As you read these words, copies of you are being created. Sean Carroll, theoretical physicist and one of this world’s most celebrated writers on science, rewrites the history of 20th century physics. Already hailed as a masterpiece, Something Deeply Hidden shows for the first time that facing up to the essential puzzle of quantum mechanics utterly transforms how we think As you read these words, copies of you are being created. Sean Carroll, theoretical physicist and one of this world’s most celebrated writers on science, rewrites the history of 20th century physics. Already hailed as a masterpiece, Something Deeply Hidden shows for the first time that facing up to the essential puzzle of quantum mechanics utterly transforms how we think about space and time. His reconciling of quantum mechanics with Einstein’s theory of relativity changes, well, everything. Most physicists haven’t even recognized the uncomfortable truth: physics has been in crisis since 1927. Quantum mechanics has always had obvious gaps—which have come to be simply ignored. Science popularizers keep telling us how weird it is, how impossible it is to understand. Academics discourage students from working on the "dead end" of quantum foundations. Putting his professional reputation on the line with this audacious yet entirely reasonable book, Carroll says that the crisis can now come to an end. We just have to accept that there is more than one of us in the universe. There are many, many Sean Carrolls. Many of every one of us. Copies of you are generated thousands of times per second. The Many Worlds Theory of quantum behavior says that every time there is a quantum event, a world splits off with everything in it the same, except in that other world the quantum event didn't happen. Step-by-step in Carroll's uniquely lucid way, he tackles the major objections to this otherworldly revelation until his case is inescapably established. Rarely does a book so fully reorganize how we think about our place in the universe. We are on the threshold of a new understanding—of where we are in the cosmos, and what we are made of.

# Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

As you read these words, copies of you are being created. Sean Carroll, theoretical physicist and one of this world’s most celebrated writers on science, rewrites the history of 20th century physics. Already hailed as a masterpiece, Something Deeply Hidden shows for the first time that facing up to the essential puzzle of quantum mechanics utterly transforms how we think As you read these words, copies of you are being created. Sean Carroll, theoretical physicist and one of this world’s most celebrated writers on science, rewrites the history of 20th century physics. Already hailed as a masterpiece, Something Deeply Hidden shows for the first time that facing up to the essential puzzle of quantum mechanics utterly transforms how we think about space and time. His reconciling of quantum mechanics with Einstein’s theory of relativity changes, well, everything. Most physicists haven’t even recognized the uncomfortable truth: physics has been in crisis since 1927. Quantum mechanics has always had obvious gaps—which have come to be simply ignored. Science popularizers keep telling us how weird it is, how impossible it is to understand. Academics discourage students from working on the "dead end" of quantum foundations. Putting his professional reputation on the line with this audacious yet entirely reasonable book, Carroll says that the crisis can now come to an end. We just have to accept that there is more than one of us in the universe. There are many, many Sean Carrolls. Many of every one of us. Copies of you are generated thousands of times per second. The Many Worlds Theory of quantum behavior says that every time there is a quantum event, a world splits off with everything in it the same, except in that other world the quantum event didn't happen. Step-by-step in Carroll's uniquely lucid way, he tackles the major objections to this otherworldly revelation until his case is inescapably established. Rarely does a book so fully reorganize how we think about our place in the universe. We are on the threshold of a new understanding—of where we are in the cosmos, and what we are made of.

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4out of 5Manuel Antão–If you're into stuff like this, you can read the full review. Pascal's Triangle: "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime" by Sean Carroll “When a spin is measured, the wave function branches via decoherence [according to the MWI], a single world splits into two and there are now two people where used to be just one. It makes no sense to ask which one is ‘really me.’ Likewise, before the branching happens, it makes no sense to wonder which branch ‘I’ will end up in. If you're into stuff like this, you can read the full review. Pascal's Triangle: "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime" by Sean Carroll “When a spin is measured, the wave function branches via decoherence [according to the MWI], a single world splits into two and there are now two people where used to be just one. It makes no sense to ask which one is ‘really me.’ Likewise, before the branching happens, it makes no sense to wonder which branch ‘I’ will end up in. Both of them have every right to think of themselves as ‘me.’ [...] The world duplicates, and everything within the world goes along with it.” In “Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime” by Sean Carroll “Many-Worlds is the most falsifiable theory ever invented.” In “Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime” by Sean Carroll Let me get this out of the way first. Let me put forth the main 4 “interpretations”... Continues elsewhere.

5out of 5BlackOxford–Scientific Revelation There is more than a hint of theological method in modern physics. Carroll confirms this in his insistence that quantum physics is, in his words, not an ‘epistemic’ but an ‘ontological’ discipline His claim is that current quantum theory is a description of the way the world really is not merely a way of understanding the world. This is the traditional position of theologians who would like us all to consider God as the ultimate reality even if we find this reality to be not Scientific Revelation There is more than a hint of theological method in modern physics. Carroll confirms this in his insistence that quantum physics is, in his words, not an ‘epistemic’ but an ‘ontological’ discipline His claim is that current quantum theory is a description of the way the world really is not merely a way of understanding the world. This is the traditional position of theologians who would like us all to consider God as the ultimate reality even if we find this reality to be not what we perceive it to be. In fact Carroll defines science in general, not just physics, in theological terms. For him, the essential presumption of science is the intelligibility of the universe. This implies not just that there is a pre-existing order to be discovered but also that such order in some sense wants itself to be discovered. These implications are precisely those of what is called fundamental theology, the study of how God can be known about at all.* The similarity between Carroll’s view of quantum physics and fundamental theology is important because in both there is no distinction possible between epistemology and ontology. How we know about the world, or God, is indistinguishable from what the world, or God, actually is. Theology has a term for referring to this knowledge of being (or Being) - revelation. Essentially, you either get revelation or you don’t. It can’t be argued about because the presuppositions about what constitute both existence and knowledge about existence are contained simultaneously within it. Thomas Aquinas is perhaps the most well-known theologian to defend the presuppositions of revelation. In doing so, his preferred approach is cosmological, that is, treating the entire universe as an entity to be explained in terms of its existence and its history. At such a level of analysis, ordinary logic (like that of cause and effect and their priority in time) start to break down. Thus, Aquinas asks, if every effect must have a cause, what is the ultimate cause? And if human beings exhibit free will and purpose as an effect of that ultimate cause, is it not reasonable to attribute will and purpose to that cause. QED, the universe is a consequence of divine action with some divine purpose toward which it is drawn. Carroll makes a parallel case for quantum physics and the Many-Worlds theory of Hugh Everett, formulated in the 1950’s. First, just like Aquinas, he adopts a cosmological position. The universe, he says, is one vast quantum state, a wave function of enormous complexity. This is not inconsistent with the theory of quantum physics even if it could never be empirically verified. And it fits with the strange results of quantum experimentation. QED, reality is composed 0f an indeterminate number of simultaneous universes. In other words, Everett’s theory qualifies as a revelation. If this is the case, then this wave function will evolve according to the mathematics of the Schrödinger equations, just as it has always done. Not according to the logic of Newtonian (or Aristotelian) cause and effect but the logic of probability and entanglement. This wave function is not something temporary or local that might transform into something else, say a particle, or ‘collapse’ upon observation. Within it is not only the universe we know about but an infinite number of others that exist simultaneously. The wave function, in other words, is the very stuff, the ultimate reality of the universe; and it doesn’t make distinctions between observer and observed or between possible and actual. Our brains and the farthest galaxies as well as everything in between, including any number of other worlds, must be part of this wave function, since there can be nothing else. So the conventional ‘Copenhagen interpretation,’ despite its usefulness, is wrong. The wave function is the Alpha and the Omega, the source and giver of not just life but also existence, the Ground of Being (as modern theologians like to say). If it explicitly isn’t called godly, it’s only because the divine has suffered a significant reduction in brand-value in recent centuries. That all sounds logically fine, if more than a tad baroque. But the reason it all sounds fine is the same reason that Aquinas sounds fine to the Pope. Once ontology and epistemology are conflated, that is, when that which is is presumed to confirm that which we know, we have entered the realm of religion. At that point, we simply assume a cosmological guarantor in what we take as revelation. Revelation is its own assurance; it proves itself. And at that point Aquinas is about as credible as Carroll * The most important Christian theologian of the 20th century, Karl Barth, devoted himself almost exclusively to this issue. The intellectual machinations he had to employ in order to establish the intelligibility of God are really important for scientists like Carroll to consider before casually presuming an even more diffuse source of such an attribute. Postscript 16Sep19. Another view: https://www.sciencefocus.com/science/...

