CARLO ROVELLI ON TIME

“We are time. We are this space, this clearing opened by the traces of memory inside the connections between our neurons. We are memory. We are nostalgia. We are longing for a future that will not come.”

Carlo Rovelli is a theoretical physicist working on the physics of space and time, currently directing the quantum gravity research group of the Centre de physique theoretique in Marseille, France.   In his The Order of Time (1), he is writing for a lay readership, offering a naturalist view of time and what it means to us. His reflections draw both on his professional work and his easy familiarity with art, literature, philosophy and music.

In the first section of this book, he deconstructs the time of common-sense. “Not only is there no single time for different places – there is not even a single time for any particular place. A duration can only be associated with the movement of something, with a given trajectory”. Rovelli’s Cosmos shows itself through change, events and processes – not through entities or things. We can know it only through what happens, interacts and evolves; through becoming rather than being. The notion that the present is ‘real’, while the past and future are not, only works if we define ‘present’ locally and in an approximate way.

From the standpoint of quantum mechanics, time is ‘granular’ and not continuous. (The Goddess is a pointillist). Grains, or quanta, of time last for a hundred millionth of a trillionth of a trillionth of a trillionth of a second. “In other words, a minimal interval of time exists. Below this, the notion of time does not exist, even in its most basic meaning”. Quanta of time, like other quanta, are ‘indeterminate’. “The substratum that determines the duration of time is not an independent entity different from the others that make up the world; it is an aspect of a dynamic field. It jumps, fluctuates, materializes only by interacting, and is not to be found beneath a minimum scale”. Yet the absence of time does not mean that everything is frozen and unmoving. It is, rather, “a boundless and disorderly network of quantum events.”

Having arrived at this point in his narrative, Rovelli begins to play with different uses of temporal language in the same space and restores our sense of having ground to stand on. He tells the story of how, in 1967, physicists Bryce de Witt and John Wheeler developed an equation accounting for quantum gravity without any time variable. Rovelli first makes the appropriate science related point: “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”. Then he starts talking personally – with a long passage of reminiscence about how he knew and valued de Witt and Wheeler as his “spiritual fathers” early in his career. He is signaling that the warmth of human subjectivity, kept alive in memory, and allowing for a sense of cultural ancestry, is not after all under threat. Professionally he inhabits a world of loop quantum gravity, where time and space “are approximations of a quantum dynamic that in itself knows neither space nor time”. Personally, he relishes human life and values his human sense of time.

Rovelli then asks: what is going on that allows us even to experience such a life and sense, if time isn’t, in fact, fundamental to it? He responds by talking about ‘emergence’. He reminds us that we see the sky turning around us every day, but we are the ones who are turning. “Is the daily spectacle of a revolving universe ‘illusory’?” he asks. “No, it is real, but it doesn’t involve the cosmos alone. It involves our relation with the sun. We understand it by asking ourselves how we move. Cosmic movement emerges from the relation between the cosmos and ourselves.”. In the case of time, we inhabit a cosmic niche that depends on low entropy, which in turn depends on a forward moving time. We earthlings have a source of low entropy – the sun, which sends us hot photons. The earth radiates heat towards the black sky, emitting colder photons, increasing the level of entropy. We hold that the entropy of the early universe was very low, eventually creating the conditions for our relationship with the sun to happen. But, as with the wheel of the day, this may not reflect the precise state of the universe: “The initial low entropy of the universe, and hence the arrow of time, may be more down to us than the universe itself”. For we observe the universe from within it, interacting with a minuscule proportion of the innumerable particles of the cosmos. What we see is a blurred image. In every experience, we are situated in the world: within a mind, a brain, a position in space, a moment in time”. Our lived experience shapes our understanding, including our experience of time.

In this sense, time as we know it is our invention, with our brain’s capacity for foresight and recollection the consequence of an evolutionary advantage within our inherited habitat. Rovelli is fine with that. It is truly the time of our experience, our hopes, our memories, our awareness of our own mortality, and much of our philosophy, poetry and music. Rovelli concludes the book with an elegant reflection:

“Song, as Augustine observed, is the awareness of time. It is time. It is the hymn of the Vedas that is itself the flowering of time. In the Benedictus of Beethoven’s Missa Solemnis the song of the violin is pure beauty, pure desperation, pure joy. We are suspended, holding our breath, feeling mysteriously that this must be the source of meaning, that this is time.

“Then the song fades and ceases. ‘The silver thread is broken, the golden lantern is shattered, the amphora at the fountain breaks, the bucket falls into the well, the earth returns to dust.’ And it is fine like this. We can close our eyes, rest. This all seems fair and beautiful to me. This is time.”

Carlo Rovelli The Order of Time Allen Lane, UK: 2018 Translated by Erica Segre and Simon Carnell. (Allen Lane is an imprint of Penguin Books)