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The Emergent Multiverse: Quantum Theory according to the Everett Interpretation
The Emergent Multiverse presents a striking new account of the "many worlds" approach to quantum theory. The point of science, it is generally accepted, is to tell us how the world works and what it is like. But quantum theory seems to fail to do this: taken literally as a theory of the world, it seems to make crazy claims: particles are in two places at once; cats are alive and dead at the same time. So physicists and philosophers have often been led either to give up on the idea that quantum theory describes reality, or to modify or augment the theory.
The Everett interpretation of quantum mechanics takes the apparent craziness seriously, and asks, "what would it be like if particles really were in two places at once, if cats really were alive and dead at the same time?" The answer, it turns out, is that if the world were like that--if it were as quantum theory claims--it would be a world that, at the macroscopic level, was constantly branching into copies--hence the more sensationalist name for the Everett interpretation, the "many worlds theory." But really, the interpretation is not sensationalist at all: it simply takes quantum theory seriously, literally, as a description of the world. Once dismissed as absurd, it is now accepted by many physicists as the best way to make coherent sense of quantum theory.
David Wallace offers a clear and up-to-date survey of work on the Everett interpretation in physics and in philosophy of science, and at the same time provides a self-contained and thoroughly modern account of it--an account which is accessible to readers who have previously studied quantum theory at undergraduate level, and which will shape the future direction of research by leading experts in the field.
The Everett interpretation of quantum mechanics takes the apparent craziness seriously, and asks, "what would it be like if particles really were in two places at once, if cats really were alive and dead at the same time?" The answer, it turns out, is that if the world were like that--if it were as quantum theory claims--it would be a world that, at the macroscopic level, was constantly branching into copies--hence the more sensationalist name for the Everett interpretation, the "many worlds theory." But really, the interpretation is not sensationalist at all: it simply takes quantum theory seriously, literally, as a description of the world. Once dismissed as absurd, it is now accepted by many physicists as the best way to make coherent sense of quantum theory.
David Wallace offers a clear and up-to-date survey of work on the Everett interpretation in physics and in philosophy of science, and at the same time provides a self-contained and thoroughly modern account of it--an account which is accessible to readers who have previously studied quantum theory at undergraduate level, and which will shape the future direction of research by leading experts in the field.
548 pages, Paperback
First published July 13, 2012
About the author
David Wallace
3 books27 followersDavid Wallace was born in San Rafael, California, in 1976, but has been resident in the UK since 1977. He studied theoretical physics at Oxford University from 1994-2002, but upon realising his research interests lay mostly in conceptual and foundational aspects of physics, he moved across into philosophy of physics. For the last six years he has been Tutorial Fellow in Philosophy of Science at Balliol College, Oxford. He holds PhDs in physics and in philosophy, and his research interests span a wide range of issues on the boundary between philosophy and physics: symmetry and the gauge principle, the direction of time, the structure of quantum field theory, and of course the interpretation of quantum mechanics.
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Displaying 1 - 13 of 14 reviews
July 9, 2013
If you've been paying any attention, you must already have at least a vague idea of what the Many-Worlds Interpretation of Quantum Mechanics is about. For example,
Source Code
is a romantic movie treatment;
Transition
is an SF thriller treatment; and
The Grand Design
is a For Dummies treatment. There is not just a single universe, there are a huge number of them, and new ones are constantly splitting off.
But can something that's been this enthusiatically embraced by the SF community really be respectable? In his impressive book, David Wallace argues persuasively that it is. It's not merely a good alternative to conventional interpretations of quantum mechanics; he claims it's the only one that gives us a view of what's going on which makes intuitive sense, and doesn't involve the addition of unprovable or downright mystical ideas like "the collapse of the wavefunction" or "the essential role of consciousness". One's first reaction may well be to label this as paradoxical or willfully contrarian, but Wallace, who has PhDs in both physics and philosophy, lays out his reasoning with skill. Since it's easy to get lost in the many details, I will focus here on two clever analogies which he uses throughout. The first is the heliocentric revolution (Copernicus and Galileo); the second is dinosaurs.
Let's look first at the heliocentric hypothesis. The book opens with a thought-provoking quote from Wittgenstein: what would it have looked like if it had looked like the Earth went round the Sun? Stop and consider that for a moment. The answer, of course, is that it would have looked exactly the same. Every piece of factual evidence people had, which convinced them that the Sun went round the Earth, could equally well have been interpreted in the opposite direction. At the end of the day, the main reason why people were so slow to agree with Copernicus was a simple one. His idea was so goddamn weird that it couldn't possibly be correct.
