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Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime Hardcover – Illustrated, September 10, 2019
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Editorial Reviews
Review
“What makes Carroll's new project so worthwhile, though, is that while he is most certainly choosing sides in the debate, he offers us a cogent, clear and compelling guide to the subject while letting his passion for the scientific questions shine through every page.”—NPR
“Enlightening and refreshingly bold.”—Scientific American
“Something Deeply Hidden is Carroll’s ambitious and engaging foray into what quantum mechanics really means and what it tells us about physical reality.”—Science Magazine
“Carroll argues with a healthy restlessness that makes his book more interesting than so many others in the quantum physics genre.”—Forbes
“If you want to know why some people take [the Everett] approach seriously and what you can do with it, then Carroll’s latest is one of the best popular books on the market.”—Physics Today
“Be prepared to deal with some equations — and to have your mind blown.”—Geek Wire
“By far the most articulate and cogent defense of the ManyWorlds view in booklength depth with a close connection to the latest ongoing research.”—Science News
"Solid arguments and engaging historical backdrop will captivate scienceminded readers everywhere.”—Scientific Inquirer
"As a smart and intensely readable undergraduate class in the history of quantum theory and the nature of quantum mechanics, Something Deeply Hidden could scarcely be improved."—Steve Donoghue, Open Letters Monthly
“Readers in this universe (and others?) will relish the opportunity to explore the frontiers of science in the company of titans.”—Booklist
“Fans of popular science authors such as Neil deGrasse Tyson and John Gribbin will find great joy while exploring these groundbreaking concepts.”—Library Journal
“[A] challenging, provocative book... moving smoothly through different topics and from objects as small as particles to those as enormous as black holes, Carroll’s exploration of quantum theory introduces readers to some of the most groundbreaking ideas in physics today.”—Publishers Weekly
“A thrilling tour through what is perhaps humankind's greatest intellectual achievement—quantum mechanics. With bold clarity, Carroll deftly unmasks quantum weirdness to reveal a strange but utterly wondrous reality.”—Brian Greene, professor of physics and mathematics, director of the Columbia Center for Theoretical Physics, author of The Elegant Universe
“Sean Carroll’s immensely enjoyable Something Deeply Hidden brings readers face to face with the fundamental quantum weirdness of the universe—or should I say universes? And by the end, you may catch yourself finding quantum weirdness not all that weird.”—Jordan Ellenberg, professor of mathematics, University of WisconsinMadison, author of How Not To Be Wrong
“Sean Carroll is always lucid and funny, gratifyingly readable, while still excavating depths. He advocates an acceptance of quantum mechanics at its most minimal, its most austere – appealing to the allure of the pristine. The consequence is an annihilation of our conventional notions of reality in favor of an utterly surreal world of Many Worlds. Sean includes us in the battle between a simple reality versus a multitude of realities that feels barely on the periphery of human comprehension. He includes us in the ideas, the philosophy, and the foment of revolution. A fascinating and important book.”—Janna Levin, professor of physics & astronomy, Barnard College of Columbia University, author of Black Hole Blues
“Sean Carroll beautifully clarifies the debate about the foundations of quantum mechanics, and champions the most elegant, courageous approach: the astonishing “many worlds” interpretation. His explanations of its pros and cons are clear, evenhanded, and philosophically gob smacking.”—Steven Strogatz, professor of mathematics, Cornell University, and author of Infinite Powers
“Carroll gives us a frontrow seat to the development of a new vision of physics: one that connects our everyday experiences to a dizzying hallofmirrors universe in which our very sense of self is challenged. It's a fascinating idea, and one that just might hold clues to a deeper reality.”—Katie Mack, theoretical astrophysicist, North Carolina State University, author of The End of Everything (forthcoming)
“I was overwhelmed by tears of joy at seeing so many fundamental issues explained as well as they ever have been. Something Deeply Hidden is a masterpiece, which stands along with Feynman's QED as one of the two best popularizations of quantum mechanics I've ever seen. And if we classify QED as having had different goals, then it's just the best popularization of quantum mechanics I've ever seen, full stop.”—Scott Aaronson, professor of computer science at the University of Texas at Austin, and Director of UT’s Quantum Information Center
“Irresistible and an absolute treat to read. While this is a book about some of the deepest current mysteries in physics, it is also a book about metaphysics as Carroll lucidly guides us on how to not only think about the true and hidden nature of reality but also how to make sense of it. I loved this book.”—Priyamvada Natarajan, theoretical astrophysicist, Yale University, author of Mapping the Heavens
About the Author
Excerpt. © Reprinted by permission. All rights reserved.
