Kortewege de Vries equation

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Discussion Overview

The discussion revolves around the Korteweg-de Vries (KdV) equation, focusing on the physical origins of its linear and non-linear terms, particularly in the context of water waves. Participants explore the properties of water molecules that might influence wave profile and stability, as well as comparisons to other equations like the Sine-Gordon equation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the physical origin of the linear and non-linear terms in the KdV equation, specifically relating it to water waves and the properties of water molecules.
  • Another participant speculates that the dipole nature of molecules might be relevant to the equation, later noting the significance of soliton solutions and questioning whether water is the only material capable of such phenomena.
  • A third participant acknowledges the importance of the KdV equation but expresses uncertainty about the physical interpretation of its terms, drawing a parallel to the Navier-Stokes equation and noting a lack of consensus on interpretations.
  • A fourth participant introduces the Sine-Gordon equation as another example that admits stable wave-like solutions, suggesting that the origins of its terms might be similar to those in the KdV equation.

Areas of Agreement / Disagreement

Participants generally agree on the importance of the KdV equation and the interest in understanding its terms, but there is no consensus on the physical interpretations or origins of these terms. Multiple competing views and uncertainties remain regarding the explanations offered.

Contextual Notes

Participants express limitations in finding satisfactory physical interpretations for the terms in the KdV equation and draw parallels to other equations without resolving the underlying questions.

kurious
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In the Kortewege de Vries equation what is the physical origin of the
linear and non-linear terms? I'm thinking of the explanation of a water wave here.What properties of the water molecules cause the profile and stability of the wave?
 
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Not familiar with that equation, but I am guessing that the molecule's dipole has something to do with this.

Edit : A quick search showed how important it was. It seems soliton solutions were actually discovered with it. Is water is the only material substance capable of this? (not counting solitons in light propagation)
 
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The KdV equation is very important indeed, and one can find many references on it. Unfortunately, I don't know any which provides an actual physical interpretation for the several terms. At least not in a satisfactory manner. I also think that the exact same story goes for the Navier-Stockes equation. Everybody agree : it is a very important equation. But nobody agree on the interpretation of the several terms.

This is a very interesting question though. I wish someone will soon pop-up with a good answer.
 
Another eqn which admits stable (no dispersion) wave-like solutions is the Sine-Gordon-Eq:

D'Alembert(theta) + Sin(theta) = 0
(using suitable units)
[theta = theta(space,time), of cours]

This theta can be visualized as follows:

Consider a chain of pendulums, each being elastically connected on the top to its neighbours by springs. Theta measures the amplitude of "swinging"
of each pendulum (therefore function of space and time).
They swing in the usual way in the normal gravitational field: therefore the potential term Sin(theta).
The other term in the Sine-Gordon-EQN, namley the differential operator, comes from the continuum-limit of the elastic force modeled here by springs between the pendulums, as in the usual Klein-Gordon-EQN, or in every other harmonic oszilator.

Maybe the terms in the KdV-eqn have a similar origin.
 

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