Similarly with the Many-Worlds Interpretation. Wallace's argument is that this is just the most straightforward way to make sense of the underlying mathematics of quantum theory, which everyone agrees on. You look at what the equations tell you is going to happen when a superposed state (Schrödinger's cat, for example) is allowed to interact with other parts of the world. The result is that the cat's state becomes quantum-entangled with everything else, including any observer who may be present. The math represents this as the sum of two algebraic terms: one stands for the live cat, plus everything else in the world; the other stands for the dead cat, plus everything else in the world. The two terms rapidly "decohere", in other words cease to influence each other. The basic claim of the Many-Worlds Interpretation is that this is best conceptualized as saying that the universe splits into two copies. That's what the math seems to be telling us: why not believe it? Yet, somehow, most people seem reluctant to take this final step. It's too goddamn weird. What do they do instead? The most common alternative is "shut up and calculate": use the equations, since they certainly appear to work, but don't worry about what they mean. Indeed, throw out the question as irrelevant and positively distracting.
So over to the dinosaurs. As Wallace says, suppose people applied the same kind of reasoning to paleontology. There are fossils; everyone agrees on that. Fossils are bits of rock which you can touch. There are consistent patterns in many of these bits of rock, and the only sensible way of explaining these patterns is to say that their appearance is as it would have been if there once had been dinosaurs. Just about everyone agrees on that too. But suppose now that you're talking to a creationist petroleum geologist (I presume such people may exist), who stops at this point and says that there were in fact no dinosaurs; they are just a theoretical device that helps us categorise fossils. You would have a hard time refuting this argument. Our hypothetical geologist would agree with everything you said about the links between fossils and dinosaurs, and in fact she would probably know rather more about it than you did, since it was part of her job. She just wouldn't agree that the dinosaurs actually existed. Needless to say, you would find this person intensely irritating; you would be sure they were wrong, even if you couldn't prove it. Well: the argument here is that we've been doing exactly the same thing in rejecting the Many-Worlds Interpretation.
Quite apart from the content, the style of the book is also interesting, and is constructed as an ingenious piece of homage to Wallace's great predecessors. Stylistically, Copernicus and Galileo were polar opposites: Copernicus was a dry, technical writer, and Galileo was an entertaining polemicist. Copernicus was extremely conservative, and worked entirely within the Ptolemaic system. (As Rovelli remarks in his recent book on Anaximander, no one could have loved Ptolemy more than Copernicus did). His intention was simply to show that Ptolemy's deferents and epicycles worked even better if you moved the Sun to the center of the universe. Galileo, in contrast, wanted to shake things up and introduce genuinely new ideas.
Wallace has daringly attempted to mix these two very different styles. Rather more than two-thirds of the book is Copernican, and consists of lengthy technical proofs; the most important ones have to do with the concept of rational behavior in the quantum multiverse, where it is easy to become confused and think that, since everything is going to happen in some branch, it makes no difference what you do. Wallace shows that this is absolutely not true. In fact, the concept of "branch weight" plays a role exactly analogous to that of probability in a classical theory, and rational agents end up doing what they would have done in a classical universe. Establishing this apparently trivial conclusion unfortunately requires over fifty pages of difficult mathematics. If all the book were like this, it would have been unreadable; despite its honored place in the history of science, it is notorious that hardly anyone has ever read De revolutionibus orbium coelestium. Wallace has addressed this problem by adding a parallel thread written in an engagingly Galilean style, where he explains the intuitive consequences of the ideas in everyday language. The layman will no doubt want more Galileo; on the other hand, the Copernicus is necessary to convince the many sceptical experts, none of whom appear yet to have detected obvious holes. It's a difficult balancing act, but he pulls it off well.
Should you buy The Emergent Multiverse? On the minus side, it's long, it's heavy, it's expensive, and there are large chunks you will most likely not understand. (There were, at any rate, large chunks I didn't understand). On the plus side, it's well-written, it's often funny, it will expand your mental horizons, and it's not impossible that it will turn out to be one of the pivotal books of the twenty-first century. I don't know how to weigh up these competing factors. You will just have to decide for yourself.
__________________________________
Simon Evnine pointed me to the excellent review here. If you're interested in learning more about the technical details, this is where to go.
