Expert: Philip Stahl - 8/20/2006

Question
Hi,

Just because we can not hear any sound (due to absence of a medium), can we say that there is no sound in space? Suppose a man in space claps his hand there will be a sound but it is not heard. Similarly is there a chance of sound in the space that we do not hear? Suppose we have air in space, will we hear any sound the moment we remove the helmet of our space suit?

Thanks in advance,
Raja


Answer
Hello,

Sound exists and is transmitted in an atmosphere because of what we call a pressure wave. That is, the molecules of air (or other composition) over which the vibrations carry, cause a train of *compressions* and *rarefactions*.

E.g.


C      R
!--------!--------!-------!-------!------!


The molecules pull together at C for compressions, and spread apart at R, for rarefactions.

In space, under near perfect (lab) vacuum conditions, there is nothing to permit the compressions and rarefactions to adequately tranfer the pressure wave from one point to another.  Since there aren't enough atoms or molecules to undergo compression and rarefaction in the first place.

If the pressure wave cannot propagate, or indeed be detected by the best instruments, then to all intents it cannot exist. Irrespective of the fact that an astronaut outside the shuttle performing an EVA may clap two pieces of wood together or sound an electronic horn.

Though he may indeed have created an *initial* source for possible vibration, it cannot transfer. You will therefore not detect it, whether you have on a space suit or not. (In any case special instruments to detect vibrations - or density waves- would be the thing to use )

Putting 'air' in space to permit pressure wave propagation (and subsequent detection) in a limited region is something I cannot even imagine. For one thing, you'd have to be able to fully control the location of the molecules and ensure they don't all dissipate before you begin your experiment.

I just cannot see it occurring.

At the same time, it should be noted that a "density wave model" has been proposed to account for the persistence of the sprial arms of galaxies. In the model spiral waves (pressure waves) of higher density move through the galactic disk and produces spiral arm 'tracers' along the way.

The compressions (C) produced are alleged to squeeze small molecular clouds together to form large ones. It is the compression of the interstellar medium by the density wave that forms the features associated with spiral arms.

However, and this is important, the model falls apart on a number of points. For one thing, it never explains the *origin* of these density waves. (This is not unique since there are many "theories" that are long on predictions but leave out critical details like origins! The 'Milankovitch hypothesis' for climate change is one, since it never accounts for the origin of so-called orbital forcing over cycles of various lengths. Perhaps one reason it's never even referenced by footnote in celestial mechanics texts)

As important as the origin issue, the model never accounts clearly for what *sustains* them. As I already noted, any hypothetical sound or pressure wave that might begin - can't last since there's not the medium to sustain its propagation.

Thus, while the density model appears to show there's a place for sound waves to have a role in shaping galaxies - I'd withhold my committment rather than taking it to the bank!