Quantum Mechanics: The Physics of the Microscopic World

Quantum mechanics is one of the most successful physical theories ever devised; learning what distinguishes it from classical mechanics, Professor Schumacher explains ground rules for the course, which teaches ideas and methods of quantum mechanics.

Debating whether the physical world is discrete or continuous; by the 19th century, physicists see a clear demarcation; matter is made of discrete atoms, while light is a continuous wave of electromagnetic energy; a few phenomena are hard to explain.

Max Planck and Albert Einstein propose ideas to resolve puzzles about light and matter; Planck's discovery that light energy can only be emitted or absorbed in discrete amounts called quanta; Einstein's application of this concept to matter.

Light propagates through space as a wave, but it exchanges its energy in the form of particles; Louis de Broglie shows that this wave-particle duality also applies to matter; this relationship makes quantum mechanics inherently probabilistic.

The mystery of why atoms are stable; Niels Bohr suggests that quantum theory explains atomic stability by allowing only certain distinct orbits for electrons; Erwin Schrödinger's powerful equation reproduces the energy levels of Bohr's model.

One of the most misunderstood concepts in quantum mechanics is the Heisenberg uncertainty principle; Werner Heisenberg's route to this view, which establishes a trade-off between how precisely a particle's position and momentum can be defined.

Einstein's belief that quantum events can, in principle, be known in every detail, is pitted against Bohr's philosophy of complementarity; the view that a measurement of one quantum variable precludes a different variable from ever being known.

Presenting quantum mechanics in a simplified form; the mysteries and paradoxes of the Mach-Zehnder interferometer; a thought experiment showing that an interferometer can determine whether a bomb will blow up without necessarily setting it off.

The interferometer from the previous lecture serves as a test case for introducing the formal math of quantum theory; By learning a few symbols and rules, describing the states of quantum particles; showing how these states change over time.

Many quantum particles move through space and also have an intrinsic spin; analyzing spin gives one a simple laboratory for exploring the basic ideas of quantum mechanics; it is one of the key tools for understanding the quantum world.

Macroscopic objects obey the snowflake principle; quantum particles do not obey this principle; learning that quantum particles come in two basic types; these are bosons, which can occupy the same quantum state and fermions, which cannot.

The tendency of bosons to congregate in the same quantum state; in a laser, huge numbers of photons are created, moving in the same direction with the same energy; in superconductivity, quantum effects allow electrons to flow without resistance.

Learning why matter is solid, even though atoms are mostly empty space; the answer is the Pauli exclusion principle, which states that no two identical fermions can ever be in the same quantum state.

At the fundamental level, bosons and fermions differ in a single minus sign; one way of understanding the origin of this difference is with the Feynman ribbon trick, which Dr. Schumacher demonstrates.

When two particles are part of the same quantum system, they may be entangled with each other; Einstein and his collaborators Boris Podolsky and Nathan Rosen used entanglement to argue that quantum mechanics is incomplete; charting Bohr's response.

John Bell shows that a deterministic theory of quantum mechanics such as EPR violates the principle of locality; particles in close interaction cannot be instantaneously affected by events happening in another part of the universe.

Feynman diagrams are a powerful tool for analyzing events in the quantum world; some diagrams show particles moving forward and backward in time; other particles appear from nowhere and disappear again; all are possible quantum scenarios.

The quantum vacuum is a rapidly fluctuating medium; it can be observed as a tiny attraction between two metal plates; discovering that vacuum energy may be the source of the dark energy that causes the universe to expand at an ever-accelerating rate.

Quantum information and quantum computing is Dr. Schumacher's specialty; the concept qubit is the unit of quantum information; learning that unlike classical information, such as a musical recording, quantum information cannot be perfectly copied.

The uncopyability of quantum information raises the possibility of quantum cryptography, an absolutely secure method for transmitting a coded message; a handful of banks and government agencies use quantum cryptography to secure their data.

The laws governing quantum information; Charles Bennett proposes basic rules governing the relationships between different sorts of information; probing quantum teleportation, in which entanglement is used to send quantum information instantaneously.

Exploring the intriguing capabilities of quantum computers, which do not yet exist but are theoretically possible; using the laws of quantum mechanics, such devices could factor huge numbers, allowing them to decipher unbreakable conventional codes.

The fundamental nature of the quantum world; the Copenhagen, hidden-variable and many-worlds interpretations; the first two reflect Bohr's and Einstein's views, respectively; the last posits a vast universe encompassing every possibility.

John A. Wheeler's metaphor of the Great Smoky Dragon; a creature whose tail appears at the start of an experiment and whose head appears at the end; what lies between is as uncertain as the mysterious and unknowable path of a quantum particle.