__________________________________
It's hard to stop thinking about this book. The author makes a strong case for the reality of the quantum multiverse; if his reasoning becomes generally accepted, it is impossible to imagine how fundamentally it will change the way we view the world. At the moment, the evidence is of an indirect nature, as it was when the pioneers of the heliocentric revolution first proposed their idea. The math works out more sensibly when you posit that the Earth goes round the Sun; also, as Aristarchus had pointed out seventeen hundred years earlier, the Sun is evidently much bigger than the Earth, and it seems odd to have the big thing circle the small thing. Direct, smoking-gun proof didn't turn up until Bessel first measured stellar parallax in 1838, but by then the scientific world was already sure that Copernicus had been right. The accumulation of indirect evidence was overwhelming.
In the case of the multiverse, Wallace suggests that the next tranche of indirect evidence will probably come from quantum computing. If things progress a little further along the directions that are currently being explored, it will soon be possible in practice to solve problems with quantum algorithms that cannot be solved at all on conventional computers. People will routinely be writing quantum software and thinking about debugging and improving it. As Wallace says, the natural way to conceptualize some of these algorithms is that the computation is parallelized by sending subtasks into enormous numbers of parallel worlds, then retrieving the answer from the branch which succeeded. When tens of thousands of geeks are spending their working day manipulating the geometry of the multiverse, it will be difficult to maintain the polite pretense that it doesn't actually exist.
Wallace appears reluctant to delve too deeply into the moral and ethical aspects. He demonstrates that rational short-term betting behavior is the same in the multiverse and the classical world; given a choice between a 75% chance and a 25% chance, you should pick the 75% chance, irrespective of whether you believe that all the outcomes will happen in different branches, or that there is only a single world governed by the laws of probability. But in cases like the notorious quantum suicide thought experiment, it is not as clear that things are still the same. Wallace notes that death is "philosophically difficult", and explicitly advises philosophers not to discuss these matters in popular works. There is a striking resonance with the last chapter of Time Reborn , where Smolin expresses concern that belief in multiple universes may lead people to value less the one universe which we can directly perceive around us.
So maybe I shouldn't even be talking about this. But, as Eve said to Adam, those apples just looked so tasty...
But can something that's been this enthusiatically embraced by the SF community really be respectable? In his impressive book, David Wallace argues persuasively that it is. It's not merely a good alternative to conventional interpretations of quantum mechanics; he claims it's the only one that gives us a view of what's going on which makes intuitive sense, and doesn't involve the addition of unprovable or downright mystical ideas like "the collapse of the wavefunction" or "the essential role of consciousness". One's first reaction may well be to label this as paradoxical or willfully contrarian, but Wallace, who has PhDs in both physics and philosophy, lays out his reasoning with skill. Since it's easy to get lost in the many details, I will focus here on two clever analogies which he uses throughout. The first is the heliocentric revolution (Copernicus and Galileo); the second is dinosaurs.
Let's look first at the heliocentric hypothesis. The book opens with a thought-provoking quote from Wittgenstein: what would it have looked like if it had looked like the Earth went round the Sun? Stop and consider that for a moment. The answer, of course, is that it would have looked exactly the same. Every piece of factual evidence people had, which convinced them that the Sun went round the Earth, could equally well have been interpreted in the opposite direction. At the end of the day, the main reason why people were so slow to agree with Copernicus was a simple one. His idea was so goddamn weird that it couldn't possibly be correct.
Similarly with the Many-Worlds Interpretation. Wallace's argument is that this is just the most straightforward way to make sense of the underlying mathematics of quantum theory, which everyone agrees on. You look at what the equations tell you is going to happen when a superposed state (Schrödinger's cat, for example) is allowed to interact with other parts of the world. The result is that the cat's state becomes quantum-entangled with everything else, including any observer who may be present. The math represents this as the sum of two algebraic terms: one stands for the live cat, plus everything else in the world; the other stands for the dead cat, plus everything else in the world. The two terms rapidly "decohere", in other words cease to influence each other. The basic claim of the Many-Worlds Interpretation is that this is best conceptualized as saying that the universe splits into two copies. That's what the math seems to be telling us: why not believe it? Yet, somehow, most people seem reluctant to take this final step. It's too goddamn weird. What do they do instead? The most common alternative is "shut up and calculate": use the equations, since they certainly appear to work, but don't worry about what they mean. Indeed, throw out the question as irrelevant and positively distracting.
