Quantum Physics: an overview of a weird world (Basics)

Some preliminary comments about the aim, idea, structure and content of the course and how it distinguishes itself from other courses on Quantum Physics.

Some few historic remarks on how the nature of light was understood from the ancient Greece to Thomas Young's double slit experiment.

Introduction to the concept of force field and the interference of waves.

This lecture describes the famous double slit experiment of Thomas Young. It is one of the most classical experiments which suggest the wave-like nature of light and which remains until nowadays the paradigm experiment of Quantum Physics.

Before 1905, according to classical physics every material object that is not frozen to the absolute zero temperature should emit an infinite amount of energy. Here we describe why inside classical physics this paradox could not find a resolution.

The historical point of departure of quantum theory was Planck's derivation of the black body radiation which assumed energy to be quantized. Previously it was thought that energy is a continuous phenomenon. Its quantization was a conceptual revolution that can be compared to a sort of "Copernican revolution".

These are the quizzes relating to lectures 2-6

The photoelectric effect comes as a further validation of the fact that energy appears always quantized. The photoelectric effect was explained by Einstein introducing the notion of the light particle, the "photon".

Bohr, inspired by the result of the photoelectric effect, advances his famous "planetary model" of the atom. 

Bohr's atom model seemed to receive experimental validation by Frank-Hertz's experiment which definitely demonstrated that atoms absorb energy in quantized amounts of energy. 

The Compton scattering of photons and showed further that electromagnetic radiation has also a corpuscular nature.

Pair creation and annihilation shows how matter and anti-matter particles can transform in pure energy and back. It is another example that showed that electromagnetic radiation has a corpuscular nature.

Bragg diffraction and the de Broglie hypothesis pave the way for understanding better the wave-particle duality problem.

Are photons and electrons particles or waves? If they are both, when do they show upn as one or the other aspect? The wave-particle duality illustrated by Young's double slit experiment will shed some light on this.

Heisenberg's uncertainty principle is explained and some of its frequent misinterpretations illustrated.

The concept of the wavefunction in quantum mechanics is explained. We will address the question if the wavefunction is a mere mathematical object or if it represents a real physical entity.

The description of the quantum world in terms of a probabilistic interpretation led to a mathematical formalism which is quite different than that used in classical physics. Classical states and dynamical variables are replaced by state vectors and operators, the "observables". The measurement postulate captures the essence of how a measurement is represented in QM.

Schrödinger's equation and the time evolution operator are the formal base for a successful understanding of atomic physics.

The modern concept of the structure of atoms in quantum mechanical terms relies on a probabilistic description. Electrons around the atomic nucleus have no longer defined positions or orbits but must be described by probability distributions, the atomic orbitals.

A short introduction to angular momentum in classical physics.

Angular momentum and spin are physical quantities which we intuitively ascribe to rotating objects. Do they apply in the same way for elementary point particles?

The Stern-Gerlach experiments were decisive in demonstrating the impossibility to know the particles's spin values along two directions at the same time and laid the foundations for a new quantum algebra.

In quantum mechanics the abstract notion of "information" seems to be a much more concrete and 'real' thing as we might have expected. This reveals us also how quantum physics is contextual, that is, the answer a quantum particle or system delivers us in a measurement depends from the context we perform it.

What happens if we measure the spin of a particle along non-orthogonal axes?

Also the concept of rotation can be quite different in quantum physics than in classical physics.

Hoe does the polarization of a wave relate to the particle-picture of photons in quantum physics? Also photons have spin as electrons, but they have some specific peculiarities which must be pointed out.

Can particles spin clockwise AND anti-clockwise at the same time? In the microscopic quantum world it is a normal state of affairs.

In analogy to Heisenberg's uncertainty over position and momentum, likewise it is impossible to determine with absolute precision the energy of a system at a definite time. There are however fundamental differences between the two uncertainties.

Can a particle jump through a classically forbidden barrier? Quantum mechanics allows to tunnel through a potential barrier even if it has not the classical allowed energy to do that.

Is "empty" space really empty? According to quantum physics there can't exist no such thing. We will take a look at the vacuum zero-point energy, the concept of virtual particles and the Casimir effect.

Einstein and Bohr did not agree on how to interpret quantum physics. Einstein tried to disprove it with thought experiments and Bohr pointed out its fallacies. The Copenhagen interpretation of quantum mechanics took shape.

Two identical elementary particles are no longer distinguishable after interaction. They will form a unique indistinguishable whole.

In quantum theory particles can be entangled with each other also light years away and apparently "feel" instantly the state of the other. How should this be correctly interpreted?

A. Einstein, B. Podolsky and N. Rosen proposed a thought experiment that was supposed to show how it is possible to circumvent the commutation relations of QM and why it has to be considered therefore an incomplete theory. Were they right?

The EPR paradox is obtained with the spin observables, as it is usually illustrated in modern textbooks.

Some quantum phenomena seem to imply an action at a distance faster than light. Instant correlation between particles also light years apart are possible. Does this allow for faster than light transmission of information?

Can a cat be dead AND alive at the same time? Quantum mechanics seems to suggest this, however at a closer inspection the paradox is tricky.

Quantum decoherence solves only partially the Schrödinger's cat paradox. The measurement problem still defeats a final resolution.

This is a standalone lecture of quantum physics from a purely philosophical perspective. Philosophical idealism gives us an insight into reality which reveals how not only our senses but also our mind deceives us in seeing reality and could be potentially useful to keep in mind when we ponder about the ontology of the quantum world.

The first notions of elementary algebra, Pythagorean theorem, the Cartesian coordinate system, and the parallelogram law of vector addition.

A brief introduction on how functions can describe waves and its representation with complex numbers and complex functions.

How waves are added and the interference term appears, the notion of the derivative and differential and an elementary intuitive explanation of integration.

Momentum must be carefully be distinguished from kinetic energy!