Reality Is Not What It Seems: The Journey to Quantum Gravity

I read Carlo Rovelli’s book about quantum gravity (for the first time, it will take me a few goes, at least, to get all that is in it.) He is quite a good writer and this book, like his seven lessons in physics, is clear and extremely literate (I imagine he wrote it in Italian, but the English is smooth and demotic and lucid. It is a pleasure to read, which is not the norm in books that try to explain physics to non-specialists; God help the guy who tries to read the specialist literature. After a review from Democritus to Einstein et al, he gives us three big conclusions. At the smallest level, the universe is granular, relational, and indeterminate. He makes some other amazing statements like that ‘time’ disappears at this level and that things only exist when they collide into each other (or as ‘events’ as he puts it.) I have a notion about these other statements, but I have to determine if I understand the big three first. Everything (like Democritus and Feynman told us) is made of “atoms” or actually irreducible ‘quanta.’ Each of which is a unit of stuff that cannot be further divided; matter is not infinitely divisible (NB; big point.) Eventually, you get to a tight-pack of Plank scale bits of somethingness that all fit together. They in their constellation are gravity, space, and at bottom, everything else. There is no overarching, organizing anything outside these quanta. Time is absolutely a characteristic of the situation of the observer and the variable being measured “in” or as “time;” it measures differently at different altitudes and in different circumstances of proximity to matter and because of other factors. There are times all over the place and they do not generalize. At the level of the granular quanta, it disappears as a factor entirely. The stuff of the universe is not strictly determined in terms of how things interact and the results of any given intervention in it. We can pretty much depend on certain things happening as if by cause and effect on the macro level, but on the basic level, you get all kinds of stuff going on that is not absolutely predictable based on the setting conditions. This is the quantum probability/uncertainty thing, but it has to be understood in one of two ways; either it means our tools or our theory is inadequate and we don’t understand what is going on entirely, or the way the universe works is not determined by rules associated with forces, etc., and compatible with mathematics but instead things do their own thing, which usually results in rule governed outcomes, but doesn’t always. I am here confronted with the issue of the void that keeps on giving me a problem; there is no such thing as nothing and stuff cannot move around in it. Nothing cannot function either as a nominative, nor accusative, nor prepositional object in a sentence relating to stuff that exists except insofar as it is used to designate and absence that serves no purpose (e.g. “nothing happened,” or “you know nothing,” or “it is surrounded by nothing,” none of which are statements to be taken literally.) Therefore, matter cannot be conceptualized as floating around or moving in nothing or a ‘void’ (which is either nothing something and cannot function as both.) Democritus knew this right at the start; “space” he explained both is and isn’t nothing. He was just being gnomic and communicating that his atomic theory needed more work. If that is so, and how can you say anything else and be sane? Then certain conclusions follow. The quanta, for example, that make up everything are the whole show. There is nothing else in the cosmos but them, configured as they be. They are not in nothing (the statement doesn’t mean anything.) Nor are they in ‘space’ since they are space. They are not held together by gravity because they are gravity. More to the point, they are not held together by gravity because they are not “held together” at all. Since there is only these quanta irreducible and adhesive upon each other, they relate to no other cohesive force, they just are together with nowhere else to go. That means the quanta do not move; they cannot. There would have to be some medium of environment into which they could go and there isn’t anything but they themselves. They are irreducible so they cannot split into smaller chunks to let others slither through them. Since there is nothing but them, they have no interstices; there isn’t anything else in the cosmos that could come between them. Thus, you have inseparable grains and nothing else and these grains are where they are in relation to each other, but there is no force or principle or anything else that affects them all, like time or gravity or space or motion. They are not determined by any law or cause or force because no such thing exists outside of them (I am deliberately repetitious because the notion blows my mind.) For that reason, the prediction of occurrences among them is hit or miss. This is the part the author doesn’t exactly state, but if I am following him, the cosmos works something like this: There is no Aristotelian/Newtonian ‘time’ at the level of the quanta, but they configure according to the warped, curved, four-dimensional morphology of space-time. That is to say the that way the quanta fit together is not only according to the three axes of a prism, but also in relationships of sequence within the prism’s extension. That means that the entirety of the universe, including what we call past and present and future exists with all the quanta co-existent in all parts of space-time. Should this be the state of play of the cosmos, and I believe it is both in general relativity and in Rovelli’s construction (he calls it “loop” theory to distinguish it from the feckless “string” theory he deprecates) then, there is no determinacy or any causes or effects or any changes at all; just all the bits configured exactly how they are and the positions they have relative to each other and the observer are not caused by anything but just are. So, the discoveries of things are like looking at a map of twelve inches by six inches but only being able to see it a centimeter at a time from the left-hand margin. You guess what is coming in the next centimeter(s) based on what you already have seen of the terrain on the map, but a sink-hole or an inexplicable mountain peak can turn up on the map that relates to nothing else on the landscape, it is just there. The cosmos is like that; things don’t pop in and out of existence; the quanta that are there are just the quanta that are there, the observer has inferred the quark or electron out of stuff that he saw in one part of the map, but it isn’t to be inferred as in the next slice because the (existing) configuration of space-time just isn’t like that. The notion that only events exist is basically pretty anthropomorphic if you ask me, almost solipsistic. You only measure something by looking at it, which is a kind of collision. Since you cannot talk about things you cannot measure, only measured things exist according to this view. I think we can stretch and hazard that stuff exists even when not being measured, but I think the point is precious and not all that interesting. Change is something the observer infers when he looks over the cosmos along a sequence of space-time and mistakes the irreducible quanta, each of which is a grain that extends the Plank scale in ALL FOUR dimensions, as continuous, surviving unities across time. Parmenides knew this. Goedel understood this too, which is why he said that time travel is not a silly idea. As Einstein said, all parts of the cosmos are always available to the observer. They are all co-existent, which means they are all made of different stuff. Continuity is mental construction of the imaginative observer. (You can ask Descartes how the observer arises from these quanta and you will get an answer no more satisfactory than one I can give you, but consciousness is no more extraordinary than any of the rest of this stuff.) If I am right, and this is what the professor is saying, then what a great book since it made something quite clear to me that I had not understood hitherto. If I misunderstood it all (like I usually do) then I cannot blame him, and I still really enjoyed the book.