1
What's Going On:
Looking at the Quantum World
Albert Einstein, who had a way with words as well as with equations, was the one who stuck quantum mechanics with the label it has been unable to shake ever since: spukhafte, usually translated from German to English as "spooky." If nothing else, that's the impression we get from most public discussions of quantum mechanics. We're told that it's a part of physics that is unavoidably mystifying, weird, bizarre, unknowable, strange, baffling. Spooky.
Inscrutability can be alluring. Like a mysterious, sexy stranger, quantum mechanics tempts us into projecting all sorts of qualities and capacities onto it, whether they are there or not. A brief search for books with "quantum" in the title reveals the following list of purported applications:
Quantum Success
Quantum Leadership
Quantum Consciousness
Quantum Touch
Quantum Yoga
Quantum Eating
Quantum Psychology
Quantum Mind
Quantum Glory
Quantum Forgiveness
Quantum Theology
Quantum Happiness
Quantum Poetry
Quantum Teaching
Quantum Faith
Quantum Love
For a branch of physics that is often described as only being relevant to microscopic processes involving subatomic particles, that's a pretty impressive rŽsumŽ.
To be fair, quantum mechanicsor "quantum physics," or "quantum theory," the labels are all interchangeableis 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 closeup that the apparent weirdness of quantum phenomena becomes unavoidable.
One of the themes in this book is that quantum mechanics doesn't deserve the connotations of spookiness, in the sense of some ineffable mystery that it is beyond the human mind to comprehend. Quantum mechanics is amazing; it is novel, profound, mindstretching, and a very different view of reality from what we're used to. Science is like that sometimes. But if the subject seems difficult or puzzling, the scientific response is to solve the puzzle, not to pretend it's not there. There's every reason to think we can do that for quantum mechanics just like any other physical theory.
Many presentations of quantum mechanics follow a typical pattern. First, they point to some counterintuitive quantum phenomenon. Next, they express bafflement that the world can possibly be that way, and despair of it making sense. Finally (if you're lucky), they attempt some sort of explanation.
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.
We will make no effort to follow historical order. In this chapter we'll look at the basic experimental facts that force quantum mechanics upon us, and in the next we'll quickly sketch the ManyWorlds approach to making sense of those observations. Only in the chapter after that will we offer a semihistorical account of the discoveries that led people to contemplate such a dramatically new kind of physics in the first place. Then we'll hammer home exactly how dramatic some of the implications of quantum mechanics really are.
With all that in place, over the rest of the book we can set about the fun task of seeing where all this leads, demystifying the most striking features of quantum reality.
¡¡¡
Physics is one of the most basic sciences, indeed one of the most basic human endeavors. We look around the world, we see it is full of stuff. What is that stuff, and how does it behave?
These are questions that have been asked ever since people started asking questions. In ancient Greece, physics was thought of as the general study of change and motion, of both living and nonliving matter. Aristotle spoke a vocabulary of tendencies, purposes, and causes. How an entity moves and changes can be explained by reference to its inner nature and to external powers acting upon it. Typical objects, for example, might by nature be at rest; in order for them to move, it is necessary that something be causing that motion.
All of this changed thanks to a clever chap named Isaac Newton. In 1687 he published Principia Mathematica, the most important work in the history of physics. It was there that he laid the groundwork for what we now call "classical" or simply "Newtonian" mechanics. Newton blew away any dusty talk of natures and purposes, revealing what lay underneath: a crisp, rigorous mathematical formalism with which teachers continue to torment students to this very day.
Whatever memory you may have of highschool or college homework assignments dealing with pendulums and inclined planes, the basic ideas of classical mechanics are pretty simple. Consider an object such as a rock. Ignore everything about the rock that a geologist might consider interesting, such as its color and composition. Put aside the possibility that the basic structure of the rock might change, for example, if you smashed it to pieces with a hammer. Reduce your mental image of the rock down to its most abstract form: the rock is an object, and that object has a location in space, and that location changes with time.
Classical mechanics tells us precisely how the position of the rock changes with time. We're very used to that by now, so it's worth reflecting on how impressive this is. Newton doesn't hand us some vague platitudes about the general tendency of rocks to move more or less in this or that fashion. He gives us exact, unbreakable rules for how everything in the universe moves in response to everything elserules that can be used to catch baseballs or land rovers on Mars.
Here's how it works. At any one moment, the rock will have a position and also a velocity, a rate at which it's moving. According to Newton, if no forces act on the rock, it will continue to move in a straight line at constant velocity, for all time. (Already this is a major departure from Aristotle, who would have told you that objects need to be constantly pushed if they are to be kept in motion.) If a force does act on the rock, it will cause accelerationsome change in the velocity of the rock, which might make it go faster, or slower, or merely alter its directionin direct proportion to how much force is applied.
