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.