The Order of Time

by Carlo Rovelli

In 1967, an equation accounting for quantum gravity was written for the first time without any time variable. This equation was discovered by two American physicists—Bryce DeWitt and John Wheeler—and today it’s known as the Wheeler–DeWitt equation.69 At first no one could understand the significance of an equation without a time variable, perhaps not even Wheeler and DeWitt themselves. (Wheeler: “Explain time? Not without explaining existence! Explain existence? Not without explaining time! To uncover the deep and hidden connection between time and existence . . . is a task for the future.”70) The issue was discussed at great length; there were conferences, debates; rivers of ink flowed.71 I think that the dust has now settled and things have become much clearer. There is nothing mysterious about the absence of time in the fundamental equation of quantum gravity. It is only the consequence of the fact that, at the fundamental level, no special variable exists. The theory does not describe how things evolve in time. The theory describes how things change one in respect to the others,72 how things happen in the world in relation to each other. That’s all there is to it.