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
The Planck length is a unit of length in the field of quantum mechanics. It is the smallest possible distance that can be measured, and is approximately 1.6 x 10^-35 meters.
"Below the Planck length" refers to distances smaller than the Planck length. It is currently unknown whether such distances are physically meaningful or even possible to measure.
The Planck length is important because it is the limit at which our current understanding of physics breaks down. It is also believed to be the smallest scale at which the laws of physics can be applied.
Currently, it is unknown whether anything can exist below the Planck length. The laws of physics as we know them do not apply at such small distances, so it is difficult to make any conclusions about what may or may not exist.
Some theories suggest that the Big Bang occurred at a scale smaller than the Planck length. However, this is still a subject of debate and more research is needed to fully understand the connection between the two.