4out of 5Peter (Pete) Mcloughlin–This book puts up an intellectual defense of the many-worlds interpretation of Quantum Mechanics first proposed by Hugh Everett III in 1957. I was sold on this interpretation a long time ago but I like the way Carroll presents his case. The Everettian view is the most conservative metaphysical view of quantum mechanics with the fewest ad hoc additions to squeeze the theory into our common-sense notions. Because of this, the conclusions of this conservative framework are the most counterintuitive This book puts up an intellectual defense of the many-worlds interpretation of Quantum Mechanics first proposed by Hugh Everett III in 1957. I was sold on this interpretation a long time ago but I like the way Carroll presents his case. The Everettian view is the most conservative metaphysical view of quantum mechanics with the fewest ad hoc additions to squeeze the theory into our common-sense notions. Because of this, the conclusions of this conservative framework are the most counterintuitive shocking commonsense so radically that many will balk at it. The many-worlds interpretation leads to the weird idea that every possible measurement of with a probability is realized in myriad branching realities when a quantum measurement takes place and all the time in the universe. Carroll shows that the simplest assumption set leads to this conclusion and one may be tempted to dismiss this as repugnant but that is where the simplest understanding of QM leads. Maybe it is a reductio absurdum and a bridge too far but commonsense has taken a beating in 20th and 21st-century physics and mathematics maybe this time we might not want to listen to our gut. Carroll also talks about a newer approach to quantum gravity. In the past, we worked with relativity and tried to quantize it. Carroll wants to start with quantum mechanics and quantum entanglement and try to get relativistic spacetime to fall out of it. Approaching Quantum gravity from the quantum end rather than the gravity end. I always enjoyed Carroll's popularizations and this one works for me as well as his others. Here is the author speaking for himself. https://www.youtube.com/watch?v=jHLfM... He boils Many Worlds into the simplest terms in this lecture in Pennsylvania. https://www.youtube.com/watch?v=MScOp...

5out of 5Peter Tillman–Rather than the confusing publisher's blurb, I recommend starting with the author's essay about his book: https://www.sciencefocus.com/science/... I struggled with Carroll's book, which doesn't make a whole lot of sense to this (physics-impaired) old geologist. He writes well, and the history of the hostile reception to new research on the roots of quantum theory is deeply disturbing. Make no mistake, no one doubts that quantum mechanics works. And is deeply weird. Feynman once said something like Rather than the confusing publisher's blurb, I recommend starting with the author's essay about his book: https://www.sciencefocus.com/science/... I struggled with Carroll's book, which doesn't make a whole lot of sense to this (physics-impaired) old geologist. He writes well, and the history of the hostile reception to new research on the roots of quantum theory is deeply disturbing. Make no mistake, no one doubts that quantum mechanics works. And is deeply weird. Feynman once said something like "I think I can safely say that nobody understands quantum mechanics." Still true, sfaict. Carroll prefers Many-Worlds as the best theoretical basis for QM. And he is a physics prof at Cal Tech, Feynman's old home base. But gosh: one wave function for the entire universe? And a new "world" is created every time there's a subatomic interaction? The number of new "worlds" created would far exceed the number of atoms in the universe.... And no way to experimentally test any of this? Good grief. Angels dancing on the heads of pins? Well. Since the book is already overdue, I think I'll call it good and go on to something else. I read about half of the book, and kept getting lost, rereading sections and trying again. My usual problem with trying to understand theoretical physics. 2.5 stars for what I read, rounded up for the good writing, interesting science history and provocative philosophizing. Do note that I'm not qualified to judge the physics -- but I do have a finely-tuned BS detector, which kept going off in this book. Here's Manuel Antão's fine review, pointing out these problems (and many more!), which is when I realized I should give up: https://manuelaantao.blogspot.com/201... He is better qualified than me to judge the physics. Well-read guy, too. Nature's review: https://www.nature.com/articles/d4158... Sample: Six decades on, the theory is one of the most bizarre yet fully logical ideas in human history, growing directly out of the fundamental principles of quantum mechanics without introducing extraneous elements. It has become a staple of popular culture, although the plots of the many films and television series inspired by it invariably flout the theory by relying on contact between the parallel worlds, as in the 2011 movie Another Earth. In Something Deeply Hidden, Carroll cogently explains the many-worlds theory and its post-Everett evolution, and why our world nevertheless looks the way it does. Largely because of its purely logical character, Carroll calls Everett’s brainchild “the best view of reality we have”. . . . Carroll argues that the many-worlds theory is the most straightforward approach to understanding quantum mechanics. It accepts the reality of the wave function. In fact, it says that there is one wave function, and only one, for the entire Universe. Further, it states that when an event happens in our world, the other possibilities contained in the wave function do not go away. Instead, new worlds are created, in which each possibility is a reality. The theory’s sheer simplicity and logic within the conceptual framework of quantum mechanics inspire Carroll to call it the “courageous” approach. Don’t worry about those extra worlds, he asserts — we can’t see them, and if the many-worlds theory is true, we won’t notice the difference. The many other worlds are parallel to our own, but so hidden from it that they “might as well be populated by ghosts”. . . . Something Deeply Hidden is aimed at non-scientists, with a sidelong glance at physicists still quarrelling over the meaning of quantum mechanics. Carroll brings the reader up to speed on the development of quantum physics from Max Planck to the present, and explains why it is so difficult to interpret, before expounding the many-worlds theory. Dead centre in the book is a “Socratic dialogue” about the theory’s implications. This interlude, between a philosophically sensitive physicist and a scientifically alert philosopher, is designed to sweep away intuitive reservations that non-scientists might have. . . .

