Understanding the Quantum World

Course No. 9750
Professor Erica W. Carlson, PhD
Purdue University
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Course No. 9750
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What Will You Learn?

  • Reveal what distinguishes quantum physics from classical physics.
  • Discover experiments that demonstrate quantum phenomena.
  • Examine quantum paradoxes and their proposed resolutions.
  • Take a closer look at the philosophical implications of quantum mechanics.

Course Overview

Quantum mechanics has a reputation for being so complex that the word “quantum” has become a popular label for anything mystical or unfathomable. In fact, quantum mechanics is one of the most successful theories of reality yet discovered, explaining everything from the stability of atoms to the glow of neon lights, from the flow of electricity in metals to the workings of the human eye.

At the same time, quantum mechanics does have a mysterious side, symbolized by the famous thought experiment concerning the fate of Schrödinger’s cat, a hypothetical feline who is both dead and alive in a quantum experiment proposed by Austrian physicist Erwin Schrödinger.

In Understanding the Quantum World, Professor Erica W. Carlson of Purdue University guides you through this fascinating subject, explaining the principles and paradoxes of quantum mechanics with exceptional rigor and clarity—and using minimal mathematics. The winner of multiple teaching awards, Professor Carlson is renowned for her “fantastic ability to develop and implement tools that help students learn a challenging subject”—in the words of one of her admiring colleagues. With her guidance, anyone can get a fundamental understanding of this wide-ranging field.

In these 24 half-hour lectures, you discover:

  • What distinguishes quantum physics from classical physics,
  • The major breakthroughs in the field and who made them,
  • How to see quantum “weirdness” as a normal aspect of matter,
  • Experiments that demonstrate quantum phenomena,
  • Quantum theory’s many applications and physical insights,
  • The probable fate of Schrödinger’s cat, and much more.

How to Learn Quantum Physics

Custom animations and graphics, analogies, demonstrations—whatever works to convey a concept, Professor Carlson uses it. You will begin Understanding the Quantum World by covering the central paradox of the field: the wave-particle duality of matter. One of the key ideas here is that waves can come in countable “quantum” units. Dr. Carlson demonstrates this with a slinky being oscillated back and forth, which generates standing waves that can be likened to quantum waves of electrons orbiting the nucleus of an atom.

Professor Carlson has a special affinity for analogies, and she uses them frequently, noting that while scientists prefer the precision of mathematics, for non-scientists an apt analogy is often the best route to an “aha” moment of insight. For example:

  • The Copenhagen coin: A spinning coin is neither heads nor tails until an observation is made. Similarly, the Copenhagen interpretation considers a quantum particle to lack definitive properties until it is measured. Before that, it’s a matter of probabilities, just as a spinning coin can be considered 50 percent heads and 50 percent tails.
  • Quantum gear shifter: Energy levels in an atom are quantized like the gear shifter in a car, which can go from first to second to third gear, but not to second-and-a-half. For gears, the limitation is the individual teeth in a gear wheel, while atoms are limited by the possible standing wave patterns in different atomic energy states.
  • The roller coaster that could: The uncanny ability of quantum particles to pass through potential energy barriers is like a roller coaster that doesn’t have enough speed to surmount a high hill but nonetheless appears on the other side. If a coaster had a long tail to its wavefunction, then it could!
  • Surfing electrons: Next time you turn on a light, think of the electrons in the wire as surfing on quantum waves, from the outer shell of one metal atom to the next, to carry current to the light bulb. Imperfections in the metal’s atomic lattice and other factors cause occasional “wipeouts,” giving rise to electrical resistance.

One of the hardest things to picture in the quantum world is the three-dimensional shape of atomic orbitals. These shapes reveal how electrons are bound to atoms and the probability of finding electrons in specific regions. Here, Dr. Carlson draws on the visualization software that physicists themselves use, which turns atoms into multicolored animations where the probability distribution is a gauzy cloud and shifting colors signify properties such as phase. These visualizations give an eerie look into a domain trillions of times smaller than the period at the end of this sentence. And for anyone studying physics or chemistry, Professor Carlson provides a handy mnemonic for remembering the nomenclature of the different atomic orbitals.

