I don't think it is unreasonable to believe that in 40 years China will have per capita GDP half of the US (The Shanghai and Beijing areas are already close on

I don't think it is unreasonable to believe that in 40 years China will have per capita GDP half of the US (The Shanghai and Beijing areas are already close on a PPP basis) - indeed some GS research paper argues this. At that point its economy will be about double the size of the US...