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

The best book on quantum physics since Briane Greene's "Elegant Universe." A superb blend of cutting edge science, history, literature, and philosophy. A must read for anyone even remotely interested in the subject.

Rovelli takes you a on a completely enjoyable journey through the history of, well, the particle and enables us all to understand the granularity of space and the world around us. His loop quantum gravity seems to be a perfectly sensible solution to the disparity between general relativity and quantum physics. I do hope it's proven. So much simpler than strings.

It is a beautiful read, poetic, humbling and insightful. Very approachable writing for such subject! I loved every page!

"Our culture is foolish to keep science and poetry separated, there are two tools to open our eyes to the complexity and beauty of the world" Carlo Rovelli, Reality of Not What It Seems: The Journey to Quantum Gravity

Carlo Rovelli's book meanders all over the place and does not really describe anything related to quantum gravity.
He spends a lot of time narrating the amazing discoveries of the ancient Greeks, this part is interesting and it the only part of the book worth reading, but it has next to nothing to do with gravity and its possible quantum nature. He towards the end tries to convince the reader that time and space result from gravity, and talks about "loops" with no proof or argument. This is a book that will not remain in my bookshelf.

One of the smartest books I have read for entertainment while deep science us being discussed he makes it seem so easy.I fell I love with sand in the beach now I look at it different. We are just grains of sand in the universe.

Great read. It explains the complexity of quantum and general relativity in simple language. I purchased this book for my adult children and my wife. Everyone loved it even though they were reluctant to start reading it.

Carlo Rovelli manages to weave together Science, Art, History, Philosophy and even a touch of Theology. He does so in a manner that clearly elevates and concisely educates. I had not delved into the subject matter in detail for quite some time. I not only learned but was left with a sense of wonder. It doesn't get much better than that.

Carlo Rovelli manages to weave together Science, Art, History, Philosophy and even a touch of Theology. He does so in a manner that clearly elevates and concisely educates. I had not delved into the subject matter in detail for quite some time. I not only learned but was left with a sense of wonder. It doesn't get much better than that.

Rovelli's book is strongest when it considers the history of science. The author has an understanding of these things the way only a European can. Ptolemy's Almagest, Archimedes' The Sand Reckoner, Alice Calaprice's childrens' letters to Einstein, J. A. Wheeler talking on web of stories -- it's great stuff (mostly now accessible online). But where Reality Is Not What It Seems falls down is in the theory. Physicists are fond of describing their theories as beautiful, powerful, and elegant, but many of these are really ugly, weak, and awkward. If quantum gravity were a stock pick, I'm not buying it. Besides the history of science stuff, and some poetic writing, I can't understand why so many reviewers were so taken in by this simplistic book. Scientific authors can have two main goals when writing a book like this one: secondarily, to make some extra pocket cash if the book turns out to be a best seller (as this one did), but primarily to implant their idea into the public consciousness that this is good science worth the funding. Perhaps now there will be more junkets of physicists to Italy.

Lee Smolin's The Trouble With Physics was the first major work to criticize string theory. This was particularly significant because Smolin himself admitted to being a former string theorist. But by the end of that book, Smolin revealed that he had now abandoned string theory for something new that he called quantum gravity. So, now we know what has happened to some of these chagrined string theorists: they emigrated to quantum gravity.

Quantum gravity has many of the same problems as string theory. It appears in retrospect reasonably ridiculous now that anyone would ever think that the fundamental components of the universe are vibrating strings. Maybe string theorists were doing something listening to their stereos at a time when rock and roll was the biggest thing going, or maybe that would be making them sound cooler than they actually were. But it may make even less sense now to say that the fundamental component of the universe is timeless spinfoam. Quantum gravity begins with the credibly interesting idea that the world might be quantized and discrete rather than infinitesimally divisible and continuous. The intuition for this may be the relative success of digital signal processing over analog signal processing in electrical engineering through the quantization of digital information, and the widespread use of digital computers that ensued, or from real analysis in number theory and the mathematics of Cantor. But the problems begin as soon as the quantum gravity theorists step beyond the original idea and begin trying to do something with the equations. Rovelli makes a little graph with nodes and links, and calls them spin networks (never mind that what spin really is was never adequately explained by Dirac, who sidestepped the question of what may actually be spinning, and how to make the speed of the spin consistent as being under the speed of light, by just labeling the spin as intrinsic), and then makes loops between the nodes that he calls gravity, but this doesn't take us far.

Consider the Planck length -- ten to the negative thirty-third centimeters. Rovelli says this is the basic thing, it defines the smallest indivisible unit of space, and he explains how it arises out of the uncertainty principle of Heisenberg. But this sounds in the end like numerology with exponents. It could be a theoretical limit or there could be things that are smaller. The uncertainty principle could merely be a statement of a measurement approximation at a certain spatial dimension of reality if you happen to be trying to measure something by bouncing a photon against it. Perhaps there will be some future way of measuring that is more minimally invasive, but the uncertainty principle is not necessarily inconsistent with there being things that are smaller than the Planck length.