That's basically it. To figure out the entire trajectory of the rock, you need to tell me its position, its velocity, and what forces are acting on it. Newton's equations tell you the rest. Forces might include the force of gravity, or the force of your hand if you pick up the rock and throw it, or the force from the ground when the rock comes to land. The idea works just as well for billiard balls or rocket ships or planets. The project of physics, within this classical paradigm, consists essentially of figuring out what makes up the stuff of the universe (rocks and so forth) and what forces act on them.
Classical physics provides a straightforward picture of the world, but a number of crucial moves were made along the way to setting it up. Notice that we had to be very specific about what information we required to figure out what would happen to the rock: its position, its velocity, and the forces acting on it. We can think of those forces as being part of the outside world, and the important information about the rock itself as consisting of just its position and velocity. The acceleration of the rock at any moment in time, by contrast, is not something we need to specify; that's exactly what Newton's laws allow us to calculate from the position and the velocity.
Together, the position and velocity make up the state of any object in classical mechanics. If we have a system with multiple moving parts, the classical state of that entire system is just a list of the states of each of the individual parts. The air in a normalsized room will have perhaps 10 molecules of different types, and the state of that air would be a list of the position and velocity of every one of them. (Strictly speaking physicists like to use the momentum of each particle, rather than its velocity, but as far as Newtonian mechanics is concerned the momentum is simply the particle's mass times its velocity.) The set of all possible states that a system could have is known as the phase space of the system.
The French mathematician PierreSimon Laplace pointed out a profound implication of the classicalmechanics 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 machinery of classical physics is so beautiful and compelling that it seems almost inescapable once you grasp it. Many great minds who came after Newton were convinced that the basic superstructure of physics had been solved, and future progress lay in figuring out exactly what realization of classical physics (which particles, which forces) was the right one to describe the universe as a whole. Even relativity, which was worldtransforming in its own way, is a variety of classical mechanics rather than a replacement for it.
Then along came quantum mechanics, and everything changed.
¡¡¡
Alongside Newton's formulation of classical mechanics, the invention of quantum mechanics represents the other great revolution in the history of physics. Unlike anything that had come before, quantum theory didn't propose a particular physical model within the basic classical framework; it discarded that framework entirely, replacing it with something profoundly different.
The fundamental new element of quantum mechanics, the thing that makes it unequivocally distinct from its classical predecessor, centers on the question of what it means to measure something about a quantum system. What exactly a measurement is, and what happens when we measure something, and what this all tells us about what's really happening behind the scenes: together, these questions constitute what's called the measurement problem of quantum mechanics. There is absolutely no consensus within physics or philosophy on how to solve the measurement problem, although there are a number of promising ideas.
Attempts to address the measurement problem have led to the emergence of a field known as the interpretation of quantum mechanics, although the label isn't very accurate. "Interpretations" are things that we might apply to a work of literature or art, where people might have different ways of thinking about the same basic object. What's going on in quantum mechanics is something else: a competition between truly distinct scientific theories, incompatible ways of making sense of the physical world. For this reason, modern workers in this field prefer to call it "foundations of quantum mechanics." The subject of quantum foundations is part of science, not literary criticism.
Nobody ever felt the need to talk about "interpretations of classical mechanics"classical mechanics is perfectly transparent. There is a mathematical formalism that speaks of positions and velocities and trajectories, and oh, look: there is a rock whose actual motion in the world obeys the predictions of that formalism. There is, in particular, no such thing as a measurement problem in classical mechanics. The state of the system is given by its position and its velocity, and if we want to measure those quantities, we simply do so. Of course, we can measure the system sloppily or crudely, thereby obtaining imprecise results or altering the system itself. But we don't have to; just by being careful, we can precisely measure everything there is to know about the system without altering it in any noticeable way. Classical mechanics offers a clear and unambiguous relationship between what we see and what the theory describes.
Quantum mechanics, for all of its successes, offers no such thing. The enigma at the heart of quantum reality can be summed up in a simple motto: what we see when we look at the world seems to be fundamentally different from what actually is.
¡¡¡
Think about electrons, the elementary particles orbiting atomic nuclei, whose interactions are responsible for all of chemistry and hence almost everything interesting around you right now. As we did with the rock, we can ignore some of the electron's specific properties, like its spin and the fact that it has an electric field. (Really we could just stick with the rock as our examplerocks are quantum systems just as much as electrons arebut switching to a subatomic particle helps us remember that the features distinguishing quantum mechanics only really become evident when we consider very tiny objects indeed.)
Unlike in classical mechanics, where the state of a system is described by its position and velocity, the nature of a quantum system is something a bit less concrete. Consider an electron in its natural habitat, orbiting the nucleus of an atom. You might think, from the word "orbit" as well as from the numerous cartoon depictions of atoms you have doubtless been exposed to over the years, that the orbit of an electron is more or less like the orbit of a planet in the solar system. The electron (so you might think) has a location, and a velocity, and as time passes it zips around the central nucleus in a circle or maybe an ellipse.
