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Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

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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.

347 pages, Hardcover

First published September 10, 2019

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About the author

Sean Carroll

32 books2,146 followers
Sean Carroll is a theoretical physicist at the California Institute of Technology. He received his Ph.D. from Harvard in 1993. His research focuses on issues in cosmology, field theory, and gravitation. His book The Particle at the End of the Universe won the prestigious Winton Prize for Science Books in 2013. Carroll lives in Los Angeles with his wife, writer Jennifer Ouellette.

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Displaying 1 - 30 of 518 reviews
Profile Image for BlackOxford.
1,081 reviews68.1k followers
July 28, 2020
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/...
Profile Image for David Rubenstein.
804 reviews2,538 followers
March 20, 2020
This book is about the "Many-Worlds" hypothesis of quantum mechanics. It is a deep description of the hypothesis, and its context in quantum mechanics. Quantum mechanics does not violate logic; its precise predictions are correct, and among the most accurate of any scientific theory. But its foundations are still quite controversial, especially when it comes to understanding the role of gravitation.

The Many-Worlds hypothesis is a simple way to explain some of the seeming paradoxes of quantum mechanics. There are alternative hypotheses, and the book covers these as well.

I can't say that I learned anything (I am a physicist), but the book did focus my attention on a few key ideas. First, it is incorrect to say that atoms are made up mostly of empty space; particles are not tiny points, but are wave functions that are spread out in space.

Another example: The Heisenberg Uncertainty Principle does not say that the act of measuring a quantity disturbs the system. In addition it does not say that you cannot simultaneously measure position and momentum perfectly at the same time. Instead, it says that a definite position and momentum (velocity) do not even exist simultaneously. The wave function solution to the Schrodinger Equation acts as a wave, and so it can be analyzed like a Fourier Transform. Think of a sudden audible transient--like a click. The click occurs at a definite point in time, but it has no specific pitch because its spectrum is broadband. Likewise, a pure audible tone must occur over a span of time; it does not occur at a specific, definite time.

Here's the problem with the book. Like many technical books that are aimed at non-specialists, it gets deep into jargon and concepts that are totally unfamiliar. The non-specialist can understand all the words, and maybe even entire sentences. But it comes off sounding like a foreign language. And, there is an additional problem with this book. Much of the book focuses on the Schrodinger Equation, which is a typical type of partial differential equation. But unless you have studied similar equations, you cannot really understand the physical concepts described in this book. A general form of the equation is written in the book, but it is so simplified, that to a mathematician it doesn't convey much of anything, and to a non-mathematician it is gibberish.

This book is an excellent attempt at explaining some of the deepest mysteries of quantum mechanics. But the fundamentals are not covered well enough for a general reader to grasp all the arguments presented here.
Profile Image for Darwin8u.
1,559 reviews8,692 followers
May 30, 2020
"I think I can safely say that nobody understands quantum mechanics."
- Richard Feynman

"Shut up and calculate."
- David Mermin

"Sweet is by convention, bitter by convention, cold by convention, color by convention; in truth there are only atoms and the void."
- Democritus


As an amateur, I love physics. I think there is something in my brain that associates the bleeding edge of physics with poetry and art. I'm not the only one. Authors like Thomas Pynchon and Cormac McCarthy are constantly using physics as a springboard into literary ideas and explorations. I think one of the big connections between theoretical physics and literature is the fact that both seek to explain the world through imagery and metaphor. Physics are hard science's poets.

Sean Carrol does a fantastic job of describing the many-worlds interpretation (MWI) as initially suggested by Hugh Everett. The entire price of admission to this book was paid when I discovered in this book that Hugh Everett is the father of EELS' lead singer and song-writer Mark Oliver Everett (also known as E). Talk about convergence.

Anyway, the book was well written, carefully laid out, and like other topics I've flirted with (Knot theory), I'm pretty sure I just walked off with a pip of knowledge, but I'll keep coming back to the damn fruit of the tree of knowledge.
Profile Image for Peter Tillman.
3,638 reviews329 followers
December 14, 2019
Rather than the confusing publisher's blurb, I recommend starting with the author's essay about his book:

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:
He is better qualified than me to judge the physics. Well-read guy, too.