This book offers an exceptionally clear presentation of Loop Quantum Gravity(LQG) by one of its founding proponent Carlo Rovelli. It is written in Hawking style of lucid and engaging prose. It was a pleasurable read, which is a science writing achievement for a very unintuitive theory requiring you to do away with space time. The idea of doing away with space and time is probably the hardest thing for the reader to comprehend, but Rovelli did a stellar job of leading his readers through. Similar to many science books that explains a new theory, the author recaps a history of physics that has bearing to his project. Rovelli begins with Anaximander, Democritus, Plato, Aristotle, Archimedes to Newton and Einstein in part one of the book. It is delightful reading. But if you don't want to get through all that history again, you can start from part II beginning with general relativity and quantum mechanics. It is advisable to read through his relational view of Quantum mechanics because his LQG uses a relational view of reality. Rovelli sees quantum mechanics as describing quantum states of "interacting particles" (only a relationship, not as strong as entanglement). Quantum state arises from particle interacting with each other. He also sees information of quantum states as finite. The presentation of LQG is between chapter 5 to 7. LQG suggests space is made up of quanta of gravitational field. So quanta is more fundamental than space. The quanta is a node that can connect to another node or quanta by links or field lines. In LQG, space is replaced by quanta of gravitational field in a granular structure. A graph which represents how the quantas of gravitational field are connected is a spin network. This spin network is what space consists in. Not only is space is replaced. Time is also replaced by variables of activity such as heart beat, pulse, or pendulum swing. The notion of time flows by itself is less useful than noticing passage of activities such as beat or pendulum swing instantiating in the real world. Activities in the universe is more direct measure of the passage of reality. Using quanta of gravity and variables of activity, LQG presents a new representation of reality using gravity and quantum mechanics to replace space-time. When quantum mechanics is involved, events instantiate under probability and actual quantum event fluctuations. Reality is hence once again contingent upon relation of activities. Rovelli further presents the application of LQG by using it to study black holes. Heat dissipated in Hawking Radiation can be studied by the spin network that describes the gravitational field on the horizon of the black hole. Another use of LQG is to assess the black hole collapse. Quantum fluctuation and repulsion prevent collapsing to infinite pressure but offers it to bounce out. This approach is brought over to modeling the Big Bang suggesting that the initial explosion to be a big bounce from quantum fluctuation, not unlike the negative potential of Linde and Vilenkin inflation model. This book brings the reader through a journey to reconceptialise their reality in quanta of gravity, spin network and spin foam. It is conceptually challenging but Rovelli's lucid and engaging narrative made it fun and worthwhile.