Quantum mechanics suggests something different. We can measure values of the location or velocity (though not at the same time), and if we are sufficiently careful and talented experimenters we will obtain some answer. But what we're seeing through such a measurement is not the actual, complete, unvarnished state of the electron. Indeed, the particular measurement outcome we will obtain cannot be predicted with perfect confidence, in a profound departure from the ideas of classical mechanics. 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, "its location and its velocity," is therefore replaced in quantum mechanics by something utterly alien to our everyday experience: a cloud of probability. For an electron in an atom, this cloud is more dense toward the center and thins out as we get farther away. Where the cloud is thickest, the probability of seeing the electron is highest; where it is diluted almost to imperceptibility, the probability of seeing the electron is vanishingly small.
This cloud is often called a wave function, because it can oscillate like a wave, as the most probable measurement outcome changes over time. We usually denote a wave function by , the Greek letter Psi. For every possible measurement outcome, such as the position of the particle, the wave function assigns a specific number, called the amplitude associated with that outcome. The amplitude that a particle is at some position x0, for example, would be written (x0).
Product details
 Item Weight : 1.24 pounds
 Hardcover : 368 pages
 ISBN10 : 1524743011
 ISBN13 : 9781524743017
 Product Dimensions : 6.2 x 1.2 x 9.3 inches
 Publisher : Dutton; Illustrated Edition (September 10, 2019)
 Language: : English

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So it was with pleasure and interest that I came across Sean Carroll’s book that also comes down on the side of the many worlds interpretation. The MWI goes back to the very invention of quantum theory by pioneering physicists like Niels Bohr, Werner Heisenberg and Erwin Schrödinger. As exemplified by Heisenberg’s famous uncertainty principle, quantum theory signaled a striking break with reality by demonstrating that one can only talk about the world only probabilistically. Contrary to common belief, this does not mean that there is no precision in the predictions of quantum mechanics – it’s in fact the most accurate scientific framework known to science, with theory and experiment agreeing to several decimal places – but rather that there is a natural limit and fuzziness in how accurately we can describe reality. As Bohr put it, “physics does not describe reality; it describes reality as subjected to our measuring instruments and observations.” This is actually a reasonable view – what we see through a microscope and telescope obviously depends on the features of that particular microscope or telescope – but quantum theory went further, showing that the uncertainty in the behavior of the subatomic world is an inherent feature of the natural world, one that doesn’t simply come about because of uncertainty in experimental observations or instrument error.
At the heart of the probabilistic framework of quantum theory is the wave function. The wave function is a mathematical function that describes the state of the system, and its square gives a measure of the probability of what state the system is in. The controversy starts right away with this most fundamental entity. Some people think that the wave function is “epistemic”, in the sense that it’s not a real object and is simply related to our knowledge – or our ignorance – of the system. Others including Carroll think it’s “ontological”, in the sense of being a real entity that describes features of the system. The fly in the ointment concerns the act of actually measuring this wave function and therefore the state of a quantum system, and this socalled “measurement problem” is as old as the theory itself and kept even the pioneers of quantum theory awake.
The problem is that once a quantum system interacts with an “observer”, say a scintillation screen or a particle accelerator, its wave function “collapses” because the system is no longer described probabilistically and we know for certain what it’s like. But this raises two problems: Firstly, how do you exactly describe the interaction of a microscopic system with a macroscopic object like a particle accelerator? When exactly does the wave function “collapse”, by what mechanism and in what time interval? And who can collapse the wave function? Does it need to be human observers for instance, or can an ant or a computer do it? What can we in fact say about the consciousness of the entity that brings about its collapse?
The second problem is that contrary to popular belief, quantum theory is not just a theory of the microscopic world – it’s a theory of everything except gravity (for now). This led Erwin Schrödinger to postulate his famous cat paradox which demonstrated the problems inherent in the interpretation of the theory. Before measurement, Schrödinger said, a system is deemed to exist in a superposition of states while after measurement it exists only in one; does this mean that macroscopic objects like cats also exist in a superposition of entangled states, in case of his experiment in a mixture of half deadhalf alive states? The possibility bothered Schrödinger and his friend Einstein to no end. Einstein in particular refused to believe that quantum theory was the final word, and there must be “hidden variables” that would allow us to get rid of the probabilities if only we knew what they were; he called the seemingly instantaneous entanglement of quantum states “spooky action at a distance”. Physicist John Bell put that particular objection to rest in the 1960s, proving that at least local quantum theories could not be based on hidden variables.
Niels Bohr and his group of followers from Copenhagen were more successful in their publicity campaign. They simply declared the question of what is “real” before measurement irrelevant and essentially pushed the details of the measurement problem under the rug by saying that the act of observation makes something real. The cracks were evident even then – the physicist Robert Serber once pointedly pointed out problems with putting the observer on a pedestal by asking if we might regard the Big Bang unreal because there were no observers back then. But Bohr and his colleagues were widespread and rather zealous, and most attempts by physicists like Einstein and David Bohm met with either derision or indifference.
