If the gravitational force between the electron (of mass 9.11 × 10−31 kg) and the proton (of mass 1.67 × 10−27 kg) in a hydrogen atom is 1.5 × 10−47 N, how far apart are the two par- ticles? The universal gravitational constant is 6.673 × 10−11 N · m2/kg2. Answer in units of m.