Last night I couldn't sleep and started to think about a few things I thought that I had read in New Scientist last year. When I got up I took all my dads New Scientists and quickly sifted through them and I didn't find what I was looking for. I thought I had read about that if you try to predict the strength of a gravitational field of a large object using quantum mechanical models you get answers that are total rubbish (I can't remember the exact example, I think it was something like that the earth would be a lot less dense or that it wouldn't even be held together by it's gravitational force). Is that true? Are these models totally useless for large scale predictions? Can anyone tell me (or just post a link or a name of a book) of how they calculate these things (what models are used). My understanding of maths and physics is surely far from enough to understand those models, but maybe some day I will get there.