Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-L] 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. It’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 doesn’t know or care whether
we are moving. It’s OK to say that 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*.


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?
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l