# Superposition principle

From David J.Griffiths Introduction to Electrodynamics, I understood superposition principle is not obvious but always found to be consistent with the experiments. So I was wondering have we found some physics quantities which do not follow superposition principle? If we have not till now why can't we generalize and make it into a law?

edit: He was talking about electromagnetic force. But my question was existence of something like mass,charge and which doesn't follow this superposition principle?

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Could you include a quote - without it, it's hard to guess what Griffiths was talking about - maybe the linearity of Maxwells equations, or the superposition of quantum states...? –  twistor59 Dec 23 '12 at 14:27
@sree: Learn physics and you will see which quantities can be naturally added and which cannot. –  Vladimir Kalitvianski Dec 23 '12 at 17:04
@VladimirKalitvianski: My question was something else. –  Inquisitive Dec 23 '12 at 17:53
@sree: Sorry to hear that. –  Vladimir Kalitvianski Dec 23 '12 at 17:55
I was unfortunately taught out of Griffiths too. The course had a lot of quirks, the most egregious being this insistence that theory was unable to logically do anything, and that every single, separate equation in physics needed to be verified experimentally, which is of course ridiculous. Superposition is just a part of the theory, and you can't accept electrodynamics as formulated without accepting it. –  Chris White Dec 23 '12 at 18:46

Superposition principle is normally valid for weak fields. It is implemented in the Maxwell equations that are linear in fields with constant (field-independent) coefficients.

Deviation from linearity occurs for strong fields (non-lienar equations due to field-dependent coefficients). For example, a dielectric breakdown can be described as an essential change in the dielectric conductivity starting from some field value (threshold behavior).

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I think we are speaking of different things. In nonlinear interactions, you are dealing with tensor spaces, which does not change the fact that the abstract field operations do not change. –  Antillar Maximus Dec 23 '12 at 15:28
Tensor spaces are not obligatory in our case. Imagine that some fields cannot exist due to breakdown they cause. You cannot simply add field strengths. The discharge is a mechanism of decreasing the field. –  Vladimir Kalitvianski Dec 23 '12 at 16:24

The answer is that there are "physics quantities" that do not obey a superposition principle. The energy density of the electric field is proportional to $E^2$ so if $\bf{E}=\bf{E}_1+\bf{E}_2$, the energy density is not the sum of the energy densities due to $E_1$ and $E_2$ separately.

Superposition is far from being a universal principle. Suppose that a physical quantity $f$ depends another physical quantity $x$ and that $f$ obeys a superposition law in $x$ so that $f(x_1+x_2)=f(x_1)+f(x_2)$. Then I can always define $\bar{x}=x^3$. Then I can choose to regard $f$ as linear in $x$ or nonlinear in $\bar{x}$.

Perhaps there is some way to phrase what you are really trying to get at with your question that doesn't fall into these obvious loopholes, but my feeling is that it's wrong to think of superposition as being a "universal law".

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Any physical quantity that can be organized as a vector space obeys the superposition principle. I would go as far as to say that the superposition principle arises from the fact that a vector space is closed under the weak operation $+$ of the field $\mathbb{(R,+,\cdot)}$.