An Astonishing Range of Applications

Quantum physics is more than just a fun intellectual exercise. It is the key to countless technologies, and also helps to explain how the natural world works, including living systems. Professor Carlson discusses many such examples, among them:

  • Color vision: What we perceive as color has its origin in quantum events in the outside world, which produce photons of visible light. Color-sensitive cones in our eyes detect some of these photons. Depending on their wavelength, the photons trigger quantum reactions that our brains interpret as different colors.
  • Global Positioning System (GPS): GPS satellites are essentially atomic clocks in orbit, sending out very accurate time signals based on tiny transitions in energy states of cesium atoms. The time for the signal to reach Earth gives the distance to the satellite. Signals from four GPS satellites suffice to fix a position exactly.
  • Flash memory: Smart phones, solid-state hard drives, memory sticks, and other electronic devices use flash memory to store data with no need for external power to preserve information. When it’s time to erase the information, quantum tunneling allows electrons that encode the data to be quickly discharged.
  • Superconductivity: Dr. Carlson covers the crucial difference between the two classes of subatomic particles—fermions and bosons. Then, in a later lecture, she shows that, under special conditions, fermions can be induced to behave like bosons, leading to a frictionless state of zero electrical resistance known as superconductivity.

These and other successes in understanding and manipulating nature make the mysteries and paradoxes of quantum theory seem almost like a scientific detour into a strange new world. This is what Nobel Prize–winning physicist Richard Feynman had in mind when he urged, “I think it is safe to say that no one understands quantum mechanics. Do not keep saying to yourself …‘but how can it be like that?’ because you will go … into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.”

On the other hand, even as scientists invent new uses for this astonishingly powerful tool, they can’t help but speculate on how it can be like that—as you do as well in this remarkable course.

