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[Phys-L] Re: "moving clock runs slower" (yes)



John D., in response to my post, said that the question of what is real
is less important than it might appear. That may or may not be true,
but isn't it the question that started this discussion? Do clocks
really slow down?

As John said, projections and/or appearances become important in the
twin situation only if one wants a blow-by-blow account of what each
twin is thinking during the trip, but not necessary in the final
analysis when the two twins are standing side by side in the same
reference frame. Yes... I agree with that. But I thought all or part
of the original question was trying to get an answer to the question of
whether the twin who travelled truly ends up being younger. The answer
to that question is yes. The next question then becomes, does that mean
the travelling twin's biological clock really slowed down. I think I
would answer that question as no. I continue to like John's description
that in the end the twins arrived at the same place by different
spacetime paths. That seems less problematic than saying the biological
clock slowed down. So what does that say about the idea of relativistic
time dilation? If it means the clock slows down, maybe it's not good
wording to use. If it means the travelling twin ends up younger, then
maybe it's okay wording to use. If we decide "time dilation" is not
good wording, what concise wording would we want to use to describe the
fact that the travelled twin ends up younger? Do we just leave it as,
"Oh, the travelling twin ended up younger because she took a different
spacetime path than her brother." I'm okay with that, and probably like
that, but we do need to change a lot of textbooks that use "time
dilation" wording.

Likewise, I agree with John that the two lab notebooks look the same in
the end after each person has done the proper analysis of the data. My
point is that the raw data are different. In one sense this is no
different than non-relativistic differences in raw data when two people
make observations from different frames for which the relative velocity
between the frames is zero or not very fast. In another sense this is
different because the non-relavistic reckoning between the notebooks can
use a Galilean transformation and the relativisitic reckoning requires a
Lorentz transformation. If time dilation, relativistic mass versus rest
mass, and length contraction are/were terms that described the various
aspects of the necessary relativistic transformations needed to make the
notebooks become reconciled (i.e. to come to the same laws of physics),
then time dilation, relativistic mass versus rest mass, and length
contraction serve some sort of usefull purpose. If we decide to abandon
those words it is okay with me, but we still need to recognize that
until the relativistic adjustments/corrections/analysis are performed,
the lab notebooks look different just like the slower-relative-motion
observers have different notebooks until the Galilean transformation
reconciles the notebooks with each other.

Another example I always have thought intriging is that for two
observers having large relative velocity between them there is a sizable
portion of their space-time graphs for which they will disagree on the
ordering of events. One observer will record event A happened before
event B. The other will record event B happened before event A. Both
lab notebooks are correct, and they do disagree... until the
relativisitic analysis ie performed. You have to do the analysis before
they agree.

John ought to respond to this, "Of course. That's no different than
reconciling different raw data taken from different but non-relativistic
frames." I agree, except the reconciliation for the relativistic case
is a different level of complexity, especially for students not used to
thinking that way. The need to make and how to make those
transformations is what teaching relativity involves.

It's not difficult to convince students the picture of Saturn is real
and that Saturn's rings are really nearly circular even though they
appear elliptical in the picture. Students are used to that reckoning.
Convincing them of the need and process for relativistic reckoning is
much more difficult.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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