4out of 5Eric–TL;DR Sean Carroll’s Something Deeply Hidden tackles the difficult many worlds theories of quantum mechanics. It’s weird; it’s funny; it’s deeply philosophical and worth reading. Highly recommended. Disclaimer: I received a free copy of this as an ebook from the publisher in exchange for an honest review. Find this and other reviews at my website Primmlife.com Review According to quantum mechanics, it’s entirely possible that there are multiple copies of you reading multiple copies of this TL;DR Sean Carroll’s Something Deeply Hidden tackles the difficult many worlds theories of quantum mechanics. It’s weird; it’s funny; it’s deeply philosophical and worth reading. Highly recommended. Disclaimer: I received a free copy of this as an ebook from the publisher in exchange for an honest review. Find this and other reviews at my website Primmlife.com Review According to quantum mechanics, it’s entirely possible that there are multiple copies of you reading multiple copies of this review. The many worlds approach to quantum mechanics says that the world decoheres into various branches. Branching reality is a difficult subject, but it is one that makes sense when interpreting exactly what quantum physics represent. Physicist, author, and podcaster, Sean Carroll attempts to explain these subtle and difficult philosophical questions in his latest book, Something Deeply Hidden, from Dutton. This is a book of big ideas explained to an audience of anyone. It doesn’t spoon feed the reader answers, but nor does it put concepts too far out of reach. For anyone interested in quantum mechanics, this is a must read. Review: Something Deeply Hidden I’m a fan of Sean Carroll. I like his podcasts and his appearances on Joe Rogan’s podcast. He’s entertaining while still conveying complex knowledge. So, this review is biased from the start. I don’t understand quantum mechanics, and for most of my studies, I’ve been told I don’t need to understand it because the math works. It’s an odd way to approach physics. To quote Richard Feynman, “…I think I can safely say that nobody understands quantum mechanics.” Certain physicists like Sean Carroll have decided to change that. Something Deeply Hidden largely succeeds for our current best understanding. It doesn’t rely on the fact that the math works out; it attempts to explain reality, which was physics original purpose. The book reads well; it’s not full of equations, though there are some. Dr. Carroll’s style of explanation is clear enough without equations. He’s funny and fills the book with good examples and easy to follow illustrations. Dr. Carroll lays down a foundation of quantum mechanics history before moving onto cutting edge physics and then to the weird stuff. Something Deeply Hidden is an intensely philosophical book that I’m still thinking about. The book focuses on Schrödinger’s equation and the Everettian interpretation, which is also known as the many worlds interpretation. In short, Schrödinger’s equation describes the wave function of the universe, and there is no collapsing of the equation. Instead of superpositions collapsing into a measured reality, the measurement causes a branching of the universe. Let me repeat that a branching of the universe. One where outcome A happens and another where outcome B happens. And guess what, we branch when the universe does as well. The Many Worlds Interpretation Decoherence, branching, and superposition are difficult concepts to understand. Honestly, I’m not sure I grasp it fully. Dr. Carroll does a good job explaining it in a way that I could start to understand. (This is a book that I will have to reread.) The idea that the universe branches has long been a popular idea in science fiction (see the TV show Sliders). But it’s much more complex than simply a person’s decision causes the universe to split. In fact, Dr. Carroll deliberately debunks this idea. The universe branches, but an individual’s decision doesn’t cause the branching. Dr. Carroll explains the many worlds interpretation in plain terms that at the same time make you scratch your head. In Chapter Seven, Dr. Carroll writes a short story that’s a dialogue between father and daughter physicists. In a way, it reminded me of What We Talk About When We Talk About Love by Raymond Carver. This chapter was unexpected yet effective in conveying difficult topics around probability. It was an odd chapter in a physics nonfiction book, but it helped convey the information. Something Deeply Hidden is well written. Part Three Something Deeply Hidden is organized into three parts with a prologue, epilogue, and appendix. I kept up easily with part one; part two stretched the limits of my intellect; and part three simultaneously blew my mind and broke my brain. I don’t think I can adequately review this section without reading it again. And I will definitely read it again. In my review copy, one of the chapters in part three is titled, “Breathing in Empty Space.” A chapter title like that deserves re-reading. Multiple Me's One consequence of branching is that when the universe decoheres and branches, so does the person. In other words, there are many copies of each of us on various branches out in the multiverse. Maybe. Dr. Carroll treats this as no big deal, and really after thinking about it for a while, it isn’t. Since we can’t interact with these other branches, contemplating the other me’s that exist is much the same as contemplating how many angels dance on the head of a pin. But I never did shake the weirdness of me branching with the universe. This branching has direct consequences to conservation of energy and the concept of entropy. I’m not entirely convinced of the answer provided, but it’s an interesting answer. This is one of the rare moments in the book where I don’t think the answer conveys a physical meaning. Or, at the very least, one that I can understand. If the universe branches enough, does that mean it’s possible to lower the energy of the many worlds to almost zero? If so, what happens to all the me’s in those branches? Competing Theories Dr. Carroll states plainly that he subscribes to Hugh Everett III’s interpretation of quantum mechanics. But he does devote time to competing theories and gives them fair treatment. Then, he explains why he thinks the alternate interpretations are wrong but in respectful manner. Maybe I’ve been reading too much politics lately, but this was really refreshing. It’s important to see a thoughtful summary of and argument against a competing philosophy without a need to ‘win’ – whatever that means in physics circles. This section also serves as a starter for investigating more about the interpretation of quantum mechanics. In this section, I learned the phrase quantum Bayesianism, which is just fun to say. Dr. Carroll’s description is quite interesting, and I might look into the topic in the future. Conclusion Sean Carroll’s Something Deeply Hidden broke my brain in the best way possible. This insightful, philosophical book explains difficult, complex concepts in understandable language. Based on the arguments, I’m now an Everettian convert. Somewhere out in the multiverse, there’s an Eric writing a better review of this book. In a different branch, there’s an Eric who didn’t get to read this book, and he’s all the poorer for it. 9 out of 10!

5out of 5Deniz Yuret–I eagerly waited for this book for a year. Having read Deutsch, Albert, Aaronson, Becker, I had very high expectations about the insights Carroll would add. The book fell short on introducing and justifying quantum concepts. Entangled pairs are presented without the obvious comparison to correlated classical objects (like two pieces of a torn card, as soon as I look at mine the one you have is determined faster than the speed of light ;) Bell’s theorem is not explained at all, when there are so I eagerly waited for this book for a year. Having read Deutsch, Albert, Aaronson, Becker, I had very high expectations about the insights Carroll would add. The book fell short on introducing and justifying quantum concepts. Entangled pairs are presented without the obvious comparison to correlated classical objects (like two pieces of a torn card, as soon as I look at mine the one you have is determined faster than the speed of light ;) Bell’s theorem is not explained at all, when there are so many simple examples. Basically if you didn’t know why the world has to be different from our classical conceptions, you still wouldn’t know after reading this book. Becker’s book does a better job at this. I also have a beef with some of the terminology that is not specific to this book. I don’t know why physicists still use the word “measurement” when it has confused so many generations - wouldn’t “interaction” work just as well without invoking conscious observers in lab coats? Similarly “wave function” misleads when you are not talking about the position of a single particle: in what way does the spin of an electron wave? How about just calling it a “state vector”? Aaron’s book does a better job at this. I would recommend this book for the speculative but provocative ideas about tying quantum theory with spacetime. It is a pity, Sean is usually such a great explainer. I suspect some draft of the book had parts that gave good intro insights but were somehow voted off. I wish I had access to earlier drafts.

5out of 5Paperclippe–This was definitely one of Carroll's more technical works. While his language as always as simple as it can be for the layman, there's only a certainly level of simplicity to which quantum theory can be broken down. That said, Carroll does good work interspersing all of the necessary technicalities with a more story-form description of the ideas behind quantum gravity, Many Worlds, and quantum physics, so if only half of the book sticks with you, you're still bound to learn something. Carroll's This was definitely one of Carroll's more technical works. While his language as always as simple as it can be for the layman, there's only a certainly level of simplicity to which quantum theory can be broken down. That said, Carroll does good work interspersing all of the necessary technicalities with a more story-form description of the ideas behind quantum gravity, Many Worlds, and quantum physics, so if only half of the book sticks with you, you're still bound to learn something. Carroll's trademark humor, too, shines through in a lot of places, and serves as a good anchor point to bring even the most baffled reader back from the brink. Definitely not for beginners to the ideas behind quantum theory, but an excellent book to build on what a fan of popsci might already know.

4out of 5Anton–First third is fantastic! The latter bits get quite meandering... It seems I am always very curious about the scope of Carroll’s books but never quite getting what I expect. Nowhere near as engrossing as Rovelli’s stuff. So... a very reserved recommendation.

4out of 5Gary Beauregard Bottomley–Do not multiple entities unnecessarily. The Copenhagen Interpretation necessitates the additional entity of an observer or a detection device, take away that added entity you will have the world described by the wave function and that’s how Hugh Everett III (remember that name, if you don’t already know who he is) gets at in his MWI (multi-world-interpretation). Is gravity real? Or is it just a label we put on the mathematics which aids us in understanding the world. Do we appeal to the Do not multiple entities unnecessarily. The Copenhagen Interpretation necessitates the additional entity of an observer or a detection device, take away that added entity you will have the world described by the wave function and that’s how Hugh Everett III (remember that name, if you don’t already know who he is) gets at in his MWI (multi-world-interpretation). Is gravity real? Or is it just a label we put on the mathematics which aids us in understanding the world. Do we appeal to the epistemological or the ontological in scientific exploration? Hume believes we only see the effect and we conjecture the cause through habit, tradition and expectations. The math/physics explains the phenomena but is what we know about what we see the thing-behind-the-thing itself. Most people lean towards thinking that gravity is a real thing, a-thing-in-itself. That logic and Occam’s razor (essentially, the first sentence in this review) will make the Schrodinger’s Wave function real and give us Hugh Everett III’s MWI. Not as far out from standard physics as some who have not read this book will naively believe. I would rank this book as an outstanding pop-science book. I would highly recommend it to all. I usually get bored by most of the pop-science crap I read (Sturge’s Law: 90% of everything is crap). This book fired on all eight cylinders and didn’t miss a beat while explaining complicated physics with a focus toward understanding MWI and the epistemological and ontological (his words) foundations of physics including whether space and time are fundamental to the universe or are emergent properties. [A bracketed aside: the themes within in this book surprisingly correlate highly with the book German Idealism by Beiser].

