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



The distinction John makes may be important for higher level courses, but I
think it is not something we need (or want) to deal with in introductory
courses. That the clock moving at high speed relative to me runs slower
than my clock is easily demonstrated with the classic 'light clock'--light
beam bouncing between two mirrors with the motion of the mirrors at right
angles to the (at rest) path of the light. Of course, the example depends
on the speed of light being independent of the motion of the source. That
the observer riding with the clock sees the normal (up down) motion of the
light is also not difficult to 'see'. That the moving observer IN ALL WAYS
agrees that the clock is keeping normal time IS the problem for students
(well most of us), but is supported by observation and experiment--including
the atomic clocks around the world 'stunt'. I just would never introduce
the description "the projection of the clock's world line onto our field of
view projects tick marks that are more widely spaced, but there has been no
real change in what the clock *is* or what it *does*" in any of _my_ intro
courses. ;-)

Rick (there are some simple animations for intro special relativity at the
site below)

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
Animations for Lectures
Photo-realistic Lab simulations
Energy management simulations
www.saintmarys.edu/~rtarara/software.html
Energy 2100--class project
www.saintmarys.edu/~rtarara/ENERGY_PROJECT/ENERGY2100.htm
********************************************************

----- Original Message -----
From: "John Denker" <jsd@AV8N.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Friday, September 09, 2005 1:00 AM
Subject: "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 one
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. Itâ?Ts OK to say that the projection of the 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 slowly
if we are moving relative to it. The clock doesnâ?Tt know or care
whether
we are moving. Itâ?Ts OK to say that the projection of the clockâ?Ts
world
line onto our field of view projects tick marks that are more widely
spaced, but there has been no real change in what the clock *is* or
what it *does*.


This issue came up in an off-list discussion of the infamous travelling
twins. It caused me to expand, revise, and re-organize my web page on
the subject:
http://www.av8n.com/physics/twins.htm

Comments, anyone?
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