Review of: “REALITY IS NOT WHAT IT SEEMS” by Carlo Rovelli For those who have an even casual interest in modern physics or “science” this is a truly remarkable book. Rovelli is able to take highly complex problems and express them in a way which can make them accessible to even those who think that mathematics is simply something you use to figure out how much change you should get at the supermarket or doing the highly difficult calculation of how much to tip on a restaurant bill. Over the years I have read literally dozens of books written for the nonscientist about special and general relativity, quantum mechanics, Brane theory, String Theory, Information Theory and quantum loop gravity. This is the first time I have encountered one which makes some real sense about these abstract concepts and clearly answers a question which has bothered me throughout this quest for knowledge: Is there such a thing as quantum time or is time simply a construct, an invention we use to measure the concurrence between different events. Spoiler Alert. Not only does quantum time exist it appears to be a requirement for healing the conflict between relativity and quantum mechanics. As most readers are aware Einstein’s theories of both general and special relativity are remarkably accurate and have passed every test to which they have been subjected as long as they apply to very large things. Quantum mechanics, the strange physics of the very small, is also exceedingly accurate in its predictions as long as it stays within its “range.” However these two great theories fail miserably when combined. This failure results from the combined mathematics giving results that result in answers that are infinite. A big no no in both mathematics and reality. According to Rovelli this can be overcome by the basic understanding that all “things” are quantized. That is there is a smallest possible size for the most basic of concepts including space and time. You may of heard about something called Zeno’s paradox. This can be stated in many ways but let’s take something that we do every day. Most of us when we get out of bed go to the bathroom. A rather basic task, which some may find harder than others to achieve and yet we are almost always able to achieve that goal. So what is paradoxical about that? Well when I get up in order to get to the bathroom I have to go half way say to my chair. That seems easy enough. But then from my chair I once more have to go half way to my final goal, the bathroom. So far I am seem to be doing okay, but am I? Now, unfortunately, I start thinking; how many half ways are there from the point I have reached to that elusive bathroom? Oh oh I exclaim I can keep on cutting this distance in half an infinite number or times. If so not only can I not reach the bathroom I could not have even begun to get out of bed, the initial place I began. What’s the solution? Am I forever stuck at one place and if not why not. The solution is the quantum of space. Space itself cannot be infinitely divided in half because there is indeed a smallest piece of space which is called the Plank length or unit. The Plank unit is very, very small but is greater than zero or in other words finite. No matter how hard I try to divide this unit it stays the same length and the concept of half of it is impossible and I am therefore not bound by infinity of lengths. I am free to do one of my most basic obligations of the day. Well if there is indeed a smallest possible piece of space how about time. Most of us have heard about the concept of space time. I had until recently mistakenly believed that the concept of space time was an invention of Einstein. It was actually proposed by another mathematician, Minkowski and is therefore referred to as Minkowski’s space time. The rational for this is highly complex but if this postulate had a bearing on reality could time itself actually be quantized. That would mean that time is not simply a way of measuring things but has a physical reality as well. I assume by now you have guessed that indeed there in a quantum of time beyond which there is no way to divide time. That length of time is the time it takes for a photon traveling at the speed of light (c) to travel on plank unit. A very, very, very small bit of time but a bit that is indeed finite. Thus we have removed from the equations of physics the two things that have always been considered infinite and make them finite. The result is that there are no infinities no matter what the conditions are in the universe, everything is finite. Whether at the Big Bank itself or at any place in the universe there are no infinities only finites are allowed. If you are interested in an accessible (understandable) description of how this comes about read Reality Is Not What It Seems. I doubt this will change anything in my life or yours except that I sleep better since I no longer have to worry about not being able to get to the bathroom the next morning.

