What do we know about time? Language tells us that it “passes”, it moves like a great river, inexorably dragging us with it, and, in the end, washes us up on its shore while it continues, unstoppable. Time flows. It moves ever forwards. Or does it? Poets also tell us that time stumbles or creeps or slows or even, at times, seems to stop. They tell us that the past might be inescapable, immanent in objects or people or landscapes. When Juliet is waiting for Romeo, time passes sluggishly: she longs for Phaethon to take the reins of the Sun’s chariot, since he would whip up the horses and “bring in cloudy night immediately”. When we wake from a vivid dream we are dimly aware that the sense of time we have just experienced is illusory.
Carlo Rovelli is an Italian theoretical physicist who wants to make the uninitiated grasp the excitement of his field. His book Seven Brief Lessons on Physics, with its concise, sparkling essays on subjects such as black holes and quanta, has sold 1.3m copies worldwide. Now comes The Order of Time, a dizzying, poetic work in which I found myself abandoning everything I thought I knew about time – certainly the idea that it “flows”, and even that it exists at all, in any profound sense.
We meet outside the church of San Petronio in Bologna, where Rovelli studied. (“I like to say that, just like Copernicus, I was an undergraduate at Bologna and a graduate at Padua,” he jokes.) A cheery, compact fellow in his early 60s, Rovelli is in nostalgic mood. He lives in Marseille, where, since 2010, he has run the quantum gravity group at the Centre de physique théorique. Before that, he was in the US, at the University of Pittsburgh, for a decade….
Rovelli’s work as a physicist, in crude terms, occupies the large space left by Einstein on the one hand, and the development of quantum theory on the other. If the theory of general relativity describes a world of curved spacetime where everything is continuous, quantum theory describes a world in which discrete quantities of energy interact. In Rovelli’s words, “quantum mechanics cannot deal with the curvature of spacetime, and general relativity cannot account for quanta”.
Both theories are successful; but their apparent incompatibility is an open problem, and one of the current tasks of theoretical physics is to attempt to construct a conceptual framework in which they both work. Rovelli’s field of loop theory, or loop quantum gravity, offers a possible answer to the problem, in which spacetime itself is understood to be granular, a fine structure woven from loops.
String theory offers another, different route towards solving the problem. When I ask him what he thinks about the possibility that his loop quantum gravity work may be wrong, he gently explains that being wrong isn’t the point; being part of the conversation is the point. And anyway, “If you ask who had the longest and most striking list of results it’s Einstein without any doubt. But if you ask who is the scientist who made most mistakes, it’s still Einstein.”
How does time fit in to his work? Time, Einstein long ago showed, is relative – time passes more slowly for an object moving faster than another object, for example. In this relative world, an absolute “now” is more or less meaningless. Time, then, is not some separate quality that impassively flows around us. Time is, in Rovelli’s words, “part of a complicated geometry woven together with the geometry of space”.
For Rovelli, there is more: according to his theorising, time itself disappears at the most fundamental level. His theories ask us to accept the notion that time is merely a function of our “blurred” human perception. We see the world only through a glass, darkly; we are watching Plato’s shadow-play in the cave. According to Rovelli, our undeniable experience of time is inextricably linked to the way heat behaves. In The Order of Time, he asks why can we know only the past, and not the future? The key, he suggests, is the one-directional flow of heat from warmer objects to colder ones. An ice cube dropped into a hot cup of coffee cools the coffee. But the process is not reversible: it is a one-way street, as demonstrated by the second law of thermodynamics…. read more: