . . . . . . . . . . . . . . . .206 SUPPLEMENTARY MATERIAL Quiz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 NAVIGATION TIP To go back to the page you came from, PRESS Alt + ← on a PC or ⌘ + ← on a Mac. On a tablet, use the bookmarks panel. iv Understanding the Quantum World COURSE SCOPE UNDERSTANDING THE QUANTUM WORLD T he microscopic world of particles—such as atoms, electrons, and photons—is ruled by quantum mechanics. It’s a wild world where particles become waves, waves become particles, and our understanding of how physical objects behave is challenged to the core. This course explores the fascinating findings scientists have uncovered about how quantum particles operate. The first segment of this course dives into the basic weirdness of quantum phenomena: Based on measurements in laboratories, we know that small particles like electrons can be in 2 places at once. You’ll learn how the 2-slit experiment reveals that even a single electron exhibits what’s called interference effects when it has 2 possible paths it could follow. You’ll also learn how this foundational experiment reveals both the particle and wave nature of quantum objects. The second segment of the course explores the meaning of these strange findings. Can a particle really be in 2 places at once? Can Schrödinger’s cat really be both dead and alive at the same time? The mathematics of probabilities underlies much of quantum theory. Does this mean that particles really behave in random ways, or do we use probabilities to account for our own lack of knowledge? You’ll learn what scientists think these probabilities mean and how to interpret Heisenberg’s uncertainty principle for position/momentum and for energy/time. The third segment of the course explores the physical consequences of the idea that electrons are waves. Through analogies to physical systems such as Slinkies and drumheads, you’ll gain an intuitive understanding of the standing wave shapes that electrons take in atoms, molecules, and materials. You’ll also learn how these beautiful geometric shapes determine much of the structure in the periodic table of the elements as well as that of molecules and materials. 1 Understanding the Quantum World COURSE SCOPE The fourth segment of the course tackles the consequences of quantization of energy and angular momentum. You’ll learn the strange behavior of angular momentum at the quantum level, including what it means that electrons and other quantum objects have quantized spin. You’ll also learn how the photoelectric effect reveals that light—which was first thought to be only a wave—is comprised of quantum particles known as photons. In the final segment of the course, you’ll learn about the fundamental quantum distinction between particles known as fermions and bosons as well as about how fermions have been used to test the Einstein- Podolsky-Rosen paradox. The course finishes by building on several of the foundational quantum concepts presented earlier in the course in order to explain why metals are metallic and why superconductors can conduct electricity without losing energy. Throughout the course, you will discover multiple applications of quantum mechanics to your everyday life, including magnets, color vision and lighting, and the exquisitely accurate quantum clocks that govern the global positioning system (GPS). By the time you complete this course, you will gain an appreciation for the incredible beauty and mystery that underlie quantum phenomena, and you will gain an understanding of what we know—and what we don’t yet know—about the quantum world. 2 Lecture 1 Particle-Wave 01 Duality T he world of quantum mechanics is a world unlike the one we observe every day. It’s a world of very small objects—things like atoms, or the things inside of atoms, that just don’t seem to play by the rules that apply to large objects. In fact, some of the ideas in quantum mechanics sound like they can’t possibly be true. For example, consider particle-wave duality: Quantum mechanical objects sometimes act like a wave and sometimes act like a particle. But although such ideas seem strange and even contradictory, learning the inner workings of quantum physics will help resolve some of these paradoxes. Scientists tend to prefer the language of mathematics and equations to explain quantum physics because it is a much more precise language than spoken languages, such as English. Most of these lectures will not use any equations; instead, analogies will be heavily relied on. But because analogies are rarely perfect, there will be ambiguities. 3 Understanding the Quantum World LECTURE 1 Particle-Wave Duality Are Particles Waves? • Quantum mechanics is the world of the very tiny. It deals with things about as small as atoms or smaller. So, atoms obey quantum mechanics; the protons, neutrons, and electrons inside of atoms also obey quantum mechanics; the things that make up protons and neutrons obey quantum mechanics, too; and so on. • In this tiny world, things aren’t always what they seem. It turns out that we shouldn’t think of particles—such as protons and electrons—as tiny balls or even as tiny points. When we look at such particles at the quantum level, they look much more smeared out. In fact, at some level of zooming in on such particles, they exhibit some properties that are very much like waves. • Whatever these quantum objects are, they have some wave properties and some particle properties. Here’s how you can get started thinking about this conundrum: Picture a wave that is not wiggling everywhere. To the right, it’s flat and not wiggling. In the middle, there’s a wavy portion, and then it flatlines again, and the wave is not wiggling on the left. • Here’s a key concept for waves in quantum physics: Where the wave is not wiggling and is flat, there is no chance of finding the particle when you go to look for it—when you go to measure its position. In the middle of the wiggly part, where the wave is biggest, is where you’re most likely to find the particle. In fact, the bigger the amplitude, or height, of the wave, the more likely you are to find it in that spot. You’re less likely to find the particle on the edge of the wiggly part, and there’s no chance of finding it where the amplitude of the wave is zero, which is the flat part. • So, if we take a wave that looks kind of like the one pictured above and then shrink it down really small, it’s going to look like a particle to you from a long distance. This is one way to think about how it could be possible that something could be both a particle and a wave. That’s 4

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