Enter Hugh Everett who was a student of John Wheeler at Princeton. Everett essentially applied Occam’s Razor to the problem of collapse and asked a provocative question: What are the implications if we simply assume that the wave function does not collapse? While this avoids asking about the aforementioned complications with measurement, it creates problems of its own since we know for a fact that we can observe only one reality (dead vs alive cat, an electron track here rather than there) while the wave function previously described a mixture of realities. This is where Everett made a bold and revolutionary proposal, one that was as courageous as Einstein’s proposal of the constancy of the speed of light: he surmised that when there is a measurement, the other realities encoded in the wavefunction split off from our own. They simply don’t collapse and are every bit as real as our own. Just like Einstein showed in his theory of relativity that there are no privileged observers, Everett conjectured that there are no privileged observercreated realities. This is the socalled manyworlds interpretation of quantum mechanics.
Everett proposed this audacious claim in his PhD thesis in 1957 and showed it to Wheeler. Wheeler was an enormously influential physicist, and while he was famous for outlandish ideas that influenced generations of physicists like Richard Feynman and Kip Thorne, he was also a devotee of Bohr’s Copenhagen school – he and Bohr had published a seminal paper explaining nuclear fission way back in 1939, and Wheeler regarded Bohr’s Delphic pronouncements akin to those of Confucius – that posited observergenerated reality. He was sympathetic to Everett but could not support him in the face of Bohr’s objections. Everett soon left theoretical physics and spent the rest of his career doing nuclear weapons research, a chainsmoking, secretive, absentee father who dropped dead of an unhealthy lifestyle in 1982. After a brief resurrection by Everett himself at a conference organized by Wheeler, manyworlds didn’t see much popular dissemination until writers like Casti and the physicist David Deutsch wrote about it.
As Carroll indicates, the MWI has a lot of things going for it. It avoids the prickly, convoluted details of what exactly constitutes a measurement and the exact mechanism behind it; it does away with especially thorny details of what kind of consciousness can collapse a wavefunction. It’s elegant and satisfies Occam’s Razor because it simply postulates two entities – a wave function and a Schrödinger equation through which the wave function evolves through time, and nothing else. One can calculate the likelihood of each of the “many worlds” by postulating a simple rule proposed by Max Born that assigns a weight to every probability. And it also avoids an inconvenient split between the quantum and the classical world, treating both systems quantum mechanically. According to the MWI, when an observer interacts with an electron, for instance, the observer’s wave function becomes entangled with the electron’s and continues to evolve. The reason why we still see only one Schrödinger’s cat (dead or alive) is because each one is triggered by distinct random events like the passage of photons, leading to separate outcomes. Carroll thus sees manyworlds as basically a logical extension of the standard machinery of quantum theory. In fact he doesn’t even see the many worlds as “emerging” (although he does see them as emergent); he sees them as always present and intrinsically encoded in the wave function’s evolution through the Schrödinger equation.
A scientific theory is of course only as good as its experimental predictions and verification – as a quote ascribed to Ludwig Boltzmann puts it, matters of elegance should be left to the tailor and the cobbler. Does MWI postulate elements of reality that are different from those postulated by other interpretations? The framework is on shakier ground here since there are no clear observable predictions except those predicted by standard quantum theory that would truly privilege it over others. Currently it seems that the best we can say is that many worlds is consistent with many standard features of quantum mechanics. But so are many other interpretations. To be accepted as a preferred interpretation, a theory should not just be consistent with experiment, but uniquely so. For instance, consider one of the very foundations of quantum theory – waveparticle duality. Waveparticle duality is as counterintuitive and otherworldly as any other concept, but it’s only by postulating this idea that we can ever make sense of disparate experiments verifying quantum mechanics, experiments like the doubleslit experiment and the photoelectric effect. If we get rid of waveparticle duality from our lexicon of quantum concepts, there is no way we can ever interpret the results of thousands of experiments from the subatomic world such as particle collisions in accelerators. There is thus a necessary, onetoone correspondence between waveparticle duality and reality. If we get rid of manyworlds, however, it does not make any difference to any of the results of quantum theory, only to what we believe about them. Thus, at least as of now, manyworlds remains a philosophically pleasing framework than a preferred scientific one.
