Light Always Faster Than Sound?

name         Amanda
status       educator
grade        other
location     TX

Question -   Will any radio wave, in every case, without 
exception, always travel faster than any sound wave? Why or why not?
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One has to be very cautious in giving a response to this for two 
reasons: 1. Your requirement of "without exception" is always an 
impossible one to meet in the physical sciences. There is no 
physical law or theory that holds "in every case, without 
exception." In fact, just the opposite is generally accepted. A 
scientific law or theory MUST BE FALSIFIABLE. Falsifiable means 
that there must be some condition(s) where it breaks down, or needs 
modification. Otherwise, one would have the "eternal truth", which 
is not domain of physical science. 2. What you may be thinking of 
is a strange state of matter called a Bose-Einstein condensate 
(BEC), which behaves very differently than "normal" matter formed 
from the same element. A (BEC) behaves as a single atom even though 
it may be composed of many atoms.
   In "normal" matter sound is propagated by the physical 
oscillations of atoms or molecules in the material. Under these 
conditions the speed of sound varies as d^-1/2 or one divided by 
the square root of the density. For any practical densities, this 
means that the speed of sound will be less than the speed of 
electromagnetic radiation IN A VACUUM (This caveat is often 
omitted) which causes a lot of confusion. Electromagnetic radiation 
does travel at slower speeds through normal matter. In fact the 
ratio of the speed in a vacuum to the speed in the matter is the 
definition of the index of refraction of the material. Usually the 
index of refraction,  I  
1 which means its speed is slower through the media than through 
a vacuum. For certain materials under certain conditions I < -1 
which does not violate any laws of optics or physics (see reference below).
   At the other extreme, as density ---> 0 that is, a vacuum, sound 
in the usual sense no longer exists. One could imagine a collection 
of atoms moving in a vacuum by turning a valve on and off, and do 
this at a high frequency and call that sound, but again such pulses 
of matter are not sound in the usual sense. This is a series of shock waves.
   In (BEC's) it is possible to retard the propagation of 
electromagnetic radiation, with some restrictions on wave length, 
until the speed of the electromagnetic radiation through the BEC is 
essentially zero. But this is NOT a normal state of matter and 
occurs only at very low temperatures under very specialized 
experimental setups. In addition, this behavior is described by 
quantum mechanics, not classical optics.

Bose-Einstein condensates

http://physicsweb.org/articles/world/10/3/3/1

negative index of refraction

http://www.ee.duke.edu/~drsmith/nim_pubs.htm

Vince Calder
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