Re: [Phys-l] The behavior of light

[John Denker <jsd@av8n.com>]

On 05/04/2008 10:18 AM, Fink Trevor M wrote:
> I finally understand how this works with the light clock.  Thank you
> very much for your diligence in answering my questions! 

:-)

>  I am still
> confused on how a biological clock is affected by traveling at high
> speeds,

The modern (post-1908) answer is that *neither* the biological clock
nor the light clock is affected by traveling at high speed.  *Both*
of them keep time (proper time!) just fine in their own reference
frame.  Neither of them cares what observers see in other reference 
frames, if/when events are projected onto those frames.

On the other edge of the same sword, special relativity is all 
about the _projection_ of events from one frame to another.  SR
does not care in the slightest what the clocks are made of ... 
biology, radiation, or whatever.  SR knows how to handle events, 
and clock-ticks are a kind of event.  

To repeat:  SR is nothing more or less than the geometry and
trigonometry of spacetime.

  I remain quite aware that there are other (pre-1908) ways of
  explaining SR.  However, this business of "biological clocks"
  versus "light clocks" is a veritable poster child, illustrating
  the power and simplicity of the modern (post-1908) approach, 
  i.e. the spacetime approach.  The question of "how a biological 
  clock is affected" does not come up -- or if it does, the answer 
  is obvious and trivial: it's not affected.  Proper time is
  invariant.

Actually, I'm not sure invariance counts as the "modern" answer.
It's the same answer Galileo would have given in 1632: neither 
a clock nor anything else is affected by uniform straight-line 
motion.  

There's a one-line proof that biological clocks *must* tell time
the same way as light clocks and all other clocks:  Otherwise it
would violate Galileo's principle of relativity.  This is diagrammed
at:
  http://www.av8n.com/physics/twins.htm#fig-two-clocks