Similarly, the gravitational interaction can be pictured as an exchange of gravitational field quanta, called gravitons. And indeed, this description works rather well, as long as the interacting particles are far apart. In this case, the gravitational force is weak and the spacetime is nearly flat. (Remember, gravity is related to the curvature of spacetime.) The gravitons can be pictured as little humps bouncing between the particles in this flat background.
At very small distances, however, the situation is completely different. As we discussed in Chapter 12, quantum fluctuations at short distance scales give the spacetime geometry a foamlike structure. We have no idea how to describe the motion and interaction of particles in such a chaotic environment.
The picture of particles moving through a smooth spacetime and shooting gravitons at one another clearly does not apply in this regime. Effects of quantum gravity become important only at distances below the Planck length