So over to the dinosaurs. As Wallace says, suppose people applied the same kind of reasoning to paleontology. There are fossils; everyone agrees on that. Fossils are bits of rock which you can touch. There are consistent patterns in many of these bits of rock, and the only sensible way of explaining these patterns is to say that their appearance is as it would have been if there once had been dinosaurs. Just about everyone agrees on that too. But suppose now that you're talking to a creationist petroleum geologist (I presume such people may exist), who stops at this point and says that there were in fact no dinosaurs; they are just a theoretical device that helps us categorise fossils. You would have a hard time refuting this argument. Our hypothetical geologist would agree with everything you said about the links between fossils and dinosaurs, and in fact she would probably know rather more about it than you did, since it was part of her job. She just wouldn't agree that the dinosaurs actually existed. Needless to say, you would find this person intensely irritating; you would be sure they were wrong, even if you couldn't prove it. Well: the argument here is that we've been doing exactly the same thing in rejecting the Many-Worlds Interpretation.
Quite apart from the content, the style of the book is also interesting, and is constructed as an ingenious piece of homage to Wallace's great predecessors. Stylistically, Copernicus and Galileo were polar opposites: Copernicus was a dry, technical writer, and Galileo was an entertaining polemicist. Copernicus was extremely conservative, and worked entirely within the Ptolemaic system. (As Rovelli remarks in his recent book on Anaximander, no one could have loved Ptolemy more than Copernicus did). His intention was simply to show that Ptolemy's deferents and epicycles worked even better if you moved the Sun to the center of the universe. Galileo, in contrast, wanted to shake things up and introduce genuinely new ideas.
Wallace has daringly attempted to mix these two very different styles. Rather more than two-thirds of the book is Copernican, and consists of lengthy technical proofs; the most important ones have to do with the concept of rational behavior in the quantum multiverse, where it is easy to become confused and think that, since everything is going to happen in some branch, it makes no difference what you do. Wallace shows that this is absolutely not true. In fact, the concept of "branch weight" plays a role exactly analogous to that of probability in a classical theory, and rational agents end up doing what they would have done in a classical universe. Establishing this apparently trivial conclusion unfortunately requires over fifty pages of difficult mathematics. If all the book were like this, it would have been unreadable; despite its honored place in the history of science, it is notorious that hardly anyone has ever read De revolutionibus orbium coelestium. Wallace has addressed this problem by adding a parallel thread written in an engagingly Galilean style, where he explains the intuitive consequences of the ideas in everyday language. The layman will no doubt want more Galileo; on the other hand, the Copernicus is necessary to convince the many sceptical experts, none of whom appear yet to have detected obvious holes. It's a difficult balancing act, but he pulls it off well.
Should you buy The Emergent Multiverse? On the minus side, it's long, it's heavy, it's expensive, and there are large chunks you will most likely not understand. (There were, at any rate, large chunks I didn't understand). On the plus side, it's well-written, it's often funny, it will expand your mental horizons, and it's not impossible that it will turn out to be one of the pivotal books of the twenty-first century. I don't know how to weigh up these competing factors. You will just have to decide for yourself.
__________________________________
Simon Evnine pointed me to the excellent review here. If you're interested in learning more about the technical details, this is where to go.
__________________________________
It's hard to stop thinking about this book. The author makes a strong case for the reality of the quantum multiverse; if his reasoning becomes generally accepted, it is impossible to imagine how fundamentally it will change the way we view the world. At the moment, the evidence is of an indirect nature, as it was when the pioneers of the heliocentric revolution first proposed their idea. The math works out more sensibly when you posit that the Earth goes round the Sun; also, as Aristarchus had pointed out seventeen hundred years earlier, the Sun is evidently much bigger than the Earth, and it seems odd to have the big thing circle the small thing. Direct, smoking-gun proof didn't turn up until Bessel first measured stellar parallax in 1838, but by then the scientific world was already sure that Copernicus had been right. The accumulation of indirect evidence was overwhelming.
In the case of the multiverse, Wallace suggests that the next tranche of indirect evidence will probably come from quantum computing. If things progress a little further along the directions that are currently being explored, it will soon be possible in practice to solve problems with quantum algorithms that cannot be solved at all on conventional computers. People will routinely be writing quantum software and thinking about debugging and improving it. As Wallace says, the natural way to conceptualize some of these algorithms is that the computation is parallelized by sending subtasks into enormous numbers of parallel worlds, then retrieving the answer from the branch which succeeded. When tens of thousands of geeks are spending their working day manipulating the geometry of the multiverse, it will be difficult to maintain the polite pretense that it doesn't actually exist.
