[Phys-L] Re: "moving clock runs slower" (not)

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

> Is the twin who made the near-light-speed trip really younger when he
> returns?  If he is, didn't his clock HAVE to run slow (again relative to
> that of his brother--since their clocks ran at the same speed just before
> and just after his trip)?   Something real has happened to create the
> biological age difference!

To amplify (I hope) on John Denker's response: Something real has
clearly happened, but it simply can't be explained by saying that
"moving clocks run slow" or even, more specifically that "the
traveling twin's clock ran slow."

To the extent that the phrase "moving clocks run slow" means anything
(and I am on record defending the idea that it does!), one would have
to acknowledge that BOTH twins find that the other's clock runs slow
during those periods that they are behaving as inertial observers.
The difference is that the stay-at-home twin finds that the traveling
twin's clock runs slow or at the same rate at EVERY instant during
the journey.

It's not quite as clear how to describe what the traveling twin
"finds" in similar terms.  It is common to invoke an apparent
gravitational blue shift during the acceleration phase, but this can
be problematic for a number of reasons.

Apropos of all this, some might find an article by Minguzzi in the
September issue of AJP ["Differential aging from acceleration: An
explicit formula," AJP, V73, p.876] to be of interest.

John Mallinckrodt
Cal Poly Pomona
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