I used to waste a lot of time thinking and theorizing about Einstein's theory relativity. how it was a more accurate descriptor than conventional Newtonian mechanics, and why Einstein's core theory seemed incomplete. No matter how much math I took I just could not get any headway. And, apparently, no matter how much advanced math anyone took, no one could get anywhere. Well, a group of physicists have finally broken through, and Carlo Rovelli is one such man who has written a really magnificent book explaining the current thinking of quantum gravity, the graviton, and how at a very basic and fundamental level the universe actually works. Thank goodness. If you're looking for "why" or other philosophical "wherefores", this is not the book for you. This is essentially a lay person's book to understanding the history and conclusion of how and why current physicists think that quantum gravity is the ultimate culmination in physics. I've bought and purchased various books on the topic over the years, seen all of the PBS and World Science programs and videos on the matter and matters related, and I think I can safely say that Carlo Rovelli has provded me and others the answers us scientifically curious individuals who are not professional scientists, with a really cool, easy to read, fun to read, summation of quantum gravity. Honestly I feel relieved. Not only have my nagging questions about space, time and gravity been answered, I now understand why the physical world is the way it is, and how future engineering projects may or may not be possible, or possible in principle but limited by other factors relating to quantum gravity. Honestly, I was one of those young science fans who had stars in their eyes when it came to all kinds of cool and futuristic concepts, and this books explains why some of those are doable, why others are not, what reality is, and what time is and is not, and again pretty much what you and I experience on a day to day basis. In a kind of odd sense it's almost a heartwarming book by presenting science as a work based effort that required some imagination to accept where observation and data were leading scientists. That is it reaffirms the scientific method, doesn't delve too much into political history of science, cuts right to the matter, and explains the system of nodes and links as constituting quantum gravity. Thank you Mister Carlo Rovelli. Your book is much appreciated. I'm just sorry your book didn't come out thirty years before because you would have saved me a lot of intellectual headaches, but I'm glad to have read your really magnificent book all the same. If you have a craving for theoretical physics, are tired of a lot of fluff about string theory, parallel universes, derivatives from quantum field theory and the like, then read this book. Carlo Rovelli explains quantum field theory, gives you some physic's history from the ancient Greeks and Egyptians up through the middle ages and renaissance to the present day, to conclude with how current thinking of quantum gravity got to where it is today. Really a great and easy to read book. Check it out and enjoy.

This is the third book I read by Carlo Rovelli. He is a poet of cosmology. The words on the page flow together like the glorious Colorado river. I consume his books much like I consume an outdoor adventure; it’s exhilarating. I finish each book thinking I am ready to change careers and work in the field of theoretical physics. This book handles the history of cosmological thought and development as though it was elementary science. Granted, the book does not work through the mathematical formulas, and that permits the reader to focus on the concepts of gravity, space and time, spacetime, fields, probabilities, and spin foams - a daunting task without the math. But the author has a command of the concepts, the terminology, and the historical development. He marshals the words on the page like Caesar marshaled his troops to victory at the Battle of Alesia. A well intentioned reader with some basic understanding of physics - particularly if reading this author in order of publication - will finish the book with a broader and deeper understanding of our physical world. But I confess that the author always ends his books with a pointed attack. The author is an atheist - serenely atheist - as he described. In fact, the last chapter of each of his books express the serenity he experiences as an atheist. He at times outright attacks the concepts of religion or metaphysics in general. In this book, he refers to the immortality of the soul as nonsense. And curiously, the author’s scientific rigor disappears when discussing anything metaphysical. And some statements bear a deep animosity to the Catholic Church, which I think is based on an improper judgement that the Church is anti-science. In summary, the author continues to fail to convince me that a person cannot wield both a sincere appreciation and thorough understanding of the physical world and a sincere appreciation and understanding of that which is metaphysical. Nonetheless, the next book by Carlo Rovelli beckons.

Carlo Rovelli's REALITY IS NOT WHAT IT SEEMS is a wonderful exploration of the history of the concept of the atom/quanta, the evolution of how humans see the world, and the interesting alternative to string theory for marrying quantum mechanics and general relativity: loop quantum theory. I think when reviewing/recommending non fiction, there are a few things to take into account: 1. ACCESSIBILITY: This book is not for a physics novice. You will need a basic understanding of physics and some chemistry (mostly pchem) to grasp some concepts. The first half of the book that explores the history is relatively accessible. However, once you get into particle/wave duality, it will help to have at least general chemistry/physics under you belt. Although I was completely lost when we got to "information theory." 2. BIAS/REPRESENTATION: This is where I think this book falls short. I would have loved a direct comparison of loop quantum theory with string theory. What are advantages/disadvantages to both? Where does on shine more? How can both be improved? 3. CLARITY: Concepts were brilliantly explained. I often have trouble with visualization, but the alien concepts of loops, spin foam, and time as a construct of thermodynamics wee effectively portrayed. I loved this book. From the history to the new concepts, I could feel it pushing and pulling my brain like putty in different directions. Although the last chapter became a little preachy, this is a wonderfully written exploration of the big and small and how to meld the two. If you are in the mood for a mind-bending experience that will fundamentally alter how you perceive the world, pick this one up!

