I put this in the atomic physics rather than classical because i want to know what happens on the atomic level that causes one billiard ball to move another. bryan greene said that if took all the empty space out of the empire state building that it would be the size of a grain of rice. so what forces another ball to move another? i'm thinking it's because the protons are surrounded by negative electrons and when the negative electrons approach the other negative electrons that that they repel each other. am i right?