3 ms·

I don't understand what the author means. Maybe someone can help me understand. > Now, what we do when we want to explain what a galaxy does, or a galaxy clust

I don't understand what the author means. Maybe someone can help me understand. > Now, what we do when we want to explain what a galaxy does, or a galaxy cluster, or even the whole universe, is not to plug the matter and energy of every single planet and star into the equations. This would be computationally unfeasible. Instead, we use an average of matter and energy, and use that as the source for gravity. Does this mean instead of individual objects we make our calculations with x units of mass and energy in every given patch of space and this may be an issue, or am I getting this wrong?

Cogito 7y ago
> Does this mean instead of individual objects we make our calculations with x units of mass and energy in every given patch of space and this may be an issue, or am I getting this wrong? Pretty much yes, as we can only take observations like that. More importantly, the GR equations are non-linear, so taking these 'average' observations and feeding them into our models may introduce significant error. Or another way of saying that, it's possible that the model would give accurate predictions if you were able to feed it precise and complete information (the details of every single point mass) but not give accurate predictions if you feed it the sampled data we have (observations about whole galaxies, for example). People have discussed this elsewhere in this thread as the N-body problem. It's also saying that even if we did have that level of detail, our numerical simulations aren't currently capable of calculating at that fine a level of detail.