By analogy to the other three forces of nature, we assume that the gravitational interaction is mediated by a particle called the "graviton." Gravitonal interactions in a traditional particle theory can be represented by Feynman diagrams like the one on the left, in which two massive particles, M1 and M2, interact by exchanging a graviton, represented by the squiggly line. The gravitational interaction changes the energy and momentum of M1 and M2. This graviton type of exchange emerges naturally from string theories, where the lowest mode of the string vibration gives rise to a set of particles that includes a graviton. We can represent the string theory version of graviton exchange with the diagram on the right, where point particles have been replaced by closed loops of string. In both diagrams, time increases in the vertical direction. (Unit: 4)