Nature's review: https://www.nature.com/articles/d4158...
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. . . .
Profile Image for Max.
343 reviews311 followers
July 6, 2020
In this book Carroll fully embraces the reality of quantum mechanics. He doesn’t accept that it’s just useful for calculations. Carroll says that the quantum world is the real world. Rather than proceed from classical physics to explain the quantum world Carroll starts with the quantum world to find out how it builds the world described by classical physics. Carroll’s approach leads to what many consider outlandish conclusions.

Carroll is a theoretical physicist at Caltech and an award winning author of physics books. Here he makes a case for the Many-Worlds theory of quantum mechanics calling it “the most promising formulation of quantum mechanics”. Quantum mechanics is proven science. It is fundamental to the way the universe works. But what it tells us about the nature of reality is disputed. The different takes on this question have been commonly called interpretations, but Carroll considers this term misleading since it implies the answer is subjective. He uses the more recent description, the foundations of quantum mechanics. Here, I’ll stick with the more familiar and less cumbersome word, interpretation.

The Many-Worlds interpretation of quantum mechanics was first espoused in 1957 by physicist Hugh Everett in his PhD thesis under the guidance of Noble Prize winner John Wheeler. The Many-Worlds interpretation was at first widely dismissed as nonsense but has gained more support over time. In the quantum state particles are in superposition meaning they are in a combination of positions all at the same time, both here and there, an electron spinning both up and down, a photon both vertically and horizontally polarized, etc. While strange we know it is true. Quantum computers are being built that operate taking advantage of those extra states to process calculations at unheard of speed.

Every quantum object has a wave function which tells us the probability of finding it in a particular state. When we measure we get a distinct answer. But what is the fundamental reality, the quantum state of superposition or the measurement yielding a specific position? Carroll believes it is the quantum state. The measurement is just what we observe. This belief pits Carroll against the standard theory also known as the Copenhagen interpretation.

Carroll says the Many-Worlds theory is consistent with the science while the traditional interpretation is not. First Many-Worlds avoids the measurement problem that plagues the Copenhagen interpretation. The Copenhagen interpretation holds that the wave function collapses when we measure yielding a specific value. Carroll holds there is no reason to believe that the wave function does collapse. Physicists commonly accept that the evolution of the wave function is defined by Schrodinger’s equation. That equation does not indicate the collapse of the wave function. Nor is there any other support for the collapse. According to Carroll it was just made up to explain why measurement gives a specific outcome.

An implication of the Copenhagen interpretation is that particles in the quantum world only become real when they are measured. This makes no sense. Carroll holds that all the possible outcomes are real, but in different universes. The Many-Worlds theory holds that for every possible outcome the universe splits accommodating each. Carroll believes there is one wave function for the universe and we are all in superposition with it. So if we are betting on a coin toss, in one universe we win but in another we lose. We are entangled with the coin and go together in both universes, each universe forever completely separate from the other.

A second problem with the Copenhagen interpretation is that it claims the quantum world only exits at the micro level of fundamental particles, even though experiments now show that larger objects display quantum characteristics. So where is the dividing line between where quantum mechanics rules and classical physics rules. Any such line is arbitrary. Carroll considers the entire universe to be part of the quantum world defined by a quantum wave function, just as are fundamental particles. He holds that the everyday world we encounter, the universe described by classical physics, is an emergent property of the universe’s wave function. Carroll says that the Many-Worlds interpretation best explains the reality of the quantum. There is no measurement problem, no arbitrary distinction between quantum and classical worlds, and no mysterious making something real by measuring it. Every outcome is real and consistent with the quantum world.

Carroll points to entanglement, an idea that came out of quantum mechanics that has been proven in experiments. When two particles become entangled they become one quantum system, a change in one means the other also changes instantaneously even if they are light years apart. Carroll believes every object can become entangled. For example the equipment or people measuring particles or anything else become entangled with them and also are in superposition with them. Photons, electrons or other particles will inevitably interact with a macroscopic object. Thus macroscopic objects are necessarily entangled with their environment and thus with the entire universe and are part of its wave function.

The Many-Worlds formulation also offers an explanation for the famous two slit experiment. When electrons pass through both slits of the testing apparatus they hit a screen forming an interference pattern because they are waves. But when we set up a detector to measure them they pass straight through to the screen showing up as if they were particles. Carroll holds that this is because any interaction including measurement, in this case between the detector and electron, causes what is known in quantum mechanics as decoherence. The electron’s quantum state has been altered. In the Many-Worlds theory decoherence causes the electron and its shared wave function with the universe to branch into different universes thus the electron can no longer interfere with itself as it passes through the detector.

