Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

The crucial difference between photons, the quanta of the electromagnetic field, and the nodes of the graph, the “quanta of gravity,” is that photons exist in space, whereas the quanta of gravity constitute space itself. Photons are characterized by “where they are.”* Quanta of space have no place to be in, because they are themselves that place. They have only one piece of information that characterizes them spatially: information about which other quanta of space they are adjacent to, which one is next to which other. This information is expressed by the links in the graph. Two nodes connected by a link are two nodes in proximity. They are two grains of space in contact with each other: this “touching” constructs the structure of space. The quanta of gravity, that is, are not in space; they are themselves space. The spin networks that describe the quantum structure of the gravitational field are not immersed in space, they do not inhabit a space. The location of single quanta of space is not defined with regard to something else, but only by the links, and the relation these express.