In article ,
Yousuf Khan writes:
We're still using Newtonian gravity in this day and age because we still
don't have computers strong enough to do a GR calculation for an entire
galaxy.

That's not the problem. GR is mathematically identical to Newtonian
gravity provided gravity is not "strong" and speeds are "low"
compared to the speed of light. The magnitude of the errors can be
quantified for the actual gravitational potentials and speeds in any
particular calculation.

The real problems are 1) even Newtonian gravity is too hard to
calculate when the system has too many bodies, and 2) treating stars
and gas together in a single calculation is difficult. These
problems require approximations to be made, and in some cases these
approximations may be inaccurate enough to affect results. That's
not to say every calculation ever done is hopelessly wrong, but any
result that depends critically on what happens when two stars come
very close together is uncertain.

People who do these calculations are well aware of the uncertainties,
but those seldom get transmitted in popular articles.

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