Carroll briefly explores some alternative quantum theories: the GRW theory of dynamical collapse, the Bohmian mechanics model using pilot waves and QBism an epistemic model that holds the wave function is purely informational and not real. Carroll sticks with Many-Worlds calling it “simple and elegant” but indicates he will switch if something better comes along. In the final chapters he explains quantum field theory describing quantum fields that fill the universe creating locality and becoming entangled. In a fascinating discussion he shows how entanglement of quantum fields could yield space. He also discusses the search for quantum gravity and looks at black holes from a quantum perspective. These sections were quite involved, but I enjoyed his unique presentation.

Central to all of Carroll’s thinking is starting with quantum mechanics to create the world of classical physics. In his own words:

Nature is quantum from the start, described by a wave function evolving according to an appropriate version of the Schrodinger equation. Things like ‘space’ and ‘fields’ and ‘particles’ are useful ways of talking about that wave function in an appropriate classical limit. We don’t want to start with space and fields and quantize them; we want to extract them from an intrinsically quantum wave function.

From a Many-Worlds perspective that treats quantum states as fundamental and everything else as emergent, this suggests that we should really turn things around, ‘positions in space’ are the variables in which interactions look local. Space isn’t fundamental; it’s just a way to organize what’s going on in the underlying wave function.

I don’t have any idea whether Many-Worlds turns out to be a great contribution to physics or just a bizarre dead end. But I don’t dismiss an idea just because it sounds crazy. Regardless of the fate of Many-Words, I learned much about quantum mechanics from Carroll’s presentation. For example, he offered the clearest explanation I have read for why it is not possible to measure position and momentum at the same time in quantum mechanics. I’ve read several of Carroll’s books and enjoyed them all. In this one I found myself frequently paging back and forth. It definitely is for someone with a strong interest in the subject and an open mind.
Profile Image for Laura Noggle.
677 reviews388 followers
February 2, 2020
This book is like taking acid, be warned—it's a total trip.

Not that I'd know, but I'm guessing based on Rick and Morty episodes I've never watched.

In other words, don't take my word for it, I need to brush up on my quantum mechanics apparently.

“On the other hand, in the memorable words of Richard Feynman, 'I think I can safely say that nobody understands quantum mechanics.'”

“Should the branching of our current selves into multiple future selves affect the choices we make? In the textbook view, there is a probability that one or another outcome happens when we observe a quantum system, while in Many-Worlds all outcomes happen, weighted by the amplitude squared of the wave function. Does the existence of all those extra worlds have implications for how we should act, personally or ethically? It’s not hard to imagine that it might, but upon careful consideration it turns out to matter much less than you might guess.”
Profile Image for Eric.
172 reviews31 followers
August 27, 2019

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


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.


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!
Profile Image for Deniz Yuret.
32 reviews41 followers
November 4, 2019
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.
Profile Image for Paperclippe.
522 reviews104 followers
August 30, 2019
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.
Profile Image for Anton.
273 reviews85 followers
December 26, 2019
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.
Profile Image for Alexander.
68 reviews49 followers
June 19, 2022
This book has resulted in a nonzero improvement in my grasp of many worlds, but to be honest, I still don’t actually truly get it. Sean Carroll reminds me of this friend I had who was incredibly good at mathematics and when I asked them to help me, they literally just restated the theorem I needed help with verbatim. This is an unfair comparision but this is sort of how Sean Carroll explains many worlds. He just states the Schrödinger equation and expects the learner to just magically get many worlds because "many worlds only assumes the Schrödinger equation.”

I guess I might be approaching this whole thing wrong. I went into this book expecting to have the question “Why is reality made of waves?” answered. I was a little disappointed because this is a philosophical question and Sean Carroll is a scientist, who is primarily interested in describing the world rather than explaining why it is the way it is.

Out of the competing theories of quantum physics, many worlds adheres most strongly to Occam’s Razor. It makes the least assumptions, yet it explains the experimental observations better than the rest. Sean Carroll tells me that I am entangled with the wave function of my light-cone and this wave function represents my world and it branches a lot (like many times a second). Sean Carroll tells me that those other worlds aren’t somewhere else physically, they are all somehow existing simultaneously. He also tells me that there’s an infinitesimal probability of these branches converging rather than diverging but because of entropy’s tendency to increase, these worlds tend to diverge. Branch merging is so unlikely that it might’ve never happened since the Big Bang.