Wallace appears reluctant to delve too deeply into the moral and ethical aspects. He demonstrates that rational short-term betting behavior is the same in the multiverse and the classical world; given a choice between a 75% chance and a 25% chance, you should pick the 75% chance, irrespective of whether you believe that all the outcomes will happen in different branches, or that there is only a single world governed by the laws of probability. But in cases like the notorious quantum suicide thought experiment, it is not as clear that things are still the same. Wallace notes that death is "philosophically difficult", and explicitly advises philosophers not to discuss these matters in popular works. There is a striking resonance with the last chapter of Time Reborn , where Smolin expresses concern that belief in multiple universes may lead people to value less the one universe which we can directly perceive around us.
So maybe I shouldn't even be talking about this. But, as Eve said to Adam, those apples just looked so tasty...
Read
October 9, 2013( I understood the introduction , and that was about it )
November 12, 2013
David makes some slightly dubious assumptions in a couple of the proofs, and they could be more elegant, but all in all a remarkably impressive piece of work. I can't understand why there isn't more buzz. He needs to get himself a better agent.
Read
May 27, 2015Convincingly and engagingly lays out the case for and consequences of the Everettian interpretation of quantum mechanics.
Probably kind of a niche interest ;)
Probably kind of a niche interest ;)
October 25, 2022
I suspect that this is THE book to read if you want to understand multiverse (or many worlds) theory. Wallace is thorough, precise, technical, and formal. And the subject matter is fascinating.
Hugh Everett was a Princeton Physics PhD student when he developed the “Everett Interpretation” of quantum mechanics, now known as the multiverse theory, or the many worlds theory. Despite support from John Wheeler, Everett got a cold reception from the physics world for his published papers and actually left academic science for a commercial career after completing his PhD.
In more recent years, the Everett Interpretation has gained credibility in the physics world, and Wallace’s treatment gives some good answers to account for its resurgence.
The traditionally dominant Copenhagen Interpretation of quantum theory has a problem with realism. It’s not even a sophisticated philosophical problem. It just doesn’t seem to render what’s really going on at the quantum level understandable, at least not in conceptual or visualizable terms. Even theorists we commonly associate with quantum mechanics, like Schrödinger, or more recently Richard Feynman, have said as much.
Wallace’s account of the Everett Interpretation aims to restore realism to quantum theory. Wallace, who has very enviable facility in both theoretical physics and philosophy of science, undertakes two daunting tasks — a detailed mathematical elaboration of the Everett interpretation itself, and a defense of realism at the quantum level when understood by that interpretation.
I should say before I go on that I’m not a physicist. My academic background is in philosophy, and, although I have certainly studied philosophy of science, a lot of this is new and uncomfortable to me. So I’m sure my account (of both Wallace and Everett) will have mistakes — hopefully they will be quibbles, not howlers. I’ll try to stick to my most solid ground.
Everett’s core idea is actually fairly straightforward. Schrödinger’s wavefunction provides successful mathematical tools for calculating and predicting behaviors of particles at the quantum level.
But it does so in terms of probabilities — the attributes of particles are expressed as objective probabilities. We cannot say what a particle’s spin or momentum or position, for example, is, only what possible values those attributes may have, and what weights or probabilities to attach to those possible values.
It’s not until a measurement is made that those attributes take on determinant values. Upon measurement, the quantum wavefunction “collapses” to a determinant state.
Wallace stresses, as has traditionally been maintained in the Copenhagen and other interpretations, that the indeterminacy prior to measurement is not epistemic, it is objective. It’s not that the particle’s spin is unknown, for example, but that the particle’s spin is objectively undetermined until the measurement takes place.
That leads traditionally to what is called the “measurement problem.” Why would a measurement determine the values of a particle’s attributes, and what becomes of all the possible but unrealized values the attributes had prior to measurement?
On Wallace’s account, the Everett Interpretation doesn’t solve the measurement problem, it eliminates it. All of the possible values for the particle’s attributes are realized. The measurement changes nothing.
The catch of course is that while all of the values are realized, they are realized in different “worlds” (or “universes”). Each possible value represents a branching of reality upon its realization. If a particle’s spin has possible values of up and down, both are realized in subsequently different branches of reality.
Straightforward but mind-boggling. We’ve rid ourselves of the nasty measurement problem, but we got the multiverse in trade.
