In the realm of quantum physics, observing something actually influences the physical processes taking place. Light waves act like particles and particles act like waves (called wave particle duality). Matter can go from one spot to another without moving through the intervening space (called *quantum tunnelling*). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities. Fortunately, it breaks down when dealing with large objects, as demonstrated by the Schroedinger’s Cat thought experiment.

### What is Quantum Entanglement?

One of the key concepts is quantum entanglement, which describes a situation where multiple particles are associated in such a way that measuring the quantum state of one particle also places constraints on the measurements of the other particles. This is best exemplified by the EPR Paradox. Though originally a thought experiment, this has now been confirmed experimentally through tests of something known as Bell’s Theorem.

### Quantum Optics:

*Quantum optics * is a branch of quantum physics that focuses primarily on the behavior of light, or photons. At the level of quantum optics, the behavior of individual photons has a bearing on the outcoming light, as opposed to classical optics, which was developed by Sir Isaac Newton. Lasers are one application that has come out of the study of quantum optics.

### Quantum Electrodynamics (QED):

*Quantum electrodynamics* (QED) is the study of how electrons and photons interact. It was developed in the late 1940s by Richard Feynman, Julian Schwinger, Sinitro Tomonage, and others. The predictions of QED regarding the scattering of photons and electrons are accurate to eleven decimal places.

### Unified Field Theory:

Unified field theory is a collection of research paths that are trying to reconcile quantum physics with Einstein’s theory of general relativity, often by trying to consolidate the fundamental forces of physics. Some types of unified theories include (with some overlap):

- Quantum Gravity
- String Theory / Superstring Theory / M-Theory
- Grand Unified Theory
- Loop Quantum Gravity
- Theory of Everything
- Supersymmetry

### Other Names for Quantum Physics

Quantum physics is sometimes called *quantum mechanics* or *quantum field theory*. It also has various subfields, as discussed above, which are sometimes used interchangeably with quantum physics, though quantum physics is actually the broader term for all of these disciplines.

### Major Figures in Quantum Physics

- Niels Bohr
- Richard Feynman
- Albert Einstein

### Major Findings – Experiments, Thought Experiments, & Basic Explanations

- Earliest Findings
- Black Body Radiation
- Photoelectric Effect

- Wave Particle Duality
- Young’s Double Slit Experiment
- De Broglie Hypothesis

- The Compton Effect
- Heisenberg Uncertainty Principle
- Causality in Quantum Physics – Thought Experiments & Interpretations
- The Copenhagen Interpretation
- Schroedinger’s Cat
- EPR Paradox
- The Many Worlds Interpretation