My understanding is that gravitational systems are not
predictable in the long term (this is called the N-body problem
in astronomy). As the SI has perhaps a 600-billion-body problem
to solve at least 100 billion years into the future, where some
of the bodies may not follow "natural laws" (i.e. they are SIs
that decide to change course for independent reasons), this
could very well take a significant amount of computation.
A system with a load of variables that we can't track at this tech level is not "unpredictable", it is "very hard to predict", if you want to be correct.
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