Its often said QM is for the very small and GR for the very large. This brings to mind that there should be some limit at which one starts to apply and the other stops. Now I know there are more substantial differences, GR being a continuous description and QM using Planck length, and Planck time, and so on...
But keeping only to the distance scale difference, is there a hard limit where either theory suddenly stops working? Is there an overlap where both theories have similar predictive accuracy? Or is there a bad gap, a distance scale where neither theory is very useful?
Secondly, I imagine QM calculations can quickly become cumbersome when the number of particles increase. Is this image correct, is QM merely computationally impractical for considering planets and satellites etc. Are there other conceptual/practical problems, such as entanglement with something in another galaxy?
Edit: The suggested duplicate question asks whether a theory is quantum. This question asks whether an application of a theory (should) be QM or GR. I can now say from answers that the question is really more about gravity than the dimension of distance.
However it is also clear that although I tried to restrict it to a length scale, there are inextricable considerations. For instance, collision dynamics of very large molecules, large group behavior of particles, gravitational effects on quantum scales inside dense matter structures, etc.
The second part can be clarified: If all relevant particles could be accurately tracked, and all relevant interaction calculations performed, would QM give similar, or better, results over GR? (regarding the orbit of the moon aroundEarth)
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