Aphysical 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 thanatoms andmolecules to macroscopic andastronomical objects, 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 completedeterminism, 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. Historically, light was reconciled with classical mechanics by assuming the existence of a stationary medium through which light propagated, theluminiferous aether, which was later shown not to exist.
Mathematically, quantum physics equations are those containing thePlanck constant.[citation needed] According to thecorrespondence principle andEhrenfest's theorem, as a system becomes larger or more massive the classical dynamics tends to emerge, with some exceptions, such assuperfluidity. 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]
^Barut, Asim O. (1980) [1964]. "Introduction to Classical Mechanics".Electrodynamics and Classical Theory of Fields & Particles. New York:Dover Publications.ISBN9780486640389.
