On Sep 29, 2009, at 4:13 PM, Gunther Schadow wrote: (01)
> Pat Hayes wrote:
>>> I was asking if 1 N.m = 1 N.m and the answer is ambiguous.
>>
>> No, it is not. If you had asked whether 1 N.m moment of force = 1 N.m
>> energy then the answer would clearly be no. But those are two
>> different
>> questions, not one ambiguous question.
>
> so we agree that 1 N.m = 1 N.m when we talk about units and there
> is no such thing as "N.m momoent of force" as a unit? (02)
I was taking your question to be based on this presupposition, yes.
However, that leaves open the question of whether it might make sense
to declare that such things as moments might be treated as distinct
dimensions in some other, enlarged system of units. UCUM has angle as
a dimension, which isn't in the SI system. (03)
> There is of
> course the Quantity moment of force 1 N.m, but the unit is still
> N.m without knowing anything about torque vs. energy. (04)
I tend to agree that this is the most workable stance to take, indeed. (05)
>
>>> The unit is newton-meter, it is not
>>> newton-meter-of-energy, therefore, I would argue, that the unit
>>> is the same even if the kinds of quantity are different. Unless
>>> we agree on this (by either one of us changing our mind) I don't
>>> see a value at looking at ontological constructs.
>>
>> Well, we clearly agree. But I suspect we would have agreed if you had
>> simply asked me this directly :-)
>
> If I remember correctly, I just asked "1 N.m = 1 N.m : true or
> false?" (06)
Yes, but that question is trivial and has a trivial answer which can
be determined by pure logic, unlike the intended actual question. You
bury too much meaning in these equations, more than they can bear. (07)
>
>>> The question remains what we believe jointly that UoM concepts
>>> should do for us.
>>
>> That isnt necessarily a welldefined question to ask of an ontology,
>> but
>> my answer would be, to support a useful connection between the purely
>> metrical notions of units, quantities and so forth and the wider
>> medium-to-high level concepts of other ontologies, such things as
>> 'physical object' and 'event'.
>
> Can you give examples for what that would mean? Just add to the
> list of test cases. (08)
For example, in the case of temperature, we ought to be able to say
that the temperature of a body of liquid is less than its boiling
point; that the temperature at one a point on a surface is less than
the temperature at another; and that the working temperature of a
device has exceeded its maximum safe working temperature. We ought to
be able to support concepts such as temperature gradients. (Not
*define* them, but not impose conditions that make them impossible or
artificially difficult to describe.) (09)
>
>>> You may want them to preserve the difference
>>> between torque and energy, I don't. So the question remains
>>> open on the list. But there is no point in proceeding if we
>>> don't agree on this.
>>
>> I disagree. If we try to establish agreement on the ultimate
>> purpose of
>> an ontology, we will never stop arguing. If it is any good, it will
>> get
>> used for things that we havn't yet imagined. That is part of the
>> point
>> of building it, in fact.
>
> All I am asking for is a list of test cases. This is not an
> "ultimate purpose", just a list of statements which our ontology
> should be able to affirm or deny. (010)
Well, OK, though test cases isn't really what they are. The ontology
should establish ways to state that a physical object has some
dimensional property whose value is expressed in the appropriate
units, and support the equivalent assertions using other units. (This
goes slightly beyond the pure UCUM units calculus, in that it also
maintains the attachment to the object being described.) It should
allow the expression of such things as 'maximum working temperature'
or 'triple point' or 'maximum tare weight'. It should allow for non-
metric scales such as Rockwell hardness and lumber quality. Will that
do for a start? (011)
>>> This is why around UCUM implementation I use the concept of
>>> a "DimensionedQuantity". A Quantity is any set of values
>>> where at least some values have a difference operation. A
>>> DimensionedQuantity is essentially a number with a dimension.
