Gerhard Wesp wrote:
On Wed, Jun 07, 2006 at 10:04:20PM +0100, Reece Dunn wrote:
so, which value is correct? The one in Joules (sp?) or the one in
Well, both are probably correct. :) Now I'm not a theoretical physicist, but I'm pretty sure that even there all but the most trivial calculations become cleaner once you use SI units. Seems as if I'd want to mildly encourage even those folks to use SI.
As to your rounding error concerns, I can reassure you, multiplication and division is very well-behaved in floating point.
Not attempting to sound snooty, but I am a theoretial physicist (astrophysics and cosmology, to be complete) and you are incorrect in thinking that SI units make everything simpler. The whole point of any of the specialized unit systems is to reduce complexity. If I want to talk about the energy of a fast moving sub-atomic particle using SI units, I have to carry around multiplicative factors that hold the scales of the speed of light, the strength of the electromagnetic interactions and the mass of the particle, among other things. If I use eV based unts instead, some of these constants become 1, and so I never need to remember to write them, and I never have to pay clock ticks in the calculation to compute them. In the inner loops of intense calculations, losing those extra multiplications can make hours or days of difference in how long the calculation takes to complete. So, the code becomes both less likely to contain errors and faster by using non-SI units. For research simulations, a restriction to SI only is a game breaker for the library. Also, moving to a well chosen set of non-SI units can frequently reduce the chance of round-off errors and make a calculation more stable. In some cases, it can even change a calculation from floating point to integer, and so remove all round off concerns. To go with that, most human beings are far more likely to be able to interpret numbers in the range of about 0.01 to 100 instead of numbers far away from one. I could calculate the age of the universe in seconds (the SI unit of time), but it is far more menaingful to most people when reported as ~14Gyr. (To get seconds from that, multiply by ~3.15*10^7.)
(3) it will have greater runtime penalties, especially if used in a loop;
I fail to see this.
(4) can have inacuracies when dealing with large or small physical constant-based values;
Why/how? In what way different from non-SI?
(5) is bad if those constants are changing!
Well... If the Lord decides to change the gravitational constant, I guess we're toast anyway.
Very few physical constants are actually constant. First, most physical constants are measured values, so as our ability to measure them improves, the accepted value changes. In some cases, these changes are frequent enough and important enough that a system without the ability to accomidate them is not useful. Second, and less intuitive, some physical constants change based on the environment in which they are measured. For example, the strength of the electromagnetic interaction is not actually constant, though in your day to day life you never see it change. It depends on the energies of the interacting particles. If you leave this out of a particle physics calculation, you will be very wrong. The gravitational constant is also expected to have an energy dependence (though it is harder to measure), by the way.
(6) can penalize you if intervals come into play.
I fail to see this, too.
Regards -Gerhard
For me, coming from my background, forcing SI units is a very bad idea. I understand that many people can work their entire careers and never run into any of these problems, but they are every day things for me, so I would like to see a design that supports what I need to do. John Phillips