Editing Physics (section)

==Core theories==
{{further|Outline of physics}}
Physics deals with a wide variety of systems, although certain theories are used by all physicists. Each of these theories was experimentally tested numerous times and found to be an adequate approximation of nature.
These central theories are important tools for research into more specialized topics, and any physicist, regardless of their specialization, is expected to be literate in them. These include classical mechanics, quantum mechanics, thermodynamics and [[statistical mechanics]], [[electromagnetism]], and special relativity.

===Distinction between classical and modern physics===
{{merge|section=yes|Classical and modern physics|discuss=Wikipedia_talk:WikiProject_Physics#Classical_and_modern_physics|date=February 2025}}
{{further|History_of_physics#Division_into_classical_and_modern}}
[[File:Modernphysicsfields.svg|thumb|upright=1.5|left|Classical mechanics works for larger and slower objects; modern theories are needed otherwise.]]

In the first decades of the 20th century physics was revolutionized by the discoveries of quantum mechanics and relativity. The changes were so fundamental that these new concepts became the foundation of "modern physics", with other topics becoming "classical physics". The majority of applications of physics are essentially classical.<ref name="Thorne-2017">{{Cite book |last1=Thorne |first1=Kip S. |author-link1=Kip Thorne |title=Modern Classical Physics Optics, Fluids, Plasmas, Elasticity, Relativity, and Statistical Physics |last2=Blandford |first2=R.D. |publisher=Princeton University Press. |year=2017 |location=United States}}</ref>{{rp|xxxi|q=The resulting quantum mechanics and relativity occupied physicists for much of the succeeding century and today are viewed very differently from each other. Quantum mechanics is perceived as an abrupt departure from the tacit assumptions of the past, while relativity-though no less radical conceptually-is seen as a logical continuation of the physics of Galileo, Newton, and Maxwell.}} 
The laws of classical physics accurately describe systems whose important length scales are greater than the atomic scale and whose motions are much slower than the speed of light.<ref name="Thorne-2017"/>{{rp|xxxii}} Outside of this domain, observations do not match predictions provided by classical mechanics.<ref name=Krane-2019/>{{rp|6}}
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===Classical theory===
{{Main|Classical physics}}

Classical physics includes the traditional branches and topics that were recognized and well-developed before the beginning of the 20th century—classical mechanics, thermodynamics, and electromagnetism.<ref name=Krane-2019/>{{rp|2|q=These three successful theories-mechanics, electromagnetism, and thermodynamics-form the basis for what we call "classical physics."}} Classical mechanics is concerned with bodies acted on by [[force]]s and bodies in [[motion (physics)|motion]] and may be divided into [[statics]] (study of the forces on a body or bodies not subject to an acceleration), [[kinematics]] (study of motion without regard to its causes), and [[Analytical dynamics|dynamics]] (study of motion and the forces that affect it); mechanics may also be divided into [[solid mechanics]] and [[fluid mechanics]] (known together as [[continuum mechanics]]), the latter include such branches as [[hydrostatics]], [[Fluid dynamics|hydrodynamics]] and [[pneumatics]]. Acoustics is the study of how sound is produced, controlled, transmitted and received.<ref name="britannica-acoustics">{{cite encyclopedia |title=acoustics |url=http://www.britannica.com/EBchecked/topic/4044/acoustics |encyclopedia=[[Encyclopædia Britannica]] |access-date=14 June 2013 |url-status=live |archive-url=https://web.archive.org/web/20130618235952/http://www.britannica.com/EBchecked/topic/4044/acoustics |archive-date=18 June 2013 }}</ref> Important modern branches of acoustics include [[ultrasonics]], the study of sound waves of very high frequency beyond the range of human hearing; [[bioacoustics]], the physics of animal calls and hearing,<ref>{{cite web |url=http://www.bioacoustics.info/ |title=Bioacoustics – the International Journal of Animal Sound and its Recording |publisher=Taylor & Francis |access-date=31 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20120905120546/http://www.bioacoustics.info/ |archive-date=5 September 2012 }}</ref> and [[electroacoustics]], the manipulation of audible sound waves using electronics.<ref>{{cite web |publisher=[[Acoustical Society of America]] |title=Acoustics and You (A Career in Acoustics?) |url=http://asaweb.devcloud.acquia-sites.com/education_outreach/careers_in_acoustics |archive-url=https://web.archive.org/web/20150904010934/http://asaweb.devcloud.acquia-sites.com/education_outreach/careers_in_acoustics |url-status=dead |archive-date=4 September 2015 |access-date=21 May 2013 }}</ref>

Optics, the study of light, is concerned not only with [[visible light]] but also with [[infrared]] and [[ultraviolet radiation]], which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light. Heat is a form of energy, the internal energy possessed by the particles of which a substance is composed; thermodynamics deals with the relationships between heat and other forms of energy. Electricity and [[magnetism]] have been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th century; an [[electric current]] gives rise to a [[magnetic field]], and a changing magnetic field induces an electric current. [[Electrostatics]] deals with [[electric charge]]s at rest, [[Classical electromagnetism|electrodynamics]] with moving charges, and [[magnetostatics]] with magnetic poles at rest.

===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]]