Editing Physics (section)

===Modern theory===
{{Main|Modern physics}}

The discovery of relativity and of quantum mechanics in the first decades of the 20th century transformed the conceptual basis of physics without reducing the practical value of most of the physical theories developed up to that time. Consequently the topics of physics have come to be divided into "classical physics" and "modern physics", with the latter category including effects related to quantum mechanics and relativity.<ref name=Krane-2019/>{{rp|2|q=The designation modern physics usually refers to the developments that began in about 1900 and led to the relativity and quantum theories, including the applications of those theories to understanding the atom, the atomic nucleus and the particles of which it is composed, collections of atoms in molecules and solids, and, on a cosmic scale, the origin and evolution of the universe.}}
Classical physics is generally concerned with matter and energy on the normal scale of observation, while much of modern physics is concerned with the behavior of matter and energy under extreme conditions or on a very large or very small scale. For example, [[Atomic physics|atomic]] and [[nuclear physics]] study matter on the smallest scale at which [[chemical element]]s can be identified. The [[Particle physics|physics of elementary particles]] is on an even smaller scale since it is concerned with the most basic units of matter; this branch of physics is also known as high-energy physics because of the extremely high energies necessary to produce many types of particles in [[particle accelerator]]s. On this scale, ordinary, commonsensical notions of space, time, matter, and energy are no longer valid.<ref>{{harvnb |Tipler|Llewellyn|2003|pp=269, 477, 561}}</ref>

The two chief theories of modern physics present a different picture of the concepts of space, time, and matter from that presented by classical physics. Classical mechanics approximates nature as continuous, while quantum theory is concerned with the discrete nature of many phenomena at the atomic and subatomic level and with the complementary aspects of particles and waves in the description of such phenomena. The theory of relativity is concerned with the description of phenomena that take place in a [[frame of reference]] that is in motion with respect to an observer; the special theory of relativity is concerned with motion in the absence of gravitational fields and the [[General relativity|general theory of relativity]] with motion and its connection with [[gravitation]]. Both quantum theory and the theory of relativity find applications in many areas of modern physics.<ref>{{harvnb |Tipler|Llewellyn|2003|pp=1–4, 115, 185–187}}</ref>

Fundamental concepts in modern physics include:
* [[Action (physics)|Action]]
* [[Causality (physics)|Causality]]
* [[Principle of covariance|Covariance]]
* [[Particle]]
* [[Physical field]]
* [[Physical interaction]]
* [[Quantum]]
* [[Statistical ensemble]]
* [[symmetry (physics)|Symmetry]]
* [[Wave]]