Re: [Phys-l] Teaching Special Relativity

[John Denker <jsd@av8n.com>]

On 07/30/2009 05:59 AM, Bob Sciamanda wrote:
> Just as the physics of a moving photon clock explains (below) the mechanism 
> which gives it a longer period, it is the assertion of the relativity 
> principle that there will be a corresponding physical mechanism behind the 
> slowing of any well constructed moving clock.

But according to a modern (post-1908) understanding of relativity,
"the" period of the clock is not longer.  It is not changed.

"The" length of a ruler is invariant under ordinary rotations, even
though the projection of the ruler onto this-or-that reference frame
will in general change.  In a profoundly analogous way, "the" length
of rulers and "the" timing of clocks is invariant under rotations in 
the XT plane, i.e. boosts, i.e. changes in velocity.  It is only the 
shadow cast on this-or-that reference frame that changes.

Do not confuse a shadow with the real object that casts the shadow.
Plato had something to say about this, 2400 years ago.

> Even before 1905 - Lorentz, Fitzgerald, et al argued that a real shortening 
> of a moving rigid rod follows from Maxwellian electrodynamics. 

Or so they thought before 1905, i.e. before they understood relativity.

The year 1905 has been over for a while now.  We are smarter now than we
were then.  Why do some people want to relive 1905 over and over?  Is
this some weird sequel to the Groundhog Day movie?  Words fail to express 
how much I don't care how things were done in 1905 or earlier.  The story 
changed in 1908, and it will never change back.


    Von Stund′an sollen Raum für sich und Zeit für sich 
    völlig zu Schatten herabsinken 
    und nur noch eine Art Union der beiden 
    soll Selbständigkeit bewahren.

                             Hermann Minkowski (1908)



    From now on, space by itself and time by itself 
    are to totally sink into shadows, 
    and only a kind of union of the two 
    is to maintain an independent existence.