John, I think you make too much universal worth of these "proper" frames.
What is the "proper" frame for specifying the time interval between an Earth
event and an Andromeda event. What is the proper distance between Io and
Earth's moon? Proper length is an idea which arises in the narrow
consideration of a rigid body - a very special (and in principle impossible)
case.
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Monday, September 12, 2005 12:00 PM
Subject: Re: "moving clock runs slower" (yes)
| As I see it, this thread is revolving around two semantic concerns:
|
| 1. What is the meaning of "real" (length, time, etc.)?
|
| 2. What is the meaning of "apparent" (length, time, etc.)?
|
| Arguing, correctly in my opinion, from the perspective of a
| sophisticated practitioner of physics, John Denker argues
| persuasively that "real" should be essentially synonymous with what
| is commonly called "proper," that the "real lengths" of rods and the
| "real rates" of clocks are their "proper" lengths and rates. Thus,
| moving clocks do NOT "run slow" and moving lengths do NOT "contract."
| When we say that they do, we are talking only about "appearances."
|
| I disagree with Denker, not because he is wrong, but because, I am
| more concerned with pedagogical issues in the teaching of relativity
| and my experience tells me that one of the biggest hurdles for
| students being introduced to relativity, is appreciating the
| difference between what something moving at high speed would look
| like and what we would measure it to be, a difference I would like to
| and do characterize as being between "appearance" and "reality."
|
| In an introductory course, I think it is critically important, at
| least briefly, to discuss very explicitly the role of the finite
| velocity of light in determining what things look like and how we can
| unravel those "appearances" to determine that moving clocks "really"
| do run slow and that moving rods "really" do contract, that it is
| decidedly NOT a matter of "appearance." Note, for instance, that a
| rod moving generally toward you will "appear" shorter because of the
| finite velocity of light whether or not it "really" is. It is only
| after doing some calculations, that we determine that not ONLY does
| it "appear" shorter, but it "really" is shorter. Similarly, as it
| moves away from us, it "appears" longer, but calculations reveal that
| it is "really" shorter.
|
| On the other hand, we certainly DO want students to understand that
| there IS a preferred reference frame for making length and time
| measurements and we want to have a word to describe those
| measurements that distinguishes the "reality" in that frame from the
| DIFFERENT "reality" in other frames. Fortunately, we DO have such a
| word--"proper."
|
| Later on in the introductory or a later course we can and should IMO
| have the sort of philosophical discussions that Denker has provoked
| here.
|
| John Mallinckrodt
| Cal Poly Pomona
|
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