It is important that we distinguish between many worlds and the cosmological multiverse. They’re completely different things. Also, no one knows how many worlds there are in many worlds but Sean Carroll thinks it’s either a VERY large number or infinite.

I guess I need to learn more about how reality works, maybe by studying proper physics and math books so that I can finally reach the Nirvana of seeing the world in the same way Sean Carroll does. I’ll keep reciting this quote by Eliezer Yudkowsky to remind myself that I am the weird one, not many worlds.

Reality has been around since long before you showed up. Don't go calling it nasty names like "bizarre" or "incredible". The universe was propagating complex amplitudes through configuration space for ten billion years before life ever emerged on Earth. Quantum physics is not "weird". You are weird. You have the absolutely bizarre idea that reality ought to consist of little billiard balls bopping around, when in fact reality is a perfectly normal cloud of complex amplitude in configuration space. This is your problem, not reality's, and you are the one who needs to change.
Profile Image for Gary  Beauregard Bottomley.
979 reviews579 followers
September 30, 2019
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].
Profile Image for D.
504 reviews62 followers
September 9, 2020
The first part is easier to read and comprehend than the following parts. The latter, while containing many gems and food for thought as well, was much more muddled and difficult to follow. Chapter 8, the ‘dialogue’, is silly and should have been left out.

In my opinion, Rovelli is a better teacher.
Profile Image for David.
580 reviews140 followers
October 5, 2020
"Rather than talking about you at 5:01 pm, we need to talk about the person at 5:01 pm who descended from you at 5 pm, and who ended up on the spin-up branch of the wave function, and likewise to the person on the spin-down branch. In many worlds, the lifespan of any person should be thought of as a branching tree, with multiple individuals at any one time, rather than as a single trajectory."

Intrigued? Read on.

This should not be your first book on the topic of quantum theory. High School science curriculums typically places chemistry prior to physics. Chemistry talks of ‘orbitals’ around the atom, but can’t really dive into the analysis of the electromagnetic attraction of the electron and the proton since that’s a physics topic.

The IB HL (International Baccalaureate High Level) 2-year high school course can have you ready for this book – but you better have been paying attention! AP (Advanced Placement) C Physics is strong on math, but very weak on these Modern Physics topics. AP B is weaker on math, but its Quantum section will have you better prepared to hear what Cal Tech Professor Sean Carroll is saying here.

Most US University engineering curriculums don’t require particle physics in the Physics I and Physics II courses. Quantum Theory is a topic for “Modern Physics” taken by physics majors. So unless you took IB HL in high school, you should maybe read one of the primer books I list at the bottom of this review.

Let’s start small...

If the Hydrogen atom (one proton, one electron) obeyed classical physics, then an orbiting electron particle would de-orbit and spiral into the proton in the nucleus in a few trillionths of a second. So why has Hydrogen been stable since the beginning of time?

What if we combine our 11th grade concept of chemistry orbitals with our 12th grade understanding of waves and think of the first orbital as having a circumference of one wavelength of the electron. The second orbital is two wavelengths. Etc. This explains why there are quantized energy levels inside the atom. The electron jumps from one orbit to the next, but cannot exist in between. So the electron cannot spiral into the proton since the electron does not exist inside the lowest first orbital.

"Particles (electrons, protons, neutrons) do not exist. It is more accurate to think of them as diffuse fields."

The famous double-slit experiment done with shooting one electron at a time shows the wave nature of an electron. If you look at an ice-cream cone from the side, it looks like a triangle. If you look at the cone from the top, all you see is a circle. Similarly, you can do experiments that show electrons (and even photons of light) look like waves, and other experiments that make them look like particles.

If the electron is a wave, then since a wave is spread out in space, there are high and low probabilities of where you would say the wave is right now. The Copenhagen Interpretation of Quantum Physics treats the Schrodinger wave equation of a particle as a probability. The Many World Interpretation pioneered by Hugh Everett in 1957 says let’s apply the

So if an electron can be thought of as a wave, and protons are waves, then we humans are waves. :) Or at least we have both particle and wave properties.

“To be fair, quantum mechanics – or “quantum physics,” or “quantum theory,” the labels are interchangeable – is not only relevant to microscopic processes. It describes the whole world, from you and me to stars and galaxies, from the centers of black holes to the beginning of the universe. But it is only when we look at the world in extreme close-up that the apparent weirdness of quantum phenomena becomes unavoidable.”

