Solve Wave Equation with D'Alembert's Solution

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Discussion Overview

The discussion centers around the application of D'Alembert's solution to the wave equation, particularly the implications of changing coordinates to u=x+ct and v=x-ct. Participants explore the conceptual and mathematical aspects of this approach, questioning its validity and implications in different contexts, such as sound and light waves.

Discussion Character

  • Debate/contested
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • Some participants express skepticism about solving the wave equation by changing coordinates to u=x+ct and v=x-ct, suggesting it implies moving at the speed of light.
  • Others propose using the relativistic D'Alembert's solution without addressing the concerns raised about the coordinate transformation.
  • A participant clarifies that 'c' in the wave equation could represent different speeds, such as the speed of sound, and questions the implications of changing variables in this context.
  • There is a discussion about the mathematical validity of changing variables and whether it constitutes a change of frame, with some arguing it does not violate relativity.
  • One participant raises a hypothetical scenario about changing to a frame moving at 0.99c, questioning the ethical implications of such a transformation.
  • Another participant emphasizes that introducing new variables is a mathematical tool and does not equate to changing physical frames, suggesting that the mathematical form remains valid regardless of the speed assigned to 'c'.
  • A participant expresses confusion regarding the application of D'Alembert's solution to the Schrödinger equation after initially using the same coordinate transformations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of changing coordinates in the wave equation. Multiple competing views remain regarding the interpretation of 'c' and the validity of the transformations used in different physical contexts.

Contextual Notes

There are unresolved assumptions about the physical meaning of 'c' in the wave equation and the implications of changing variables for different wave types. The discussion reflects varying interpretations of mathematical transformations in relation to physical theories.

actionintegral
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Friends,

I have seen the wave equation solved by changing the coordinates to
u=x+ct and v=x-ct.

This is preposterous! Solve the wave equation by moving at the speed of light! Outlandish!
 
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Use the relativistic D'Alembert's solution then..
 
Solution to what?
 
actionintegral said:
Friends,

I have seen the wave equation solved by changing the coordinates to
u=x+ct and v=x-ct.

This is preposterous! Solve the wave equation by moving at the speed of light! Outlandish!
Um, the 'c' is just the speed appearing in the wave equation:

[tex]\frac{\partial^2 f}{\partial x^2}=\frac{1}{c^2}\frac{\partial^2 f}{\partial t^2}[/tex]

Could be the speed of sound or whatever. It depends on what the speed 'c' in your wave equation is.
 
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Yes. Suppose c is the speed of sound. What is the effect of setting
u=x-ct? You are changing to a frame that is traveling at the speed of sound. Suppose c is the speed of light. What is the effect of setting u=x-ct? You are changing to a frame that is traveling at the speed of ... of ...
 
But so what? It's just a change of variable that will help you solve the equation. It has nothing to do with violating relativity if that's what you're thinking of.
 
Fair enough. Let me counter. Suppose I solve a physics problem by changing to a frame where
x= .99ct Can I do so with a clear conscience?
 
You're not 'changing to a frame'. You introduce new variables u and v to help solve the differential equation. It's mathematics, not physics. Mathematically you can make c 20 times the speed of light and you'll still get a solution to the differential equation in the form F(x+ct)+G(x-ct). In a relativistic physical theory you wouldn't find something that obeys that wave equation with c greater than lightspeed so that's not an issue.
 
Galileo said:
You're not 'changing to a frame'.

I understand perfectly now. Saying "my friend is at x=5" and then saying "let u=x-5. My friend is at u=0" is distinct from saying "I am where my friend is".

That said, maybe someone can help me understand the d'alembert solution to the schroedinger equation. I did this with u=x+ct, v=x-ct.
Then I got nervous and changed the "c" to a "v".

Now I don't know what the h******ell to think.:confused:
 
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actionintegral said:
Yes. Suppose c is the speed of sound. What is the effect of setting
u=x-ct? You are changing to a frame that is traveling at the speed of sound. Suppose c is the speed of light. What is the effect of setting u=x-ct? You are changing to a frame that is traveling at the speed of ... of ...

...light, because you're solving the wave equation for light. The solution is a light wave moving with the speed of light "c".
 

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