proper time

["John S. Denker" <jsd@MONMOUTH.COM>]

On 04/29/2003 08:32 AM, Mark Sylvester wrote:
> I don't get why we must go to the photon's frame when using the term proper
> time. Is French's usage of "proper time" at variance with the rest of the
> world?

To my ears, "proper" in SR (and GR) refers
to a particular observer's description of
what happens to himself.

Proper distance is zero, because the
observer cannot be located away from
himself.

As a consequence of the foregoing, the
Lorentz interval along the observer's
world line is equal to the proper time.
And indeed "proper time" is primarily
used as a shorthand for exactly that:
it is used to mark off lengths along the
world-line of some object.

For an unaccelerated observer, this is
pretty much trivial.  Proper time is just
time in the inertial frame of the observer.

Proper time becomes nontrivial for an
accelerated observer, such as the infamous
travelling twin.  In such cases, there is
no such thing as "the" inertial frame of
the observer.  Instead, you can use a
plurality of instantaneously comoving
frames.
  http://www.monmouth.com/~jsd/physics/twins.htm

Proper time enters the discussion only if
we can find some observer whose world line
goes through the events of interest.  In
the PV train puzzle, the only world line
that goes through the events in question
is the photon's world line.  So, to answer
the question above, that's why I brought it
up.

Of course, photons make lousy observers.  So
AFAICT the topic of proper time is a topic
that needn't come up in the discussion of
this puzzle.  The Lorentz interval between
the two events is zero.  That's mildly
interesting, but AFAICT it doesn't really
help explain what needs explaining, namely
the breakdown of simultaneity at a distance.