Wave second order derivative equation

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Discussion Overview

The discussion centers on the relationship between second order derivatives of physical quantities and wave propagation in a medium. It explores the conditions under which such relationships imply the existence of waves, including considerations of different types of wave equations.

Discussion Character

  • Debate/contested, Technical explanation

Main Points Raised

  • One participant asserts that whenever the second order derivative of a physical quantity relates to its second order spatial derivative, a wave must travel in a medium.
  • Another participant introduces the concept of evanescent waves, which satisfy the wave equation but do not propagate.
  • A different participant challenges the initial claim by presenting two forms of wave equations, arguing that only one represents a true wave, while the other does not, citing differences in their solutions.
  • There is a suggestion that the relationship between derivatives can be more complex than linear, indicating that generalizations may be difficult.

Areas of Agreement / Disagreement

Participants express differing views on the implications of second order derivatives for wave propagation, with no consensus reached on the conditions under which waves exist.

Contextual Notes

Participants note that generalizations of the relationships between derivatives can be complex and may depend on specific definitions or contexts.

shiromani
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Whenever the second order derivative of any physical quantity is related to its second order space derivative a wave of some sort must travel in a medium, why this is so?
 
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Evanescent wave which does not propagate also satisfies wave equation.
 
It's not so.

\frac{\partial^2 u}{\partial t^2} = v^2 \nabla^2 u

is a wave, but

\frac{\partial^2 u}{\partial t^2} = -v^2 \nabla^2 u

is not. The solution of the top equation is in sines and cosines, and the second is sinh and cosh.
 
Thanks.
 
And this is just for a linear relation. They can be "related" in more complicated ways.
Generalizations are tricky.:)
 

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