Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime by Sean Carroll (PDF)

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INSTANT NEW YORK TIMES BESTSELLERA Science News favorite science book of 2019As 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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⭐Sean Michael Carroll (born 1966) is a cosmologist and research professor in the Department of Physics at the California Institute of Technology. He has also written

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⭐.He wrote in the Prologue to this 2019 book, “This book has three main messages. The first is that quantum mechanics should be understandable—even if we’re not there yet—and achieving such understanding should be a high-priority goal of modern science… The second message is that we have made real progress toward understanding. I will focus on the approach I feel is clearly the most promising route, the Everett or Many-Worlds formulation of quantum mechanics… The third message is that all this matters, and not just for the integrity of science… We don’t know for sure… But the time has come to take the fundamental nature of reality seriously, and that means confronting quantum mechanics head-on.” (Pg. 5-7)Later, he adds, “One of the themes of this book is that quantum mechanics doesn’t deserve the connotation of spookiness, in the sense of some ineffable mystery that it is beyond the human mind to comprehend… Our theme is prizing clarity over mystery, so I don’t want to adopt that strategy, I want to present quantum mechanics in a way that will make it maximally understandable right from the start. It will still seem strange, but that’s the nature of the beast. What it won’t seem, hopefully, is inexplicable or unintelligible.” (Pg. 12-13)He explains, “Quantum mechanics suggests… We can MEASURE values of the location or velocity … But what we’re seeing through such a measurement is not the actual, complete, unvarnished state of the electron… The best we can do is to predict the PROBABILITY of seeing the electron in any particular location or with any particular velocity. The classical notion of the state of a particle… is therefore replaced in quantum mechanics by something utterly alien to our everyday experience: a cloud of probability… This cloud is often called a ‘wave function,’ because it can oscillate like a wave, as the most probable measurement outcome changes over time… So if you measure a quantum system, in general the best you can do is predict probabilities for various outcomes, but if you were to immediately measure the same quantity again, you will always get the same answer—the wave function has collapsed onto that outcome.” (Pg. 19, 22)He suggests that we should “abandon the idea that the electron has some particular location. An electron is in a SUPERPOSITION of every possible position we could see it in, and it doesn’t snap into any one specific location until we actually observe it to be there. ‘Superposition’ is the word physicists use to emphasize that the electron exists in a combination of all positions, with a particular amplitude for each one. Quantum reality is a wave function; classical positions and velocities are merely what we are able to observe when we probe that wave function.” (Pg. 34)He asserts, “what we’ve described … [is] commonly known as the Everett, or Many-Worlds, formulation of quantum mechanics… The Everett view … suggests… that there is just a single kind of quantum evolution… the theory describes many copies of what we think of as ‘the universe,’ each slightly different, but each truly real in some sense. Whether the benefit is worth the cost is an issue about which people disagree. (It is.)… at no point did we take ordinary quantum mechanics and tack on a bunch of universes. The potential for such universes was always there—the universe has a wave function, which can very naturally describe … superpositions of the whole universe… this potential is naturally actualized in the course of ordinary quantum evolution… [This] leaves many questions unanswered. When exactly does the wave function split into many worlds?… Are the other worlds really ‘real’? How would we ever know, if we can’t observe them?… How does this explain the probability that we’ll end up in one world rather than another one? All of these questions have good answers… and much of the book to come will be devoted to answering them. But we should admit that the whole picture might be wrong, and something very different is required.” (Pg. 39-40)He says, “The point of the famous Schrödinger’s Cat thought experiment, in which the wave function of a cat evolves … into a superposition of ‘alive’ and ‘dead,’ was not to make people say, ‘Wow, quantum mechanics is really mysterious.’ It was to make people say, ‘Wow, this can’t possibly be correct.’ But to the best of our current knowledge, it is.” (Pg. 66)He states, “Decoherence causes the wave function to split, or ‘branch,’ into multiple worlds. Any observer branches into multiple copies along with the rest of the universe. After branching, each copy of the original observer finds themselves in a world with some particular measurement outcome… We don’t know how often branching happens, or even whether that’s a sensible question to ask. It depends on whether there are a finite or infinite number of degrees of freedom in the universe, which is currently an unanswered question in fundamental physics. But we do know that there’s a lot of branching going on: it happens every time a quantum system in a superposition becomes entangled with the environment. In a typical human body, about 5,000 atoms undergo radioactive decay every second. It every decay branches the wave function in two, that’s 2[to the 5000th] new branches every second. It’s a lot.” (Pg. 119-120) He continues, “And there’s nothing special about what constitutes ‘a measurement’ or ‘an observer’… Consciousness, in particular, has nothing to do with it. The ‘observer’ could be an earthworm, a microscope, or a rock…. The price we pay for such powerful and simple unification of quantum dynamics is a large number of separate worlds.” (Pg. 122-123)He suggests, “If Everett is right, there is a 100 percent probability that each possibility is realized in some particular world… there really are such worlds… According to Many-Worlds, when I generated [a] random number [he printed in this book], the universe split into … approximately 1 quadrillion [copies], each of which carries a slightly different number. If all of the copies of me in all of those different worlds stuck with the plan of including the [random] number into the text of this book, that means there are over a quadrillion different textual variations of [this book] out there in the wave function of the universe…” (Pg. 134-135) He adds, “Every one of those people has a reasonable claim to being ‘you.’ None of them is wrong. Each of them is a separate person, all of whom trace their beginnings back to the same person. In Many-Worlds, the life-span of a person should be thought of as a branching tree, with multiple individuals at any one time, rather than as a single trajectory… And nothing… really hinges on … being a person rather than a rock. The world duplicates, and everything within the world goes along with it.” (Pg. 139-140)He includes a fictional dialogue between a physicist/philosopher at a major university and her physicist father, in which the daughter explains, “Many-Worlds doesn’t say ‘everything possible happens’; it says ‘the wave function evolves according to the Schrödinger equation.’ Some things don’t happen, because the Schrödinger equation never leads to them happening. For example, we will never see an electron spontaneously convert into a proton. That would change the amount of electric charge, and charge is strictly conserved. So branching will never create, for example, universes with more or less charge than we started with. Just because many things happen in Everettian quantum mechanics doesn’t mean that everything does.” (Pg. 167)Her father objects, “‘How can you hold up Many-Worlds as a respectable scientific theory if not only can’t you observe them, but you don’t even have a definite criterion for how it happens? Branching is just… your opinion[?]…[She replied] ‘In a sense, sure. But there are better and worse opinions to have.’” (Pg. 170) Later, she concludes, “I personally find Everettian quantum mechanics, and all the many worlds it implies, to be by far the most plausible version of quantum theory that I know of. If that means I must accept that my present self will evolve into a number of slightly different future selves who will never be able to talk to each other, I’m willing to accept that.” (Pg. 174)Carroll summarizes, “Many-Worlds offers a perspective on quantum mechanics that is not only simple and elegant at its core but seems ready-made for adapting to the ongoing quest to understand quantum field theory and the nature of spacetime. That’s enough to convince me that I should learn to live with the annoyance of other copies of me being produced all the time. But if it turns out that an alternative approach answers our deepest questions more effectively, I’ll happily change my mind.” (Pg. 204) But he also acknowledges, “If the real world is truly this radically different from the world of our everyday experience, does this have any implications for how we live our lives? Largely—no… life goes on as if they lived in a single world with truly stochastic quantum events. But the issues are worth exploring.” (Pg. 206)He clarifies, “We might care about what happens to the versions of ourselves who live on other branches, but it’s not sensible to think of them as ‘us.’ … You are not them, and they are not part of you. Post-branching, you’re two different people. Neither your experiences nor your rewards should be thought of as being shared by various copies of you on different branches.” (Pg. 209) He continues, “No, you do not cause the wave function to branch by making a decision… A decision… is a purely macroscopic phenomenon. There are no decisions being made by the electrons and atoms inside your brain; they’re just obeying the laws of physics… Your decisions don’t cause the wave function to branch, because ‘the wave function branching’ is a relevant concept at the level of fundamental physics, and ‘your decisions’ is a relevant concept at the everyday macroscopic level of people. So there is no sense in which your decisions cause branching. But we can still ask whether there are other branches where you made different decisions. And indeed there might be…” (Pg. 214)He acknowledges, “Up to this point in the book, basically everything we’ve talked about is either well understood and established doctrine … or at least a plausible and respectable hypothesis (the Many-Worlds approach). Now we’ve reached the edge of what is safely understood, and will be venturing out into uncharted territory. We’ll be looking at speculative ideas that might be important to understanding quantum spacetime and cosmology. But they might not be. Only… further investigation will reveal the answer with any confidence.” (Pg. 268)This book will be of great interest to those studying speculative theories in physics (particularly the Many-Worlds idea).

