Classical physics

Classical physics refers to physics theories that 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 relativity.^[1] However relativity is based on classical field theory rather than quantum field theory and is often categorized with "classical physics".^{[citation needed]}

Overview

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 Newtonian, Lagrangian, or Hamiltonian formulations), as well as classical electrodynamics and relativity.^[2]^[3] Alternatively, the term can refer to theories that are neither quantum or relativistic.^[4]

Depending on point of view, among the branches of theory sometimes included in classical physics are:^[5]^: 2

Classical mechanics
- Newton's laws of motion
- Classical Lagrangian and 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.^[5]^: 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 atoms and molecules, 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 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 tbe Planck constant.^{[citation needed]} 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.^{[citation needed]}

References

^ Weidner and Sells, Elementary Modern Physics Preface p.iii, 1968
^ Morin, David (2008). Introduction to Classical Mechanics. New York: Cambridge University Press. ISBN 9780521876223.
^ Barut, Asim O. (1980) [1964]. "Introduction to Classical Mechanics". Electrodynamics and Classical Theory of Fields & Particles. New York: Dover Publications. ISBN 9780486640389.
^ Einstein, Albert (2004) [1920]. Relativity. Robert W. Lawson. New York: Barnes & Noble. ISBN 9780760759219.
^ ^a ^b Krane, Kenneth S. (2020). Modern physics (4 ed.). Hoboken, New Jersey: John Wiley & Sons, Inc. ISBN 978-1-119-49548-2.

[1] Weidner and Sells, Elementary Modern Physics Preface p.iii, 1968

[2] Morin, David (2008). Introduction to Classical Mechanics. New York: Cambridge University Press. ISBN 9780521876223.

[3] Barut, Asim O. (1980) [1964]. "Introduction to Classical Mechanics". Electrodynamics and Classical Theory of Fields & Particles. New York: Dover Publications. ISBN 9780486640389.

[4] Einstein, Albert (2004) [1920]. Relativity. Robert W. Lawson. New York: Barnes & Noble. ISBN 9780760759219.

[Krane-2019-5] Krane, Kenneth S. (2020). Modern physics (4 ed.). Hoboken, New Jersey: John Wiley & Sons, Inc. ISBN 978-1-119-49548-2.

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v t e Major branches of physics
Divisions	Pure Applied Engineering
Approaches	Experimental Theoretical Computational
Classical	Classical mechanics Newtonian Analytical Celestial Continuum Acoustics Classical electromagnetism Classical optics Ray Wave Thermodynamics Statistical Non-equilibrium
Modern	Relativistic mechanics Special General Nuclear physics Particle physics Quantum mechanics Atomic, molecular, and optical physics Atomic Molecular Modern optics Condensed matter physics Solid-state physics Crystallography
Interdisciplinary	Astrophysics Atmospheric physics Biophysics Chemical physics Geophysics Materials science Mathematical physics Medical physics Ocean physics Quantum information science
Related	History of physics Nobel Prize in Physics Philosophy of physics Physics education research Timeline of physics discoveries

v t e History of physics (timeline)
Classical physics	Astronomy timeline Electromagnetism timeline Electrical engineering Maxwell's equations Fluid mechanics timeline Aerodynamics Field theory Gravitational theory timeline Material science timeline Metamaterials Mechanics timeline Variational principles Optics Spectroscopy Thermodynamics timeline Energy Entropy Perpetual motion
Modern physics	Computational physics timeline Condensed matter timeline Superconductivity Cosmology timeline Big Bang theory General relativity tests Geophysics Nuclear physics Fission Fusion Power Weapons Quantum mechanics timeline Atoms Molecules Quantum field theory Subatomic physics timeline Special relativity timeline Lorentz transformations tests
Recent developments	Quantum information timeline Loop quantum gravity Nanotechnology String theory
On specific discoveries	Cosmic microwave background Graphene Gravitational waves Subatomic particles timeline Higgs boson Neutron Speed of light
By periods	Copernican Revolution Golden age of cosmology Golden age of physics Medieval Islamic world Astronomy Noisy intermediate-scale quantum era
By groups	Harvard Computers The Martians Oxford Calculators Via Panisperna boys Women in physics
Scientific disputes	Bohr–Einstein Chandrasekhar–Eddington Galileo affair Leibniz–Newton Joule–von Mayer Shapley–Curtis Relativity priority Special relativity General relativity Transfermium Wars
Category

Overview

Comparison with modern physics

Comparison to quantum physics

See also

References