Are Solitons the Only Type of Waves in Wave Equations?

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Discussion Overview

The discussion revolves around the nature of wave equations and whether solitons are the only type of waves that can be described by these equations. Participants explore the characteristics of wave-like solutions, the implications of linearity and non-linearity in equations such as the Schrödinger equation, and the definitions of wave equations versus other types of differential equations.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that any wave equation, whether linear or non-linear, should have solutions of the form \(\Psi(x,t) = f(x-ct)\).
  • Others argue that not all wave equations necessarily admit such solutions, suggesting that the existence of wave-like solutions is not guaranteed.
  • One participant clarifies that while functions of the form \(f(x-ct)\) are wave-like, they are not necessarily solitons, which maintain their shape as they propagate.
  • It is noted that the linear Schrödinger equation does not support soliton solutions for finite waves, while the non-linear Schrödinger equation does allow for soliton solutions, indicating that non-linearity is essential for soliton formation.
  • There is a discussion about the relationship between space and time derivatives in wave equations, with some questioning whether such relationships must hold for all differential equations.
  • Participants express confusion regarding the distinction between wave equations and other types of differential equations, particularly in terms of their classification and characteristics.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether solitons are the only type of waves in wave equations. Multiple competing views remain regarding the definitions and characteristics of wave equations and the nature of their solutions.

Contextual Notes

There are limitations in the discussion regarding the definitions of wave equations and the assumptions underlying the existence of wave-like solutions. The implications of linearity and non-linearity in the context of soliton solutions are also not fully resolved.

Klaus_Hoffmann
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Solitons=Only waves??

given any wave equation (linear or not) could we always find a solution.

[tex]\Psi (x,t)= f(x-ct)[/tex]

is this the so-called only wave or soliton ?? , and what would be the shape of f(r) r=x-ct given a certain wave equation ?.. does the Schröedinguer equation admit such solutions or only if this SE is non-linear ??
 
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Klaus_Hoffmann said:
given any wave equation (linear or not) could we always find a solution.

[tex]\Psi (x,t)= f(x-ct)[/tex]

no.

67890
 
I would say that the term "wave-equation" implies the existence of wave-like solutions (i.e. functions of the form f(x-c.t)). Olgranpappy, I'm a little confused by your post - I would think that you would at least attempt to qualify such a statement! (either that or I am missing something clever).

Functions of the form f(x-c.t) are not solitons, though they must satisfy this condition, as they are propagating waves. A solition is a wave that does not change its shape as it propagates, hence not only must it be of the form f(x-c.t), its profile in the y and z directions cannot vary with x.

The Schrödinger equation being a linear equation does not have any soliton solutions for finite waves (i.e. not including the artificial examples of infinite plane waves and so forth). The non-linear Schordinger equation does have soliton solutions. As I understand it, nonlinearity is crucial for soliton formation and propagation.

Claude.
 
Claude Bile said:
I would say that the term "wave-equation" implies the existence of wave-like solutions (i.e. functions of the form f(x-c.t)). Olgranpappy, I'm a little confused by your post - I would think that you would at least attempt to qualify such a statement! (either that or I am missing something clever).

What?...

The form [tex]\psi(x,t)=f(x-ct)[/tex] necessarily implys a relation between the space and time derivatives. Namely,

[tex] \frac{\partial \psi}{\partial x}=\frac{-1}{c}\frac{\partial \psi}{\partial t}[/tex]

Why would such a thing be true for a solution of a general differential equation? For example, I could explicitly forbid such solutions in the differential equation.
 
olgranpappy said:
What?...

The form [tex]\psi(x,t)=f(x-ct)[/tex] necessarily implys a relation between the space and time derivatives. Namely,

[tex] \frac{\partial \psi}{\partial x}=\frac{-1}{c}\frac{\partial \psi}{\partial t}[/tex]

Why would such a thing be true for a solution of a general differential equation? For example, I could explicitly forbid such solutions in the differential equation.
Ah, I see where you are coming from now - though I think the OP was talking specifically about wave-equations, not DEs in general.

Claude.
 
oh, he did say "wave equation." But then what exactly is the difference between a "wave equation" and some other type of differential equation? Are "wave equations" hyperbolic? I don't get it.
 
Yes, this is a little confusing for me too - I thought that the term wave-equation is defined on the basis of having solutions of the form f(x-vt) because such functions describe a propagating disturbance, though this is more of a physicists classification than one a mathematician would use.

Claude.
 
Claude Bile said:
Yes, this is a little confusing for me too - I thought that the term wave-equation is defined on the basis of having solutions of the form f(x-vt)...

In that case, the answer to the original question is: "Yes, by definition."
 

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