Wave questions and Čerenkov radiation

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Rarefaction occurs in acoustic waves as a result of pressure changes, with the trough of the wave representing this rarefaction on a graph. The force causing rarefaction is linked to the behavior of air particles, which move towards the center after being displaced. In explosions, rarefaction does occur as the momentum of air pushes outward and pressure drives it back. Čerenkov radiation involves phase velocity exceeding the speed of light in a medium, while group velocity can also surpass light speed, although this does not imply that information or energy travels faster than light. Understanding these concepts requires a clear distinction between pressure, force, and the mathematical nature of wave velocities.
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I have a couple questions about waves and Čerenkov radiation:

1. I know in all acoustic waves, rarefaction occurs after a wave passes through a medium. On paper, is the trough of a wave its rarefaction? What is the force that causes rarefaction? I've heard the "atoms are like springs" explanation, but I don't quite understand it. I am trying to understand why the air particles in this http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/monopolfinalptpt.gif" rush towards the center. Why doesn't rarefaction occur in explosions (which have just one gigantic wave followed by much smaller ones) or in light waves? (or does it?)

EDIT: Explanation of picture can be found http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-basics-pointsources.htm" , but it doesn't explain why rarefaction occurs.

2. In Čerenkov radiation, I understand that the phase velocity in a charged particle exceeds the speed of light (in that medium). I also understand that it is possible for lasers to have a higher group velocity than light. How can phase and group velocities (or even frequencies in some cases I believe) be faster than the signal (speed) velocity of a wave? Does the group/phase velocity dissipate when it gets to the end of a signal?

Thanks!
 
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Anybody? Should I post this in another part of a messageboard, or is my question not clear?

Help!
 
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I admit an initial reluctance to respond. Your original questions are not very clear to me- for example, question #1 indicates some conceptual confusion between pressure (extrapolated to a discrete, particle model) and force, and question #2 seems like a 'word salad'. For example, I don't know what you mean by 'I also understand that it is possible for lasers to have a higher group velocity than light.'.

Maybe framing the questions with a coherent framework would help: Is there a particular physical phenomena you are thinking about?
 
In question #1 I was asking why rarefaction occurs, if it is represented on a graph by the wave trough, and what other types of waves besides sound waves (explosion, light, ocean etc.) encounter rarefaction.

In question #2 I was asking how group or phase velocity can be faster than the wave signal. I was also asking what would happen if the group/phase velocity (or frequency) was fast enough to get the end of a signal because it was faster than the speed of the signal.
 
rarefaction does occur in explosions. the momentum of the air carries it outward then the pressure drives it back.

phase velocity can exceed c because nothing is actually moving at that speed. its a mathematical artifact. nothing more. group velocity on the other hand is usually descrided as the rate at which energy moves. why it can exceed c is beyond me.
 
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