4out of 5Kyle–I think this is probably closer to a 4.5 star than 5 star rating for me, but I only have integer values. I have a lot of things I don't agree with Professor Carroll, but he has great explanations and makes it clear where he is speculating. That gets it the final star for me. Carroll is a very clear and lucid writer, and that is very helpful for a book like this. He goes over quantum mechanics, quantum interpretations, and a possible way to look at quantum gravity. This is all written excellently I think this is probably closer to a 4.5 star than 5 star rating for me, but I only have integer values. I have a lot of things I don't agree with Professor Carroll, but he has great explanations and makes it clear where he is speculating. That gets it the final star for me. Carroll is a very clear and lucid writer, and that is very helpful for a book like this. He goes over quantum mechanics, quantum interpretations, and a possible way to look at quantum gravity. This is all written excellently and engagingly. Carroll is a proponent of the Many Worlds Interpretation (MWI) or Everett interpretation. This interpretation says that we have the wavefunction, and when we apply the Schrödinger equation and get superposition, that is what happens. Our entire universe is a superposition of eigenstates of the Schrödinger equation. There are many branches that effectively never affect each other, and so we call each branch a separate universe. Hence many worlds/universes. I tend not to find this to be as convincing. I like Chad Orzel's Metaphorical Worlds Interpretation better, because this seems like a better way of understanding what is going on. MWI advocates tend to claim that this version is the simplest, because all we have is the Schrödinger equation. I think that they add extra suppositions about how to understand measurements as axioms. When we do measurements we don't see superpositions (whatever that would even mean), we see something happen with 100% probability. As Sabine Hossenfelder (and Peter Woit, and others) point out, the MWI proponent doesn't really solve this problem by saying on some branch the detector will see the measurement with 100%. But then, you need to update your calculations using the detector on the branch where the process did happen, then do calculations from there. Notice that figuring out which branch you are on does not involve solving the Schrödinger equation. There are reasonable ways of doing calculations from looking at branches, but this requires rules. Thus, saying that MWI only uses the Schrödinger equation seems incorrect to me. You still have to figure out which branch you are on in order to make predictions. The main point that everything is quantum (so we should try and derive classical results from quantum phenomena) seems sound though. I also am not so sure on the quantum foundations investigating people being looked down on angle. I'd like more historical proof than the anecdotes given in the book. Also, Peter Woit and others have shown that the quote about quantum foundations papers being blanket rejected by Physical Review is missing context that I think is crucial. It was not just about quantum foundations papers, but any papers and it is not clear it was implemented because of quantum foundations. (See the blog Not Even Wrong on "Regarding Papers about Fundamental Theories") My other critiques are that "taking equations seriously" seems like a slogan rather than a good thing. Maybe it is because I was trained as a computational physicist, but trusting equations often leads to numerical gobbledygook for solutions. You trust number crunching like that at your own risk, as you need initial conditions and all sorts of things to be right to ensure your equations go to the correct solution in the regime you are interested in. Equations seem to be approximations of reality no matter how deep we go, and so I remain distrustful of those who say we should trust the Schrödinger equation because it is "exact" (how would we know? clearly it is only an approximation, anyway, because of gravity...). Besides, if we think there is more than the Schrödinger equation to nature, then why would we take it as the equation to base all our physical intuitions off of? While I think looking at quantum foundations is a good thing, I don't think differing quantum interpretations seem all that much of a problem yet. When they yield different experimental results that can be tested, then I think it's a problem. Also, despite people loving to say that Newtonian physics doesn't have interpretational issues, I think there are big interpretational issues, it's just they have been superseded because we have better theories (special and general relativity and quantum physics) . Newtonian physics allows infinite velocities (thus you must know the state of everything in the universe to be sure that you can figure out what happens in the future or the past. Also, I mean that objects can be accelerated to arbitrary velocities in finite time), it can have solutions that are not unique (see Norton's dome), and as touched on in the book, force at a distance is a problem for gravitation in Newtonian physics if you believe in a certain type of locality. The quantum suicide discussion and problems of identity struck me as okay but not totally convincing. If you are okay with a being with essentially the same history and memories as you continuing to be conscious (whatever that means to you), then that will occur in the MWI and you can exploit that. If you are the type that is okay with a being with your memories ("you" in probably all relevant senses), then the suicide experiment will work, but I see no reason why you would want to do it. You'd still hurt others that care about you and if you don't care about that, then you can just wait it out as you get old. You should expect to be the oldest person because the only beings with your memories that will remain will be those who grow to be the oldest person in their universe. A side question I have is why people don't worry about merging of branches. The book answers my question in one way, using an entropy metaphor (entropy must increase). But that is for the whole universe, and I can believe that about the MWI. But aren't there local cases where similar branches "recohere"? Just as I can have a subsystem have its entropy decrease, but the overall entropy of the system increases? Despite my critiques, I would definitely recommend the book. It comes from an MWI advocate angle, but Carroll is usually careful about what he says and is simply an advocate and doesn't straw man his opponents. He even talks about superdeterminism without calling it a "cosmic conspiracy" which wins points from me (I'm not really an advocate of superdeterminism, but I think it gets an undeserved bad reputation). Plus the quantum gravity part is excellent. It points out it is speculative when it needs to, and explains fields and particles very well. Carroll answers many questions I have had about the MWI, and he certainly outlines why it's a reasonable position to take. I just don't think it's clear that it is the best interpretation. If you'd like to know more about MWI and quantum mechanics, this book is an excellent addition.

4out of 5Roy–The problems of quantum worlds and the emergence of spacetime are essentially mathematical. If you don't look at the math, you don't have a problem. So although I have loved several of Carroll's other books, I think this one is a failure. He spends the whole book trying to describe mathematical problems, and their (possible) mathematical solutions, without any mathematics. Which can't be done, so the reader learns very little about either the problems or the solutions. Read "The Big Picture," The problems of quantum worlds and the emergence of spacetime are essentially mathematical. If you don't look at the math, you don't have a problem. So although I have loved several of Carroll's other books, I think this one is a failure. He spends the whole book trying to describe mathematical problems, and their (possible) mathematical solutions, without any mathematics. Which can't be done, so the reader learns very little about either the problems or the solutions. Read "The Big Picture," and skip this one.

5out of 5Thomas Ray–Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, Sean Carroll, 2019, 347pp, ISBN 9781524743017 Dewey 530.12 The author is physicist Sean M. Carroll, b. 1966, https://en.m.wikipedia.org/wiki/Sean_... NOT biologist Sean B. Carroll, b. 1960, https://en.m.wikipedia.org/wiki/Sean_... "I think I can safely say that nobody understands quantum mechanics." --Richard P. Feynman. p. 2. Epistemology is the study of knowledge; ontology is the study of what is real. p. 30. "an initially Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, Sean Carroll, 2019, 347pp, ISBN 9781524743017 Dewey 530.12 The author is physicist Sean M. Carroll, b. 1966, https://en.m.wikipedia.org/wiki/Sean_... NOT biologist Sean B. Carroll, b. 1960, https://en.m.wikipedia.org/wiki/Sean_... "I think I can safely say that nobody understands quantum mechanics." --Richard P. Feynman. p. 2. Epistemology is the study of knowledge; ontology is the study of what is real. p. 30. "an initially unentangled situation--the electron is in a superposition of various possible locations, and you haven't looked at the electron yet--evolves smoothly into an entangled one--a superposition of each location the electron could have been observed, and you having seen the electron in just that location." p. 38. No. There is /never/ a moment when the /observer/ can be described by such a superposition. The instant of observation is an /interaction/ occurring with a particular observable outcome. The cat is /never/ half dead. /Before/ the interaction there's a superposition of states. The interaction occurs in a particular observable outcome. The photon goes through both slits, but hits a /particular/ point on the screen behind the slits. The interaction is between quanta of matter and energy. A consciousness isn't required for a quantum-mechanical observation, any more than for a tree falling in the forest to make a sound. "afterward, you and the system you interacted with are in a superposition, in each part of which you have seen the electron in a slightly different location." p. 38. Carroll wants us to believe that nothing ever happens: there's only ever potential events, no possibilities ever disappearing; no possibilities ever actually happening. He's saying the photon never encounters the silver nitrate, never darkens the photographic plate at a particular spot. He's sticking straw in his hair and running into the woods. "the observer evolves into an entangled superposition of the different possible measurement outcomes." p. 39. No. just one of them. Carroll goes on to posit many worlds, having denied the existence of this one.

