Making Light:  Tracking Nielsen Haydens in their habitat ::: August 28, 2004, 12:14 AM

Chad:

  That sounds pretty dodgy to me-- being "suddenly older," in
  particular, strikes me as unlikely (discontinuous jumps are not
  common in real systems).

The "suddenness" is entirely an artifact of how suddenly the traveling
twin adopts a new inertial frame at the turnaround.  All that happens
is that the traveler's calculation of what "now" is on Earth has
advanced by a significant amount.  There is no discontinuous jump in
any observed quantity, only in what is calculated to be simultaneous
in a distant place.

  The only good textbook description I have of what each twin
  sees at each point in the journey is one that also includes the
  time-of-flight delays of light signals sent from one to the other
  (in which case, they each "see" slower aging at first (in the sense
  of annual signals from one to the other arriving more slowly),
  followed by a burst of rapid aging at the end of the round trip.

I think you are remembering a description equivalent to the short
one already cited.  However, the "burst of rapid aging" is what the
traveling twin "sees" happening to the earthbound twin when turning
around (and adopting a new inertial frame) in the middle of the round
trip (the turnaround), not the end.  That aging is of course not
directly observed, since the twins are far apart when it occurs.  The
earthbound twin does not change frames, and so does not observe any
"rapid aging" in the traveler.  On the web page, the burst of "rapid"
(or "sudden") aging is shown in the right-hand diagram, where "Joe 2"
in one frame becomes "Joe 6" in the other.

As for your other remarks, I don't find acceleration to be necessary
to resolving the paradox, I don't believe I'm applying formulae
blindly, and I don't believe I'm confused.  But you may feel free
to disagree.