Bronwyn wrote:
> "Tom Roberts" <tjroberts137@sbcglobal.net> wrote in message
> news:JU9Ij.3626$p24.1942@nlpi061.nbdc.sbc.com...
>> The point is that nothing at all changes about the clock, due to its
>> motion relative to some inertial frame. That is, motion does not change
>> THE CLOCK, but it can (and does) change how measurements in that frame
>> relate to the clock and its readings.
>> 
> So if I could see a large digital clock moving away from me at v, its
> reading would appear
> different from its true reading by the factor (c-v)/c. Is that right?

No. First note that we must carefully separate the propagation delay (of
light or other signals) from the intrinsic timekeeping of a clock. To do
that, pre-position a number of assistants along the path of the moving
clock, making sure they are all at rest in your inertial frame, and that
they each have their own clock synchronized with yours. Instruct each of
them to wait for the moving clock to pass, and when it does, to write
down both its displayed value and the value displayed on their own clock
[@]. Afterwards, collect all the notes from the assistants and you can
compute the rate of the moving clock compared to the time coordinate in
your inertial frame [#]. SR predicts it will be slower than your clock
by a factor sqrt(1-v^2/c^2), where v is the speed of the moving clock
relative to your inertial frame; many experiments confirm this prediction.

[@] As each assistant is reading both clocks "right here",
there are no issues about signal propagation to worry about.

[#] from the description, it should be clear that this is NOT
comparing the moving clock to your clock. This approach
constructs a time coordinate in your inertial frame, and
compares the moving clock to it. A key aspect of this is that
each assistant must be at rest in your inertial frame, or this
would not make sense.


>> The ambiguity about what "time" means is mere words: in physics we often
>> say "time is what clocks measure", or better: in physics we use a time
>> coordinate determined by the measurements of clocks. This last shows
>> that it really does not matter what one means by the word "time". It
>> also avoids confusion about "time varying", because it is the various
>> TIME COORDINATES that vary, and that does not violate ingrown prejudices
>> about "time".
> 
> But clocks are just devices humans have made to try to measure intervals of
> time accurately.

Right. So this approach does not violate anything.

With enhanced understanding comes a need for refinement and enhanced
precision in descriptions. So careful physicists rarely discuss "time",
instead they discuss time coordinates and clocks.

Yes, many popular books are written by authors who are
not being careful....


> I don't see that this defines TIME itself any more than a ruler tells us
> what space is.

Fine. But as I said, EXPERIMENTS use a time coordinate defined by clock
measurements. So to compare theory to experiment one must use the
appropriate value. To many/most people it comes as a surprise that
moving clocks are measured to "run slow", and therefore clock time is
not universal. Regardless of what one thinks "time" means, this variance
in clock rates is an inherent aspect of the world we inhabit.

Frankly, I doubt you (or anyone else, myself included)
really knows what time is. Or space. Regardless of rulers
and clocks. The point is to relate the tools we use to
measure these things to the theories we use to model the
world. What space and time "really are" is a question for
theologists, not physicists.


Tom Roberts