## What is Quantum Theory?

Quantum theory superceded the classical theory of Newton and Maxwell in the early 20th century. It is still often believed that the effects predicted by quantum theory are subtle and hard-to-measure. In fact, there are numerous easy-to-observe phenomena that classical physics is unable to explain and that quantum theory explains beautifully, such as:

- blackbody radiation
- the stability of atoms
- line spectra of atoms
- radioactive decay
- the photoelectric effect

In other words,** classical physics fails to describe the real world!**

Explaining these phenomena required the fundamental insight that on the scale of atoms **energy is quantised**, i.e. it can only be transferred in discrete packets, called **quanta** (the Planck Hypothesis). For example, ordinary light comes in discrete packets of energy, called **photons**.

Further development of quantum theory led to the realization that our physical intuition, developed in everyday life, fails significantly at the atomic and subatomic level. One encounters totally new and unfamiliar phenomena such as:

- the uncertainty principle
- wave-particle duality
- anti-particles and anti-matter
- quantum tunnelling
- superconductors
- quantum entanglement

**The quantum world is indeed weird!**

Despite the fact that quantum theory operates on the atomic level, it manifests itself in many ways on the macroscopic level, i.e. in everyday life. Indeed, over the past 50 years, quantum phenomena have been exploited repeatedly in making **technological advances**, for example:

- nuclear reactors
- transistors
- lasers
- photovoltaic cells (solar panels)
- electron microscopes
- Magnetic Resonance Imaging (MRI)
- Charge-Coupled Devices (CCD)

Through devices such as these quantum theory is having an impact in areas ranging from power generation, through information and communication technology, to medicine. In other words, **Quantum phenomena are changing the world we live in.**