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Re: [ontolog-forum] Model or Reality

To: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Ed Barkmeyer <edbark@xxxxxxxx>
Date: Tue, 14 Aug 2007 14:22:39 -0400
Message-id: <46C1F2EF.4070602@xxxxxxxx>
Jon Awbrey wrote:    (01)

> 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.    (02)

Pat Hayes answered:    (03)

> 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.    (04)

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.    (05)

Part of that is mathematics, part of that is physics, and part of that is 
engineering.  Pat is correct:    (06)

> 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.    (07)

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.    (08)

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".    (09)

So there is a flow of separable concepts here:  abstract quantity, reference 
physical quantity, reference measurement and accuracy (uncertainty), actual 
measurement practice.    (010)

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.)]    (011)

-Ed    (012)

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    (013)

"The opinions expressed above do not reflect consensus of NIST,
  and have not been reviewed by any Government authority."    (014)

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