MadSci Network: Physics

Re: Sound in a vacuum.

Area: Physics
Posted By: Georg Hager, Grad student Theorie III
Date: Thu Jun 6 06:12:09 1996

Dear Melody!

Although light and sound waves may be described by similar mathematical
equations, their physical origin is quite different. The fact that
sound waves cannot travel trough vacuum but light waves can has nothing
to do with light being `waves and particles' at the same time (a notion
that is confusing anyway and should, in my opinion, not be taught).

Sound waves are density fluctuations in a gas, a liquid or a solid. When 
a sound wave travels through air, a `snapshot' of the density of air molecules
at a certain time shows a modulation which is, in the simlplest case,
periodic. Without the presence of a gas or a solid, there are no density
fluctuations and thus no sound waves. In other words, sound needs a
`carrier' material to propagate. No carrier, no sound.

Light consists of electromagnetic waves, i.e. electric and magnetic
fields arranged in such a fashion as to propagate over large
distances. These fields do not need a `carrier' like sound waves, they
`live' in pure vacuum. One could say that an electromagnetic field
is a certain state of the vacuum (physicists call it an `excited state',
and that's where the notion of light being particles begins, but that
is another story).

In the time before Einstein's special theory of relativity, physicists
believed that light needed a carrier which they called `ether'. Einstein
showed that the ether was not necessary, or even obstructive, in the
description of light.

As to the second part of your question, I think that you want to know
what happens if e.g. an explosion happens in vaccum. In air, a fraction
of the energy goes into sound waves which we perceive as `booooom'
or something like that. In a vacuum, this energy is available
for the kinetic energy of the bomb fragments, because the explosion
does not have to accelerate air molecules to produce sound. So,
the bomb fragments would just travel a little faster in a vacuum
than they would in air.

I hope that my comments have provided satisfactory answers to your
questions. Feel free to submit subsequent questions to the MSN! You
can also send e-mail directly to my address if you prefer.


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