I read a book which has rekindled my passion for the subject of physics. Reality Is Not What It Seems: The Journey To Quantum Gravity, by Carlo Rovelli was originally published in 2014. While the book is over ten years old now, Rovelli's explanatory power and simple and clear writing stand out. Even the book's core idea, what he refers to as thermal time, remains in the zeitgeist. Physicist Sean Carroll recently mentions Rovelli and thermal time by name in a podcast episode (January 6, 2025): https://www.preposterousuniverse.com/podcast/2025/01/06/300-solo-does-time-exist/ . In fact, Rovelli also appears as a guest on episode #2 of Mindscape (July 10, 2018): https://www.preposterousuniverse.com/podcast/2018/07/10/episode-2-carlo-rovelli-on-quantum-mechanics-spacetime-and-reality/ . I highly recommend listening to them.
In this book, Rovelli starts us at the beginning. Reality Is Not What It Seems is perhaps aimed at readers who are inexperienced in physics. He takes us on the journey of physics and science from Democritus through Kepler, Galileo, Newton, and then finally a number of 19th and 20th century scientists many of us have at least heard of. Some more knowledgeable readers may assume this would be tedious. I didn't find it tedious at all. First, it really did remind me of all of the reasons why I love science. Although I frequently read news articles, blog posts, listen to podcasts, and even read books on specific topics, those sources only ever cover a narrow aspect of science. Getting to enjoy a full book-length rendition of why science is great has been refreshing. Rovelli accomplishes this very well. Second, the journey he takes us on is actually meant to elucidate an important aspect of reality that we don't think much about in everyday life. Because the topic is both foundational and obscure, Rovelli's intertwining of it with his telling of physics since Democritus is exceptional in its didactic qualities.
For Rovelli, it all comes down to whether reality is discrete or continuous. Democritus believed that reality could not be divided up infinitely. At some point you reach the smallest unit of reality and cannot divide it further. As science progressed historically, mathematical models of different aspects of reality were developed: the orbits of the planets, gravitational acceleration, and eventually Newtonian mechanics. These advancement were accompanied by real explanatory power. However, the mathematical models often led to strange conclusions and infinities when taken to extremes. Even Einstein's relativity led to infinities in cases like singularities (black holes). Naturally, each generation of scientists wondered if these infinities are reflected in reality or are they artifacts of incomplete models.
Rovelli is like Democritus and believes that reality is discrete. The reason he believes this, as he carefully details in this book, is because discreteness resolves many of the aforementioned strange conclusions and infinities. Specifically, from Rovelli's perspective quantum theory and experimental evidence shows that matter and energy are discrete. The missing piece then is a quantum theory of gravity. Today, gravity remains under the regard of Einstein's theory of relativity and its weird infinities in areas like singularities. According to Rovelli, this is because Einstein treats space as a continuous background known as spacetime. A more accurate theory of gravity would entail space itself being discrete. In this kind of theory, particles of space do not have a background on which to measure things. After all, that would mean that there was something more fundamental in which space itself exists.
Einstein himself considered this, but didn't like it. It is hard to imagine how a useful model of reality could be built on a universe made up of discrete quanta of space that are relational, changing or moving constantly. Indeed, when some physicists pursued this further they arrived at loop quantum gravity and found that time itself was not there. It seems bizarre to us, because as mortal beings at our scale of reality, time seems unavoidable and of primary importance. However, loop quantum gravity suggests that time is an emergent phenomenon. This is why Sean Carroll mentions Rovelli and the concept of thermal time in his podcast episode about time.
Interestingly, this is where Rovelli devotes a chapter to information theory. He writes, "[...] in order to grasp the basic grammar of the world, we need to merge three basic ingredients, not just two: not just general relativity and quantum mechanics, but also the theory of heat, that is, statistical mechanics and thermodynamics, which we can also describe as 'information theory'." According to him, it may be possible that time "comes from averages of many microscopic variables." As he points out, heat and time are two relative phenomena that we cannot reverse. Conceptually we can imagine there might be a connection there. Perhaps time emerges from some aspect of reality that is also fundamental to heat. In the end, I'm not sure how to succinctly describe thermal time. I'll let Rovelli speak for himself:
"As long as we have a complete description of a system, all the variables of the system are on the same footing; none of them act as a time variable. That is to say, none is correlated to irreversible phenomena. But as soon as we describe the system by means of averages of many variables, we have a preferred variable that functions like common time. A time along which heat dissipates. The time of our everyday experience."
