New UCDs for VOEvent please

[Rob Seaman]

I said:

>> The challenge here is not only that the list will never be complete,  
>> it is that we should be encouraging researchers to actively augment  
>> and improve the list.  A workable classification scheme is often the  
>> first step in organizing a research program.  But the result of a  
>> research program is often to overturn the original classification  
>> scheme.  We don't want to provide a mechanism that is only useful for  
>> describing objects far removed from the cutting edge.

Rick Hessman says:

> The "cutting edge" will always be such that one won't have a useful  
> classification scheme other than unusual combinations of the more  
> general bits and pieces.  That's why the UCD has to be hierarchical  
> (to emphasize/extract general properties even when given specifics)  
> and easily combined.  Let's see.....
>
> 		cosmology.dark_energy	<==	cosmology.background;em.microwave
> 								cosmology.background.polarization;em.microwave
> 								stars.supernova.Type_Ia;process.eruption.hydrodynamic; 
> process.eruption.thermonuclear
> 								galaxies.velocity_curve
> 								cosmology.abund
> 								....
>
> (Aha - already discovered a few important UCD's to add....!)

I'm becoming more skeptical that IVOA standard UCDs are appropriate for  
representing the often slippery nature of astronomical "processes" and  
"objects".  (As I pointed out at in a similar context at the VOEvent  
workshop - one might choose to regard an object simply as a long lived  
process.  A main sequence star is a ~10 Gy thermonuclear reaction.)   
But a UCD is targeted differently (from UCD-1.9.9b):

	"The Unified Content Descriptor (UCD) is a formal vocabulary for  
astronomical metadata that is controlled by the International Virtual  
Observatory Alliance (IVOA)."

The question is whether the characterization of astronomical  
objects/processes represents "metadata".  The whole point of the  
precision of a UCD specification (e.g.,  
phot.flux;em.optical;meas.error;stat.max) is to provide a solid  
foundation for building a sound scientific argument - but such an  
argument typically results in drawing new insights and building new  
logical connections - often using new vocabulary.  On the other hand,  
even the most well established nomenclature regarding astronomical  
objects and processes is subject to revision and extension as better  
data and more profound theory collide.

Namespaces provide a possible way to address this.  For instance, IVOA  
is developing a very basic namespace and various astronomical  
constituencies then support richer, and perhaps shorter latency,  
vocabularies of their own.  However, we have this bit of constraining  
boilerplate to muddy the waters:

	"4.1.2 Namespaces.
	The use of namespaces, indicated by the presence of a colon in the  
word is possible, but should be avoided as far as possible. The  
namespace is defined by the string before the colon and the word  
follows. The words in the non-standard namespace must be distinct from  
all words currently in the IVOA namespace.  While developers may need  
local namespace, they should be used only temporarily, for words that  
are not yet included into the UCD validated by the IVOA. New words  
should be added using the procedures discussed in section XX."

What is the justification for trying to require that words in a  
non-standard namespace be distinct?  For instance, we couldn't possibly  
require that two different non-standard namespaces not overlap.  Why  
regard the IVOA namespace as *necessarily* preeminent?   "Standard"  
does not equate with "compulsory".  Either the standard makes sense or  
you won't be able to force folks to use it anyway.  In any event, one  
would think that alternate interpretations of the same vocabulary would  
be one of the strongest justifications for alternate namespaces.  This  
isn't an opportunity to lose control of alternate vocabulary - it's the  
way to enforce control of alternate usages.  I also find the word  
"temporarily" to be curiously undefined for a science in which  
observations and inferences from decades and even centuries gone by may  
be revisited.

To the list of UCD commandments:

	1. UCDs should be short.
	2. They should suggest the concept being labeled.
	3. Only a single UCD should be appropriate for a given concept.
	4. UCDs should be complete, describing all concepts of interest.
	5. The vocabulary used within UCDs should be as small as possible.
	6. Related concepts should have related UCDs.
	7. Quantities with the same UCD should be comparable.

I might suggest adding:

	8.  For IVOA to control its vocabulary, it must provide the  
opportunity for others to control their own.

Ralph Waldo Emerson may shed some light:

	"The poets made all the words, and therefore language is the archives  
of history, and, if we must say it, a sort of tomb of the muses For,  
though the origin of most of our words is forgotten, each word was at a  
stroke of genius, and obtained currency, because for the moment it  
symbolizes the world to the first speaker and to the hearer. The  
etymologist finds the deadest word to have been once a brilliant  
picture. Language is fossil poetry."

However much effort is expended to constrain and mandate standard  
usage, the users will seek ways to subvert the dominant paradigm.  This  
isn't just the nature of users, it is the nature of science as poetry.   
Careful support for alternate usage is the key to controlling anarchy.

Rob Seaman
NOAO
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