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

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

I have some sympathy for this point of view.
"Proper" measurements are called "proper" for a
reason.  Nevertheless, I think it is fair to say
that a "moving clock" REALLY DOES run slow.  We
determine reality by making observations and
observations of a "moving clock" reveal beyond
doubt that it does not keep pace with "stationary
clocks."

I'd be more inclined to take issue with the
description of the clock as "moving" rather than
the less misleading "moving relative to the
observer."  Once one accepts the description of
the clock as "moving," however ...

I imagine you would say the same thing regarding
Lorentz contraction; I think that would be the
only consistent position.  But am I alone in
thinking that the case seems even weaker in that
situation?  After all, I REALLY can get an
oversized pole completely inside a barn as long
as it is moving fast enough.  Are you going to
tell me that it isn't REALLY shorter?

John Mallinckrodt
Cal Poly Pomona

> 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*™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 doesnâ*™t know or care whether
>    we are moving. Itâ*™s OK to say that the projection of the clockâ*™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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