[LEAPSECS] the big artillery

Michael Deckers michael.deckers at yahoo.com
Thu Nov 6 16:58:14 EST 2014

   On 2014-11-04 22:26, Steve Allen quoted Bernard Guinot
   about the unit for the difference TAI - UT1:

> Guinot explained this using the term "graduation second"
> in section 2.2 of 1995 Metrologia 31 431
> http://iopscience.iop.org/0026-1394/31/6/002
> He points out that the way the IAU has written the definitions of the
> time scales uses a subtly ambiguous notation.  He writes
>    The numerical value of UT1(IERS)-TAI does not
>    of course, express a duration. In this context, the "s"
>    only conveys the information that the readings of the
>    two time scales are expressed in graduation seconds.

   Guinot comes back to this question, and revises his position,
   in [Guinot 2011, section 7.a, p 4139], where he exposes the
   underlying fundamental question: how can the set of spacelike
   and timelike coordinates be given consistent dimensions
   (invariant under the Minkowski group). He writes:

      (a) Unit of relativistic coordinates

          Some authors consider the relativistic coordinates as dimensionless,
          others give a special name to their unit, such as the ‘TCB second’ or
          a global name such as ‘graduation unit’. I was myself
          in favour of the latter name. However, after long discussions with
          eminent metrologists, Quinn and de Boer, I agreed that it was
          simpler to name ‘second’ the graduation unit. Thus, more generally,
          all quantities having the dimension of time have the second (without
          any qualifier) as their unit, even if they have different natures,
          such as time interval and reading of a time scale. If the logic of
          this point of view seems rather obscure, then it is possible to
          consider it as a convention which has the merit of being in
          agreement with the quantity calculus. It also agrees with the
          metrological rule that the unit does not define a quantity.

    While I can only agree with Guinot's position, I am not sure whether
    space coordinates and relativistic change of coordinates can be modeled
    neatly in that way. Amazing that simple questions about time scales
    can lead to such really fundamental conceptual issues!

     [Guinot 2011] Bernard Guinot: "Time scales in the context of general
        relativity". in: Philosophical Transactions of the Royal Society A.
        vol 369 p 4131..4142. 2011-09-19. online at:

    Michael Deckers.

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