Carlo Rovelli

In 1988, Rovelli, Lee Smolin and Abhay Ashtekar introduced a theory of quantum gravity called loop quantum gravity. In 1995, Rovelli and Smolin obtained a basis of states of quantum gravity, labeled by Penrose's spin networks, and using this basis they were able to show that the theory predicts that area and volume are quantized. This result indicates the existence of a discrete structure of space at a very small scale. In 1997, Rovelli and Michael Reisenberger introduced a "sum over surfaces" formulation of the theory, which has since evolved into the currently covariant "spinfoam" version of loop quantum gravity. In 2008, in collaboration with Jonathan Engle and Roberto Pereira, he has introduced the spin foam vertex amplitude which is the basis of the current definition of the loop quantum gravity covariant dynamics. Loop theory is today considered a candidate for a quantum theory of gravity. It finds applications in quantum cosmology, spinfoam cosmology and quantum black hole physics.

In his 2004 book, Quantum Gravity, Rovelli developed a formulation of classical and quantum mechanics that does not make explicit reference to the notion of time. The first step towards a theory of quantum gravity without time variable is described by Wheeler-DeWitt equation. The timeless formalism is used to describe the world in the regimes where the quantum properties of the gravitational field cannot be disregarded. This is because the quantum fluctuation of spacetime itself makes the notion of time unsuitable for writing physical laws in the conventional form of evolution laws in time.

This position led him to face the following problem: if time is not part of the fundamental theory of the world, then how does time emerge? In 1993, in collaboration with Alain Connes, Rovelli proposed a solution to this problem called the thermal time hypothesis. According to this hypothesis, time emerges only in a thermodynamic or statistical context. If this is correct, the flow of time is an illusion, one deriving from the incompleteness of knowledge. Similar conclusions had been reached earlier in the context of nonequilibrium statistical mechanics, in particular in the work of Robert Zwanzig, and in Caldeira-Leggett models used in quantum dissipation.[10][11]

In 1994, Rovelli introduced the relational interpretation of quantum mechanics, based on the idea that the quantum state of a system must always be interpreted relative to another physical system (like the "velocity of an object" is always relative to another object, in classical mechanics). The idea has been developed and analyzed in particular by Bas van Fraassen[12] and by Michel Bitbol. Among other important consequences, it provides a solution of the EPR paradox that does not violate locality.[13] Rovelli has exposed the main idea of relational quantum mechanics in the popular book Helgoland.

Rovelli won the second prize in the 2013 FQXi contest "It From Bit or Bit From It?" for his essay about "relative information". His paper, Relative Information at the Foundation of Physics, discusses how "Shannon’s notion of relative information between two physical systems can function as [a] foundation for statistical mechanics and quantum mechanics, without referring to subjectivism or idealism...[This approach can] represent a key missing element in the foundation of the naturalistic picture of the world."[14] In 2017, Rovelli elaborated further upon the subject of relative information, writing that:

In nature, variables are not independent; for instance, in any magnet, the two ends have opposite polarities. Knowing one amounts to knowing the other. So we can say that each end “has information” about the other. There is nothing mental in this; it is just a way of saying that there is a necessary relation between the polarities of the two ends. We say that there is "relative information" between two systems anytime the state of one is constrained by the state of the other. In this precise sense, physical systems may be said to have information about one another, with no need for a mind to play any role. Such "relative information" is ubiquitous in nature: The color of the light carries information about the object the light has bounced from; a virus has information about the cell it may attach; and neurons have information about one another. Since the world is a knit tangle of interacting events, it teems with relative information. When this information is exploited for survival, extensively elaborated by our brain, and maybe coded in a language understood by a community, it becomes mental, and it acquires the semantic weight that we commonly attribute to the notion of information. But the basic ingredient is down there in the physical world: physical correlation between distinct variables. The physical world is not a set of self-absorbed entities that do their selfish things. It is a tightly knitted net of relative information, where everybody’s state reflects somebody else’s state. We understand physical, chemical, biological, social, political, astrophysical, and cosmological systems in terms of these nets of relations, not in terms of individual behavior. Physical relative information is a powerful basic concept for describing the world. Before “energy,” “matter,” or even “entity.”[15]

Carlo Rovelli, Rome 2015

Rovelli has written a book on the Greek philosopher Anaximander, published in France, Italy, US[16] and Brazil. The book analyses the main aspects of scientific thinking and articulates Rovelli's views on science. Anaximander is presented in the book as a main initiator of scientific thinking.