Some readers, like some physicists, will bail at this point. Wallace, following Everett, has posited the existence of an indefinitely large number of universes (the terms “universe” and “world” are kind of awkwardly used interchangeably) — indefinitely large given that our reality is after all quantum reality, and the branchings are going to take place and compound upon one another on a literally countless scale.
Those universes do not interact, or technically may do so only very minimally. For all intents and purposes, there are countless independently existing universes, following their own courses, themselves producing new branchings and new universes.
You and I exist in countless of those universes, as branchings happen from the universe we are currently in. We experience only one — we are “branch-bound” in Wallace’s term, but we are instances among countless instances of ourselves across the multiverse.
Wallace says, “The ‘actual physical Universe’ is the multiply branching structure generated by unitary evolution under the Schrödinger equation. The branch weights are physical features of the structure; they are represented mathematically with the quantum wavefunction.”
It’s critical to Wallace’s argument that what Everett offers is a “literal” interpretation of quantum mechanics, in some sense the simplest interpretation. He even balks at the term “interpretation,” preferring its use restricted to such accounts as the Copenhagen Interpretation that add something to quantum mechanics (to Schrödinger’s wavefunction) rather that taking it “literally” as he believes the Everettian quantum theory does.
I would have liked to hear a little more discussion from Wallace of what constitutes a “literal” interpretation of a mathematical equation like Schrödinger’s, especially given how central the claim is to his argument.
That is the core idea, and the remainder of Wallace’s book is elaboration and defense of the Everett Interpretation. The longest treatment is a formal construction of the notion of “probability” in an Everettian universe, in which Wallace claims advantages for Everettian theory over the Copenhagen and other interpretations, as well as over classical physics itself.
Throughout, Wallace’s strategy is to show that what may appear paradoxical or weird about quantum theory as a whole is mitigated by Everettian multiverse theory, and that any apparent paradoxes or weirdness generated by Everettian physics itself dissolve under scrutiny or at least pale by comparison to alternatives.
Of course, there’s still the elephant in the room (or the many elephants in many rooms). That idea of “many worlds” or the “multiverse.”
I can’t even come close to presuming to pass judgment on Everettian theory, or on Wallace’s account of it, but here are some areas I’m left puzzling about. I know this review is going to get long, so I’ll try to be brief, especially since I don’t have answers, just questions.
Ontology:
Occam's Razor tells us that "entities should not be multiplied beyond necessity." But Occam’s Razor doesn’t always cut cleanly. Wallace would have it that the Everett universe has the advantage in that it doesn’t require the existence of unrealized probabilities, but it does so, arguably, at the cost of multiplying entities indefinitely, across its “many” worlds. I don’t think there’s a clear winner on the Occam scale.
Maybe more importantly, I’m unsure what to make of the notion of an Everettian “world.” In keeping with his emphasis on realism, Wallace treats the branched worlds of Everettian physics as physical, not logical worlds. The very physical world we live in right now is one of those Everettian worlds. To ask “where” the others are places a pretty severe stress on the word “where” since they aren’t anywhere in our universe.
Wallace asks us though to consider that the Everett universe is not so strange after all. For example, the extension of reality into “many worlds” is not so different from the expansion of our concept of space — once we considered the solar system to be the universe, but now we’ve expanded that concept to encompass countless solar systems in countless galaxies.
But the extension that the Everett universe requires is of a different kind — it’s not an expansion of the concept of physical space, it’s an uncontrolled multiplication of discrete, non-interacting physical spaces.
Personal Identity:
Each of us experiences continuous existence in only one Everettian world — we are, as Wallace says, branch-bound.
But there are indefinitely many worlds in which “I” also exist, alongside the one I experience. And, as time goes along, with more branching, there will be more and more “me’s” populating them. All of those “me’s” will continue a first person experience, in multiple branches of the future, but first person experience itself is singular, not multiple, so only one of those branches will be experienced by (this) me.
Our branch-boundedness guarantees that nothing really changes in terms of our singular first person experience. As Wallace says, “Despite appearances, the branching structure of Everettian quantum mechanics has few or no consequences for our everyday beliefs and actions.”
It is only from a “God’s eye view” that there are multiple me’s in multiple branches, with their own first person experiences.
Some ethical questions struggle to rise up. What are my responsibilities to those branched versions of myself, or the branched versions of other persons? Once branched, each is responsible for its own future, but, in the future branchings, my actions now have consequences for all of my branched selves and the branched selves of everyone I affect in my present world.
