Physics is concerned with the basic principles of the Universe. Contemporary research in physics is divided into several distinct fields that study different aspects of the material world.
- Condensed matter physics, by most estimates the largest single field of physics, is concerned with how the properties of bulk matter, such as the ordinary solids and liquids we encounter in everyday life, arise from the properties and mutual interactions of the constituent atoms.
- The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light.
- The field of particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed.
- Finally, the field of astrophysics applies the laws of physics to explain celestial phenomena, ranging from the Sun and the other objects in the solar system to the universe as a whole.
- Since the 20th century, the individual fields of physics have become increasingly specialized, and nowadays it is not uncommon for physicists to work in a single field for their entire careers.
- "Universalists" like Albert Einstein (1879—1955) and Lev Landau (1908—1968), who were comfortable working in multiple fields of physics, are now very rare.