Re: [Phys-l] physics lessons from wikipedia

[Brian Whatcott <betwys1@sbcglobal.net>]

John Denker wrote:
> /snip/  There is some interesting
> physics to be found, in a perverse way, in this
> wikipedia passage:
>   http://en.wikipedia.org/wiki/Wave_equation#Spherical_waves
>
> On 03/11/2010 08:46 AM, curtis osterhoudt wrote:
>   
> >  If you want to follow a dit with "sharp edges" by another
> > such dit in an even-dimensioned space, you have to wait until the
> > first dit has died out via dissipation, or until the wake formed by
> > the first dit is of a significantly different shape./snip/
> >     
>
> That's all fine, but what's true for a baseband dit isn't
> necessarily true for a wave packet in the far field.
Hmmm...the hair on my neck visibly rises when I see the "physics" 
attitude on display.
(Not to say it isn't correct...)        At times like these, I hasten to 
the
last redoubt of all conviction: physical example.

It's easy to recall the quasi one-dimensional surface wave examplar -
the canal wave soliton, which let a two horse hitch pull hundreds of tons
of long-boat up to speed, where a single horse could then maintain the 
unusually fast pace
assisted by the non-dissipating stern wave.

The quasi-one dimensional sound wave is easily demonstrated in a tube - 
or along a rail -
with similarly long and non-dissipating transmission.

A quasi-two dimensional example for a sound wave or I suppose a 
microwave source ought
to look like two close spaced parallel plates, with a launch point in 
the middle of the channel.

The assertion is that such a 2-D channel behaves differently than a 1-D 
channel in
response to a Dirac type pulse.  Does it? Probably: Not sure - haven't 
tried.

Brian W