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On Apr 9, 10:33 pm, Tom Roberts <tjroberts...@sbcglobal.net> wrote in
sci.physics.relativity:
> bill wrote:
> > A person is located on a mountain top and is looking at a (very large)
> > clock (B) at sea-level.
> > He notices that clock B is ticking over at a slower rate than his own
> > clock (A) as theorized by Einstein in general theory and as ratified
> > by the Wallops Island experiment where a clock at sea-level, being
> > located in a strong gravitational tidal area, will tick over at a
> > slower rate than an identical clock on top of a mountain that is in a
> > weaker gravitational tidal area.
>
> Note that what you call "gravitational tidal area" is really called
> "Newtonian gravitational potential", valid in the Newtonian
> approximation to GR. Indeed, tidal forces/effects are not involved
> (tidal forces are second derivatives of the Newtonian potential).
>
> Note that while A _SEES_ B tick at a slower rate, this does not imply
> that clock B actually ticks at a slower rate than clock A. Indeed, in GR
> this is modeled as an artifact of the COMPARISON [#], not as an effect
> or modification of the clocks themselves: both clocks tick at their
> usual (proper) rates, but their tick rates appear different when
> situated and compared as you describe.
>
>         [#] This comparison is via EM signals in curved spacetime.
>
> > Is he entitled to be of the opinion that if he were to move to sea-
> > level his clock would be subjected to the same 'law' of physics thus
> > it will then be ticking over at a slower rate than it is before he
> > starts his descent?
>
> No. See above -- this is not an effect ON THE CLOCKS, but rather OF THE
> COMPARISON. Note that he is entitled to expect that if he carries his
> clock down and puts it right next to B that the two clocks will tick at
> the same rate.
>
> Your statements are expressed with the implicit assumption there there
> is some "global", "universal", or "absolute" way to determine a clock's
> tick rate. In GR there is no such thing -- all you can do is compare
> clocks to other clocks; the method of comparison can, and usually does,
> affect the result.
>
> Tom Roberts

Honest Roberts, your cleverer brothers Einsteinians expose the
"artifact of the COMPARISON" in a somewhat clearer way:

http://www.people.fas.harvard.edu/~djmorin/book.html
Chapter 14: "The equivalence principle has a striking consequence
concerning the behavior of clocks in a gravitational field. It implies
that higher clocks run faster than lower clocks. If you put a watch on
top of a tower, and then stand on the ground, you will see the watch
on the tower tick faster than an identical watch on your wrist. When
you take the watch down and compare it to the one on your wrist, it
will show more time elapsed."

However, Honest Roberts, even your cleverer brothers Einsteinians
would never answer the following question:

Is the "artifact of the COMPARISON" (that is, gravitational time
dilation) consistent with Einsteiniana's dicovery that the speed of
light is CONSTANT in a gravitational field, or is it consistent with
Einsteiniana's discovery that the speed of light is VARIABLE in a
gravitational field and obeys e.g. Einstein's 1911 equation c'=c(1+gh/
c^2) given by Newton's emission theory of light?

Pentcho Valev
pvalev@yahoo.com