Editing Classical physics

{{short description|Category of theories}}
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[[File:Modernphysicsfields 2.svg|thumb|upright=1.75|Impact of relativity and quantum mechanics on classical mechanics]]

'''Classical physics''' refers to [[physics]] theories that are non-quantum or both non-quantum and non-relativistic, depending on the context. In historical discussions, ''classical physics'' refers to pre-1900 physics, while ''[[modern physics]]'' refers to post-1900 physics, which incorporates elements of [[quantum mechanics]] and [[theory of relativity|relativity]].<ref>{{cite book|first1=Richard T. |last1=Weidner|first2=Robert L. |last2=Sells|title=Elementary Modern Physics|chapter=Preface|page=iii|year=1968}}</ref> However, relativity is based on [[classical field theory]] rather than [[quantum field theory]] and is often categorized with "classical physics."{{cn|reason=ref needed for relativity based on classical field theory|date=February 2025}}

== Overview ==
{{Classical mechanics}}
''Classical theory'' has at least two distinct meanings in physics. It can include all those areas of physics that do not make use of [[quantum mechanics]], which includes [[classical mechanics]] (using any of the [[Newton's laws of motion|Newtonian]], [[Lagrangian mechanics|Lagrangian]], or [[Hamiltonian mechanics|Hamiltonian]] formulations), as well as [[classical electrodynamics]] and [[Theory of relativity|relativity]].<ref>{{cite book |last=Morin |first=David |title=Introduction to Classical Mechanics |year=2008 |publisher=Cambridge University Press |location=New York |isbn=9780521876223 |url-access=registration |url=https://archive.org/details/introductiontocl00mori }}</ref><ref>{{cite book |last=Barut |first=Asim O. |title=Electrodynamics and Classical Theory of Fields & Particles |chapter=Introduction to Classical Mechanics |year=1980| orig-year=1964 |publisher=[[Dover Publications]] |location=New York |isbn= 9780486640389}}</ref> Alternatively, the term can refer to theories that are neither quantum or relativistic.<ref>{{cite book|last=Einstein|first=Albert|author-link=Albert Einstein|others= Robert W. Lawson |title=Relativity|year=2004|orig-year=1920|publisher=Barnes & Noble|location=New York|isbn=9780760759219}}</ref>

Depending on point of view, among the branches of theory sometimes included in classical physics are:<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]]
** [[Newton's laws of motion]]
** Classical [[Lagrangian mechanics|Lagrangian]] and [[Hamiltonian mechanics|Hamiltonian]] formalisms
* [[Classical electrodynamics]] ([[Maxwell's equations]])
* Classical [[thermodynamics]]

== Comparison with modern physics ==
In contrast to classical physics, "[[modern physics]]" is usually used to focus on those revolutionary changes created by [[quantum physics]] and [[theory of relativity]].<ref name=Krane-2019>{{Cite book |last=Krane |first=Kenneth S. |title=Modern physics |date=2020 |publisher=John Wiley & Sons, Inc |isbn=978-1-119-49548-2 |edition=4 |location=Hoboken, New Jersey}}</ref>{{rp|2}}  

A [[physical system]] can be described by classical physics when it satisfies conditions such that the laws of classical physics are approximately valid.

In practice, physical objects ranging from those larger than [[atom]]s and [[molecule]]s, to objects in the macroscopic and astronomical realm, can be well-described (understood) with classical mechanics. Beginning at the atomic level and lower, the laws of classical physics break down and generally do not provide a correct description of nature. Electromagnetic fields and forces can be described well by classical electrodynamics at length scales and field strengths large enough that quantum mechanical effects are negligible. Unlike quantum physics, classical physics is generally characterized by the principle of complete [[Scientific determinism|determinism]], although deterministic interpretations of quantum mechanics do exist.

From the point of view of classical physics as being non-relativistic physics, the predictions of general and special relativity are significantly different from those of classical theories, particularly concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light. Traditionally, light was reconciled with classical mechanics by assuming the existence of a stationary medium through which light propagated, the [[luminiferous aether]], which was later shown not to exist.

== Comparison to quantum physics ==

Mathematically, quantum physics equations are those containing the [[Planck constant]].{{cn|date=February 2025}} According to the [[correspondence principle]] and [[Ehrenfest's theorem]], as a system becomes larger or more massive the classical dynamics tends to emerge, with some exceptions, such as [[superfluidity]]. This is why we can usually ignore quantum mechanics when dealing with everyday objects and the classical description will suffice. [[Decoherence]] is the field of research concerned with the discovery of how the laws of quantum physics give rise to classical physics.{{cn|date=February 2025}}

== See also ==
{{Portal|Physics}}
* [[Glossary of classical physics]]
* [[Semiclassical physics]]

== References ==
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