Classical physics

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The classical physics includes the branches of physics that without the concepts of quantization and the four-dimensional spacetime get along. These are classical mechanics (including celestial mechanics and classical statistical mechanics ), classical electrodynamics (including optics ) and classical thermodynamics or thermodynamics. The corresponding theories were set up from the 17th century and have been continuously developed since then. For the macroscopic physical processes in nature and technology, classical physics enables an almost complete understanding in many areas. But it fails to describe the microscopic (elementary particles, atoms, molecules ...) and the astronomical large. Therefore, physics has been expanded since around 1900 by radically new concepts, which are collectively referred to as modern physics and contrasted with classical physics. In the context of modern physics, it turns out that some, sometimes fundamental, terms and theories of classical physics, which seem to be absolutely valid when observed under macroscopic conditions, actually only apply approximately.


With the expansions and corrections of the last hundred years or so, classical physics has by no means lost its importance, rather it has the same validity as before in its established field of application, i.e. above all in macroscopic physics. Classical physics emerges from modern physics as a description of reality that is approximately correct. Many questions in physics, especially the structure and properties of matter, can only be explained by quantum theory and relativity theory.

Classical mechanics including classical statistical mechanics and continuum mechanics , electrodynamics , classical thermodynamics and optics are included in classical physics . Sometimes the special theory of relativity is also included because it was developed from electrodynamics. The changes that the theory of relativity triggered in physics go far beyond electrodynamics.

Classical physics is based on a number of assumptions which, according to modern physics, are approximately correct in our closer experience, but generally do not apply strictly:

Classical physics Modern physics
Coordinate transformations Times and lengths are absolute quantities; H. independent of the choice of the reference system . Consequently, every speed, including the speed of light , depends on the state of motion of the observer (see Galileo transformation ). The speed of light is an absolute quantity; H. independent of the choice of the reference system. Consequently, times and lengths depend on the state of motion of the observer (see time dilation , length contraction and Lorentz transformation )
Structure of the room All physical processes take place in a three-dimensional Cartesian space . The laws of Euclidean geometry apply . Time passes regardless of space. The three dimensions of space and time are interwoven and together form a four-dimensional space - time .
Nature of gravity According to Isaac Newton, gravitation is an action at a distance that is described by the law of gravitation . Inertial forces and gravitational forces are equivalent to each other . They are expressed in the curvature of space.
Conservation of mass and energy Mass and energy are conserved quantities . Energy is a conserved quantity, but mass is not. Because of the mass-energy equivalence , a system also loses mass when it radiates energy, although it does not give off any matter.
Quantization According to Maxwell's equations of electrodynamics , electromagnetic waves , which also include light, can exist with any energy content. Light energy always occurs in quantized form , i.e. H. in discrete portions of energy ( photons ).
Physical measurement accuracy In principle, the location and impulse of a physical object can be determined at any point in time with any precision. There is only a practical limit due to the maximum technically achievable precision. The maximum achievable accuracy when determining position and momentum at the same time is not only limited in practical measurements, but also in principle when defining both quantities , according to Heisenberg's uncertainty principle.
determinism With sufficiently precise knowledge of all natural laws and parameters, the behavior of a physical system can be predicted exactly ( determinism of classical physics). According to the laws of quantum physics , statements can only be made about probabilities ( Copenhagen interpretation of quantum mechanics ).

In practice, when it comes to physical questions, it is often decided on the basis of the required accuracy or the relevant orders of magnitude whether a classical treatment is possible or whether quantum or relativistic effects have to be taken into account. Explanatory models that only partially give up the classic ideas are referred to as "semi-classic", such as B. Bohr's atomic model .


The epoch of classical physics spans around the 17th, 18th and 19th centuries. It was justified by Galileo Galilei with the introduction of the experimental method and the mathematical description of physical processes. He studied movements and tried to describe them systematically and quantitatively, creating kinematics as the first sub-area of ​​classical mechanics. The actual foundation of the mechanics, however, was laid by Isaac Newton . He not only introduced the infinitesimal calculus into physics, but with Newton's laws also provided a uniform basis for all dynamic processes by establishing a connection between forces and movements. In addition, it was he who set up the law of gravity , which Henry Cavendish was able to check quantitatively in laboratory experiments. Newton's findings were later theoretically deepened by d'Alembert , Euler , Lagrange and Hamilton and extended to fluids by Bernoulli , Navier and Stokes .

