# Introduction

*Modern physics* can be said to date from the beginning of the twentieth century. In 1905, German-born physicist *Albert Einstein* proposed his revolutionary *special theory of relativity*, which changed our ideas about space and time. In 1916 he published his *general theory of relativity*, which generalised special relativity to take into account gravitational effects. These achievements, together with his theories on photons and their implications for the quantum theory first proposed in 1900 by German physicist *Max Planck*, earned him the Nobel Prize for Physics in 1921. Meanwhile, New Zealand-born physicist *Ernest Rutherford* was formulating his model of the atom, which was later improved upon by the Danish Physicist *Neils Bohr*. Rutherford is also credited as having been the first scientist to "split the atom" in 1917.

In its original form, Planck's quantum theory left many questions unanswered. It did however pave the way for *quantum mechanics*, a more complete set of theories that provide a mathematical description of the *wave-particle duality* of energy and matter. The thinking behind quantum theory, and later quantum mechanics, is that certain types of particle at the sub-atomic level can exist in a finite number of discrete states. A change of state only occurs when a discrete amount (or *quantum*) of energy is either gained by the particle or lost to it. One of the best known pioneers in the field of quantum mechanics is the German theoretical physicist *Werner Karl Heisenberg*, whose contributions earned him the Nobel Prize for Physics in 1932. The list of scientists who have played a significant part in the development of modern physics is a long one, and we have mentioned only a few of them here.

Today, astrophysicists probe the mysteries of the cosmos, continually extending the range of their instruments and finding new techniques to analyse the data collected. At the other end of the scale, particle physicists explore the fundamental properties of matter and energy using powerful particle accelerators. In between these two extremes, the vast majority of physicists work in far less esoteric but nevertheless important fields of endeavour. Many work in industry. Some teach physics in schools, colleges or universities. Others work in hospitals or specialised medical facilities. In fact, physicists of one kind or another can be found almost anywhere. Very few are actively engaged in splitting the atom or working as rocket scientists. You are more likely to find them creating more efficient industrial processes, looking for more effective ways to treat cancer, or designing more efficient computer memory.