Simply using classical physics, it may seem that since we can predict the trajectory of a golf ball, then we could predict everything (knowing its current motion), and even back-track into the past.

“The French mathematician Pierre-Simon Laplace pointed out a profound implication of the classical mechanics way of thinking. In principle, a vast intellect could know the state of literally every object in the universe, from which it could deduce everything that would happen in the future, as well as everything that had happened in the past. ‘Laplace’s demon’ is a thought experiment, not a realistic project for an ambitious computer scientist, but the implications of the thought experiment are profound. Newtonian mechanics describes a deterministic, clockwork universe.”

The ‘interpretation of quantum mechanics’ is studied due to the ‘measurement problem’: “what we see when we look at the world (measure) seems to be fundamentally different from what actually is.”

Classically, we think of the electron as this particle in orbit around the nucleus. But “the best we can do is to predict the probability of seeing the electron in any particular location or with any particular velocity.”

So, the electron particle is replaced with a cloud of probability. This cloud is the ‘wave function’. The amplitude of this wave function involves a complex number (a+bi). Amplitude^2 = a^2 + b^2 (think ‘Pythagoreus’)

The Schrodinger Equation now governs how the wave function will evolve. Schrodinger: “The rate of change of a wave function is proportional to the energy of the quantum system.”

“When you perform a measurement, such as the position or spin of a particle, quantum mechanics says there are only certain possible results you will ever get. You can’t predict which of the results it will be, but you can calculate the probability for each allowed outcome. And after your measurement is done, the wave function collapses to a completely different function, with all of the new probability concentrated on whatever result you just got.”

We need to think of the entire world as a wave function, that is in a constant state of being measured as our wave nature interferes with the wave nature of everything around us.

This is an austere view of quantum mechanics. You will often hear: “Atoms are mostly empty space.” Utterly wrong! This is our classical mind viewing an electron particle. Rather, there is simply the quantum state of the electron described by wave functions that stretch throughout the extent of the atom.

Everything is quantum. So not only is the electron orbit a superposition of all its possible states, but the electron+us+earth+universe is all its own SINGLE wave function in a superposition of all its possible states. This is the quantum phenomenon known as ‘entanglement’.

"Fortunately the Schrodinger Equation is straight forward and definite in what is says about how the wave function behaves. Once we understand what’s going on for two particles, the generalization to 10^88 particles (the universe) is just math."

Enter “Many Worlds”

Before a measurement happens (like camera sees an electron), there was one electron and one observer (camera, or you). “After they interact, however, rather than thinking of that one observer having evolved into a superposition of possible states, we could think of them as having evolved into multiple possible observers. The right way to describe things after the measurement, in this view, is not as one person with multiple ideas about where the electron was seen, but as multiple worlds, each of which contains a single person with a very definite idea about where the electron was seen.”

This is known as the Everett, or Many-Worlds, formulation of quantum mechanics. It was put forth by Hugh Everett in 1957. “The price we pay for theis vastly increased elegance of theoretical formalism is that the theory describes many copies of what we think of as “the universe,” each slightly different, but each truly real in some sense.

My summary so far (above) takes me to page 39. There are 300 more pages to help you understand Many Worlds, and even dive into other interpretations of quantum mechanics.

Sean Carroll covers this all carefully and clearly, but you had better enter this book having read about quantum mechanics prior to this book.

Other 'primer' books before this one might include:
A Short History of Nearly Everything
The Fabric of the Cosmos: Space, Time, and the Texture of Reality
A Brief History of Time
In Search of Schrodinger's Cat: Quantum Physics And Reality

15 reviews
October 17, 2019
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.
Profile Image for Steve.
375 reviews72 followers
October 29, 2021
One of the most difficult ‘popular science’ books I’ve read in recent years, and by an author I admire for his clarity, usually, on cutting edge physics. I don’t think this is a read for the ‘lay reader’ totally unfamiliar with Quantum Mechanics (QM), Relativity or sub-atomic physics. Some familiarity with the topics from thorough popular books on those subjects is probably required. I did QM and Relativity modules at university some decades ago, and the book challenged me.

The aim of the book is to clarify the underlying meaning of Quantum Mechanics (QM), the modelling of the sub-atomic world by apparently hazy wave functions and probabilistic outcomes to experiments. The mathematics behind QM is indisputably accurate but can we give a meaning to the equations and what they seek to describe? This ‘underlying meaning’ issue has been around for the best part of 100 years...

