Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Simulating a disturbance of a stable planetary system.

On Jan 2, 2008, at 12:08 AM, John Denker wrote:

On 01/01/2008 08:19 PM, Ludwik Kowalski wrote:
..... The simulation software
I am using , I.P., seems to be highly reliable (consistent with
underlying physics).

"Highly reliable"? That statement would be more informative if
it were more specific and more quantitative:
-- What tests have been run?
-- Quantitatively, how good was the agreement with analytical
results?
-- Are these tests designed to be incisive? What classes
of bugs are they likely to detect? Are they appropriate to
the numerical methods IP is actually using?
-- What are the /limits/ of validity?
-- How do we know that IP did not simply incorporate the
analytic solution for simple cases? (That's what I would
have done.) Doesn't that mean that the results for non-
simple cases will be incomparably less accurate?

1) I do not have answers to all these questions. But I am probably not the only teacher to compare answers to various textbook problems with results of simulations. I did this many times and I was never disappointed. That was the basis for my statement.

2) Do you agree, John, that the definition of stability of a rigid body (a wooden cone on a table or an airplane in moving fluid) is not the same concept as stability of motion of only two or three stars interacting with 1/r^2 forces? What is your definition of stability of motion of such particles in otherwise empty space?
_______________________________________________________
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/