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{{About|the superposition principle in linear systems|other uses|Superposition (disambiguation)}}
[[File:Anas platyrhynchos with ducklings reflecting water.jpg|thumb|right|Superposition of almost [[plane wave]]s (diagonal lines) from a distant source and waves from the [[wake]] of the [[duck]]s. [[Linearity]] holds only approximately in water and only for waves with small amplitudes relative to their wavelengths.]]

In [[physics]] and [[systems theory]], the '''superposition principle''',<ref>The Penguin Dictionary of Physics,  ed. Valerie Illingworth, 1991, Penguin Books, London</ref> also known as '''superposition property''', states that, for all [[linear system]]s, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually. So that if input ''A'' produces response ''X'' and input ''B'' produces response ''Y'' then input (''A'' + ''B'') produces response (''X'' + ''Y'').

The homogeneity and additivity properties together are called the superposition principle. A linear function is one that satisfies the properties of superposition. Which is defined as 
:<math>F(x_1+x_2+\cdots)=F(x_1)+F(x_2)+\cdots  \,</math>{{pad|3em}}'''Additivity'''

:<math>F(a x)=a F(x) \,</math>{{pad|19em}}&nbsp;'''Homogeneity'''

:for scalar {{mvar|a}}. 

This principle has many applications in [[physics]] and [[engineering]] because many physical systems can be modeled as linear systems. For example, a [[beam (structure)|beam]] can be modeled as a linear system where the input stimulus is the [[structural load|load]] on the beam and the output response is the deflection of the beam.  The importance of linear systems is that they are easier to analyze mathematically; there is a large body of mathematical techniques, [[frequency domain]] [[linear transform]] methods such as [[Fourier transform|Fourier]], [[Laplace transform]]s, and [[linear operator]] theory, that are applicable.  Because physical systems are generally only approximately linear, the superposition principle is only an approximation of the true physical behavior.

The superposition principle applies to ''any'' linear system, including [[algebraic equation]]s, [[linear differential equations]], and [[system of equations|systems of equations]] of those forms. The stimuli and responses could be numbers, functions, vectors, [[vector field]]s, time-varying signals, or any other object which satisfies [[vector space|certain axioms]]. Note that when vectors or vector fields are involved, a superposition is interpreted as a [[vector sum]].

==Relation to Fourier analysis and similar methods==

By writing a very general stimulus (in a linear system) as the superposition of stimuli of a specific, simple form, often the response becomes easier to compute,

For example, in [[Fourier analysis]], the stimulus is written as the superposition of infinitely many [[Sine wave|sinusoid]]s. Due to the superposition principle, each of these sinusoids can be analyzed separately, and its individual response can be computed. (The response is itself a sinusoid, with the same frequency as the stimulus, but generally a different [[amplitude]] and [[phase (waves)|phase]].) According to the superposition principle, the response to the original stimulus is the sum (or integral) of all the individual sinusoidal responses.

As another common example, in [[Green's function|Green's function analysis]], the stimulus is written as the superposition of infinitely many [[impulse function]]s, and the response is then a superposition of [[impulse response]]s.

Fourier analysis is particularly common for [[wave]]s. For example, in electromagnetic theory, ordinary [[light]] is described as a superposition of [[plane wave]]s (waves of fixed [[frequency]], [[Polarization (waves)|polarization]], and direction). As long as the superposition principle holds (which is often but not always; see [[nonlinear optics]]), the behavior of any light wave can be understood as a superposition of the behavior of these simpler [[plane wave]]s.

==Application to waves==
{{main|Wave|Wave equation}}

[[File:Standing wave 2.gif|thumb|right|Two waves traveling in opposite directions across the same medium combine linearly. In this animation, both waves have the same wavelength and the sum of amplitudes results in a [[standing wave]].]]

Waves are usually described by variations in some parameter through space and time—for example, height in a water wave, [[pressure]] in a sound wave, or the [[electromagnetic field]] in a light wave. The value of this parameter is called the [[amplitude]] of the wave, and the wave itself is a [[function (mathematics)|function]] specifying the amplitude at each point.

In any system with waves, the waveform at a given time is a function of the [[wave equation|sources]] (i.e., external forces, if any, that create or affect the wave) and [[initial condition]]s of the system. In many cases (for example, in the classic [[wave equation]]), the equation describing the wave is linear. When this is true, the superposition principle can be applied.
That means that the net amplitude caused by two or more waves traversing the same space is the sum of the amplitudes which would have been produced by the individual waves separately. For example, two waves traveling towards each other will pass right through each other without any distortion on the other side. (See image at top.)

===Wave interference===
{{main|Interference (wave propagation)}}

The phenomenon of [[Interference (wave propagation)|interference]] between waves is based on this idea. When two or more waves traverse the same space, the net amplitude at each point is the sum of the amplitudes of the individual waves. In some cases, such as in [[noise-cancelling headphone]]s, the summed variation has a smaller [[amplitude]] than the component variations; this is called ''destructive interference''. In other cases, such as in [[Line Array]], the summed variation will have a bigger amplitude than any of the components individually; this is called ''constructive interference''.

