Re: [boost] Re: runtime unit-modifiablephysical-quantitiesusingpqs1-00-01

From: Matthias Schabel (boost_at_[hidden])
Date: 2004-01-04 22:50:44

I had pretty much given up trying to make this point, so it's nice to  
see that
I'm not alone in making the observation that there's nothing fundamental
about the seven base units chosen by SI.  Also, Andy's argument that  
there
is something more fundamental about length and time than units such as
velocity is seriously flawed - in fact, you can choose any linearly  
independent
set of combinations of the seven base units as your unit system.  If  
anything,
from the standpoint of measurable fundamental physical constants, good  
base
units might be better based on quantities which can be precisely  
measured
experimentally (Planck constant, Bohr magneton, von Klitzing constant,  
etc...)
Ultimately, dimensional analysis is useful from a pragmatic standpoint  
as a
means of tracking powers of quantities in mathematical expressions  
first and
foremost - physical quantities are a somewhat arbitrary subset of a  
larger
space of prospective quantities...

Matthias

> Surely this is ridiculous?  I can see an extreme reductionist arguing
> that you only need dimensions for space, time, and the fundamental
> forces.  And if you really want to get anal, then you could equate
> space and time, and end up with four dimensions (distance, mass,
> electroweak charge, strong charge).  Everything else is derived,
> right?  And if you buy superstring theory or quantum gravity, then
> you could probably get away with just one or two dimensions, right?
> But how useful is that?  Who cares if luminous intensity is really
> just photons/area?  It's still a useful dimension, right?  And even
> though temperature and energy are related, I think there's a lot of
> physicists that would be unhappy to not have a natural energy
> dimension without having to state it in derived terms.  Isn't it about
> expressing the solution in the problem domain?  Physicists write
> equations with Joules and Watts, not just N m and lbs.-ft./s.
------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
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mschabel at ucair med utah edu