Theoretical physicist Carlo Rovelli is best known among physicists for his work on loop quantum gravity, a mathematical theory that quantizes spacetime (see the article by Martin Bojowald, Physics Today, March 2013, page 35). His broader appeal comes from his book Seven Brief Lessons on Physics, which introduced general audiences to the physics of black holes, general relativity, and quantum mechanics.
In his latest book, The Order of Time, Rovelli explores what he calls “perhaps the greatest remaining mystery” in physics: the nature of time. “Why do we remember the past and not the future?” he asks. “What does it really mean to say that time ‘passes’? . . . What am I listening to when I listen to the passing of time?”
To answer those questions, Rovelli takes readers on a tour of what physicists know—and what they don’t know—about time. In the February issue of Physics Today, cosmologist Anthony Aguirre calls the book “lovely, thoughtful, and poetic” and says it “will give all readers a taste of the mysteries of time.”
PT: What inspired you to write a book about time?
ROVELLI: To work in quantum gravity, one must face questions about the nature of time. General relativity tells us that the amount of time between two events is determined by gravity, and therefore time is affected by the quantum behavior of gravity. There can be quantum superpositions of different temporal states. A clock can be in a quantum superposition of two different times. So I have been thinking about the nature of time and the many problems it raises all through my scientific life. I thought that the moment had arrived to try to connect the dots and write down what I think we do and do not understand about time.
PT: In the introduction to your book, you argue that “the growth of our knowledge has led to a slow disintegration of our notion of time.” What are some of the advances or revelations that challenge the idea that time flows neatly from past to present to future?
ROVELLI: I cover several in the first part of the book. We have learned that time passes at different rates depending on altitude and on speed. We have learned that the fundamental equations of physics do not distinguish the past from the future. And we have learned that our very strong intuition about the present is valid only in a relatively small bubble around us; there is no objectively defined present in the large universe. Those are not speculations. They are established physics.
Then there is the speculative research in quantum gravity that further questions the nature of time. In loop quantum gravity, for example, there is no time variable in the fundamental equations of the theory. The theory describes the relative evolution of physical variables rather than their evolution in time.
PT: If the universe is fundamentally atemporal, what do you think explains the phenomenon that humans experience as time? Is time an illusion?
ROVELLI: I do not think that the universe is fundamentally atemporal. The main point of the book is that there isn’t a single notion of time that is either true or false. What we call time is a rich, stratified concept; it has many layers. Some of time’s layers apply only at limited scales within limited domains. This does not make them illusions.
For instance, the distinction between up and down is not an illusion, but it has no meaning away from Earth. There is no up and down for astronauts during interplanetary travel. Many properties of time are similar. In particular, there are aspects of our own human experience of time that are very much tied to the specific way our brain works: the fact that we have memories, that we anticipate the future, and so on. It is the human brain, not fundamental physics, that determines what we call the flowing of time and the sense of the speed at which it flows.
PT: What is your next project?
ROVELLI: I always have too many projects going on at the same time. I am mostly focused on white holes right now. A white hole, like a black hole, is a solution of the Einstein field equations but reversed in time. I am studying the possibility that black holes end their lives by becoming white holes.
The time from the formation of the black hole to its evaporation, transformation into a white hole, and final dissipation can be extremely long as observed from the outside but extremely short as measured from inside the hole. It’s an intriguing scenario that I developed with Eugenio Bianchi and colleagues in a recent paper. If that scenario is correct, the black holes we see in the sky are stars that are collapsing and then bouncing out, but we see that in extremely slow motion because of gravitational time dilation.
PT: Can white holes be observed?
ROVELLI: Yes, perhaps. One hypothesis is that their formation is the cause of fast radio bursts, mysterious super-violent signals captured by radio telescopes. Francesca Vidotto and I recently suggested another possibility that I find intriguing: that small white holes left over by black holes at the end of evaporation could be stable, and they could form an important component of dark matter.
PT: What are you reading right now?
ROVELLI: An extraordinary book by Alexander Bogdanov, Tectology. Bogdanov was a great Russian intellectual at the beginning of the 20th century. His ideas anticipated aspects of cybernetics, system theory, and contemporary structural realism.
