Re: [Phys-L] Sound waves in modern physics

[John Denker <jsd@av8n.com>]

On 03/23/2013 10:04 AM, I wrote:
> The physics here is easy
> to understand:  Sound is grossly attenuated unless the wavelength
> is long compared to the mean-free-path in the medium.

As always, "easy" is relative.  It is context-dependent.

This makes contact with a discussion some folks were having in 
another forum.  The question was, what should be included in a 
"modern physics" course for physics majors.

There was a faction (including me) that argued there should
not be any such thing as a "modern physics" course per se.
Instead, modern ideas should be integrated into the entire
educational system ... from the earliest grades on up.

For example, the idea that time is the fourth dimension is so 
well integrated that more-or-less everybody has heard of it.
They don't necessarily take it as seriously as they should,
but that's a fixable problem.

Closer to the topic of this morning's discussion:  The idea 
that atoms exist could perfectly well be considered a "modern 
physics" idea.  Ostwald and Mach -- who were not exactly the 
village idiots -- held out against atomic theory until well 
into the 20th century.  However, modern atomic ideas and 
even some sub-atomic ideas are now well integrated into the 
curriculum from 3rd grade onward, so we don't need to worry 
about it too much.

THAT is the result I'm looking for.  I would prefer the modern 
ideas to be so well integrated that everybody takes them for 
granted, and nobody would even dream of corralling them into 
a specialized "modern physics" course.

There are plenty of top-notch universities that do not offer
any modern-physics-by-itself course.  If you don't believe
me, check the catalogs.  They're online.

This is relevant because if you think of the sound-carrying
medium as a continuous fluid -- without any notion of atoms -- 
you can understand all sorts of basic fluid properties such as 
the pressure, density, speed of sound, et cetera ... but you 
will not get much traction on /transport/ properties such as 
the diffusion constant, viscosity, or thermal conductivity.
The latter are sensitive to the size of atoms, the number of
atoms, the mean free path, et cetera.

So, to spiral back to where we started:  If a high-school
student asks why there is no sound in space, my answer would
be that there's plenty of sound in space.  It's long-wavelength
sound.  The short stuff is grossly overdamped, because the
wavelength isn't long enough compared to the mean free path.

This may seem like a stretch in high school, but it's not
thaaaat much of a stretch.  The kids should have heard of
thermal conductivity and viscosity ... and if they haven't,
it's high time they did ... whether or not it's on the 
almighty state-mandated test.

Presumably they don't have a very clear idea about mean free
path, but it wouldn't hurt them to hear it mentioned.  It is
fairly easy to visualize.  They should learn that it is verrry 
much longer than the interparticle spacing.  There is a nifty 
non-dimensional scaling argument that one can make about this, 
leading to a simple and interesting result:
  http://www.av8n.com/physics/scaling.htm#sec-mfp

This illustrates the unity and grandeur of physics:  Ideas we 
know about the small-scale stuff (atoms) tell us interesting
things about the large-scale stuff (interplanetary medium).