Manyworlds also raises some thorny questions about the multiple worlds that it postulates. Is it really reasonable to believe that there are literally an infinite copies of everything – not just an electron but the measuring instrument that observes it and the human being who records the result – splitting off every moment? Are there copies of me both writing this post and not writing it splitting off as I type these words? Is the universe really full of these multiple worlds, or does it make more sense to think of infinite universes? One reasonable answer to this question is to say that quantum theory is a textbook example of how language clashes with mathematics. This was wellrecognized by the early pioneers like Bohr: Bohr was fond of an example where a child goes into a store and asks for some mixed sweets. The shopkeeper gives him two sweets and asks him to mix them himself. We might say that an electron is in “two places at the same time”, but any attempt to actually visualize this dooms us, because the only notion of objects existing in two places is one that is familiar to us from the classical world, and the analogy breaks down when we try to replace chairs or people with electrons. Visualizing an electron spinning on its axis the way the earth spins on its is also flawed.
Similarly, visualizing multiple copies of yourself actually splitting off every nanosecond sounds outlandish, but it’s only because that’s the only way for us to make sense of wave functions entangling and then splitting. Ultimately there’s only the math, and any attempts to cast it in the form of everyday language is a fundamentally misguided venture. Perhaps when it comes to talking about these things, we will have to resort to Wittgenstein’s famous quote – whereof we cannot speak, thereof we must be silent (or thereof we must simply speak in the form of pictures, as Wittgenstein did in his famous ‘Tractatus’). The other thing one can say about manyworlds is that while it does apply Occam’s Razor to elegantly postulating only the wave function and the Schrödinger equation, it raises questions about the splitting off process and the details of the multiple worlds that are similar to those about the details of measurement raised by the measurement problem. In that sense it only kicks the can of complex worms down the road, and in that case believing what particular can to open is a matter of taste. As an old saying goes, nature does not always shave with Occam’s Razor.
In the last part of the book, Carroll talks about some fascinating developments in quantum gravity, mainly the notion that gravity can emerge through microscopic degrees of freedom that are locally entangled with each other. One reason why this discussion is fascinating is because it connects many disparate ideas from physics into a potentially unifying picture – quantum entanglement, gravity, black holes and their thermodynamics. These developments don’t have much to do with manyworlds per se, but Carroll thinks they may limit the number of “worlds” that many worlds can postulate. But it’s frankly difficult to see how one can find definitive experimental evidence for any interpretation of quantum theory anytime soon, and in that sense Richard Feynman’s famous words, “I think it is safe to say that nobody understands quantum mechanics” may perpetually ring true.
Very reasonably, manyworlds is Carroll’s preferred take on quantum theory, but he’s not a zealot about it. He fully recognizes its limitations and discuss competing interpretations. But while Carroll deftly dissects manyworlds, I think that the real value of this book is to exhort physicists to take what are called the foundations of quantum mechanics more seriously. It is an attempt to make peace between different quantum factions and bring philosophers into the fold. There’s a huge number of “interpretations” of quantum theory, some more valid than others, being separated by each other as much by philosophical differences as by physical ones. There was a time when the spectacular results of quantum theory combined with the thorny philosophical problems it raised led to a tendency among physicists to “shut up and calculate” and not worry about philosophical matters. But philosophy and physics have been entwined since the ancient Greeks, and in one sense, one ends where the other begins. Carroll’s book is a hearty reminder for physicists and philosophers to eat at the same table, otherwise they may well remain spooky factions at a distance when it comes to interpreting quantum theory.
The huge success of QM comes from using the wavefunction to compute mathematical probabilities via the Born Rule. However in Many Worlds, every possible outcome occurs in some "world" when everything occurs, there is no probability that it won't. Ay, there's the rub with Many Worlds. Philosophers and some physicists have spent the last 60 years unsuccessfully trying to reintroduce the probabilistic Born Rule into their deterministic Many Worlds approach because without the Born Rule, Many Worlds is a Dud.
The biggest bragging right of MWs is that it is a minimalist theory using only the deterministic Schrodinger Equation without auxiliary baggage. Carroll attempts to insert the Born Rule into MWs (see p.140141) by imagining "selflocating observers " who can "see" the wavefunction (which resides in Hilbert space, not ordinary 3D) and make Bornlike probability computations. This all supposedly happening in an infinitesimal interval between the branching of the wavefunction and an observation. That infinitesmal interval is Carroll's only excuse for jamming probability into seemingly deterministic MWs. In fact, he says it"... gives us an opening to talk about probabilities. In that moment after branching, both copies of you are subject to selflocating uncertainty...." (pp. 140141). However, other than Carroll's assertion that two copies of us exist in this infinitesimal interval and that these imaginary observers can somehow see the wavefunction branching, what justification is there for these totally outlandish claims? None whatsoever! I'll continue my critique below, but really folks, the game is up Carroll is just pulling all of this out of his derriere (there is no science here at all) in a last gasp attempt to save Many Worlds!
