Re: [Phys-l] special ed/relativity

[John Denker <jsd@av8n.com>]

On 02/29/2008 07:56 AM, Rick Tarara wrote in part

> ..... slow clocks, masses 
> increasing, and lengths contracting (probably didn't say this last), IS that 
> these are the perceived phenomenon for someone coming from the Newtonian 
> model. 

That seems like a weak argument.  Why should we let the inmates
run the asylum?  Why not capitulate entirely and teach Aristotelian
mechanics, since that makes the most sense to them, based on where
they are "coming from"?


Bob LaMontagne wrote in part:

> The basic concepts of Special Relativity are invariance (tying together all 
> the energy and momentum concepts from earlier in the course) and the 
> mass-energy identity.  

OK.

> Length contrraction and time dilation as a consequence 
> of the constancy of the speed of light 

That violates the just-stated emphasis on invariance.  When 
we emphasize invariance, that should include invariant mass, 
invariant length, and invariant time.

If you look at a ruler at a funny angle, the /appearance/ of
the ruler is foreshortened, and the /projection/ of the ruler
is foreshortened.  It is neither conventional nor practical
to say that "the length" of the ruler is decreased.

The same applies to boosts.  The length of the ruler is unchanged;
it is only the /projection/ of the ruler onto the lab frame
that is contracted.  The same goes for boosted clocks:  only
the /projection/ is dilated.

  Note that there are two issues here:
   1) There is the vastly more important question of emphasis.
    -- Do we emphasize intrinsic length, or apparent projected length?
    -- Do we emphasize proper time, or apparent projected time?
   2) There is a much less important question of terminology,
    Which length-like quantity should be called "the" length?

  Once you figure out the answer to question (1), the answer to
  question (2) becomes obvious.

It has been known since the time of Plato that the projection
of an object is not the same as "the" object.

This is important, because the motion of the projection is much 
messier and more complicated than motion of the object itself.