This is an email I sent to the author: ========================================== Dear Dr. Rovelli, I just wanted to take a couple of minutes to tell you how much your book, "Reality is Not What it Seems," moved me. I'm kind of an old fart interdisciplinary computer scientist, who was first moved to pursue science when watching Carl Sagan's "Cosmos" in the mid 1980's. He lit a spark that drove me to finish my BS and go on to a PhD, and his "Demon Haunted World" was, in my view, one of the seminal works on what science is all about. I turned to your book some months ago, having read a little on relativity over the years, and barely understanding quantum mechanics. Immediately, I loved that you started with an historical perspective dating back to the Greek philosophers, and you maintained that connection to the very end. I usually juggle several books, so I had time to think about it over the months, and I love that you hinted early on at "information" being a potential foundation of the physical world. I spent many early morning runs with my dogs here in Alaska, under the aurora, contemplating the way information is exchanged at all levels, from the single cells (and molecules) to our current environment. So, I was really moved to see you touch on this in depth in your last two chapters. One analogy that really moved me was your comparison of the covariant quantum fields to our view of the ocean from space, and then, coming lower, viewing the turbulence that really exists. To this day, I now attempt to see my beautiful Alaska from the perspective of the Planck length! Your final chapter, I think, did a beautiful job of bringing it all together and, more importantly, describing the uncertainty and humility that serves as a foundation for the pursuit of science. I think that, perhaps, this chapter could well stand alone from the others, yet, the previous chapters made it so much more meaningful. I sincerely hope that young people are motivated by your splendid work to pursue science in its purest and noblest form, as I was motivated to pursue science after reading much of Carl Sagan's writings. You have a gift, which is apparent throughout the book, but really shines in that last chapter. My only complaint is that after reading your book, I still don't "get" it all! But, that's the point of it all, isn't it? Many thanks for igniting a spark again. Best Regards, Don Morton

If you're interested in the current efforts to combine general relativity and quantum mechanics, read this book. Einstein's theory of general relativity and quantum mechanics as developed by a number of physicists in the early 20th century are considered incompatible absent the development of a theory of quantum gravity, which many physicists, including Professor Rovelli, have devoted their careers. In a complex area, the author writes well. More importantly, the author understands that much of "reality" as currently explained by theoretical physics, and supported by much experimental testing, is hugely inconsistent with the ways in which ordinary humans experience their world. Before I go too far, Professor Rovelli has a t-shirt on page 191 of the paperback version of the book with the equations for loop quantum gravity on the shirt. I've got to get one of these t-shirts. He's right when he says "the reader will not be able to decipher [the equations]", and I certainly can't, but I want the t-shirt. The book explores the idea that spacetime is quantum. It also advances the idea that time may not really exist, an idea that I find really challenging. The book is challenging, but the writing is clear enough that I thought I had some grasp of the efforts to understand a quantum world. The big bang, quantum black holes, space as quanta, information theory and infinity's end all are considered in the book. And considered in a way that a non-scientist such as myself can at least dimly understand what's being proposed. Professor Rovelli also introduces the scientific insight of a young Belgian priest named Georges Lemaitre who first recognized that Einstein's equations could include what Lemaitre called the "primordial atom" and we now call the "Big Bang." As Professor Rovelli shows, Father Lemaitre possessed sufficient intellect, courage and gravitas to persuade both Einstein and the Pope, at different times, to back down from positions that each took on matters of physics. An impressive achievement that enhances the author's story. One minor quibble is that the author seems to think that no scientific effort was made between the ancient Greeks, Anaximander, Leucippus, Democritus, Aristotle and others, and the time of Galileo, Kepler, Newton and other well-known scientific luminaries. A French theoretical physicist of the late 19th and early 20th centuries named Pierre Duhem who also had an interest in the history of science and read Latin studied the works of Roger Bacon, Nicole Oresme and others from the Middle Ages and concluded that Bacon, et al, had done some careful scientific work that informed later scientific investigation. So, maybe the Middle Ages weren't completely dark after all. The book is well worth your time. Professor Rovelli writes clearly. The topic is obviously complex and focus on the book will be required. Your attitude about the reality that you experience daily will be tested.