At first, electricity was examined purely phenomenologically. On Benjamin Franklin knowledge goes back, that there is only one available type of cargo that can certainly be positive or negative. The attractive and repulsive forces between the charges were described by Coulomb by a law that is formally similar to Newton's law of gravitation. The laws of the electrical circuit originate from Ohm and Kirchhoff . It is true that magnetostatics had already been researched by Gilbert in the 16th century . The connection between electrical and magnetic forces was only gradually discovered, among others by Ampère and Faraday . Maxwell succeeded in summarizing these relationships in four equations . From these equations it could be deduced that there must be electromagnetic waves that Hertz could prove in experiments. The correspondence of the speed of these waves with the speed of light suggested that light is an electromagnetic wave.

Until then, the nature of light had long been a matter of dispute. Newton had described it as a particle, but Huygens already suspected that light is a wave. This was confirmed by Young's double slit experiments.

Thermodynamics was initially primarily concerned with changes in the state of gases, e.g. B. by the physicists Gay-Lussac , Boyle , Mariotte and Amontons , which finally led to the general gas equation . In the 19th century the idea emerged that the “ living force ” of mechanics and the “warmth” of thermodynamics were related terms. Among other things, Joule was able to measure the “mechanical heat equivalent”. This gave birth to the idea that there is a universal physical quantity that we call energy today. Mayer realized that it was a matter of preservation. This is the essential content of the first law of thermodynamics. The second law states, among other things, that although mechanical forms of energy can be converted into thermal forms of energy at will, the reverse is not possible. This law goes back to Clausius . A deeper understanding of thermodynamics was only achieved when one began to describe thermodynamic processes at the particle level. Because of the unmanageably large number of particles, one had to do this with the means of statistical mechanics, which goes back to Boltzmann , among others .


The following areas are included in classical physics:


At the end of the 19th century, physics was considered to be almost complete, although physicists already knew that certain phenomena in nature could not be reconciled with the laws of classical physics known at the time. Here are a few examples, without claiming to be exhaustive:

  • In classical calculations, the accelerated electron seemed to have different masses depending on the measurement setup . One spoke of a “transversal” and a “longitudinal” mass. In 1905 Einstein showed in his special theory of relativity that the mass of the body is invariant, but that lengths, times and impulses and thus the inertial behavior of the body do depend on the choice of the reference system.
  • The perihelion of Mercury's orbit was 0.43 arc seconds per year greater than classical calculations could explain. An exact calculation of Mercury's orbit was only possible with the help of the general theory of relativity by Albert Einstein in 1915.
  • The intensity of the radiation from a black body could only be explained well in the range of low frequencies. For the high frequencies, on the other hand, classical physics provided absurdly high numerical values, which was referred to as an " ultraviolet catastrophe ". Nothing of the kind was observed in the experiment. Max Planck succeeded in solving this problem in 1900 with the introduction of the quantum hypothesis (see Planck's law of radiation ).
  • The structure of the matter could not be explained with classical methods. In particular, the idea of ​​an atom in which electrons revolve around an atomic nucleus in stable orbits contradicted the laws of classical electrodynamics. Such an arrangement would have to continuously emit energy until the electrons fall into the atomic nucleus after a short time. In 1926 Erwin Schrödinger succeeded in treating the hydrogen atom mathematically by describing the electron not as a classic, circling particle, but as a standing wave in the electric field of the atomic nucleus.
  • The radioactivity was already known since 1896, but did not let himself fit into classical matter concepts. In order to understand them, one needs both the mass-energy equivalence from the theory of relativity and quantum physical approaches to describe interactions and particles.

Individual evidence

  1. Galileo Galilei: Discorsi e dimostrazioni matematiche , Leiden 1638, German: Conversation and mathematical demonstration about two new branches of knowledge relating to mechanics and the laws of fall, online .
  2. ^ Isaac Newton: Philosophiae Naturalis Principia Mathematica , 1687.
  3. James Clerk Maxwell: A Dynamical Theory of the Electromagnetic Field . In: Philosophical Transactions of the Royal Society . Volume 155, 1865, pp. 459-512, doi: 10.1098 / rstl.1865.0008 .
  4. James Prescott Joule: About the mechanical heat equivalent . In: Annals of Physics and Chemistry. Volume 4, Verlag JA Barth, 1854, pp. 601ff. (German version of its publication published in 1850). Available on Google Books .
  5. Rudolf Clausius: About the moving power of heat and the laws that can be derived from it for the theory of heat itself in JC Poggendorff (Ed.): Annalen der Physik und Chemie , Vol. 79, 1850, online .