4out of 5Gendou–This book addresses the incompleteness and alternative interpretations of quantum mechanics. Carroll is an unapologetic MWI proponent, but he doesn't let this ruin his impartial delivery of the science. For the most part. Unfortunately, he does oversell the strengths of MWI, claiming it solves problems. It may very well be the framework under which those solutions are someday found, but he significantly misrepresents "promises, in my opinion, to solve" as "solves".

4out of 5Realms & Robots–Something Deeply Hidden is that rare science nonfiction book that’s both easy to understand and incredibly complex. This is quantum mechanics like you’ve never seen, laid out in an understandable fashion. With a combination of history, basic explanations, and visual aids that simplify its complexities, Carroll presents an essential guide to this mysterious field. I’ll admit I was nervous as I started reading the book. At first glance, the subject matter seems too dense for a basic human without Something Deeply Hidden is that rare science nonfiction book that’s both easy to understand and incredibly complex. This is quantum mechanics like you’ve never seen, laid out in an understandable fashion. With a combination of history, basic explanations, and visual aids that simplify its complexities, Carroll presents an essential guide to this mysterious field. I’ll admit I was nervous as I started reading the book. At first glance, the subject matter seems too dense for a basic human without any scientific background. As you read, it slowly starts to make sense until you’re nodding along at things you never thought you’d learn. One of the most fascinating aspects of the book is the history behind quantum physics and how it came to be. It’s nearly impossible to imagine a group of people coming up with these kinds of theories but here we are. Above all, you’ll learn so many things about the mysteries of the universe. I continue to have trouble wrapping my head around this fascinating field of science but I feel a big step closer after reading this thoughtfully written guide to everything quantum mechanics. NOTE: I was provided a free copy of this book via NetGalley in exchange for my honest, unbiased review.

4out of 5Todd–This one fell a bit short of getting the balance between story and science exactly right for me. Most of this imbalance is a result of Carroll being caught in some ineffective middle ground between not enough technical information at times and jumping ahead to too much detail at others. The decisions about when to provide visual explanations or equations were not effectively calibrated in my mind. The book is not without its charms. Carroll's earnestness in wanting to move the reader through the This one fell a bit short of getting the balance between story and science exactly right for me. Most of this imbalance is a result of Carroll being caught in some ineffective middle ground between not enough technical information at times and jumping ahead to too much detail at others. The decisions about when to provide visual explanations or equations were not effectively calibrated in my mind. The book is not without its charms. Carroll's earnestness in wanting to move the reader through the complexities of the many worlds interpretation of quantum is evident throughout and anyone familiar with his podcast will recognize Carroll's friendly tone. Ultimately, Carroll is the professor you like a lot but "Something Deeply Hidden" is the course that never quite clicks into place.

4out of 5Galen Weitkamp–Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime By Sean Carroll. Review by Galen Weitkamp. Sean Carroll is a Research Professor of Physics at Caltech and the Santa Fe Institute. I remember waiting, back in 2004, for the publication of his textbook (Spacetime and Geometry) on general relativity. In spite of the mathematical intricacies of tensor calculus, Professor Carroll’s exposition is clear and intuitive. It has become my standard reference on matters of gravity. Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime By Sean Carroll. Review by Galen Weitkamp. Sean Carroll is a Research Professor of Physics at Caltech and the Santa Fe Institute. I remember waiting, back in 2004, for the publication of his textbook (Spacetime and Geometry) on general relativity. In spite of the mathematical intricacies of tensor calculus, Professor Carroll’s exposition is clear and intuitive. It has become my standard reference on matters of gravity. Something Deeply Hidden has no mathematics. It is both a book for a more general audience, and also a serious book addressed to current researchers. Regardless of the audience to which you belong, if this book is on your reading list, then you are likely already aware that quantum theory is associated with a number of perplexing interpretative issues, paradoxes and problems: the measurement problem, wave function collapse, cats in superposition, non-locality, hidden variables and multiple worlds to list a few. These are often considered annoying philosophical problems which have little bearing on the practical applications and the everyday work of research in physics. In this book, Dr. Carroll suggests this is not the case and urges his colleagues to pay closer attention to the foundational issues. Most physicists will agree that the spin of an electron may simultaneously be both clockwise and counterclockwise around a given axis. Until the spin around that axis is measured the spin of such an electron is neither one nor the other but a weighted superposition of both. On the scale of ordinary objects such descriptions make little sense. We’ve never seen a spinning figure skater in that kind of quantum superposition. Yet one can imagine a physicist figure skater who measures the spin of just such an electron to decide whether to twirl clockwise or counterclockwise. If the measurement and pirouette are executed in an isolated setting did the measurement collapse the electron’s superposition or is the figure skater too (like Schrodinger’s Cat) now in quantum superposition? Is she spinning clockwise and counterclockwise until her colleague walks in observes the direction of her spin? Does the colleague collapse the superposition or is the colleague now in quantum superposition too; i.e., both in the state of seeing his colleague twirl in one direction and the state of seeing her spin in the other direction? It’s easy to see that wave function collapse is a problematic concept. Is it even a thing? Dr. Carroll would say no, it isn’t a thing; that wave collapse, hidden variables and epistemic approaches to resolving the measurement problem offend Occam’s maxim of minimality either by making additional assumptions (in the case of wave function collapse) or by making additional ontological commitments (in the case of hidden variables). For Dr. Carroll, the only two things quantum theory requires is a state space (of wave functions) and a deterministic transition function (the Schrodinger equation) that determines how a given state evolves through time. All else is at best superfluous, at worst problematic or inconsistent. If this is the case, then measurements don’t collapse wave functions. Observing that your colleague is spinning clockwise doesn’t change the fact that her spin-state is actually a weighted superposition of both clockwise and counterclockwise. But how does one square this description of her spin with your observation that she’s spinning clockwise? The answer is that you’re in superposition too, and the two components of your state are demonstrably incommunicado. Technically you, your colleague, the skating rink , etc., have become entangled with the electron’s spin and the wave function has decohered. Entanglement and decoherence are not extra-quantum concepts added to the theory to explain the measurement problem but are phenomena that are derivable from the minimal apparatus consisting of the state-space and the Schrodinger equation. So there is one component of the evolving wave-function that describes you seeing your colleague twirling clockwise and another component that describes you observing the opposite. This is what is known as the many-worlds interpretation of quantum theory (first proposed by Hugh Everett in 1955). What I like (what Dr. Carroll likes) about the many-worlds scenario is its logical efficacy. It uses a minimal number of assumptions. It may seem, however, that it has a maximal ontology (i.e., lots and lots of branching worlds). But the existence of these worlds is not presumed. The only ontological presumption is that there is only one wave-function. It just happens to evolve deterministically and predictably via the Schrodinger equation into multiple branching components that are best interpreted, treated or thought of as distinct worlds. I must admit that it’s tempting to treat all the other components of the wave function (i.e., all the other worlds) as unimportant mathematical fictions. Just ignore them. Pretend only the observed component exists and go on from there. Dr. Carroll disagrees. We have been able to formulate field theories consistent with quantum theory by tweaking the classical formulations. However, all attempts to ‘quantize’ Einstein’s classical theory of gravity have, so far, failed to reconcile general relativity with quantum theory. Nevertheless the two theories have had some friendly encounters, Hawking’s analysis of blackhole evaporation for example. In 1995 Ted Jacobson demonstrated that Einstein’s field equations can be derived from an assumption relating quantum entanglement entropy to the areas of infinitesimal volumes. Carroll suggests we may make progress by starting with quantum theory first and deriving a theory of the gravitational field from it. Perhaps the spatial metric (which is the classical gravitational field) can be derived from the wave-function using Jacobson’s assumption that connects the entanglement entropy of the wave-function with the metric via the areas of infinitesimal volumes. Perhaps, and this is Carroll’s greater hope, the notion of space itself emerges from the wave-function of the universe when viewed on the appropriate scale. But, Carroll warns, such an approach requires researchers to take the foundations of quantum theory seriously. As always, Carroll’s writing is cheerful, lucid, insightful and captivating. Something Deeply Hidden is a worthwhile read.