⭐Sadly, it seems, the quest to make sense of quantum mechanics is not a high-status item in modern physics, and in some quarters, it is even disparaged, according to Carroll. Let this not stop the author from pursuing this matter in more detail. There are three main messages conveyed by the book: it should be understandable, real progress has been made toward understanding it, and thirdly, all of this really matters.We are introduced to some basic concepts such as the measurement problem, the wave function, probabilities, and amplitudes associated with the wave function, Schrodinger’s equation, which then leads to a discussion of what is called austere quantum mechanics or AQM. This is more commonly known as the Everett or Many-Worlds model. In a measurement, we are dealing with one wave function, which includes the object of measurement (perhaps an electron) and the measurement device. The Everett formulation basically states every version of quantum mechanics features just two things: the wave function and Schrodinger’s equation, which governs how wave functions evolve in time. These ingredients “provide a complete empirically adequate account of the world.” Reality is described by a smoothly evolving wave function and nothing else. There is a lot to learn here. We gain an understanding of Planck’s discovery of the connection of energy to frequency, which leads to Planck’s constant, the Planck length, and Planck time. In a subsequent chapter, we learn that position and momentum don’t even exist at the same time. There is a spread of possibilities for both quantities, neither having a definite value- very strange. This leads to what is called the uncertainty principle. “If position is concentrated near some location, momentum is completely undermined, and vice versa.” Then there is complementarity, which allows more than two ways of looking at a quantum system. For example, electrons can have particle-like or wave-like properties, just not at the same time. One of the degrees of freedom (explained in the book) is spin. The author’s discussion on this is interesting, if not a bit confusing to me. The next discussion is on entanglement and a feature of this is the no-signaling theorem.In section two, we delve more deeply into the Everettian world. Here every part of the universe is treated according to the rules of quantum mechanics, including the observers within it. There is a single quantum state, what Everett called the “universal wave function.” There is no Copenhagen collapse of the wave function or classical realms. The measuring apparatus itself evolves into a superposition, entangled with the state of the thing being observed. In this case, the other possible measurement outcomes exist. This is ultimately interpreted to mean branching into separate never-to-interact worlds. The collapse we see is only apparent, due to dechoerence splitting of the wave function. In a subsequent chapter the author discusses why probabilities enter into things considering that the Schrodinger equation is perfectly deterministic. In Chapter 8, we are given more insight to this many-worlds idea via a discussion between a hypothetical Alice and her father. Following this, we learn of some alternatives to the many-worlds theory. Three competitors are discussed: dynamical collapse, hidden variables, and epistemic theories. There some other theories that the author discusses, such as Bohmian mechanics, which posits hidden variables; and there is the epistemic approach, an example of which is QBism.By section III, we get into a discussion of space and the concepts of emergence and locality. We obtain an interesting take on what the “vacuum” means in quantum field theory. There is a lot to learn here if you are unfamiliar. I could probably reread some of these sections numerous times until it all sinks in. An interesting point made here is called the cosmological constant problem: the quantum contribution to the vacuum energy (including only contributions of wavelengths longer than the Planck length) gives a finite answer which is ten to the 122nd power greater than the value actually observed. Another interesting point is that empty space is “described by a stationary, unchanging quantum state, where nothing is happening from moment to moment.” This is contrary to the notion that the vacuum is full of “quantum fluctuations.” By page 268 of the book, we begin to leave the edge of what is safely understood and venture into uncharted territory. There is some discussion of black holes and entropy; we learn of the Bekenstein-Hawking entropy, for example, and that black holes have a special property: “they represent the highest-entropy states we can have in any given size region of space.” We also learn that gravity changes things. The author states that gravity leads to a “finite number of quantum degrees of freedom,” which “implies a finite-dimensional Hilbert space,” that in turn, “implies that there is some finite number of branches of the wave function.” This would make the number of worlds in the many-worlds theory finite not infinite. Piece of cake!As you can see, the reading level for this is a bit difficult unless you have some learning in the field. So, in conclusion, it seems that “we reached a point where it is no longer practical to draw a bright line between the quantum and classical realms. Everything is quantum.”