Testability:
Since the many worlds of Everettian physics don’t interact and aren’t directly detectable (see exception below), there is no sense in which we can detect the presence of other worlds than our own, or detect effects events in our own world could have on those others. We don’t directly observe branchings, and once branched, other worlds continue separately and independently.
Testability — some way in which we can observe and test whether the branching actually happens — is a core element of scientific method. I wouldn’t be the first to point out this weakness in multiverse theory.
This is where Wallace’s insistence that Everettian theory is a literal interpretation of quantum mechanics needs to bear weight. He, I think, would have it that the burden of proof lies not on Everettian theory but on other interpretations of quantum mechanics that add such features as wavefunction collapse or hidden variables. As he says, “. . . the Everett interpretation just is quantum mechanics. If you’re after some experiment to distinguish Everett-interpreted quantum mechanics from an operational interpretation of quantum mechanics [i.e., a non-realist interpretation], then sure, I haven’t got anything to offer.”
Given that, according to Wallace, Everettian theory is a literal interpretation of quantum mechanics, all tests of quantum mechanics are for him tests of Everettian theory. Therefore, Everettian theory is testable, despite our inability to directly detect the presence of the many worlds it predicts.
The one exception to the indetectability of other Everettian worlds, by the way, is discussed by Wallace in Chapter 10, specifically in a discussion of David Deutsch’s work on neutron interference. I admit I wasn’t fully able to follow that discussion. More homework for me.
Probability:
Remember that this is an attempt to construct a “realist” interpretation of quantum theory
Part II of the book is an extensive construction of probability in an Everett universe.
I won’t (and couldn’t reliably) go into the detail. The gist is that Wallace believes that Everettian quantum theory can not only make proper sense of the probabilities of events, but do so in a way that is in some respects superior to classical physics or non-Everettian quantum theory.
Intuitively, I can see it arguable that, since all non-zero probabilities are realized in the Everettian universe, there is a superior sense to the ‘reality” of those probabilities than for a universe in which some of those non-zero probabilities are not realized and simply vanish.
The suspicion I had about probability going into Wallace’s discussion was this: Schrödinger’s wavefunction contains weights assigned to potential attributes of a particle, awaiting realization by measurement. If we interpret those weights as probabilities (e.g., that the spin of a particle will be measured with one value rather than another), as is normally done (and as Wallace does under Everettian quantum theory), what effect do those probabilities have on branching? For example, do higher probability outcomes produce more worlds with that outcome that lower probability ones?
But Wallace has ruled out, for reasons I won’t go into and don’t fully understand, counting worlds — thus there is no sense in which there are “more” worlds with higher probability outcomes than lower probability ones.
So the problem, phrased in the way that I just did to anticipate some such resolution, is a pseudo-problem.
What becomes of probability in an Everettian universe is exactly what one would want to become of it — for any observer, the weights in the wavefunction can be interpreted as the likelihood that that observer will obtain the relevant value for the particle’s attribute when measured. The existence of other branched observers subsequent to the measurement is irrelevant to that observer’s own experience.
Measurement:
The dissolution of the measurement problem is one of the chief advantages that Wallace sees with Everettian theory. No wavefunction collapse, no magic happening when a measurement takes place, because all possible values of the measurement are realized, just in branched worlds.
Fine. But I’m still left with a question. Does the branching take place when measurement takes place? If so, although the measurement doesn’t generate a wavefunction collapse, it does seem to generate branching.
I may be misunderstanding the relationship between measurement and branching, so the gap here may be my own.
I’ve gone on for a long time. This is obviously a fascinating, provocative book. I’d dare anyone to read it and not be challenged by it.
I can’t say Im convinced — after all, my understanding of what I would be convinced of has a lot of gaps. But I won’t stop thinking about this.
I should mention that there are more accessible treatments of Everettian theory out there. See Sean Carroll’s Something Deeply Hidden for example. Wallace’s book is challenging not only conceptually but technically — his treatment is formal and mathematical.
Hugh Everett was a Princeton Physics PhD student when he developed the “Everett Interpretation” of quantum mechanics, now known as the multiverse theory, or the many worlds theory. Despite support from John Wheeler, Everett got a cold reception from the physics world for his published papers and actually left academic science for a commercial career after completing his PhD.