The beauty of the writing, the clarity of the exposition, the presentation of cutting edge research on quantum gravity and the nature of physical reality, the creative thinking of things to come along with the enlightened spirit which imbues the text are some of the gratifying characteristics of the book. The author takes us on a fascinating journey spanning 2,500 years of evolution in physics and the human quest for understanding the nature of physical reality from Democritus of Abdera to loop quantum gravity and beyond. Democritus comes something of a hero in the book in anticipating modern thinking on the nature of physical reality overshadowing in this regard Plato and Aristotle. Plato is given due credit, however, in realizing that Mathematics is the language best adapted to understand and describe the world; the fact that Mathematics forms the basis of modern science owes much to Pythagoras and Plato. Mathematics assisted in the evolution of planetary science from classical antiquity through Ptolemy in Alexandria in the second century AD. Ptolemy provided accurate predictions on the movements of the planets with his geocentric system. Then the dark ages ensued. It was in the Renaissance that Nicolaous Copernicus (1473-1543) who made the revolutionary transition from geocentric to heliocentric cosmology but poor mathematics did not improve the accuracy of prediction of planetary movements. It was Johannes Kepler (1571-1630) similarly adopting a heliocentric system and better Mathematics that for the first time humanity finds how to do something better than what was done in Alexandria more than a thousand years earlier. Galileo Galilei (1564-1642) discovered the constancy of pendulum's swing and formulated the law of constant acceleration of falling bodies. And from the earth of Galileo, the genius of Isaac Newton (1642-1727) jumped to heavens and formulated the universal laws of gravity. Then come Michael Faraday and James Clerk Maxwell with the discovery of o electromagnetism and the concept of the field. Electromagnetism is the single force apart from gravity that governs virtually all phenomena we see: this is the force which holds together the matter that forms solid bodies; holds atoms in molecules and electrons in atoms; this is what makes chemistry and living matter work. The two and apparently incompatible pillars of twentieth century physics which radically transformed our ideas about the cosmos were Einstein's general theory of relativity and quantum mechanics: space and time in relativity; matter and energy in quantum mechanics. General relativity provides a simple and coherent vision of gravity, space and time. Einstein appreciated that gravity like electricity must be conveyed by a gravitational field. But his insight of a genius was his visualization that the gravitational field is not diffused through space but is the space itself. Space and time should not be viewed as two distinct entities but as an integrated whole - space-time. The theory provides an enormous simplification of the world. The predictions of the theory of general relativity were verified experimentally with a precision of one part to one hundred billion. The theory describes a world where universes explode, space collapses into black holes, time slows near a planet, interstellar space ripples like the surface of a sea while the space expands. Quantum mechanics, the second pillar, achieves unequaled experimental success and leads to applications which have transformed our every-day life e.g the computers, the advancement of molecular chemistry and biology, lasers and semiconductors. Planck was the first to correctly assume that the energy of the electric field is distributed in quanta (small packets) of energy; the size of the quanta depends on the frequency of the electromagnetic waves. Einstein came to the same conclusion for the energy of the quanta of light (photons). Bohr realized that the fact that the spectra of atoms are discrete and not continuous could be explained if the energy of electrons in atoms (electron orbitals) could only assume certain quantized values - certain specific values as hypothesized by Planck and Einstein. It becomes clear that granularity (distinct and finite as opposed to continuous and infinite) is something widespread in nature. Then comes Heisenberg's audacious theory: a fundamental description of the movements of particles, in which they are described not by their position at every moment but only by their position at particular instants: the instants in which they interact with something else. This is the second cornerstone of quantum mechanics, the relational aspect of things. The strange genius of Paul Dirac provides a final refinement producing the equations of quantum theory of unique abstract beauty. Dirac's equations is a recipe for calculating the spectra (the values) of the variables and a recipe for calculating the the probability that one or another value will appear in the next interaction. What happens between one interaction and another does not appear in the equations. The indeterminacy (the probabilistic character in the equations) is the third cornerstone of quantum mechanics: the discovery that chance operates at the atomic level. Dirac realizes that the theory can be directly applied to fields such as electromagnetic ones and can be made consistent with special relativity (making it consistent with general relativity, a cutting edge research topic in the book, will appear next and conclude this review). Dirac, in the process, discovers an ulterior, profound simplification of the description of nature: the convergence between the notion of particles used by Newton and the notion of fields introduced by Faraday. Einstein understood that space and time are manifestations of a physical field. Bohr, Heisenberg and Dirac understood that physical fields have quantum character: granular, probabilistic, manifesting through interactions. The theory to address the problem of quantum space and time is quantum gravity which reconciles the theories of general relativity and quantum mechanics. The first to intuit the existence of a minimum length in the gravitational field already in the thirties was a young Russian, Matvei Bronstejn who died tragically in the Stalinist regime. Matvei's reasoning was that at a very tiny length, a particle would collapse into a microscopic black hole thus rendering granular the gravitational field. This minimum length (Planck's length) was computed at 10 in the minus 33 centimeters. From this starting point, the Wheeler-Dewitt equation evolved and its solutions had the peculiarity that they depended on closed lines in space; a closed line is a 'loop'. The closed lines that appear in the solutions of the Wheeler-Dewitt equation are Faraday lines of the gravitational field. But now the continuous spiderweb of Faraday's lines become a finite number of distinct lines. The key to understanding the physics of these solutions lies on the points where these lines intersect. These points are called nodes, and the lines between nodes are called links. A set of intersecting lines forms a graph (a combination of nodes connected by links). A calculation demonstrates that without nodes, physical space has no volume. It is in the nodes of the graph, not in the lines, that the volume of space resides. These nodes representing the discrete packets (volumes) can be computed using Dirac's general quantum equation. Each node in the graph has its own volume, one of the numbers which collectively comprise the spectrum of the volume. Similarly the links joining the nodes represent collectively the spectrum of areas adjoining the volumes. What about time? Time no longer exists. The Wheeler-Dewitt equation, the fundamental equation no longer contains the time variable. The absence of the variable time from the fundamental equation does not imply that everything is immobile and that change does not happen. On the contrary, it means that change is ubiquitous. Only; elementary particles cannot be ordered along a rhythm. At the extremely small scale of the quantum space, the dance of nature does not develop to a sequential rhythm: every process dances independently with its neighbors, following its own rhythm. I shall conclude the long review by presenting the progression of our understanding of physical reality from Newton to quantum gravity which constitutes an enormous simplification but at the same time is profoundly counter intuitive: Newton: Space, Time, Particles. Faraday- Maxwell: Space, Time, Fields, Particles. Einstein 1905: Space -time, Fields, Particles. Einstein 1915: Co -variant fields*, Particles Quantum mechanics: Space -time, Quantum fields. Quantum gravity: Co -variant quantum fields. *Co -variant quantum fields are fields that live on themselves, without the need of a space-time to serve as a substratum , and which are capable by themselves of generating space-time.