I have to say that conceptually I like Rovelli's interpretation of quantum theory more than ones like the many worlds interpretation. When Rovelli wrote this book in 2014 he said, "the thermodynamics of general relativity - that is to say, the statistical mechanics of quanta of space - is as yet only in its early infancy." After reading this book, I am very intrigued and excited to read more about loop quantum gravity and where the theory stands today.
In this book, Rovelli starts us at the beginning. Reality Is Not What It Seems is perhaps aimed at readers who are inexperienced in physics. He takes us on the journey of physics and science from Democritus through Kepler, Galileo, Newton, and then finally a number of 19th and 20th century scientists many of us have at least heard of. Some more knowledgeable readers may assume this would be tedious. I didn't find it tedious at all. First, it really did remind me of all of the reasons why I love science. Although I frequently read news articles, blog posts, listen to podcasts, and even read books on specific topics, those sources only ever cover a narrow aspect of science. Getting to enjoy a full book-length rendition of why science is great has been refreshing. Rovelli accomplishes this very well. Second, the journey he takes us on is actually meant to elucidate an important aspect of reality that we don't think much about in everyday life. Because the topic is both foundational and obscure, Rovelli's intertwining of it with his telling of physics since Democritus is exceptional in its didactic qualities.
For Rovelli, it all comes down to whether reality is discrete or continuous. Democritus believed that reality could not be divided up infinitely. At some point you reach the smallest unit of reality and cannot divide it further. As science progressed historically, mathematical models of different aspects of reality were developed: the orbits of the planets, gravitational acceleration, and eventually Newtonian mechanics. These advancement were accompanied by real explanatory power. However, the mathematical models often led to strange conclusions and infinities when taken to extremes. Even Einstein's relativity led to infinities in cases like singularities (black holes). Naturally, each generation of scientists wondered if these infinities are reflected in reality or are they artifacts of incomplete models.
Rovelli is like Democritus and believes that reality is discrete. The reason he believes this, as he carefully details in this book, is because discreteness resolves many of the aforementioned strange conclusions and infinities. Specifically, from Rovelli's perspective quantum theory and experimental evidence shows that matter and energy are discrete. The missing piece then is a quantum theory of gravity. Today, gravity remains under the regard of Einstein's theory of relativity and its weird infinities in areas like singularities. According to Rovelli, this is because Einstein treats space as a continuous background known as spacetime. A more accurate theory of gravity would entail space itself being discrete. In this kind of theory, particles of space do not have a background on which to measure things. After all, that would mean that there was something more fundamental in which space itself exists.
Einstein himself considered this, but didn't like it. It is hard to imagine how a useful model of reality could be built on a universe made up of discrete quanta of space that are relational, changing or moving constantly. Indeed, when some physicists pursued this further they arrived at loop quantum gravity and found that time itself was not there. It seems bizarre to us, because as mortal beings at our scale of reality, time seems unavoidable and of primary importance. However, loop quantum gravity suggests that time is an emergent phenomenon. This is why Sean Carroll mentions Rovelli and the concept of thermal time in his podcast episode about time.
Interestingly, this is where Rovelli devotes a chapter to information theory. He writes, "[...] in order to grasp the basic grammar of the world, we need to merge three basic ingredients, not just two: not just general relativity and quantum mechanics, but also the theory of heat, that is, statistical mechanics and thermodynamics, which we can also describe as 'information theory'." According to him, it may be possible that time "comes from averages of many microscopic variables." As he points out, heat and time are two relative phenomena that we cannot reverse. Conceptually we can imagine there might be a connection there. Perhaps time emerges from some aspect of reality that is also fundamental to heat. In the end, I'm not sure how to succinctly describe thermal time. I'll let Rovelli speak for himself:
"As long as we have a complete description of a system, all the variables of the system are on the same footing; none of them act as a time variable. That is to say, none is correlated to irreversible phenomena. But as soon as we describe the system by means of averages of many variables, we have a preferred variable that functions like common time. A time along which heat dissipates. The time of our everyday experience."
I have to say that conceptually I like Rovelli's interpretation of quantum theory more than ones like the many worlds interpretation. When Rovelli wrote this book in 2014 he said, "the thermodynamics of general relativity - that is to say, the statistical mechanics of quanta of space - is as yet only in its early infancy." After reading this book, I am very intrigued and excited to read more about loop quantum gravity and where the theory stands today.