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24 lectures
 |  Average 30 minutes each
  • 1
    Particle-Wave Duality
    Begin your journey into the quantum world by focusing on one of its most baffling features: the behavior of quantum entities as both particles and waves. Following her approach of presenting analogies over equations, Professor Carlson gives a handy way of visualizing this paradox. Then she takes you further into quantum weirdness by using a slinky to show how waves can be quantized. x
  • 2
    Particles, Waves, and Interference Patterns
    Investigate one of the most famous demonstrations in physics: the double-slit experiment. See how electrons behave as both particles and waves when passing through two parallel slits in a plate and then striking a screen. Bizarrely, the wave properties disappear when the electrons are monitored as they pass through each slit, showing our inability to have complete information of a quantum state. x
  • 3
    Observers Disturb What They Measure
    Consider what life would be like if quantum effects held at our everyday scale. For instance, there would be no trouble sitting in three chairs at once! Learn what happens when a particle in such a mixed state is forced by measurement to assume a definite position—a situation known as wave function collapse. This leads to the important quantum principle that observers disturb what they measure. x
  • 4
    Bell’s Theorem and Schrödinger’s Cat
    Ponder two celebrated and thought-provoking responses to the apparent incompatibility of quantum mechanics and classical physics. Bell’s theorem shows that attempts to reconcile the two systems are futile in a certain class of theories. Next, Schrödinger’s cat is a thought experiment implying that a cat could be both dead and alive if the standard interpretation of quantum mechanics holds. x
  • 5
    Quantum Paradoxes and Interpretations
    Review the major theories proposed by physicists trying to make sense of the paradoxes of the quantum world. Look at the Copenhagen interpretation, Einstein’s realist view, the many worlds interpretation, quantum Bayesianism, non-local hidden variables, and other creative attempts to explain what is going on in a realm that seems to be governed by probability alone. x
  • 6
    The Position-Momentum Uncertainty Relation
    Heisenberg's uncertainty principle sets a fundamental limit on how much we can know about an object's position and momentum at the same time. Professor Carlson introduces this simple equation, showing how it explains why atoms have structure and come in the diverse forms of the periodic table of elements. Surprisingly, the stability of our everyday world rests on uncertainty at the quantum level. x
  • 7
    Wave Quantization
    Electrons don't just orbit the nucleus—they simultaneously exist as standing waves. Go deeper into what standing wave modes look like in one, two, and three dimensions, discovering that these shapes explain the quantization of energy states in an atom. As usual, Professor Carlson introduces useful analogies, including the standing waves produced in a vibrating drum head. x
  • 8
    Quantum Wave Shapes and the Periodic Table
    Focus on standing waves of electrons around nuclei, seeing how the periodic table of elements results from what electrons do naturally: fall into the lowest energy state given the total electric charge, existing electron population, and other features of an atom. Learn the Pauli exclusion principle and a handy mnemonic for remembering the terminology for atomic orbitals, such as 1s, 2p, 3d, etc. x
  • 9
    Interference of Waves and Sloshing States
    Watch what happens when electrons are put into wave forms that differ from standing waves. Your goal is to understand why some of these superposition states are unstable. Professor Carlson notes that the sloshing of an electron back and forth in an unstable state causes it to act like an antenna, radiating away energy until it falls to a lower energy level. x
  • 10
    Wave Shapes in Diamond and Graphene
    What accounts for the dramatic difference between diamond and graphene (a sheet of graphite one atom thick), both of which are composed of pure carbon? Study the role of electrons in molecular bonds, applying your knowledge of electron standing waves. In carbon, such waves make possible several types of bonds, which in diamond and graphene result in remarkably different physical properties. x
  • 11
    Harmonic Oscillators
    A clock pendulum is an example of a classical harmonic oscillator. Extend this concept to the atomic realm to see how quantum waves behave like harmonic oscillators. Then learn how quantum physics was born at the turn of the 20th century in Max Planck’s solution to a paradox in the classical picture of oscillating atoms. His conclusion was that the energies of oscillation had to be quantized. x
  • 12
    The Energy-Time Uncertainty Relation
    Return to the Heisenberg uncertainty principle from Lecture 6 to see how quantum uncertainty also extends to energy and time. This has a startling implication for energy conservation, suggesting that short-lived “virtual” particles can pop into existence out of nothing—as long as they don’t stay around for long. Consider evidence for this phenomenon in the Lamb shift and Casimir effect. x
  • 13
    Quantum Angular Momentum and Electron Spin
    Continue your investigation of the counterintuitive quantum world by contrasting angular momentum for planets and other classical objects with analogous phenomena in quantum particles. Cover the celebrated Stern–Gerlach experiment, which in the 1920s showed that spin is quantized for atoms and can only take on a very limited number of discrete values. x
  • 14
    Quantum Orbital Angular Momentum
    Having covered electron spin in the previous lecture, now turn to orbital angular momentum. Again, a phenomenon familiar in classical physics relating to planets has an analogue in the quantum domain—although with profound differences. This leads to a discussion of permanent magnets, which Professor Carlson calls “a piece of quantum physics that you can hold in your hand.” x
  • 15
    Quantum Properties of Light
    Among Einstein’s insights was that light comes in discrete packets of energy called photons. Explore the photoelectric effect, which prompted Einstein’s discovery. See a do-it-yourself project that demonstrates the photoelectric effect. Close by surveying applications of the quantum theory of light to phenomena such as lasers, fluorescent dyes, photosynthesis, and vitamin D production in skin. x
  • 16
    Atomic Transitions and Photons
    Dive deeper into the interactions of light with matter. Starting with a hydrogen atom, examine the changes in energy and angular momentum when an electron transitions from one orbital to another. See how the diverse possibilities create a “fingerprint” specific to every type of atom, and how this is the basis for spectroscopy, which can determine the composition of stars by analyzing their light. x
  • 17
    Atomic Clocks and GPS
    Peer into the structure of a cesium atom to see what makes it ideal for measuring the length of a second and serving as the basis for atomic clocks. Then head into space to learn how GPS satellites use atomic clocks to triangulate positions on the ground. Finally, delve into Einstein’s special and general theories of relativity to understand the corrections that GPS must make to stay accurate. x
  • 18
    Quantum Mechanics and Color Vision
    Probe the quantum events that underlie color vision, discovering the role of the retinal molecule in detecting different frequencies of photons as they strike cone cells in the eye’s retina. Also investigate the source of color blindness, most common in men, as well as its inverse, tetrachromacy, which is the ability to see an extra channel of color information, possessed by some women. x
  • 19
    A Quantum Explanation of Color
    Now turn to the sources of color in the world around us, from the yellow glow of sodium street lights to the brilliant red of a ruby pendant. Grasp the secret of the aurora, the difference between fluorescence and phosphorescence, and the reason neon dyes look brighter than their surroundings. It turns out that our entire experience of color is governed by the quantum world. x
  • 20
    Quantum Tunneling
    Anyone who makes use of a memory stick, a solid-state hard drive, or a smartphone relies on one of the most baffling aspects of the quantum world: quantum tunneling. Professor Carlson uses a roller coaster analogy, combined with your newly acquired insight into wave mechanics, to make this feat of quantum sorcery—the equivalent of walking through walls—perfectly logical. x
  • 21
    Fermions and Bosons
    Investigate why two pieces of matter cannot occupy the same space at the same time, reaching the conclusion that this is only true for fermions, which are particles with half-integer spin. The other class of particles, bosons, with integer spin, can be in the same place at the same time. Learn how this feature of bosons has been exploited in lasers and in superfluids such as liquid helium. x
  • 22
    Spin Singlets and the EPR Paradox
    Study the most celebrated challenge to the Copenhagen interpretation of quantum mechanics: the paradox proposed by Albert Einstein and his collaborators Boris Podolsky and Nathan Rosen—later updated by David Bohm. Is quantum mechanics an incomplete theory due to hidden variables that guide the outcome of quantum interactions? Examine this idea and the experiments designed to test it. x
  • 23
    Quantum Mechanics and Metals
    Analyze how metals conduct electricity, discovering that, in a sense, electrons “surf” from one metal atom to the next on a quantum mechanical wave. Probe the causes of electrical resistance and why metals can never be perfect conductors. Finally, use the Pauli exclusion principle to understand the optimum distribution of electrons in the different quantum states of metal atoms. x
  • 24
    Close with one of Professor Carlson’s favorite topics: superconductivity. As noted in Lecture 23, when electrons flow through a metal, they lose energy to resistance. But this is not true of superconductors, whose amazing properties trace to the difference between bosons and fermions. Learn how quantum stability allows superconductors to conduct electricity with zero resistance, then step back and summarize the high points of your quantum tour. x

