Physics, the science that deals with matter and energy and the relationships that exist between them. Physics is the most comprehensive of the natural sciences because it includes the behavior of all kinds of matter—from the smallest particles to the largest galaxies.

The word physics originates from a Greek word meaning natural things. Physics was originally called natural philosophy and included all natural science. As a large amount of knowledge was collected on a particular subject within natural philosophy, that subject branched off and developed into a separate science. This trend is still continuing. Those sciences, such as chemistry, that branched off long ago have developed theories and procedures that seem to have little direct connection with physics. Those, such as electronics, that branched off recently have theories and procedures that still show their close connection to the parent science. Regardless of when the branching took place, however, all the natural sciences exist within the framework of the laws of physics.

Although such sciences as biology and geology have their own viewpoints and experimental procedures, the viewpoint and procedures of physics can also be directly applied to them. Where this application has been made, a new series of sciences has developed. To separate them from their parent sciences, they are known by such names as biophysics (the physics of living things) and geophysics (the physics of the Earth). The sciences of chemistry and physics sometimes overlap in subject matter as well as in viewpoint and procedure. The result is physical chemistry. In astrophysics, the techniques of physics are applied to astronomical observations to determine the properties of celestial objects.