Consider Rovelli's argument in favor of quantum gravity as explaining the Big Bang. There are several problems with this. One is a central problem with the Big Bang itself that physicists don't ever really logically give due consideration. If the initial state of the universe is what physicists appear to be saying it is, with all this matter present in one place ready to explode, then why wasn't the universe initially a black hole? There are all kinds of problems as to why the universe would be in the initial state that they say it was in ready to explode, and one would have to at least completely buy into an iffy spontaneous symmetry breaking to resolve any of them, but this is another big problem. Rovelli distracts us from this with an interesting idea of gravitational time dilation in the black hole during which occurs a Big Bounce. If the universe began somehow from some prior universe's black hole in a gravitational Big Crunch, Rovelli then says the Big Bounce occurs in slow motion over many billions of years by our temporal frame of reference due to the gravitational time dilation. However, the problem here is that there is no explanation as to why any Big Bounce -- which is presumably going to be relatively weak because it is only due to quantum effects -- is in any way going to be strong enough to overcome the gravity of a black hole to begin a new universe again. Never mind that all the evidence is that the universe is only expanding, and will never reverse into a Big Crunch. Moreover, let's assume that it is possible that our universe began from a black hole of a prior universe as an exploding Big Bounce. Then the universe would be getting smaller every time this happens because there would be less matter in it. Okay, now. Let's try to add Penrose's Conformal Cyclic Cosmology. Then the universe begins again with everything scaled down to a smaller size, with all the subatomic particles being regenerated at a conformally smaller spatial dimension of scale in a new Big Bang. But we would all like to know how this 'phase transition' to a new universe can ever actually happen, why whatever laws of physics would necessitate that the ratios of the sizes of the particles and the physical constants and the forces would be replicated at a smaller scale again in a new universe. String theory's initial motivation was to address this issue, but it never made any headway, instead getting bogged down in the mathematics, and the program was forgotten. All of our notions of what might constitute a Big Bang are tied up in our observations of chemical explosions, or in the historicity of physics, where the development of the atomic bomb in the same decades as the development of the idea of the Big Bang fused and confused these ideas together.

There is a role for quantum gravity, but Rovelli's isn't it. There is quantum gravity in trying to understand what is happening to the soup of atomic nuclei in a neutron star where all the empty space is compressed out of the electron orbitals of the atom, or maybe in a quark soup at the core of a black hole where all the empty space is compressed out of the distances between the protons and neutrons of the atomic nucleus. There are actually some concepts in astrophysics called spaghettification and nuclear pasta that describe some of this, where neutron stars have a 'gnocchi phase', a 'lasagna phase', a 'bucatini phase', and an 'antispaghetti phase'. In the words of Dave Barry, "I'm not making this up!" Perhaps there is an even denser state, a super black hole if you will, where all the empty space is compressed out of the quarks, leaving a soup of whatever the components of quarks are. It is a fact that the universe as we now know it is almost all empty space at every dimensional scale -- this is true of the space in the solar system, between the stars, or between the galaxies. I read somewhere that an atomic nucleus is like a twenty-five cent piece on the fifty yard line of a football stadium that is an atom, and that protons and neutrons are similarly proportioned with mostly empty space between the quarks.

Consider the sum of over histories. Rovelli says that quantum gravity resolves this problem because everything is now discrete and there are no infinities. However, there isn't any evidence presented to show that physicists have actually made any progress with this. Yes, you could see how conceptually making things discrete might resolve divergences of a sum over histories, but the book is too thin on the ground to show that quantum gravity actually does this. The sum over histories has been a royal pain to physicists for some time now, and maybe the problem is somewhere in the application of the sum over histories itself, which was not very well thought out from its beginning. (Feynmann, who developed this theory, was maybe never a really deeply logical thinker.)

Rovelli almost gets it right in my opinion when he begins to talk at the end about information theory. But he uses the thermodynamic entropy equation of phase space instead of the information entropy equation of Shannon, and gets everything confused and backwards when he puts thermodynamics -- historically reflected by the industrial revolution -- superior to time, and so has to conclude that time doesn't really exist. (Maybe we should blame this on Hawking's theory of the thermodynamic entropy of black holes.)

The book's provocative title shouldn't let it hook you in. Sometimes reality is just what it seems to be if you are looking at it carefully. Sometimes it's not. But for every school of the ancient world with a Democritus, there were likely hundreds that believed that the fundamental elements of the universe were turtles, or elephants, or string music.

Well done on the clear writing and especially the derivation of current physics from ancient thought -- humankind has always had a more or less correct intuition about reality (how could we not? we are of reality, of physics, of the universe). The challenge has always been to express that intuition in some kind of concrete terms -- something expressible in language or mathematics that can be communicated between people with some degree of clarity. I read this right after a third reading of Tor Norretranders "The User Illusion" which might have influenced my perspective.