John wrote:
| . . .but there has been no real change
| in what the pencil *is*.
The pencil *is* (at least to physics) only the aggregate of its measurable
properties. We *define* these measurable properties as useful raw material
for our models. One of these properties is its measured length, defined as
the spatial separation of its ends, measured simultaneously (all of course
in the measurer's frame).
This harks back to our earlier discussion re: the Aristotelian (and
Scholastic) doctrine of substance vs accidents (properties).
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "John Denker" <jsd@AV8N.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Friday, September 09, 2005 2:00 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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