🚀 go-pugleaf

In article <1992Dec16.235224.27550@gn.ecn.purdue.edu> prasanth@gn.ecn.purdue.edu (Ravi K. Prasanth) writes:
			
>> 			{This stuff by me...}
>> What's REALLY neat about this equation is that it says that, in a totally
>> incompressible substance, the speed of the compression wave is INFINITE!!! 
>> This fact does make sense, in a manner of speaking.  If the medium cannot be
>> compressed, then each molecule will react to the disturbance at the same time,
>> and the entire substance will move at the EXACT same time!!!
>>
>
>Hmmm. So there are some compressible mediums where sound
>travels faster than light. Where is the mistake ?

The problem you are thinking of is similar to the Cerenkhov (sp?) Radiation
found in Nuclear reactors.  The electrons in the core are moving at speed close
to that of the speed of light in the medium of the core (assume it to be air:
c about= 3E+8 m/s)  As the electrons pass through the boundary of the core, and
enter the moderator (assume water) the electrons are moving FASTER THAN THE
SPEED OF LIGHT IN WATER!  This is allowed, believe it or not, and the result is
a really nice blue glow around the core.

Nearly all of the substances with a compressibility factor which are even
1/100% as high as would be needed for the sound to move as fast (or faster)
than light are solids.  For example, copper has a bulk modulus of 132 MPa at
300 K.  (The bulk modulus is a value used in calculating the speed of sound in
a solid.  B = Rho * (dP / d (rho)) _s).  This translates to a speed of sound
equal to 3840 m/s, whiere the speed of sound = sqr(B / Rho).  If someone has
the speed of sound in a diamond (which is the most incompressible substance I
know of...), and compare it with the speed of light, I'd really appreciate it.
I'd like it even better if you could E-Mail me at GANDALF@WPI.WPI.EDU, because
I'm leaving on term break soon...and won't be back for QUITE some time :^)

Until next time...

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