>>
>> No, wait. Because it is *described* using a number and a dimension
>> does
>> not mean it *is* a number-dimension pair. In fact, it can't be. We
>> have
>> already established that we agree that 2.3 N.m = 2300 g.m2.s-2, and
>> if
>> that equation means what it says, then the same one of these can be
>> described by two different number/dimension pairs.
>
> Where do you get two different number/dimension pairs from? Both
> of them are the same in any one system of dimensions.
>
> 2.3 N.m = 2300 m2.s-2.g = (2300, [2,-2,1,0,0,0,0])
> 2300 g.m2.s-2 = 2300 m2.s-2.g = (2300, [2,-2,1,0,0,0,0])
>
> (2300, [2,-2,1,0,0,0,0]) = (2300, [2,-2,1,0,0,0,0])
>
> same thing. (012)
I agree they are the same THING, but they aren't the same <number,
dimension> pair. Just look: 2.3 =/= 2300, and N.m =/= m2.s-2.g . So
these things that are the same cannot be the actual pairs. Its a very
elementary point, rather like observing the distinction between
numbers and numerals. (013)
>
>>> Such a quantity for example is 16 N.m. Units are themselves
>>> DimensionedQuantities with a name (and the name can be complex
>>> such as N.m or even 16.N.m) So, my ontology behaves exactly
>>> like the symbols that I write on a sheet of blank paper when
>>> I compute my scientific equations.
>>
>> Again, that does not sound like good ontology engineering. It
>> certainly
>> does not generalize to more complex or more general situations. A
>> description of a commercial transaction is not isomorphic to the
>> actual
>> transaction.
>
> I feel like you're taking out some realist bat on me now. What
> does that have to do with the issue? I do not see units anywhere
> but when I manipulate them on my paper. I don't encounter them
> by any of my senses other than as constructs of the mind, I never
> touched them. (014)
Not the units, perhaps (though apparently one can actually see the
kilogram, I am told), but certainly the quantity dimensions are
directly apprehended, at least sometimes. But this is beside the main
point. (015)
> So whatever reality is the referent of those
> symbols I experience through my symbols and I trust that that
> reality behaves like what I am practicing when I manipulate my
> symbols. (016)
I think not, in fact. You trust that the reality behaves in a way that
is adequately *described* by your symbol manipulations, sure: but so
do we all, with all our symbols. That was my only point, above. (017)
>
> Only the most privileged of us will be able to experience a
> measuring with the metrologically defined "real" units. My
> clock works using a quartz not a vibrating hydrogen atom, and
> I measure distance using some tape measure, not a Caesium
> wavelength-o-meter.
>
> So to me and most scientists it's rather irrelevant whatever
> the metrological "reality" is behind those units. While the
> idea is that all measurement standards are traceable to those
> authoritative embodiments of them, practically this means little
> more than that these authoritative embodiments are just
> reconstructions of whatever the referent is of these units.
>
> That is also why the BPIM can choose to replace the embodiments.
> Just as they redefine the second and the meter, they will one
> day redefine the second again, and the kilogram also. But
> whenever they re-define, they will choose some odd constant
> factor so as to make sure that the new standard approximates
> the referent of what we understand as the meaning of our unit
> symbols. (018)
Yes, yes, I entirely agree. I'm not even an amateur metrologist, and
my intuitions about units and measures come from carpentery rather
than science. I wouldnt want to suggest for a moment that the use of
units implies that one is referring to or implicitly reasoning about
the currently accepted way to 'define' (ie to authoritatively measure)
the actual magnitudes of those units. I'm happy to just take units as
given things that it is our job to describe well enough to use them.
But then we do need to at least give a nod to how they will in fact
get used: we have to at least look outside the metrological ivory tower. (019)
Pat (020)
>
> -Gunther
>
> --
> Gunther Schadow, M.D., Ph.D. gschadow@xxxxxxxxxxxxxxx
> Associate Professor Indiana University School of Informatics
> Regenstrief Institute, Inc. Indiana University School of Medicine
> tel:1(317)423-5521 http://aurora.regenstrief.org
> (021)
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