I have been reading a Weinberg paper (the cosmological constant problem), where he writes "...Zeldovich assumed that these were cancelled leaving only the higher order effects: in particulal, the gravitational force between the particles in the vacuum fluctuations. (In Feynman diagram terms, this corresponds to throwing away te one-loop graphs, but keeping those with two loops" Can someone help me understand this. I thought QFT said nothing about gravity. Why do the two loops correspond to the gravitational force??