I find the kind of logics used by John here, rather unusual. It boils=
down to this:
A moving car does not move. Why? Because the true properties of an =
object can be observed only in the object's rest frame, and any drive=
r knows that in the rest frame of a car the car does not move. Welcom=
e back to Zeno!
Moses Fayngold,
NJIT
-----Original Message-----
=46rom:=09Forum for Physics Educators on behalf of John Denker
Sent:=09Fri 9/9/2005 2:00 AM
To:=09PHYS-L@LISTS.NAU.EDU
Cc:=09
Subject:=09"moving clock runs slower" (not)
Hi --
There are a lot of references that try to explain relativity on an
elementary level by saying "a moving clock runs slower".
Some authors seem to take that as one of the axioms -- or at least on=
e
of the theorems -- of relativity. It's not. It describes only part
of what's happening, and doesn't even do a very good job at that.
I reckon most people on this list already know this, but it seems
worth mentioning anyway. I confess I recently caught myself having
used the "moving clock runs slower" notion. I mentioned it only in
passing, not as an important part of any argument, but still I was
embarrassed.
To make amends, I cooked up the following analogy:
It would be unwise to say that a pencil gets shorter if we look at
it nearly end-on. It=E2=80=99s OK to say that the projection of th=
e pencil
on our field of view is shorter, or perhaps that the appearance of
the pencil is foreshortened -- but there has been no real change
in what the pencil *is*.
By the same token it would be unwise to say that a clock runs slow=
ly
if we are moving relative to it. The clock doesn=E2=80=99t know or=
care whether
we are moving. It=E2=80=99s OK to say that the projection of the c=
lock=E2=80=99s world
line onto our field of view projects tick marks that are more wide=
ly
spaced, but there has been no real change in what the clock *is* o=
r
what it *does*.
This issue came up in an off-list discussion of the infamous travelli=
ng
twins. It caused me to expand, revise, and re-organize my web page o=
n
the subject: http://www.av8n.com/physics/twins.htm