all those constraints

[Boud Roukema]

Witam,

On Tue, 14 Jan 2003, Michal Frackowiak wrote:

> szajtan wrote:
>
> >Boud, I got your point about those constraints ;)
> >
> >(Maeybe there arose a little confiusion because I treat everything from the
> >CMB point of view (CMB & rest of the world) as everything  else (other
> >methods) were just a kind of supplementary thing, some aid of narrowing our
> >choices in space of parameters.)
> >
> >When we have a theory describing sth. with some free parameters, it doesnt
> >give any constraints on  unknown values until we know the values of those
> >parameters. In that moment measurments enter the scene as some kind of
> >calibration. Then we have a model and can derive whatever we want
> >quantitatively.
> >(e.g. LSS constraint - from assumption on model + "calibration"
> > measurments )
> >
> >But what if we had such a good theory that it doesnt need any additional
> >measurmenst (except some well known physical constants) (nb measurments that
> >introduce additional uncertanities) to predict  the thing we're looking for.
> >That would be a constraint of purely theoretical nature.
> >(e.g. BBN constraint - only from physics of high energy particles - which
> >happens to be more less consistent with constraints from observations of
> >Ly_\alpha forest)
> >
> >Do you agree with this approach to the word "constraint" in astronomy ?

My understanding of how the word "constraint" is used in astronomy
is that you should still not use the word "constraint" here.

The tradition is to use "constraint" to mean an observational constraint.

You may choose to try to fight this tradition, but often it's not worth
the effort to try to change words which are used in a confusing way when
the community that uses the words has no wish to make it easy for outsiders
to enter into "the priesthood".  We still talk about "metals" in astronomy.
This was apparently a divergence between astronomers and chemists in the
early 20th Century. The chemists won for the general public.

If you really wish to fight ambiguity and lack of clarity, you should
try contributing to the http://www.wikipedia.org  . This is a community
which naturally dislikes ambiguity. In fact, a concept has developed of
"disambiguation pages" in wikipedia...


> holy grail of physics? gut? theory of everything? that's how they should
> work. they predict values of all constansts instead on depending on
> measurements. good way...
> but at the moment we have only parametrizable theories. :-(

OK, i think here we have a use of language where there is some sort of
consensus, at least among "theorists".

"theory" - is the sort of thing we all hope for, a few
fundamental principles and laws and constants *imply* everything else.

"phenomenology" - is what happens in practice (it's what i do,
mostly), where you have some laws based on theory but with the
addition of some simple, though arbitrary laws, motivated from
observation, and usually also some arbitrary free parameters.
All of this is then compared to observation.

And we then talk about "constraints" on a phenomenological model. This
means observational constraints.

If the phenomenological side seems to work correctly, such as the
SNeIa method, then it may also "constrain" theory. Once we redo the DE
method, we'll hopefully (maybe) have some "constraints" which are
clear and robust.

The method itself (SNeIa or comoving standard ruler) is a
phenomenological method, but it may constrain theory.


However, among "observers", theory and phenomenology are both called
"theory", and the observers wish to "constrain theory".

BTW, Michał just used the word "predict" in the way many people do,
to "predict" a value we already have measured. A more careful word that
some people now use is "postdict". Or you can simply say "imply",
which is a purely logical word, without any connotations on philosophy
of science.

Think of "I predict that the ZSRR will collapse around 1989-1991" or
"I predict that there will be a major political revolution in France
in 1789." Those are not predictions.

pozd
boud

PS: Michał - will you be in Piwnice for the
http://adjani.astro.uni.torun.pl:9673/zwicky/WinterWorkshop   ?