Now, for a moment, imagine that an amoeba splits and takes two separate, independent paths just as is supposed to happen in MWs. There isn't a 50% chance the amoeba went to the left and a 50% that it went to the rightit does Both. In Many Worlds probability on different branches doesn't sum to 1 like we are accustomed to. Nevertheless, Carroll assumes probability sums to 1 in his arguments on p.142150. In fact, his entire reason for introducing "selflocating observers" is to manufacture an excuse for inserting this assumption, because then the Born Rule directly follows.
Perhaps it could be argued that these are only meant to be mythical selflocating observers and shouldn't be taken literally. But if so, it follows that Carroll's version of Many Worlds rests firmly upon a myth not a good look in physics! Philosophers might happily spend decades debating elaborate imaginary "selflocating observers", but physicists expect no, demand observers to perform and produce verifiable results. The major complaint about the Copenhagen interpretation has been its ad hoc rules, but is replacing those rules with Carroll's SciFi characters an advancement?
Remarkably, Heisenburg seemed to anticipate wild inventions such as 'selflocating observers' when he wrote "There is no use discussing what could be done if we were other beings than we are." Tell me that doesn't hit the mark when it comes to this book! Nevertheless, Carroll pretends that his mythical observers answer everything. In fact, on p.142, Carroll congratulates himself saying, in essence, "Nothing more to see here, just have Faith! This is what Carroll leaves us with  have faith in invisible beings  sound familiar?
Usually, in my experience, hiding behind nonexplanatory lingo like this is a sign the writer doesn't really understand the subject matter.
The Mr. Carroll should take a good, hard look at "Relativity Visualized" by Lewis Carroll Epstein if he wants to get an inkling of how to actually explain scientific phenomena to those who aren't members of the club. Much as he seems to think relativity is merely an adjunct of Newtonian physics and not of great significance compared to quantum mechanics, maybe he'd get a clue if he'd learn a bit more about it.
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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 nonlocal 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.
The author is obsessed with the MultiWorlds 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.
Sean Carroll ist theoretischen Physiker am Caltech in Pasadena, er befasst sich mit fundamentalen Fragen der Physik und Kosmologie, darunter zum Ursprung der Universums, zur Quantengravitation und der Natur der dunklen Materie und Energie. Aber Carroll beschäftigt sich auch mit der Popularisierung aktueller Erkenntnisse seines Fachgebiet, nicht nur als Autor von Büchern wie 'From Eternity To Here', 'The Particle at the End of the Universe' und 'The Big Picture', sondern er ist auch ein bekannter Protagonist von Wissenschaftssendungen auf History Channel, PBS und in der von Morgan Freeman moderierten Serie 'Through the Wormhole' (dt. 'Mysterien des Weltalls').
Anlass für das vorliegenden Buch sind die erwähnten Arbeiten von Carroll und Cao zur Quantengravitation. Zwar gibt es seit geraumer Zeit verschiedene Konzepte, die in der Regel dem üblichen Verfahren der Quantisierung einer klassischen Theorie folgen – das stößt im Fall der Gravitation auf diverse Schwierigkeiten, so dass die entstehende Theorie für starke Gravitationsfelder scheitert. Dem gegenüber erinnert der Autor daran, dass die Welt intrinsisch quantenmechanisch ist, er meint, dass man das Problem der Quantengravitation nur dann auflösen kann, wenn man versteht, wie Raum und Zeit aus Quantenphänomenen entstehen und die Gravitation auf natürliche Weise einschließen.
Seiner Darstellung stellt der Autor eine allgemein gehaltene Einführung in die Quantenmechanik voran, die direkt und präzise ausgefallen ist. Nach einem knappen historischen Exkurs, kommt er recht zügig auf Wellenfunktionen und Schrödingers Gleichung, die deren zeitliche Entwicklung beschreibt, und die wesentlichen Phänomene, wie Überlagerung, Unschärfe und Verschränkung, zu sprechen. Die Interpretation der Quantenmechanik, die weitgehend in Standard Lehrbüchern vertreten wird, mit ihrer a priori Abgrenzung von Quanten und klassischen Objekten und dem Wellenfunktions Kollaps, nennt der Autor unverständlich und orakelhaft, er folgt hingegen im Wesentlichen Hugh Everett, dessen Viele Welten Interpretation ihm auch hinsichtlich seines Ziel, der 'Ableitung' des Raum aus einer Quantenfeldtheorie, entgegen kommt. Recht ausführlich geht er deswegen auch auf die Grundzüge dieser Interpretation ein. Er betont, dass Everetts Vorstellungen zufolge, nie ein Kollaps stattfindet, die Wellenfunktion des Universums entwickelt sich ungehindert allein nach Schrödingers Gesetz, womit auch das Messproblem einfach gegenstandslos wird. Messungen sind danach nichts weiter als Wechselwirkungen zweier Subsysteme, die danach verschränkt sind. Ist eines dieser Systeme makroskopisch, kommt es unweigerlich zu einer Wechselwirkung mit der Umwelt – nach der Auffassung von Dieter Zeh führt das zur Dekohärenz, d.h. die universelle Wellenfunktion spaltet sich in verschiedene Zweige auf, die sich fortan unabhängig voneinander entwickeln, oder jedenfalls nur mit geringer Wahrscheinlichkeit miteinander interagieren – sie beschreiben insofern verschiedene parallele 'Welten'.