{|
|-
| '''combined<br> waveform'''
| colspan="2" rowspan="3" | [[File:Interference of two waves.svg]]
|-
| '''wave 1'''
|-
| '''wave 2'''
|-
| <br>
| '''Two waves in phase'''
| '''Two waves 180° out <br>of phase'''
|}

===Departures from linearity===

In most realistic physical situations, the equation governing the wave is only approximately linear. In these situations, the superposition principle only approximately holds. As a rule, the accuracy of the approximation tends to improve as the amplitude of the wave gets smaller. For examples of phenomena that arise when the superposition principle does not exactly hold, see the articles [[nonlinear optics]] and [[nonlinear acoustics]].

===Quantum superposition===
{{main|Quantum superposition}}

In [[quantum mechanics]], a principal task is to compute how a certain type of wave [[wave propagation|propagates]] and behaves. The wave is called a [[wavefunction]], and the equation governing the behavior of the wave is called [[Schrödinger's wave equation]]. A primary approach to computing the behavior of a wavefunction is to write that wavefunction as a superposition (called "[[quantum superposition]]") of (possibly infinitely many) other wavefunctions of a certain type—[[stationary state]]s whose behavior is particularly simple. Since Schrödinger's wave equation is linear, the behavior of the original wavefunction can be computed through the superposition principle this way.<ref name="QuaMech">Quantum Mechanics, [[Hendrik Anthony Kramers|Kramers, H.A.]] publisher Dover, 1957, p. 62 ISBN 978-0-486-66772-0</ref> See [[Quantum superposition]].

==Boundary value problems==
{{main|Boundary value problem}}

A common type of boundary value problem is (to put it abstractly) finding a function ''y'' that satisfies some equation
:<math>F(y)=0</math>
with some boundary specification
:<math>G(y)=z</math>
For example, in [[Laplace's equation]] with [[Dirichlet problem|Dirichlet boundary conditions]], ''F'' would be the [[Laplacian]] operator in a region ''R'', ''G'' would be an operator that restricts ''y'' to the boundary of ''R'', and ''z'' would be the function that ''y'' is required to equal on the boundary of ''R''.

In the case that ''F'' and ''G'' are both linear operators, then the superposition principle says that a superposition of solutions to the first equation is another solution to the first equation:
:<math>F(y_1)=F(y_2)=\cdots=0\ \Rightarrow\ F(y_1+y_2+\cdots)=0</math>
while the boundary values superpose:
:<math>G(y_1)+G(y_2) = G(y_1+y_2)</math>
Using these facts, if a list can be compiled of solutions to the first equation, then these solutions can be carefully put into a superposition such that it will satisfy the second equation. This is one common method of approaching boundary value problems.

==Other example applications==

* In [[electrical engineering]], in a [[linear circuit]], the input (an applied time-varying voltage signal) is related to the output (a current or voltage anywhere in the circuit) by a linear transformation. Thus, a superposition (i.e., sum) of input signals will yield the superposition of the responses. The use of [[Fourier analysis]] on this basis is particularly common. For another, related technique in circuit analysis, see [[Superposition theorem]].

* In [[physics]], [[Maxwell's equations]] imply that the (possibly time-varying) distributions of [[electric charge|charges]] and [[electric current|currents]] are related to the [[electric field|electric]] and [[magnetic field]]s by a linear transformation. Thus, the superposition principle can be used to simplify the computation of fields which arise from given charge and current distribution. The principle also applies to other linear differential equations arising in physics, such as the [[heat equation]].

* In [[mechanical engineering]], superposition is used to solve for beam and structure deflections of combined loads when the effects are linear (i.e., each load does not affect the results of the other loads, and the effect of each load does not significantly alter the geometry of the structural system).<ref>Mechanical Engineering Design, By Joseph Edward Shigley, Charles R. Mischke, Richard Gordon Budynas, Published 2004 McGraw-Hill Professional, p. 192 ISBN 0-07-252036-1</ref> Mode superposition method uses the natural frequencies and mode shapes to characterize the dynamic response of a linear structure.<ref>Finite Element Procedures, Bathe, K. J., Prentice-Hall, Englewood Cliffs, 1996, p. 785 ISBN 0-13-301458-4</ref>

* In [[hydrogeology]], the superposition principle is applied to the [[Drawdown (hydrology)|drawdown]] of two or more [[water well]]s pumping in an ideal [[aquifer]].

* In [[process control]], the superposition principle is used in [[model predictive control]].

* The superposition principle can be applied when small deviations from a known solution to a nonlinear system are analyzed by [[linearization]].

* In [[music]], theorist [[Joseph Schillinger]] used a form of the superposition principle as one basis of his ''Theory of [[Rhythm]]'' in his ''Schillinger System of Musical Composition''.

==See also==
* [[Impulse response]]
* [[Green's function]]
* [[Quantum superposition]]
* [[Interference (wave propagation)|Interference]]
* [[Coherence (physics)]]
* [[Convolution]]

== References ==
<!--See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for an explanation of how to generate footnotes using the <references/> tags-->
{{reflist}}

==Further reading==
{{refbegin}}
*{{cite book |author=Haberman, Richard |year=2004 |title=Applied Partial Differential Equations  |publisher=Prentice Hall |isbn=0-13-065243-1}}
*[http://www.acoustics.salford.ac.uk/feschools/waves/super.htm Superposition of sound waves]
{{refend}}

{{DEFAULTSORT:Superposition Principle}}
[[Category:Concepts in physics]]
[[Category:Waves]]
[[Category:Systems theory]]

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