Classical physics
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| [[Image:Drawing by Étienne-Louis Boullée (1728 - 1799) .jpg|thumb|right|200px|''[[Cenotaph for Newton]]'' ([[1784]]) by French architect [[Étienne-Louis Boullée]]]] | [[Image:Drawing by Étienne-Louis Boullée (1728 - 1799) .jpg|thumb|right|200px|''[[Cenotaph for Newton]]'' ([[1784]]) by French architect [[Étienne-Louis Boullée]]]] | ||
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| - | In [[physics]], '''motion''' means a continuous change in the position of a body relative to a reference point, as measured by a particular [[observation|observer]] in a particular [[frame of reference]]. Until the end of the [[19th century]], [[Isaac Newton]]'s laws of motion, which he posited as axioms or postulates in his famous [[Principia|''Principia,'']] were the basis of what has since become known as [[classical physics]]. Calculations of trajectories and forces of bodies in motion based on Newtonian or classical physics were very successful until physicists began to be able to measure and observe very fast physical phenomena. | ||
| - | At very high speeds, the equations of classical physics were not able to calculate accurate values. To address these problems, the ideas of [[Henri Poincaré]] and [[Albert Einstein]] concerning the fundamental phenomenon of motion were adopted in lieu of Newton's. Whereas Newton's laws of motion assumed absolute values of space and time in the equations of motion, the model of Einstein and Poincaré, now called the [[special theory of relativity]], assumed values for these concepts with arbitrary zero points. Because (for example) the special relativity equations yielded accurate results at high speeds and Newton's did not, the special relativity model is now accepted as explaining bodies in motion (when we ignore [[gravity]]). However, as a practical matter, Newton's equations are much easier to work with than those of special relativity and therefore are more often used in [[applied physics]] and [[engineering]]. | + | What "'''classical physics'''" refers to depends on the context. When discussing [[special relativity]], it refers to the [[Newtonian physics]] which preceded relativity, i.e. the branches of [[physics]] based on principles developed before the rise of [[Theory of relativity|relativity]] and [[quantum mechanics]]. When discussing [[general relativity]], it refers to the result of modifying Newtonian physics to incorporate special relativity. When discussing [[quantum mechanics]], it refers to non-quantum physics, including special relativity, and general relativity. |
| - | In the newtonian model, because motion is defined as the proportion of [[space]] to [[time]], these concepts are prior to motion, just as the concept of motion itself is prior to [[force]]. In other words, the properties of space and time determine the nature of motion and the properties of motion, in turn, determine the nature of force. | + | ==Overview== |
| + | '''Classical theory''' has at least two distinct meanings in [[Physics]]: | ||
| + | #'''In the context of [[quantum mechanics]]''', "classical theory" refers to [[theory|theories]] of physics that do not use the [[Quantization (physics)|quantisation]] [[paradigm]], particularly [[Newtonian mechanics]] (which is also known as [[classical mechanics]]). [[General relativity]] and [[special relativity]] are also considered to be "classical" in this sense. | ||
| + | #'''In the context of general and special relativity''', "[[classical theory and special relativity|classical theory]]" refers to classical mechanics, and other theories which obey [[principle of relativity|Galilean relativity]]. Light and other electromagnetic pheneomena cannot be correctly modeled in such a theory. Traditionally, light was reconciled with classical mechanics by assuming the existence of a "stationary" medium through which light propagated, the [[luminiferous aether]]. | ||
| - | In the special relativistic model, motion can be thought of as something like an [[angle]] between a space direction and the time direction. | + | The existence of these two distinct meanings of the term can lead to confusion: special relativity is a "classical theory" in the first sense, but its predictions are more accurate than "classical theory" in the second sense. |
| - | In special relativity and [[Euclidean space]], only relative motion can be measured, and absolute motion is meaningless. | + | '''In other contexts''', "classical theory" will have other meanings—if a current accepted theory is considered to be "modern", and its introduction represented a major [[paradigm shift]], then previous theory (or new theories based on the older paradigm) will often be referred to as "classical". |
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| + | === Scope === | ||
| + | {{Classical mechanics}} | ||
| + | Among the branches of theory included in classical physics are: | ||
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| + | * [[Classical mechanics]] | ||
| + | ** [[Newton's laws of motion]] | ||
| + | ** Classical [[Lagrangian]] and [[Hamiltonian mechanics|Hamiltonian]] formalisms | ||
| + | * [[Classical electrodynamics]] ([[Maxwell's Equations]]) | ||
| + | * Classical [[thermodynamics]] | ||
| + | * [[Special relativity]] and [[General relativity]] | ||
| + | * Classical [[chaos theory]] and [[nonlinear dynamics]] | ||
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| + | ===Differences=== | ||
| + | In contrast to classical physics, ''[[modern physics]]'' is a slightly looser term which may refer to just [[quantum physics]] or to [[History of physics#20th century|20th and 21st century physics]] in general and so ''always'' includes [[Quantum mechanics|quantum theory]] and ''may'' include [[theory of relativity|relativity]]. | ||
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| + | A ''physical system on the classical level'' is a [[physical system]] in which the laws of classical physics are valid. There are no restrictions on the application of classical principles, but, practically, the scale of classical physics is the level of isolated [[atom]]s and [[molecule]]s on upwards, including the macroscopic and astronomical realm. Inside the atom and among atoms in a molecule, the laws of classical physics break down and generally do not provide a correct description. | ||