5out of 5Nilesh–Something Deeply Hidden is a difficult book. I had to go through it twice back to back to understand only a small fraction of all that it tries to teach and convey. Only the first two paragraphs below are a review of the book, while the rest are my reflections on what I understood, I learned, I doubt and where I disagree. The book is not for the starters. The subject matter assumes extreme pre-knowledge of the early twentieth-century quantum mechanic ideas and evolution. For those well prepared Something Deeply Hidden is a difficult book. I had to go through it twice back to back to understand only a small fraction of all that it tries to teach and convey. Only the first two paragraphs below are a review of the book, while the rest are my reflections on what I understood, I learned, I doubt and where I disagree. The book is not for the starters. The subject matter assumes extreme pre-knowledge of the early twentieth-century quantum mechanic ideas and evolution. For those well prepared too, there is a lot in the book which could prove incomprehensible. At times, the author is deliberately vague on the concrete meaning of the Everettian interpretation, the topic at the heart of the book. At others, the discussions on highly esoteric subjects like entanglement entropy, gravity quantization, or black hole radiation are so brief that only those extremely familiar have any hope of appreciating the points made. These are the same factors that make the book a wonderful one to learn from and reflect on. There are numerous radical and intriguing points made in every section. Anything straightforward, oft-repeated or universally accepted in other popular books is almost painstakingly kept out. Even without the usual anecdotal stories of how discoveries came about or various scientists' life stories, the author can keep even the most complex subject matter quite engaging. Now on to my thoughts and importantly, disagreements based on what I understood. I am an amateur on the subject. All my knowledge is non-technical and from popular books. The arguments must be full of errors. If nothing else, the amateurish language could cause many purists to suffer heart-attacks. However, it would be an unpardonable waste if I don't put down my thoughts after reading such a gem. A. Let's start at the beginning. The world is quantum. The book makes this point thoroughly. In common parlance, at the most fundamental microscopic level, fields, particles, space, and time are all digital. Nothing is continuous or analog. Let's call these basic quantum blocks of fundamental entities generically as "quantas" for the rest of this review. B. The quantum world is also random There is a probabilistic nature to the behavior of these quantas, including fields. This, in itself, is not random as quantum mechanics does a fantastic job explaining the contours of the probable modes of most quantas. However, somewhere - or somewhen - quantas move away from a superposition of probable modes or potentialities to definite states or actualities. We have little knowledge of how a particular mode comes into existence at the elimination of all other potentialities and what drives the transformation. In simpler terms, our world is a giant, unpredictably shape-shifting wave function. The function is fairly well known/knowable when the world or its constituents are in their probabilistic modes. The reality we experience is a single manifestation at micro and macro levels. We have no methods to understand how potentialities transform into specific manifestations. C. Does everything have to have human language epistemology? Quantum Mechanics works. The world is perhaps quite sufficiently described in its mathematical equations. Yet, understanding it, or converting the mathematical equations into a human language form has been proven impossible. The book tries hard by relying on the Everettian "conversion" for a common man understanding. It imparts a lot of knowledge and fails in even more! Before we return to some of these problems, we need to ask the question not asked in the book: is quantum mechanics necessarily understandable beyond its mathematical equations? Not everything is explainable in every language. Math cannot describe human feelings through equations. We do not attempt rational, existential discussion on why certain DNA base pair combinations result in certain diseases. No philosophers have spent a long time asking why hydrogen and oxygen molecules combine to give a water molecule of characteristics we observe. Many practitioners strongly believe that quantum mechanical equations are also in similar ontological domains. In their views, one cannot do much better than observe and learn from the details. Asking why for every quantum equation evolution is not much different from asking why on every phenomenal emergence one experiences in physics, chemistry or biology. The attempted human language answers appear silly to all but most ardent fans. The skeptics also wonder whether these forced and imposed - always unproven if not unprovable - understandings have any ability to enhance the real science. The proponents strongly differ, mainly on account of the failure of the theory in explaining gravity. The proponents feel that epistemological progress would help narrow the search on how we move towards the more encompassing theories, the way early-twentieth-century scientists did. D. The collapse, the entanglement, the decoherence, and so many words! As explained above, the epistemologists are stuck on the quantas' transformation from probabilistic stages to actual states. Experiments have repeatedly thrown highly counter-intuitive or unorthodox outcomes that do not require changes in the mathematical expressions but make our human language based understandings appear like deeply flawed. The explanations of the earliest and most famous such epistemologists, widely referred as the Copenhagen interpreters, are by now comprehensively dismissed. They mumbled about the "collapse" of the "probabilistic wave function" with "observations" of a "conscious" mind into a "particular state." To a cynic, The author's favorite Many World or Everettian interpretation does little other than have a new set of words...about the "decoherence" (instead of a "collapse") of a "many world in superimpositions" (another word for probabilistic wave function) with "entanglements" with "macro environment" into "many world states" of which only one we can observe. E. What is decoherence? And what is entanglement with the macro environment? One place where many world interpretation is different is in its post-collapse outcome: the branching and the existence of many worlds after the decoherence. Let's return to this vital difference later. To eliminate the Copenhagen consciousness, Many World resorts to entanglement with the macro environment without ever defining any of the terms. Decoherence explains hardly anything more than collapse based on what I understood in the book. If any interaction with the environment causes decoherence, one or both of the following issues arise: Environment or macro environment - that causes the entanglement, which leads to the decoherence - is everywhere as such. There is no existence without environment so when do quantas actually entangle and when do they not? Why do we observe any interference in a double slit as some or the other environment element should have "entangled" the spreading wave function even in the absence of observation devices and caused the traveling quantas' wave functions to decohere the way they do with observation devices? Or why are observation devices the right enough macro "environment" with which quantas entangle and decohere but your atmosphere in the lab is not? From a decohered state, when does a single field or particle recohere and again exhibit wave-like properties? Without recoherence, everything should have decohered right at the first Big Bang moment when everything interacted with anything or everything else and it was one big, thick environment where quantas had little space to be all alone in the cohered state? F. Many Worlds: what do they do? Everettians strongly believe in branching and continuous existence of probabilistic modes post the decoherence - although no longer in superposition post the entanglement but in their splendid and parallel isolation. And they fail to see why the rest of us fail to see the futility of creations of 2^10^122 or more worlds? Our world is a state of the evolving universal wave function as it settles on a set of actualities from erstwhile probabilities. Many World either means the other potential actuality sets are in existence and evolution or simply a convenient way of thinking. The author and his Many World believing philosophers deliberately choose not to answer this simple question of belief one way or the other. If the near-infinite or infinite parallel worlds are uncountable, unobservable, and unusable, how are they any less fanciful or more useful than the Copenhageners' consciousness? One spends an enormous amount of time debating the size of the Hilbert Space rather than trying to guess the theories that might attempt to explain the collapse and the observed state after. Everettians claim that their interpretation is an austere quantum theory. It allows one to accept the quantum field equations as fully explained without the need to add any more variables. One wonders how this is different from the "shut up and calculate" practitioners when the interpretation is so much nothing more than just a set of fanciful, untestable words. Alternate theories that rely on only observations or attempt to introduce an additional probabilistic function for decoherence make far more sense. G. Entanglement entropy is not the same as ordinary entropy The author goes at great length in drawing parallels between thermodynamic entropy with a relatively new quantum concept of entanglement entropy. There is a fascinating chapter of a discussion between Alice and her Dad where the author asserts that entanglement entropy, as driven by the entanglements responsible for decoherence, was low to start with and is continuously increasing. Despite the strong arguments, this might be a promising and extremely unbaked concept. It is anything but a proven fact like the celebrated thermodynamic counterpart. Quantum theory, as we know, is yet to accommodate gravity. It is a theory of particles in a particular space rather than one of space. It treats time differently within this space. It does not have any flexibility as it stands now to equate time to space. The author does well in stretching the theory of quantum fields to speculate on how gravity could come in. But the discussion turns extremely cumbersome due to the lack of concrete evidence as the final chapters wear on. Quantum fields don't have a single wave function as a classical field. A quantum field has modes of different waves with the modes having their own probabilistic quantum wave function. They decohere randomly to assume a specific observed classical field quation with entanglements although these field entanglements/decoherence are even less specified than the much-discussed quantum particle decoherence. In quantum entropy, the author begins by describing how quantum fields are entangled with other quantum fields and defines distance (and hence space) as the level or the extent of entanglement between fields. The approach is used to conjecture how gravity could come in with this method in the same global wave function. I hope to read the author's future work, where he further develops the details of decoherence and entanglement or entanglement entropy to overcome all the doubts I have at the end of this book. A great new subject matter, but incomplete.

