Verona and Marseille, born 1956
Carlo Rovelli
Time is not one thing but a layered bundle of properties, and physics strips them away one by one. At the bottom there is no time variable at all, only events and their relations. The flow we feel belongs to our blurred view of the world.
The view
Rovelli is an Italian theoretical physicist who has spent much of his career at the Centre de Physique Théorique in Marseille. He is one of the founders of loop quantum gravity, the originator of the relational interpretation of quantum mechanics, and the author of The Order of Time (2017), which sets out his account for general readers.
The book proceeds by subtraction. Each layer of physics removes one feature of common-sense time. Relativity removes its unity: clocks at different heights and speeds run at different rates. It also removes the present: there is no "now" that spans the universe. Thermodynamics accounts for its direction, and only statistically. General relativity removes its independence from matter: time is part of the gravitational field. Quantum gravity, finally, makes it granular and indefinite.
The argument
The fundamental equations need no time. In the approach to quantum gravity Rovelli works on, the basic equations describe how physical quantities relate to one another, with no external time variable for them to evolve in. Change is real, but it is the change of variables relative to other variables.
The direction of time is a feature of blurring. The microscopic laws do not distinguish past from future. The difference shows up only in entropy, and entropy is defined relative to a coarse-grained description: it counts how many microscopic states look the same to someone who cannot see every detail. Our past looks special because, for systems like us, interacting with the world through a few variables, the past was in a state of low entropy.
Thermal time. With the mathematician Alain Connes, Rovelli proposed in the 1990s that the flow of time can be defined from a system's statistical state. On this "thermal time hypothesis", a time variable emerges when we describe a system we know only partially, and that is the time we live in.
Flowing time belongs to our blurred view, not to the world's detail. after Rovelli, The Order of Time (2017)
Neither block nor present. Rovelli rejects presentism, since there is no universal now. But he also rejects the static block of eternalism. The world, he says, is a network of events, of things happening, and becoming is real even if it is local and not ordered along a single line.
Where he meets Deutsch
Most of Rovelli's subtractions are also Deutsch's. Both say time does not flow at the fundamental level. Both take relativity to abolish any objective "now": Deutsch, like Rovelli, points out that observers in relative motion disagree about which events are simultaneous. Both expect quantum gravity to finish the job. Deutsch says the sequence of moments must break down near the Big Bang and at the Planck scale, which is close to Rovelli's granular, indefinite time.
Both also place the direction of time in thermodynamics. Deutsch traces the fact that causes precede effects to the second law, and Rovelli traces every arrow to entropy.
Where they part
Many worlds or relations? This is the deepest split. Deutsch's multiverse is an objective whole, with every outcome realised. Rovelli's relational quantum mechanics rejects the multiverse: facts are relative to the systems that interact, and there is no view of the whole from nowhere.
Is the arrow objective? For Rovelli the low entropy of the past is partly perspectival, a feature of how systems like us couple to the world. For Deutsch the growth in the number of distinct kinds of snapshot is a fact about our region of the multiverse, whoever is looking.
Is there becoming? Rovelli keeps a local, real becoming: events happen. Deutsch denies that anything happens in that sense. Every snapshot simply exists, and to exist at a moment at all means to exist there for ever.
All moments, or only events? Deutsch's ontology is close to eternalism: all moments, in all universes, are equally real. Rovelli has argued explicitly for "neither presentism nor eternalism".
| Rovelli | Deutsch | |
|---|---|---|
| Does time flow? | Not fundamentally; flow emerges from a blurred view. | No. The idea is incoherent. |
| Is there a global now? | No. | No; only observer-relative slices. |
| Where does the arrow come from? | Entropy, partly perspectival. | Growing variety of snapshots: the second law. |
| Quantum reality | Relational: facts relative to systems. | A multiverse in which every outcome occurs. |
Further reading
- Carlo Rovelli, The Order of Time (London, 2018).
- Carlo Rovelli, Helgoland (London, 2021), on relational quantum mechanics.
- Carlo Rovelli, "Neither Presentism nor Eternalism", Foundations of Physics (2019).
- Relational Quantum Mechanics, Stanford Encyclopedia of Philosophy.