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| + | Moreover, the classical theory of [[electromagnetic radiation]] is somewhat limited in its ability to provide correct descriptions, since quantum effects are observable in more everyday circumstances than quantum effects of matter. Unlike quantum physics, classical physics is generally characterized by the principle of complete [[Scientific determinism|determinism]] (although the [[Many-worlds interpretation of quantum mechanics]] is in a sense deterministic). | ||
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| + | Mathematically, classical physics equations are ones in which [[Planck's constant]] does not appear. According to the [[correspondence principle]] and [[Ehrenfest's theorem]] as a system becomes larger or more massive ([[action (physics)|action]] >> [[Planck's constant]]) 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; instead the classical description will suffice. However, one of the most vigorous on-going fields of research in physics is [[Decoherence#Loss of interference and the transition from quantum to classical|classical-quantum correspondence]]. This field of research is concerned with the discovery of how the laws of quantum physics give rise to classical physics in the limit of the large scales of the classical level. | ||
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| + | ==See also== | ||
| + | * [[Glossary of classical physics]] | ||
| + | * [[Semiclassical physics]] | ||
| - | An object is in motion when its distance from another object is changing.Whether the object is moving or not depends on your point of view. For example, a woman riding in a bus is not moving in relation to the seat she is sitting on, but she is moving in relation to the buildings the bus passes. A reference point is a place or object used for comparison to determine | ||
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What "classical physics" refers to depends on the context. When discussing special relativity, it refers to the Newtonian physics which preceded relativity, i.e. the branches of physics based on principles developed before the rise of relativity and quantum mechanics. When discussing general relativity, it refers to the result of modifying Newtonian physics to incorporate special relativity. When discussing quantum mechanics, it refers to non-quantum physics, including special relativity, and general relativity.
Contents |
Overview
Classical theory has at least two distinct meanings in Physics:
- In the context of quantum mechanics, "classical theory" refers to theories of physics that do not use the quantisation paradigm, particularly Newtonian mechanics (which is also known as classical mechanics). General relativity and special relativity are also considered to be "classical" in this sense.
- In the context of general and special relativity, "classical theory" refers to classical mechanics, and other theories which obey Galilean relativity. Light and other electromagnetic pheneomena cannot be correctly modeled in such a theory. Traditionally, light was reconciled with classical mechanics by assuming the existence of a "stationary" medium through which light propagated, the luminiferous aether.
The existence of these two distinct meanings of the term can lead to confusion: special relativity is a "classical theory" in the first sense, but its predictions are more accurate than "classical theory" in the second sense.
In other contexts, "classical theory" will have other meanings—if a current accepted theory is considered to be "modern", and its introduction represented a major paradigm shift, then previous theory (or new theories based on the older paradigm) will often be referred to as "classical".
Scope
Template:Classical mechanics Among the branches of theory included in classical physics are:
- Classical mechanics
- Newton's laws of motion
- Classical Lagrangian and Hamiltonian formalisms
- Classical electrodynamics (Maxwell's Equations)
- Classical thermodynamics
- Special relativity and General relativity
- Classical chaos theory and nonlinear dynamics
Differences
In contrast to classical physics, modern physics is a slightly looser term which may refer to just quantum physics or to 20th and 21st century physics in general and so always includes quantum theory and may include relativity.
A physical system on the classical level is a physical system in which the laws of classical physics are valid. There are no restrictions on the application of classical principles, but, practically, the scale of classical physics is the level of isolated atoms and molecules on upwards, including the macroscopic and astronomical realm. Inside the atom and among atoms in a molecule, the laws of classical physics break down and generally do not provide a correct description.
Moreover, the classical theory of electromagnetic radiation is somewhat limited in its ability to provide correct descriptions, since quantum effects are observable in more everyday circumstances than quantum effects of matter. Unlike quantum physics, classical physics is generally characterized by the principle of complete determinism (although the Many-worlds interpretation of quantum mechanics is in a sense deterministic).
Mathematically, classical physics equations are ones in which Planck's constant does not appear. According to the correspondence principle and Ehrenfest's theorem as a system becomes larger or more massive (action >> Planck's constant) 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; instead the classical description will suffice. However, one of the most vigorous on-going fields of research in physics is classical-quantum correspondence. This field of research is concerned with the discovery of how the laws of quantum physics give rise to classical physics in the limit of the large scales of the classical level.
See also