For Rovelli, science is a continuous process of exploring novel possible views of the world;[17] this happens via a "learned rebellion", which always builds and relies on previous knowledge but at the same time continuously questions aspects of this received knowledge.[18] The foundation of science, therefore, is not certainty but the very opposite, a radical uncertainty about our own knowledge, or equivalently, an acute awareness of the extent of our ignorance.[18]

Rovelli discusses his religious views in several articles and in his book on Anaximander. He argues that the conflict between rational/scientific thinking and structured religion may find periods of truce ("there is no contradiction between solving Maxwell's equations and believing that God created Heaven and Earth"),[19] but it is ultimately unsolvable because (most) religions demand the acceptance of some unquestionable truths while scientific thinking is based on the continuous questioning of any truth. Thus, for Rovelli the source of the conflict is not the pretense of science to give answers — the universe, for Rovelli, is full of mystery and a source of awe and emotions — but, on the contrary, the source of the conflict is the acceptance of our ignorance at the foundation of science, which clashes with religions' pretense to be depositories of certain knowledge.[19]

• Quantum Gravity, Cambridge University Press, 2004, ISBN 0-521-83733-2
• With Francesca Vidotto, Covariant Loop Quantum Gravity: An Elementary Introduction to Quantum Gravity and Spinfoam Theory, Cambridge University Press, 2014, ISBN 978-1107069626

• Helgoland, Penguin Randon House 2021 / Helgoland, Adelphi, 2020.
• The Order of Time, Penguin Random House, 2018 / L'ordine del tempo, Adelphi, 2017.
• Reality Is Not What It Seems: The Journey to Quantum Gravity, Penguin Random House, 2016 / La realtà non è come ci appare: La struttura elementare delle cose, Raffaello Cortina Editore, 2014.
• Seven Brief Lessons on Physics, Penguin Random House, 2015 / Sette brevi lezioni di fisica, Adelphi, 2014.
• The first scientist Anaximander and his legacy, Westholme Publishing, 2011 / Che cos'è la Scienza. La rivoluzione di Anassimandro., Mondadori, 2012.
• What is time, what is space? (interview), Di Renzo Editore, 2006 / Che cos'é il tempo, che cos'é lo spazio?, Di Renzo Editore, 2004.

• C Rovelli, L Smolin: "Discreteness of Area and Volume in Quantum Gravity", Nuclear Physics, B442, 593 (1995).
• C Rovelli, L Smolin: "Loop space representation for quantum general relativity", Nuclear Physics, B331, 80 (1990).
• C Rovelli, L Smolin: "Spin Networks and Quantum Gravity", Physical Review, D53, 5743 (1995).
• C Rovelli: "Relational Quantum Mechanics", International Journal of Theoretical Physics 35, 1637 (1996).
• C Rovelli "Loop Quantum Gravity", Living Rev.Rel. 1 (1998) 1.
• A Connes, C Rovelli, "Von Neumann algebra automorphisms and time-thermodynamics relation in general covariant quantum theories", Classical and Quantum Gravity, 11, 2899 (1994).
• M Reisenberger, C Rovelli: "Sum over Surfaces Form of Loop Quantum Gravity", Physical Review D56, 3490 (1997).
• C Rovelli "Time In Quantum Gravity: Physics Beyond The Schrodinger Regime", Physical Review D43, 442, (1991).
• J Engle, E Livine, R Pereira, C Rovelli, "LQG vertex with finite Immirzi parameter", Nuclear Physics, B799, 136 (2008).