Albro And The Twins

David Byrden wrote:
 I want an example of the usual "paradox" involving two 
 twins who split up, make some kind of journey/s and come 
 back together... when the twins get back together they 
 must EACH be younger than the other, after you add up 
 the time durations as seen from each twin respectively.

Without wishing to tax someone who is fighting the good fight, 
I think it's important to point out that the twins paradox (i.e., 
the privliged status of inertial frames) IS an epistemological 
problem for *special* relativity, and was one of the main 
considerations that motivated Einstein to abandon SR as a 
viable theoretical framework for physics.

The problem is that SR is predicated not only on the assumption 
of the existence of inertial frames, but also on the assumption 
that we can empirically identify those frames.  This is already 
somewhat problematical, but when SR is combined with the
Equivalence Principle the result is an epistemological mess.
In Einstein's words,

  "In the theory of special relativity there is an inherent
   epistemologcial defect...  The weakness of the principle 
   of inertia lies in this, that it involves an argument in 
   a circle: a mass moves without acceleration if it is 
   sufficiently far from other bodies; we know that it 
   is sufficiently far from other bodies only by the fact 
   that it moves without acceleration."

To put this in more familiar terms, Einstein would say to all the
people who claim that special relativity is adequate to "handle"
the twins paradox:  We can say that the twin who followed the
unaccelerated worldline will have aged the most, but if we are
asked which twin had the unaccelerated worldline we can only answer:
the one who aged the most!  Accelerometers can't rescue us from 
this circle, because the Equivalence Principle implies that the
lapse of proper time along a given worldline cannot be inferred
from the locally "felt" accelerations.  For example, both twins 
could spend the entire interval from A to B experiencing zero 
local acceleration, and yet the lapses of proper time could be 
vastly different.

Thus, as soon as the Equivalence Principle is adopted, it's clear 
that special relativity is epistemologically unsatisfactory, and 
can only be salvaged by a suitable theory of gravitation (e.g., 
general relativity), within which SR may serve as a useful 
approximate simplification in appropriate limiting cases.  However, 
we can only assess the appropiateness of SR in a given circumstance 
by evaluating it in the context of GR.  In other words, SR can serve 
as a set of convenient computational recipes for technicians who 
don't want or need to understand what they are doing, but from an 
epistemological standpoint there is only one modern theory of 
relativity, and that is GENERAL relativity.  Special relativity 
had already been discarded as a viable theory of knowledge by 1911.

I think it's also worth mentioning that when ordinary non-physicists
ask about relativity, they aren't hoping to become technicians or
computational experts, they are asking from a broad philosophical
and epistemological standpoint, i.e., they are curious to know, in 
broad terms, the basis of relativity as a theory of knowledge.  From 
this perspective, the custom of telling such people that special 
relativity is "the answer" to the twin's paradox is particularly 
unfortunate.  (I say this in spite of the undeniable fact that most 
people who worry about the twins paradox have actually failed to 
understand special relativity, and aren't even close to the level 
of comprehension on which the actual inadequacy of special relativity 
appears.  On the other hand, most of the people who DON'T worry
about the twins paradox are equally far from understanding the 
real issues involved.)