⭐I’m a fan of pop-sci books having recently enjoyed books like “Six Easy Pieces” by Richard Feynman, “The Particle Zoo” by Gavin Hesketh and “Reality Is Not What Is Seems: The Journey to Quantum Gravity” by Carlo Rovelli.This book has been a disappointment, or perhaps even worse: an annoyance!I don’t know if it’s the author or the subject matter. I already have a pretty good understanding of our current view of Quantum Physics and I did my best to come to this book and the “Many Worlds” theory with an open mind.The author claims Many Worlds proponents are taking what has become an almost taboo subject: understanding “the MEANING of Quantum world”. He complains that for a long time the accepted practice in the physics world has been to simply accept quantum theory’s incredible accuracy and reliability and just “Shut up and Calculate” without looking deeper. He claims Many Worlds does this but I don’t see much evidence of it.Instead, Many Worlds takes the fundamental equations from quantum theory and strips them back to the bare essentials of the Schrodinger Equation. No exceptions or special cases or unexplained reasons for the apparent collaps of the quantum wave function or “electrons deciding”.What is the cost for such streamlined, elegant, pure maths? Nothing less than entire universes being created with every single quantum wave collapse! Nothing major.These other worlds – unobservable copies of the universe – millions, trillions of them coming into existence every second – the author then repeatedly treats these entire universes as being no big deal and easily ignored. They don’t matter, they are irrelevant to us. The millions of versions of “me”, differing only by an electron here or there – they’re not the “real” me. Me in this world is “me” and the other copies are someone else.On the question of the “splitting” of the universe upon every quantum decision a local or non-local event? Does it propagate at the speed of light or does it happen everywhere at once? The answer: “Whatever is convenient for you – it’s not really a relevant question because, you see, the Schrodinger Equation is satisfied and the quantum wave function continues.About a quarter of the way through I started to get irked by the author flicking away questions like these with what I belive are poor arguments. By half way through I’m starting to view the author like a Flat Earther who believes what they believe without ever being able to convince me because they are coming from a different set of basic beliefs. The quantum wave function is preserved, the maths is pure. They can happily “shut up and calculate” and to me, haven’t really answered anything.I came with the best open mind I could, but I’m left thinking Many Worlds is simply rubbish. Maybe a different author could explain it better.

⭐I am not a physicist, just a 75-year-old retiree with an interest in science. I have read all Sean Carroll’s books and “Something Deeply Hidden” is without doubt his pièce de résistance. The book explains in an intelligible way a deeply complex theory and anyone interested in how the physical world may work will enjoy it immensely. No doubt a few theoretical physicists will take issue with some of the book’s details but for people like me it is a masterpiece. Thank you, Sean Carroll, for stepping outside of the academic bubble and taking the time to entertain and inform the rest of us (again!).

⭐The book needs more organisation and thought. Clearly the author was paid by the word, as it is over verbose, and at times, the topics merge together in the reader’s brain, causing a fog. I would have liked a multi-page summary table at the start, itemising & classifying every theory, together with summaries of their key features, strengths and weaknesses. The summary should also itemise what predictions each theory has made, and the numerical accuracy of each prediction against experiment. Without this, it is just so much waffle.The author is obsessed with the Multi-Worlds theory. If I think about tossing a coin 6 times, I can draw a tree diagram showing every possible permutation, but I do not think that the universe has split into 2 universes at each toss. The author also talks about the wave function of the whole universe, which is rather meaningless.Sadly, physics is political, where different theories have become religions, which seems to be an excuse for a lack of understanding. The research that gets funded is heavily influenced by tribal groups, which is wrong for what should be a science. Maybe that explains the lack of progress in the area.

⭐Full of spelling mistakes and grammatical errors, this makes an otherwise interesting book difficult to read.

⭐Over the past decade or so Sean Carroll has gained a worthy reputation as one of the best communicators in his sphere of scientific research (cosmology and quantum theory), and his recent books bear testament to this, being among the most comprehensible in print at the present time. Something Deeply Hidden is particularly valuable in that it explains, in the clearest terms, one of the most misunderstood and tantalising (if not to say downright weird) theories in quantum mechanics – the somewhat misnamed Many Worlds interpretation.Whilst Carroll makes no bones about the fact that this is his preferred theory, he also offers a fair – and easily understandable – overview of the alternative interpretations, so the book does a very good job at presenting the most credible options that are available at the present time.I must confess to having been a complete sceptic as to the plausibility of the Many Worlds interpretation before reading this marvellous tome, but it has at least partly shifted my perspective and I now consider the theory as at least a viable contender, somewhat more credible than the instantaneous collapse of the wave function variants which most people seem to believe in.As in his excellent series of MindScape podcasts (which I also highly recommend), Carroll takes some incredibly abstuse concepts and makes them comprehensible to just about anyone with an enquiring mind and a willingness to engage with some of the most fantastic concepts the human mind has so far had to ponder. With so few practicising scientists even remotely interested in the foundations of quantum mechanics and all too willing to regard it as a black box Carroll does an excellent job in reminding us that quantum theory still contains within it some very profound mysteries and that much work has yet to be done if we are to understand exactly what quantum mechanics is. It’s a sobering thought that our entire civilisation, our entire technological revolution appears to be based on an area of science that has the flimsiest of foundations and which no one really understands…

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