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Your professor

Erica W. Carlson

About Your Professor

Erica W. Carlson, PhD
Purdue University
Erica 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 and a Ph.D. in Physics from the University of California, Los Angeles (UCLA). A theoretical physicist, she researches electronic phase transitions in quantum materials. Widely recognized for her teaching and research, Professor Carlson received...
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Understanding the Quantum World is rated 4.4 out of 5 by 34.
Rated 5 out of 5 by from Understanding the Quantum Worls Good so far. I have only had it for a week. Gives good detail for each subject.
Date published: 2019-06-18
Rated 5 out of 5 by from Prof delivers with quantum energy... I find myself pausing the lecture to make my own notes. Wish high school had been so interesting. Her enthusiasm and sparkling personality carry the day. Her analogies are like sticky notes in my memory. I’m actually making sense of all this. Amazing.
Date published: 2019-05-31
Rated 5 out of 5 by from Enlightening Graphics! The lectures on standing waves and wave shapes were extremely insightful and the graphics definitely helped clarify them. All of Dr. Carlson's explanations were clear, concise and lucid but the lecture on superposition was particularly helpful. I will definitely refer back to this course and its guidebook.
Date published: 2019-05-20
Rated 4 out of 5 by from Good Introduction to Quantum Mechanical Concepts This set of lectures starts out slowly; I thought the first six lectures were short on content and could have been collapsed into two. Lecture seven is where it becomes interesting, and thereafter I thought it was quite good. I enjoyed the focus on wave quantization and its application to the orbital sets available for constructing the elements of the periodic table. There are spots where I would have wished for less explanation, and spots where I would have wished for more--e.g., a deeper consideration of the Bell Theorem--but that is to be expected in any set of lectures and is hardly cause for complaint. I can recommend it for anyone without substantial knowledge in the subject who is interested in a good, visual approach to quantum thought.
Date published: 2019-05-20
Rated 2 out of 5 by from QM for the non-scientist This course seemed to be an effort to relate quantum mechanics to everyday experiences like waves bouncing around the living room, avoiding the couch, etc. But quantum mechanics is a realm apart from classical mechanics, and should be loved and respected for what it is: superposition, quantization, wave-particle duality, entanglement, uncertainty. We need the math. We need Schroedinger's equation, etc.
Date published: 2019-05-19
Rated 5 out of 5 by from Extraordinary Educational Experience Professor Erica Carlson managed to pull off an difficult feat. She was able to take one of the most challenging areas of modern physics, Quantium Mechanics, and explain the underlying principles in a matter of fact manner easily understandable. Using graphics and models she showed the manner in which the smallest particles of the Universe are able to underlie everything else and provide the basis for the rest of existence. The explanation of particle/wave duality was truly exceptional in its clarity. Although It is true, that a true advocate of this area would have to become enmeshed in higher mathematical concepts, most would become bogged down if this would have been the main educational focus. Although, I personally have been introduced previously to many of the facts presented during the lectures, I must admit I was able to better understand the underlying concepts much better after watching Dr Carlson. I strongly recommend the course and congratulate the Teaching Company for presenting it in this manner. Kudos all around.
Date published: 2019-05-10
Rated 5 out of 5 by from The cat is half-dead and half-alive? I liked very much this course. The teacher provided very friendly explanations to concepts that are very difficult to understand by an average person. I also enjoyed that the impact of quantum theory on Chemistry was also included. In particular, I think lecture 18 “Quantum mechanics and color vision” is a very interesting and nice lecture.
Date published: 2019-05-09
Rated 5 out of 5 by from Quite appropriate. Dr. Carlson has the ability to take a sophisticated subject out of the clouds and present it on the understanding level of her students. This is the mark of a great teacher. I count the content of Understanding the Quantum World to be one of the best in The Great Courses portfolio
Date published: 2019-05-03
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