(I'm moving this from physics.research, because it has been
almost comatose for several days now).
__________________________________________________ ___________________________

Everything Bartosz Milewski said is correct. (I've
quoted his posting below).

An inertial observer can determine the current age of
some distant person (for brevity, I call this the "CADO",
for "the current age of a distant object") by using
the Lorentz equations. But if he receives a sequence
of transmissions from the distant person (each giving
her age at the time of transmission), then he can compute
her current age by using only first principles (plus
his own clocks and measuring sticks), even if he doesn't
know anything about special relativity. I discuss this
in detail in my paper

"Accelerated Observers in Special Relativity",
PHYSICS ESSAYS, Sept 1999, p629.

I also show why an accelerating observer, when attempting
to determine the current age of a distant person, MUST
adopt the conclusions of the "MSIRF", the inertial
reference frame in which he is momentarily stationary
at that instant. Otherwise, he will be forced to ignore
his own simple, elementary measurements.

Mike Fontenot
__________________________________________________ ___________________________

Bartosz Milewski wrote:

You are right and I think, after reading all the replies to my original
posting, I understand why. The sudden instantaneous aging is not the effect
of GR, but is an adjustment you have to make when switching inertial frames.
The problem is that the concept of "actual age" of the earthbound twin
depends on the frame of the observer.

When watching the films recorded by twins, we don't use the concept of
"actual age"--what we see is the observed, delayed, age. This age is
continuous, there is no sudden jump when turning around. But if the
traveling twin tries to keep track of the "actual age" of his brother, he
has to calculate it using SR. He has to make adjustments to translate what
he sees to what is happening "right now" to his brother. And, of course, the
definition of "right now", as applied to his brother, changes drasticly when
he does a U-turn. The hyperplane of "the present" in Minkowski space
reverses its tilt. The farther the other brother is, the bigger the sudden
adjustment to his calculated age is.