I found it a strangely structured book. The author outlines with his characteristic clarity what the problems are facing understanding QM philosophically (mainly the measurement problem). In the first half it’s an enthusiastic recommendation for the Many Worlds interpretation of QM. But I appreciated the author giving the current alternative views in a fairly neutral manner though it’s fair to point out the inelegances of the Copenhagen view, or the contradictions in the Bohm approach, etc. In fact the author did take the time to air difficult issues - for example, what exactly is the Wave Function that Schrodinger’s Equation uses?! Later in the book other difficult areas requiring work are considered, such as Quantum Gravity, and the nature of space time as emergent consequences of QM.

Unfortunately I find it difficult to view these ideas without my own pragmatic prejudices. The author gave intelligent speculations or extrapolations from what we know now to what that may mean for underlying structure. But in the end they are speculations at present - experiments in the cutting edge areas are sadly wanting, and it is experiments that have hitherto driven the original development of the major concepts that the author is trying to extrapolate from (basic QM and General Relativity).
I’m certainly amazed at how well our ‘monkey brains’ have found patterns in nature that lead to ‘natural laws’ and allow predictions. It’s how our technological age has developed, for better or worse. But I’m also concerned that our primate pattern recognition skills have their limits - what may be the simple underlying concepts we are looking for in nature may not be how nature is structured at its most fundamental levels. Try to find the unifying patterns by all means but it’s possible that they may not be so simple, or even recognisable. Everything in the sub-atomic world seems well explained by the mathematical formulations developed to date (Schrodinger’s Equation and Wave Functions) but our underlying explanations often involve analogies with things we can see or feel in our everyday world. Atoms, depending on the context, can be visualised as microscopic billiard balls rattling around in a container, or mini solar systems when we want to explain chemical processes or radiation. Sub-atomic particles are visualised sometimes as tiny dots simultaneously linked to wave like behaviour. These are our inadequate attempts at analogy that we use to give some visual meaning to things beyond our experience, even if the maths works precisely. As the author points out even cutting edge researchers can’t avoid dealing in discrete particles when they are manifestations of underlying quantum fields.

I couldn’t help feeling that the Many Worlds view is similar - a way to avoid or explain the measurement issue with analogy, albeit one that’s heavy with universes! Maybe you can see I wasn’t convinced by the author’s arguments that this has to be the most likely underlying explanation of QM because of its lack of assumptions (aside from the almost infinite multiple universes every second!).

So, in summary, difficult topics well explained but rather too evangelical on the Many Worlds approach for my taste in the first half of the book. If it’s one of the options on the table, fine, but I wasn’t convinced it’s a necessary understanding, or more naturalistic than the other current options. I was interested in the second half more, on Quantum Gravity (if only we could detect Hawking Radiation!) and space time emergence, where the Many Worlds approach is mentioned, but which are still interesting areas capable of exploration on their own. Oh, a few glasses of beer helped in this fairly heavyweight read!
Profile Image for Realms & Robots.
196 reviews4 followers
August 27, 2019
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.
Profile Image for Kyle.
328 reviews
October 3, 2019
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.
Profile Image for Lemar.
665 reviews53 followers
April 20, 2020
What you (don’t) see is what you get. Sean Carroll brings the Many Worlds interpretation of quantum field theory to the people. Something Deeply Hidden delivers what I was looking for. It strikes exactly the right tone as his breezy demeanor encompasses a deep dive that does not shy away from equations and competing theories.
Carroll is really rather heroic, clearly the in-group at physics conferences have disdain for those spending time talking about philosophical implications of discoveries on the quantum level. In addition, I can imagine his publisher reacting in horror upon seeing actual equations appear in a book meant to be friendly. Neither of these roadblocks deter him from coming through for us, his readers.
This clear-eyed courageous approach is also fundamental to the theory he adheres to. Kicking away the crutches of additional variables and boundaries meant to placate our unease at the seeming disconnect between our experience and quantum reality, Carroll goes all in. He cheerfully and logically gravitates to (sorry, had to do that) the Many Worlds theory because it is the most unencumbered and elegant.
“If we train ourselves to discard our classical prejudices, and take the lessons of quantum mechanics at face value, we may eventually learn how to extract our universe from the wave function.”
Profile Image for Tony.
402 reviews3 followers
March 19, 2022
Something Deeply Hidden clearly lays out the many worlds (or Everettian) interpretation of quantum mechanics. The book also makes a strong argument for why many worlds may be the most convincing explanation of QM. Unfortunately, Carroll does not stop there. Instead, he unsuccessfully pursues a number of other topics. Some of these--like a section on alternative interpretations of QM--are treated so summarily as to be virtually incomprehensible to the layman. Others--such as whether a person should consider his/her effect on other worlds in making their actual decisions--simply come across as ludicrous. These latter chapters turn this relatively slim volume (347 pages) into a real slog.
Profile Image for Thomas Ray.
1,009 reviews326 followers
September 29, 2019
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.

