Before I get nit-picked, I should have further specified that the pencil
length definition requires that the spatial separation of its ends be
measured along the measured dimension (not eg "head=on").
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "Bob Sciamanda" <trebor@WINBEAM.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Friday, September 09, 2005 11:15 AM
Subject: Re: "moving clock runs slower" (not)
| 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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