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.

Indrukwekkend

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 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.

Carlo Rovelli, Professor für Theoretische Physik an der Universität Marseille, leistete grundlegende Beiträge auf dem Gebiet der Schleifen Quanten Gravitationstheorie, seine Monographie 'Covariant Loop Quantum Gravity' ist eine relativ elementare Einführung in dieses Thema, und wurde bereits zu einem Standardwerk. Während dessen wurde der Autor, wie er im Vorspann bemerkt, immer wieder gebeten, auch eine populäre Darstellung zu verfassen, und in der Tat gibt es etliche allgemein verständliche Bücher zur Kosmologie und String Theorie, aber eines, zum Stand der Forschung über die Quantennatur von Raum und Zeit, fehlte bisher. Wiewohl sich Rovelli auf die Forschung konzentrieren wollte, fand er schließlich, als die Monographie abgeschlossen war, dass sein Gebiet nun eine gewisse Reife erreicht hätte, die es erlaubt, das Thema einem breiteren Publikum vorzutragen. Auf einer langen Fahrt nach Frankreich, reifte schließlich eine Idee, wie man eine so komplexe Materie entwickeln könnte – er beschloss, die Ideen von Raum und Zeit in ihrer historischen Entwicklung zu schildern, beginnend bei den griechischen Philosophen. Das Buch erschien Anfang 2014 in Italienisch; als ein Ableger davon erschienen einige Artikel in der Sonntagsausgabe einer Zeitung, diese wurden – in etwas erweiterter Form – zu einem schalen Bändchen 'Seven Brief Lessons on Physics' zusammengefasst, dieses wurde bereits in verschiedenen Sprachen (dt. 'Sieben kurze Lektionen über Physik') zu einem Bestseller, bevor noch das vorliegende Buch in Englisch vorlag. Der historische Exkurs beginnt bei Demokrit und anderen Atomisten, die sich bereits mit der Frage befassten, ob denn Dinge beliebig oft teilbar seien und wie es mit dem Raum dazwischen aussehe, Überlegungen, die Zenon zur Formulierung seiner berühmten Aporien veranlasste. In der Neuzeit greift Newton Ideen von Galilei und Kepler auf, und formuliert Gesetze für die Bewegung von Massepunkten bzw. Partikeln, damit diese mathematisch sinnvoll werden, postuliert er als 'playground' seiner Dynamik einen absoluten unveränderlichen und homogenen Raum und eine absolute gleichförmige Zeit. Faraday und Maxwell fügen noch Felder als Konstituenten der Elektrodynamik hinzu. Zu Beginn des 20. Jahrhunderts erkennt Einstein, dass man die Elektrodynamik bewegter Körper konsistent mit der Mechanik vereinigen kann, wenn man die Newtonschen Absoluta zur Raumzeit der Speziellen Relativitätstheorie vereinigt. Schließlich gelingt es Einstein 1915 die Newtonsche Gravitationstheorie und die SRT zur Allgemeinen Relativitätstheorie zu verbinden, dass gelang ihm, in dem die starre Raumzeit der SRT durch eine flexible, variable Struktur ersetzte, dabei entspricht die Gravitation gerade Krümmung dieser Raumzeit -- mit anderen Worten, in der ART entspricht die Raumzeit einem Feld. Die zweite Revolution des 20. Jahrhunderts, die Quantenmechanik, führte zu noch weiterführenden Umwälzungen in der Vorstellungswelt der theoretischen Physik – Messgrößen, wie Energien, Frequenzen etc., können diskrete Werte annehmen, für die Resultate von Interaktionen lassen sich nur Wahrscheinlichkeiten ermitteln. Die Quantentheorie konnte aber auch die Begriffe von Teilchen und Feldern vereinheitlichen – beide lassen sich als Quantenfelder beschreiben, das gelang zunächst für das elektromagnetische Feld und das Elektron, in den 1970igern dann auch für die Konstituenten der schwachen und starken Wechselwirkung, die gemeinsam zu dem äußerst erfolgreichen Standardmodel der Elementarteilchen Theorie vereinigt werden konnten. Mit Teil III verlässt