o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o (01)
Ed, (02)
I'm going to skip the part about how we had cesium
on our periodic papyri when I was at school and
just point out that everything you e-graved on
this new-fangled stele below simply confirms
the necessary connection between concepts of
physical magnitude and the mechanisms that
are devised to measure them. The logical
aspects of referring to an atomic clock
are the same as the logic of referring
to any other standard of reference,
past or future. None of that can
be said to "dissociate" the idea
of magnitude from the practice
of measurement, but all of it
goes to guarantee the link. (03)
Jon Awbrey (04)
Ed Barkmeyer wrote:
>
> Jon Awbrey wrote:
> >
> > Can we really and truly dissociate the semantics of terms like
> > "duration", "length", and "mass" from the epistemological stance
> > of a particular frame of reference, or the operational resources
> > of the apparatus that we use to measure them? I don't think so.
>
> Pat Hayes answered:
>
> > I do. In fact, we must. How would we talk about accuracy of a
> > measuring apparatus, if there were not a meaningful distinction
> > between a real magnitude and a measurement? To even discuss a
> > measuring apparatus, we need to have a theory of the physical
> > magnitudes which they are designed to measure.
>
> For Jon's edification, the function of the National Institute
> of Standards and Technology, and its counterparts in many other
> nations, is to achieve international agreement on the definitions
> of terms like "duration", "length", and "mass", define physical
> references for the units of measure in which specified values
> of those concepts can be expressed, and to define mechanisms
> for making measurements and determining their relationship
> to the reference values.
>
> Part of that is mathematics, part of that is physics,
> and part of that is engineering. Pat is correct:
>
> > More fundamentally, however, these are clearly distinct concepts.
> > Magnitude is not an epistemic notion, but measurement is. And truth,
> > perhaps unlike knowledge of truth, does not require verification or
> > measurement to be meaningfully spoken of.
>
> That is, a "length" is importantly different from every measurement
> of a length. In product engineering, we talk about "ideal dimensions" --
> those that are computed from the mathematical (geometric) model of
> a part -- as distinct from "actual dimensions" -- those that are
> determined by measurement of an instance of the part as-manufactured.
>
> The reference physical quantity for a unit of length is an abstracted
> physical phenomenon -- the wavelength of a particular band of spectral
> radiation from a Cesium atom of a particular structure. That concept
> is known to be fixed, and is independent of all means of measuring it.
> Means of measuring that quantity to a certain degree of accuracy or
> better are now well-known. The international agreement depends on
> each national body making its own measurement of that quantity to
> that degree of accuracy, and defining the means by which industrial
> measurements in that country are compared to that standard. So a
> manufacturer may measure steel thickness with a gage block that is
> known to have been calibrated against the reference standard, or with
> an instrument that is known to have been calibrated against a reference
> sample of some carefully chosen alloy that is known to be of a certain
> thickness within an acceptable degree of "uncertainty".
>
> So there is a flow of separable concepts here:
> abstract quantity, reference physical quantity,
> reference measurement and accuracy (uncertainty),
> actual measurement practice.
>
> Sorry for the soapbox, but this is NIST's raison d'etre.
> [In 1901, our job was to calibrate U.S. instruments against
> the U.S. copy of the "standard metre" -- a platinum-iridium
> metal bar made to match the international reference metal bar
> in Paris with the best technology of the time. So perhaps Jon's
> position was more tenable in 1901. But the technology has moved on,
> and the concept has changed -- we now use an absolute, reproduceable
> physical phenomenon for the international reference for every standard
> measure except the kilogram. (We still can't count atoms as well as we
> can build balances, but we are getting really close.)]
>
> -Ed
>
> Edward J. Barkmeyer Email: edbark@xxxxxxxx
> National Institute of Standards & Technology
> Manufacturing Systems Integration Division
> 100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
> Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694
>
> "The opinions expressed above do not reflect consensus of NIST,
> and have not been reviewed by any Government authority." (05)
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o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o (06)
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