Light Always Faster Than Sound?
Will any radio wave, in every case, without
exception, always travel faster than any sound wave? Why or why not?
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.
negative index of refraction
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Update: June 2012