Erica Carlson – Understanding the Quantum World

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.