# Classical physics: Wikis

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# Encyclopedia

 Classical physics ${ \partial^2 u \over \partial t^2 } = c(u)^2 \nabla^2 u$ Wave equation History of physics This box: view • talk • edit

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, often including even general relativity. In other words, it is the physics preceding the physics of interest in one's discussion.

## Overview

### Scope

Among the branches of theory included in classical physics are:

### 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.

However, when pursuing ideas of modern physics, it is often useful to look at it's classical physics analogue. The most famous example of this is the spin property in quantum mechanics, which seems in many ways to behave much like angular momentum does classically.

Classical Physics
$\left\{ \partial^2 u \over \partial t^2 \right\} = c\left(u\right)^2 \nabla^2 u$
Wave equation
History of physics

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. In other words, it is the physics preceding the physics of interest in one's discussion.

## Overview

### Scope

Among the branches of theory included in classical physics are:

### 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.

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# Simple English

Classical physics is the study of matter and energy that was first worked out by Sir Isaac Newton and later elaborated by countless scientists. Newton observed that things in nature such as the falling of an apple occur the same way each time. For instance, a cannon ball dropped from a tall building will strike the ground after the same length of time no matter how many times you drop it. It will be going the same speed each time that it hits the ground. Newton made equations that let him predict such events. He called these equations the laws of physics.

Newton's laws of physics have been tested both by intentional experiments and by using them in doing jobs. Sometimes other things interfere with an experiment and make the results come out a little wrong. For instance, in the example above, a powerful wind coming from directly below the cannon ball could slow it down a little bit. But when people make efforts to get rid of these extra factors the results of the experiments always get closer to what Newton's laws predict. Or at least that was always the case for ordinary things and events.

Classical physics turned out not to work when the things being studied were very, very small (around the size of atoms or smaller), or were moving very, very fast (at some fairly large fraction of the speed of light). So, around the beginning of the twentieth century, Albert Einstein worked out his Theory of Relativity and people like Neils Bohr, Werner Heisenberg. and Erwin Schrödinger created Quantum mechanics.