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



On 01/02/2008 12:54 AM, Ludwik Kowalski wrote:

-- 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.

OK.

Did the textbook contain problems concerning chaotic 3-body
systems? If not, then we have (so far) not the slightest
evidence one way or the other as to how well IP handles
that case.

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?


I still think we should discuss the physics first and worry
about terminology later (if at all).

I'm not the one asking the question, so don't ask me what's
important to me (irrelevant) ... and don't ask me what's
important to you (I don't know).

Is there a physics question here? If so, please show us
the physics! It should be possible to formulate objective
statements (and questions) about the physics without using
any questionable terminology. For example, you might
describe an experiment and discuss objectively observable
outcomes of the experiment.