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



If it appears to run slower then that means it runs slower. Things are what
we measure them to be.

----- Original Message -----
From: "Quist, Oren" <Oren.Quist@SDSTATE.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Friday, September 09, 2005 8:50 AM
Subject: Re: "moving clock runs slower" (not)


I have always used the phrase, "to us, a moving clock appears to run
slower." I don't know if this is really clearer, but it makes sense of the
situation to me.

Oren Quist
South Dakota State

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of John Denker
Sent: Friday, September 09, 2005 1:00 AM
To: PHYS-L@LISTS.NAU.EDU
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. Itââ,¬â"¢s 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 doesnââ,¬â"¢t know or care
whether
we are moving. Itââ,¬â"¢s OK to say that the projection of the
clockââ,¬â"¢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*.


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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