Profile Image for Gendou.
585 reviews264 followers
December 31, 2019
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".
Profile Image for Trike.
1,467 reviews153 followers
October 14, 2022
This is fine, but I didn’t really hear anything new about quantum mechanics, either in terms of the field’s history or the current state of the art. This should probably be your second or third book on the subject, as I think you need a nodding familiarity with it to follow what he’s talking about in some instances.
Profile Image for Goatboy.
180 reviews63 followers
February 3, 2020
Not as mind shattering as The Big Picture but definitely the first book to really help me understand and maybe believe Everett's Many Worlds theory of physics. Sean Carroll is my go to explainer of physics at this point. If you haven't heard his podcast yet you should check it out.
Profile Image for Nilesh Jasani.
988 reviews135 followers
September 28, 2019
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.
Profile Image for The  Conch.
278 reviews17 followers
April 17, 2021
This book tells us about a world beneath the observable one, the world of quantum. In this world laws of classical physics do not hold good. As classical physics can tell location of a particle if its velocity and direction are given, but in quantum world it is impossible to know both.

The book discusses an emerging concept in quantum universe "Many - World", where subatomic particles branch to create new worlds, though it will not reflect in visible world.

The book is difficult to read and not in category of popular science. Author may think about an easy version for general non-science background. Language could be made simple so that readers can enjoy the reading and acquire knowledge.
Profile Image for David W. W..
Author 13 books37 followers
October 25, 2019
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....
Profile Image for David Agranoff.
Author 23 books135 followers
March 11, 2020
This book is not for everyone. Sean Carroll is an author I reviewed before with his epic book about the universe the Big Picture. I feel more deeply connected to this author because I know his voice well. I listen every week to his podcast mindscape that I often describe as Sean Carrol talks to other geniuses. I was familiar with these ideas and the many-worlds theories before not just as a Carroll podcast listener but as a huge Sci-fi nerd and Philip K Dick Podcaster. We have talked about Many-worlds in a pseudo-science 60's way a lot.

I am not going to pretend for one minute that I am able to process more than the basic ideas here. I read this book quickly because whenever math or the Nitty-Gritty of how particles spin. I really enjoyed the history of discovery and how Carroll weaves the methods that the greats in science came to the various theories that make Quantum science. I know this will sound corny but my love for this topic has roots in my favorite childhood horror movie John Carpenter's Prince of Darkness that was the first place that a young me heard of Quantum psychics.

This is primarily an introduction to one of the most debated issues in the study of spacetime, as such I think it is a good introduction but the big bottom line in this review do you enjoy this stuff? I do and still felt the need to skip a few parts. I enjoy Sean Carroll's books because he helps takes universe-spanning Ideas and boils them down.

Separated into three sections, part one is where we get the majority of the history. This part is called Spooky based on the idea that even Einstein in the early days found these issues to be hard to deal with. In this section, Carroll sets up the questions that we are going to ponder. The second part is called Splitting and gets into what it all means. Part three Spacetime is where most of the interesting theories happen.

The most interesting concepts for me were near the end. "It is plausible that the symmetry between space and time that we are familiar with from relativity isn't built into Quantum Gravity." Oh no he didn't? Was there more to the universe than Einstein could see? sure and towards the end Carroll questions if Space is even part of the equation and this tiny level. Another part I enjoyed was one of the last chapters that explained more of the science behind Black holes, as Hawking was quoted in the book Black holes ain't so black.

I enjoyed Carroll's last book more than this one but that could have as much to do with the epic themes of that one. This one was designed to be a purely academic exercise, while the Big Picture dealt with the point where the rubber meets the road between cosmology and philosophy.
Profile Image for Todd.
160 reviews7 followers
November 24, 2019
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.
Profile Image for Prejudice Neutrino.
96 reviews31 followers
November 5, 2019
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.
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