Erst im letzten Drittel des Buches kommt Carroll auf sein eigentliches Thema zu sprechen. Nach einer kurzen Vorstellung der Quantenfeldtheorie, näht er sich der Frage, wie sich der Begriff des Raumes aus einer Wellenfunktion mit abstrakten Freiheitsgraden ergeben könnte, mit einem reverse engineering Ansatz. Benachbarte Gebiete eines Quantenfeldes sind verschränkt, Verschränkung steht aber mit Entropie (John von Neumanns entanglement entropy) in Zusammenhang. Nach Arbeiten von Jacobsen ist diese Entropie proportional zum Flächeninhalt der Begrenzung dieses Bereiches, wenn man die Gravitation berücksichtigt, und aus diesen Flächeninhalten lässt sich die Geometrie rekonstruieren. Der Ansatz von Carroll und Cao setzt nun umgekehrt bei verschränken abstrakten Freiheitsgraden an, denen eine Entropie zugeordnet ist – nun werden Flächeninhalte proportional zur Entropie definiert, aus denen sich die Dynamik der Allgemeinen Relativitätstheorie ableiten lässt. Die genauen Bedingungen, unter denen dieses Konzept funktioniert, liegen dabei aber noch im Dunkeln, nicht alle Voraussetzungen, die zwar plausibel sind. lassen sich auch bereits beweisen. Leider gelten die bereits besser verstandenen Fälle der Theorie nur für schwache Gravitation.
Auf einige der offenen Enden, im Zusammenhang mit Phänomene starker Gravitation, wie Schwarze Löcher und der Urknall selbst, geht Carroll In einem abschließenden Kapitel ein. Hawking hatte in einem semi klassischen Ansatz mit Mitteln der Quantenfeldtheorie nachgewiesen, dass Schwarze Löcher eine thermische Strahlung emittieren. Damit in Zusammenhang steht auch das Informations Paradoxon Schwarzer Löcher. Diese Phänomene wurden genau untersucht und gelten als stabil, d.h. sie sind unabhängig von der genauen Struktur einer Quantengravitationstheorie. Genau deswegen sind sie Prüfsteine für künftige Theorie, im Rahmen derer sie stringent verstanden werden müssen. Sie zeigen damit aber auch die Grenzen, der bisherigen Vorstellungen auf. Trotz zahlreicher offenen Probleme, ist Autor hoffnungsvoll, dass der eingeschlagene Weg zu einem besseren Verständnis des Universums führen wird.
Ergänzt wird der Text durch einen Anhang über virtuelle Teilchen, in dem der Autor versucht, die gängigen Missverständnissen im Zusammenhang mit dem Quantenvakuum auszuräumen.
Das Faszinierende an Sean Carrolls Buch ist, dass er darin auf aktuelle Entwicklungen zur Quantengravitation eingeht und allgemeinverständlich darlegt. Es ist damit vergleichbar in der Ambition mit Lee Smolins 'Einsteins Unfinished Revolution', auch wenn Smolin einen ganz anderen Weg einschlägt und die Ursache der Probleme mit der Quantengravitation in einer intrinsischen Unvollständigkeit der Quantenmechanik sieht. Beeindruckend ist die Klarheit von Carrolls Darstellung der Viele Welten Interpretation. Allerdings suggeriert er, dass diese die Bornschen Wahrscheinlichkeits Regel reproduzieren könnten. Das ist nach Steven Weinbergs Analyse des Messproblems nicht möglich (vgl. Lectures on Quantum Mechanics, 2nd ed.). Tatsächlich muss der Autor zusätzliche Annahmen verwenden, auch wenn diese plausibel erscheinen.
Leider behandelt der Autor sein Hauptthema dann etwas stiefmütterlich, am Ende befasst sich nur ein Abschnitt direkt mit den Erläuterungen zur Emergenz von Raum und Zeit, und die Ausführungen darin sind recht vage, verlichen mit dem Stil der Einführungs Kapitel.
Als populärwissenschaftliches Buch ist es etwas mager ausgestattet, neben dem obligatorischen Index, gibt es nur eine kurze Liste mit weiterführender Literatur, der Abschnitt 'References' enthält lediglich, nach Kapiteln geordnete, Quellenangaben zu Anmerkungen im Text. Leide fehlt auch eine separate Bibliographie, wie so of bei dieser Art Buch.
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