4out of 5Jake–Ill bite my tongue at the moment, and reserve my primary criticisms. This is practically a book arguing that the many worlds hypothesis is our best escape from the Copenhagen interpretation. It is evidently a much more robust theory than all of our present alternatives. Recommended for those thinking about 'quantum foundations'. Well complimented by Tegmark's :https://www.google.com/search?client=... and smolin's https://www.google.com/search?client=... There were some things about this book that Ill bite my tongue at the moment, and reserve my primary criticisms. This is practically a book arguing that the many worlds hypothesis is our best escape from the Copenhagen interpretation. It is evidently a much more robust theory than all of our present alternatives. Recommended for those thinking about 'quantum foundations'. Well complimented by Tegmark's :https://www.google.com/search?client=... and smolin's https://www.google.com/search?client=... There were some things about this book that bothered me, but it did what it needed to do pretty decently.

4out of 5Prejudice Neutrino–I don’t feel as if I learned any of the topics to the level of depth that I would have liked to. Many topics were discussed, but unfortunately only at a superficial level. I love the podcast, but I can’t really say I’d recommend this book to anyone interested in quantum mechanics. I wish each chapter had a summary at the end so I could at least have an idea of what was trying to be conveyed at some parts in the book and perhaps a glossary/further reading recommendations at the end.

5out of 5David W. W.–Sean Carroll's new book is probably the single best argument for the Everett understanding of quantum mechanics - the approach often called (although slightly misleadingly) the Many Worlds Interpretation. Carroll makes clear the powerful attractions of the Everett understanding, and persuasively counters the objections that are commonly raised against it. He highlights how this approach is the natural, straightforward response to the remarkable success of the quantum formalism. Despite its Sean Carroll's new book is probably the single best argument for the Everett understanding of quantum mechanics - the approach often called (although slightly misleadingly) the Many Worlds Interpretation. Carroll makes clear the powerful attractions of the Everett understanding, and persuasively counters the objections that are commonly raised against it. He highlights how this approach is the natural, straightforward response to the remarkable success of the quantum formalism. Despite its apparent profligacy of multiple worlds (multiple diverging branches of reality), it's actually a lean and austere interpretation of quantum mechanics. Unique among interpretations of quantum mechanics, it adds in *nothing* beyond the wave equation itself. Back in the 1980s I spent four years mulling the philosophical implications of quantum mechanics. Over time, against my initial inclinations (and hopes), I came to have an increasing respect for the Everett understanding - an outcome I wrote about at https://dw2blog.com/2008/11/16/schrod.... Alongside my grudging respect for that interpretation, I retained the view that it still faced many hard questions. However, Carroll's book has convinced me that these questions aren't particularly hard. In other words, the book has strengthened my conviction that these "Many Worlds" do come into being whenever quantum transactions are macroscopically magnified. In terms of the history of the topics covered, and the pros and cons of the different interpretations reviewed, I see Carroll as being overwhelmingly correct. I particularly liked his demolition of the idea that there's such a thing as a coherent "Copenhagen interpretation" of quantum mechanics. The only area where I wanted to see the argument extended was that more could have been said about how all the non-Everett interpretations of quantum mechanics have to accept one or other kind of radical non-locality (despite the attempts of various writers to "have their cake and eat it"). The final third of the book may be the most important. It reviews the possibility for progress in an area of physics that has long experienced troubles: quantum gravity. Carroll argues that the best hopes for us obtaining a correct quantum theory of gravity (that works at all energy scales) is to take quantum mechanics itself more seriously. This part of the book is more speculative than the earlier parts, but it has raised my interest in delving more into these topics. This final part of the book also underlines the difficulties faced by the non-Everett interpretations of quantum mechanics in dealing, not with particles, but with the relativistic fields which modern physics views as being more fundamental than particles. This part also reviews how space and time should emerge from the theory of quantum gravity, rather than being presupposed as the canvas upon which the theory would operate. Some of the potential implications for black holes (and maybe even the Big Bang) are mind-stretching. It's a shocking possibility that each of us exist alongside with numerous different versions of ourselves, in the overall multiverse - versions that have increasingly divergent experiences. This possibility is one of the most important insights to have arisen from humanity's millennia-long exploration into science. It's an insight that takes time to sink in. It's a good question how much this insight should change our day-to-day behaviour. Carroll has an answer to that too: not as much as we might first think. Personally I find it a humbling realisation. For another book that addresses some of the same topics - inside an even larger set of profound ideas - I recommend "Our mathematical universe" by Max Tegmark https://dw2blog.com/2014/01/30/a-bril....

5out of 5Jose Rodriguez–I think that Sean Carroll is one of the best “popularizers” of physics I have ever encountered. One could argue that such popularizations are like cartoon versions of literature classics, and in some ways they are. But if done well, they can bring an understanding of the “forest” that could be lost when engaged in the particularities of the mathematical “trees.” On this book, Carroll tackles the foundations of quantum mechanics, a problem from the very beginning of the field and which many I think that Sean Carroll is one of the best “popularizers” of physics I have ever encountered. One could argue that such popularizations are like cartoon versions of literature classics, and in some ways they are. But if done well, they can bring an understanding of the “forest” that could be lost when engaged in the particularities of the mathematical “trees.” On this book, Carroll tackles the foundations of quantum mechanics, a problem from the very beginning of the field and which many physicists feel is not relevant to the calculations and experiments they carry out. Who cares if we can’t figure out what is the wave function collapse that happens after a measurement, as proposed by the so-called “Copenhagen interpretation”, if we have done extremely well without worrying about it? But some people would find this attitude deeply unsatisfactory, since it results in no understanding of the nature of reality, and the judgement by Richard Feynman that nobody really understands quantum mechanics. One of the most popular solutions has been the Many Worlds interpretation that Hugh Everett III proposed in the 1950’s, which was ignored or ridiculed by many physicists, resulting in Everett working in the defense industry after his PhD, and maybe contributing to his early death due to alcohol and other unhealthy habits, But recently there has been a resurgence of interest in these ideas, partly due to their relevance to a formulation of quantum gravity. And Sean Johnson comes out in this book as a major proponent of the Everett interpretation. It is a very difficult endeavor, and I’m not sure that he was completely successful in achieving the goal. And because he only uses words, I found that some of the concepts remained unclear, which made me wish for a lot of equations on the pages. Things get worse when the discussion turns into philosophical issues of ontology and epistemology, in which I found it very difficult to “disentangle” my thinking. The rock bottom is the chapter that tackles whether this view impacts our thinking about morality (it doesn’t). I wish that he would have stayed the course along the lines of his statement at different points that we are not really required to think about many worlds in the Everett picture. The main point is that we accept a universal wave function which evolves in accordance with Schroedinger’s equation, and from which descriptions of measurements, particles, and even space emerge (emergence is discussed in detail in his previous book “The Big Picture”). What convinced me to give this book five stars is its provocative discussion about new ways of thinking about reality. The last chapters discussing space, time, black holes, holographic picture, etc. of course do not present new solutions (they do not exist at this point), but are very successful in raising questions and introducing concepts that continue to make physics such an exciting field, at least for me. Now I should look for the equations, except that I don’t think I have the time nor the energy to do so.

