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

by Carlo Rovelli

Take any region of space. For example, the room in which you are reading this, if you are in a room. How big is this room? The size of the space of the room is measured by its volume. Volume is a geometrical quantity that depends on the geometry of space, but the geometry of space—as Einstein understood, and I recounted in chapter 3—is the gravitational field. Volume is therefore a property of the gravitational field, expressing “how much gravitational field” there is between the walls of the room. But the gravitational field is a physical quantity and, like all physical quantities, is subject to the laws of quantum mechanics. In particular, like all physical quantities, volume may not assume arbitrary values, but only certain particular ones, as I described in chapter 4. The list of all possible values is called, recall, the “spectrum.” Hence, there should exist a “spectrum of the volume” (figure 6.2).