die Darstellung die etablierte Physik und wendet sich der aktuellen Forschung, und damit auch dem Arbeitsgebiet des Autors, also auch dem eigentlichen Anliegen des Buch, zu – den Quanten des Raumes und der relationalen Zeit. Der Erfolg der Quantenfeldtheorie beruhte darauf, dass man die Unendlichkeiten, die bei ihren Berechnungen auftreten, beseitigen kann – G. t' Hooft und M. Veltman bewiesen, dass Eichfeldtheorie renormiert werden können – bei der Quantisierung von Gravitationsfeldern traten jedoch unüberwindliche Schwierigkeiten auf, wie schon M. Bronstejn auffiel. Dann gelang es Wheeler und de Witt, eine Gleichung für den Raumzeit- Schaum der Quanten Gravitation aufzustellen, allerdings war diese nicht leicht zu interpretieren, nur eine ihrer Merkwürdigkeiten ist, dass sie keine Zeit Variable enthält. Ted Jacobson und Lee Smolin konnten die Wheeler – de Witt Gleichung umformulieren und zeigen, dass jedem Loop, d.h. jeder geschlossene Linie, eine exakte Lösung entspricht. Jorge Pullin und Jerzy Lewandowski fanden, dass die wesentlichen Elemente der Theorie die Schnittpunkte solcher Loops, sogenannte Nodes, und die Verbindungsstücke der Nodes, Links, sind, d.h. das Gesamtkonstrukt ist nichts anderes als ein Graph. Schließlich zeigten Rovelli und Smolin 1994, dass die quantenmechanischen Operatoren, die Fläche und Volumen beschreiben, diskrete Spektren haben. Dies Graphen repräsentieren nichts anderes als die Quantenzustände des Gravitationsfelds; wie bereits im Fall der ART – existiert der Graph aber nicht in einem externen Raum, sondern bildet just diesen Raum, und ist damit, in dem genannten präzisen Sinn, granuliert. Die Zeit ist aus diesem Bild verschwunden, die Gesetze der Quanten Gravitation ordnen, wie in der Quantenwelt üblich, den Übergängen von einem Graph zu einem anderen, Wahrscheinlichkeiten zu, die entstehenden Strukturen werden Spin- Schaum (spin foam) genannt. Es gibt also eine einheitliche Antwort der Quanten Gravitations- Theorie auf die Frage, woraus die Welt im innersten aufgebaut ist: Kovariante Quantenfelder – Teilchen sind Quanten solcher Quantenfelder, Raum ist nichts weiter als ein eben solches Feld, das auch aus Quanten besteht, und Zeit erwächst aus den Prozessen dieses Feldes. Im letzten Teil werden einige Konsequenzen der Theorie diskutiert, die die Grenzen des heutigen Wissen streifen: die Phänomene betreffen den Big Bang und Black Holes, ferner werden Experimente, die die Theorie testen sollen, erörtert. Abschließend folgen einige Reflexionen über noch unverstandene Zusammenhänge zur Thermodynamik: die Rolle von Information in einer Welt ohne Raum und Zeit, und die Emergenz der Zeit. Carlo Rovelli gelingt es, dem Leser einen faszinierenden ersten Eindruck von einem Gebiet, dass zur Zeit noch an der vordersten Forschungsfront entsteht, zu vermitteln, geschickt knüpft er an die Mainstream Physik von Relativitätstheorie und Quantenmechanik an, um schrittweise grundlegende Ideen der Loop Quanten Gravitationstheorie zu entwickeln. Der Autor fordert beim Leser sicher ein gehöriges Maß an Abstraktionsfähigkeit ein, begnügt sich dafür aber auch nicht mit billigen Gleichnissen, natürlich kann er im Rahmen eines populären Werks nicht auf technische Details eingehen und muss deswegen gelegentlich Abkürzungen nehmen, er ist aber in höchsten Maße darum bemüht, sein Thema systematisch, nachvollziehbar und gut überschaubar darzustellen, wobei er auch schwierige Gegenstände in seinen gelungenen knappen Überblick zu integrieren versteht; ohne Zweifel lohnt es sich, mache Passage mehrfach zu lesen. Das Buch gehört sicher zu den besten, allgemein verständlichen Schriften über fundamentale, moderner theoretischer Physik, die in letzter Zeit veröffentlicht wurden.