5out of 5Alex Zakharov–A good non-technical introduction to quantum mechanics with heavy emphasis on Everettian interpretation. Carroll gives a brief history of the field including the 1927 Solvay conference, Einstein-Bohr debates, and personalities of the key players (Pauli, Schrodinger, Heisenberg, de Broglie). He then quickly goes over the basics that you need to follow the rest of the book – superposition, entanglement, Born rule, Wave Function, Hamiltonian, decoherence. Carroll quickly moves on to the measurement A good non-technical introduction to quantum mechanics with heavy emphasis on Everettian interpretation. Carroll gives a brief history of the field including the 1927 Solvay conference, Einstein-Bohr debates, and personalities of the key players (Pauli, Schrodinger, Heisenberg, de Broglie). He then quickly goes over the basics that you need to follow the rest of the book – superposition, entanglement, Born rule, Wave Function, Hamiltonian, decoherence. Carroll quickly moves on to the measurement problem and describes both its well-known interpretations (“Copenhagen”, “Pilot Wave/Hidden Variables”, “Many Worlds”) and the relatively more recent Dynamical collapse and QBism theories. You get a good intuition for the theories themselves and pros/cons of each. As expected, he spends a fair amount of time on Everettian interpretation, of which he is a well-known proponent. Here we get a healthy dose of branching, decoherence, and wave function evolution. Carroll makes a case that technically the approach is pure (“austere” is his go-to word), and acknowledges that the amount of cognitive dissonance from reconciling this purity with observable reality is uncanny. He suggests a few cognitive tricks to reduce that dissonance, and some of them almost help. In the last part of the book we revisit quantum field theory (which is very nicely covered in his previous books btw), and start talking about theoretical physicists’ holy grail - quantum gravity. Carroll briefly explains how other fundamental forces were quantized, and why quantizing gravity is damn hard. We get a mention of String theory and a classy dis of AdS/CFT correspondence (field’s only lamp post in a drunken man’s parking garage). But there is hope still. Carroll is part of the wave of physicists who are becoming increasingly convinced that quantizing gravity will not happen. Instead we should derive spacetime itself from quantum mechanics, which would give us gravity on a quantum mechanical platter. Susskind & Maldacena’s 2013 “ER=EPR” paper is an obvious step in that direction, and Carroll’s thinking runs along similar lines. Space and locality is sorta-kinda like entanglement, and time is sorta-kinda like growth of the entanglement. (In Susskind’s case time is sorta-kinda like growth of quantum complexity). The first half of the book gets 5 stars – informative, mostly clear, fun to read. But, as we get into the second half, explanations start running away a bit. Carroll brings some back, but many escape. The fix is obvious - include equations in the text. Fine, if you don’t include equations in a popular science book so be it, but then providing more pictorial illustrations is a must.

4out of 5D.L. Morrese–Sean Carroll is a well-known and respected theoretical physicist, and I am in no way qualified to review his book. I did read it, however, and I did enjoy it, in a way. What I enjoyed mostly, I think, was the thought that some very clever people probably understand it. Sadly, I can't say I'm one of them. There are three reasons for this: I don't have a background in quantum mechanics, I'm not especially brilliant, and I don't think my mind is flexible enough to wrap around most of the ideas Sean Carroll is a well-known and respected theoretical physicist, and I am in no way qualified to review his book. I did read it, however, and I did enjoy it, in a way. What I enjoyed mostly, I think, was the thought that some very clever people probably understand it. Sadly, I can't say I'm one of them. There are three reasons for this: I don't have a background in quantum mechanics, I'm not especially brilliant, and I don't think my mind is flexible enough to wrap around most of the ideas presented. None of these are the fault of the book, hence my 4-star rating. The point Dr. Carroll is making, as best I can tell, is that reality, in all its apparent wild complexity and diversity is, in fact, simply the wave function of the universe evolving in accordance with Schrödinger's equation. Like, duh! Why didn't I think of that? Just kidding. I have only a vague idea what that even means. What Dr. Carrol thinks it means (or at least what I think he thinks it means) is that truly quantum phenomena cause the universal wave function to split or branch, which results in two equally real universes where before there was only one...not that anyone notices, or can notice, or even know which one they are in.... I can understand the idea of branching. Actually, it's what the late, great Terry Pratchett used as a premise in his books Jingo and Night Watch. (Excellent stories with cops, robbers, revolution, and a special, magical kind of physics set in a wonderfully imaginative fantasy world. I highly recommend them. But, I digress. Back to the subject at hand...) The creation of multiple worlds every time an entangled particle is seen to spin one way when it could equally have been observed to spin another seems to be a rather inefficient way of constructing a universe, but I'm willing to go along with it if it's all backed up with a sound theory and some supporting observations. After reading this book, I'm still not sure it is. Again, probably not the fault of the book, and it makes a great premise for a good story.

4out of 5Axel Brambila–Since the dawn of time, humans have tried to figure out how the world works. Countless theories have been created to simplify the world for us. This book talks about how the world looks quantumly, or to the smallest spectrum. For instance, we know that Earth turns counter-clockwise to the sun but what are the atoms doing when this happens. Quantum physics in this book is explained well enough for anyone to understand what we've learned. However, it is a matter of how motivated you are to learn Since the dawn of time, humans have tried to figure out how the world works. Countless theories have been created to simplify the world for us. This book talks about how the world looks quantumly, or to the smallest spectrum. For instance, we know that Earth turns counter-clockwise to the sun but what are the atoms doing when this happens. Quantum physics in this book is explained well enough for anyone to understand what we've learned. However, it is a matter of how motivated you are to learn about the subject, because you will oftentimes find yourself rereading paragraphs in order to fully understand. The book sometimes even surfaces starter physics topics, which might affect the reading of an experienced physics student. The contents of the book range from how much we don't know, different theorys, and space-time. I enjoyed the book as it was a great learning experience. The many different theories were enjoyable to learn about; with the many-worlds theory being my favorite because of its craziness. The world splitting into different realities like a cell dividing is a real possibility, and it is scary finding out how little we know about the world. The state of quantum physics now is like the average person using a phone. We know how to use it, but we don't know what is actually going on in the inside. Overall, I gave this book a 4/5 because it made learning quantum physics enjoyable.

4out of 5Cole–As some other reviewers have remarked, I found the first third (maybe even half?) of Carroll’s latest book to be an enthralling ride, but it quickly took off without me thereafter. Things become pretty impenetrable for a book marketed as a popular science read, even taking into consideration the unintuitive nature of any discussion of quantum mechanics. It’s a shame, as Carroll has proven himself capable at explaining complex topics with ease and charm, and the concept of Many Worlds is As some other reviewers have remarked, I found the first third (maybe even half?) of Carroll’s latest book to be an enthralling ride, but it quickly took off without me thereafter. Things become pretty impenetrable for a book marketed as a popular science read, even taking into consideration the unintuitive nature of any discussion of quantum mechanics. It’s a shame, as Carroll has proven himself capable at explaining complex topics with ease and charm, and the concept of Many Worlds is fascinating to say the least. What I’m missing here is that quality being consistent across the book. (To be clear, I can’t imagine what an incredible challenge this would be.) For readers with more than an armchair physics nerd’s knowledge, I’m sure this could easily be a 4 or 5 star read.

5out of 5Eric Layton–Sadly, I wasn't impressed with this one from Dr. Carroll. My interest floundered a bit in the middle of the book. I'm not sure why. The content was somewhat interesting, but "Many Worlds" reminds me a bit of String Theory in that they are both a bit far-fetched. I'm not saying that they can't be. I'm just saying that it's a bit much to swallow. Maybe classic Physics and a slight lean toward empiricism is holding me back a bit. I believe Carroll knew when he wrote this book that many would suffer Sadly, I wasn't impressed with this one from Dr. Carroll. My interest floundered a bit in the middle of the book. I'm not sure why. The content was somewhat interesting, but "Many Worlds" reminds me a bit of String Theory in that they are both a bit far-fetched. I'm not saying that they can't be. I'm just saying that it's a bit much to swallow. Maybe classic Physics and a slight lean toward empiricism is holding me back a bit. I believe Carroll knew when he wrote this book that many would suffer from my affliction. But hey... there are stranger things and all that, Horatio. ;)

5out of 5David Readmont-Walker–Excellent book in a thoroughly confusing and fascinating topic. Recommended.

5out of 5Seth–Carroll has successively convinced me of the plausibility and austerity of Many-Worlds theory. It's truly not as crazy as it sounds, and in fact may be the most parsimonious interpretation of quantum mechanics.

5out of 5Jet Khasriya–Fascinating but impenetrable content unless you have a physics degree. I tried my best. The author has done his best to give a really good grounding, however this is not for the faint hearted and you should be prepared to be utterly frustrated. Great book and I will read again to try and understand.

4out of 5Marcus Cramer–The first half of the book was very interesting and fairly easy to understand. The second half, however, was a different story. I would give the book 3 stars but I blame my own incompetence for not really comprehending anything.