I thought this book was very well written. I still have no clue about quantum physics or gravity but loved reading this book regardless.

I had read rave reviews of this book as well as Rovelli’s other books, but held off reading them for a while. I have read a lot of other popular physics books including several that cover the conundrum of trying to combine quantum mechanics and general relativity (Smolin, Greene, etc) and I had assumed Rovelli was just covering well-trodden ground. I couldn’t have been more wrong. While the majority of content in this book was relatively familiar to me, the way in which the story was told - the prose, the poetic language and the weaving together with history, philosophy and art - was just a pure pleasure to read. It reminded me why I found science interesting in the first place earlier in my life, the sparking of that sense of wonder. That is the feeling you get reading Rovelli, plus learning about a bunch of interesting science in the progress. Highly recommended.

I am an ardent reader of physics subjects, I have read many books on physics ,but only a few of them imparted a satisfactory outlook so that a person with basic awareness of physics can understand it's content, but this one is a amazingly special for the simple reason that a person with general awareness about physics can understand. It covered almost every topics of main theories of classical, relativistic and quantum mechanics with such brevity and simplicity... Thanks to the author who tried his level best to come to the strata of an average physics enthusiast. After reading this book I got immense satisfaction which no other books of the same sort could provide... This is a must read book and I assure that you will get something very special which you will experience later...

Carlo Rovelli presenta come si è evoluta la visione del mondo per arrivare attraverso la quantum gravity ad un tentativo di coniugare l’indeterminazione della teoria quantistica con la granularità delle cose della relatività generale. Ci si è riusciti? Le due teorie oggi sopravvissute string e loop sono ancora divise pure se quest’ultima sta raccogliendo indizi a suo favore. A fronte della mancanza di certezza rimane lo spirito della scienza che dubita di ogni certezza e che trova il suo valore nel dare le risposte che servono consapevole che domani queste cambieranno. Un libro molto bello da leggere.