In one form or another, physics is one of the oldest academic disciplines; through its modern subfield of astronomy, it may be the oldest of all. Sometimes synonymous with philosophy, chemistry and even certain branches of mathematics and biology during the last two millennia, physics emerged as a modern science in the 17th century and these disciplines are now generally distinct, although the boundaries remain difficult to define.

Advances in physics often translate to the technological sector, and sometimes influence the other sciences, as well as mathematics and philosophy. For example, advances in the understanding of electromagnetism have led to the widespread use of electrically driven devices (televisions, computers, home appliances etc.); advances in thermodynamics led to the development of motorized transport; and advances in mechanics led to the development of the calculus, quantum chemistry, and the use of instruments like the electron microscope in microbiology.

Today, physics is a broad and highly developed subject. Research is often divided into four subfields: condensed matter physics; atomic, molecular, and optical physics; high energy physics; and astronomy and astrophysics. Most physicists also specialize in either theoretical or experimental research, the former dealing with the development of new theories, and the latter dealing with the experimental testing of theories and the discovery of new phenomena. Despite important discoveries during the last four centuries, there are a number of open questions in physics, and many areas of active research.

Although physics encompasses a wide variety of phenomena, all competent physicists are familiar with the basic theories of classical mechanics, electromagnetism, relativity, thermodynamics, and quantum mechanics. Each of these theories has been tested in numerous experiments and proven to be an accurate model of nature within its domain of validity.

For example, classical mechanics correctly describes the motion of objects in everyday experience, but it breaks down at the atomic scale, where it is superseded by quantum mechanics, and at speeds approaching the speed of light, where relativistic effects become important. While these theories have long been well-understood, they continue to be areas of active research—for example, a remarkable aspect of classical mechanics known as chaos theory was developed in the 20th century, three centuries after the original formulation of mechanics by Isaac Newton (1642–1727).