
NEWTON, Ask A Scientist!
Name: Amanda
Status: other
Age: N/A
Location: N/A
Country: N/A
Date: N/A

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.

Bose-Einstein condensates


negative index of refraction


Vince Calder

Click here to return to the Physics Archives



Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory