Re: [Phys-l] Relativity Question

[John Denker <jsd@av8n.com>]

In the context of how to synchronize all the clocks in a frame,
On 05/15/2009 09:29 AM, LaMontagne, Bob wrote:
> Isn't the use of sound fundamentally different from electromagnetic
> signals? 

Two answers:
 -- Yes, sound provides a fundamentally different physical 
  basis for the sync process.
 -- No, the difference is not important to the end result.

Comparing these options was meant to provide some insight as 
to what assumptions are necessary in order to "derive" SR, in
contrast to assumptions that are commonly made but not really
necessary.

  Intro-level students don't care about such fine points.  They
  are content to know what's true and (maybe) some set of
  supporting evidence.  Much later they may go through the phase
  where they want to know a "minimal" set of evidence.

> For enclosed trains, any observer will agree that the speed
> of sound is the same relative to and inside each train. 

Yes, and that suffices for sync.

> With
> electromagnetic signals each observer finds the speed of light the
> same relative to themselves. 

Yes, and that suffices for sync.

> I have a feeling that this is another of
> those situations where we are all actually in agreement but talking
> past each other because we have slightly different examples in mind.
>
> John Denker Wrote: You can get along just fine without the star, if
> you are able to send a signal up and down your train in a way that is
> independent of the state of motion of the train.  A sound signal
> suffices, if your sound is very very accurate, and the medium is
> comoving with your train, and you invoke Galilean relativity.  In
> practice people often use electrical pulses.
>
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