In more recent years, the Everett Interpretation has gained credibility in the physics world, and Wallace’s treatment gives some good answers to account for its resurgence.
The traditionally dominant Copenhagen Interpretation of quantum theory has a problem with realism. It’s not even a sophisticated philosophical problem. It just doesn’t seem to render what’s really going on at the quantum level understandable, at least not in conceptual or visualizable terms. Even theorists we commonly associate with quantum mechanics, like Schrödinger, or more recently Richard Feynman, have said as much.
Wallace’s account of the Everett Interpretation aims to restore realism to quantum theory. Wallace, who has very enviable facility in both theoretical physics and philosophy of science, undertakes two daunting tasks — a detailed mathematical elaboration of the Everett interpretation itself, and a defense of realism at the quantum level when understood by that interpretation.
I should say before I go on that I’m not a physicist. My academic background is in philosophy, and, although I have certainly studied philosophy of science, a lot of this is new and uncomfortable to me. So I’m sure my account (of both Wallace and Everett) will have mistakes — hopefully they will be quibbles, not howlers. I’ll try to stick to my most solid ground.
Everett’s core idea is actually fairly straightforward. Schrödinger’s wavefunction provides successful mathematical tools for calculating and predicting behaviors of particles at the quantum level.
But it does so in terms of probabilities — the attributes of particles are expressed as objective probabilities. We cannot say what a particle’s spin or momentum or position, for example, is, only what possible values those attributes may have, and what weights or probabilities to attach to those possible values.
It’s not until a measurement is made that those attributes take on determinant values. Upon measurement, the quantum wavefunction “collapses” to a determinant state.
Wallace stresses, as has traditionally been maintained in the Copenhagen and other interpretations, that the indeterminacy prior to measurement is not epistemic, it is objective. It’s not that the particle’s spin is unknown, for example, but that the particle’s spin is objectively undetermined until the measurement takes place.
That leads traditionally to what is called the “measurement problem.” Why would a measurement determine the values of a particle’s attributes, and what becomes of all the possible but unrealized values the attributes had prior to measurement?
On Wallace’s account, the Everett Interpretation doesn’t solve the measurement problem, it eliminates it. All of the possible values for the particle’s attributes are realized. The measurement changes nothing.
The catch of course is that while all of the values are realized, they are realized in different “worlds” (or “universes”). Each possible value represents a branching of reality upon its realization. If a particle’s spin has possible values of up and down, both are realized in subsequently different branches of reality.
Straightforward but mind-boggling. We’ve rid ourselves of the nasty measurement problem, but we got the multiverse in trade.
Some readers, like some physicists, will bail at this point. Wallace, following Everett, has posited the existence of an indefinitely large number of universes (the terms “universe” and “world” are kind of awkwardly used interchangeably) — indefinitely large given that our reality is after all quantum reality, and the branchings are going to take place and compound upon one another on a literally countless scale.
Those universes do not interact, or technically may do so only very minimally. For all intents and purposes, there are countless independently existing universes, following their own courses, themselves producing new branchings and new universes.
You and I exist in countless of those universes, as branchings happen from the universe we are currently in. We experience only one — we are “branch-bound” in Wallace’s term, but we are instances among countless instances of ourselves across the multiverse.
Wallace says, “The ‘actual physical Universe’ is the multiply branching structure generated by unitary evolution under the Schrödinger equation. The branch weights are physical features of the structure; they are represented mathematically with the quantum wavefunction.”
It’s critical to Wallace’s argument that what Everett offers is a “literal” interpretation of quantum mechanics, in some sense the simplest interpretation. He even balks at the term “interpretation,” preferring its use restricted to such accounts as the Copenhagen Interpretation that add something to quantum mechanics (to Schrödinger’s wavefunction) rather that taking it “literally” as he believes the Everettian quantum theory does.
I would have liked to hear a little more discussion from Wallace of what constitutes a “literal” interpretation of a mathematical equation like Schrödinger’s, especially given how central the claim is to his argument.
That is the core idea, and the remainder of Wallace’s book is elaboration and defense of the Everett Interpretation. The longest treatment is a formal construction of the notion of “probability” in an Everettian universe, in which Wallace claims advantages for Everettian theory over the Copenhagen and other interpretations, as well as over classical physics itself.
Throughout, Wallace’s strategy is to show that what may appear paradoxical or weird about quantum theory as a whole is mitigated by Everettian multiverse theory, and that any apparent paradoxes or weirdness generated by Everettian physics itself dissolve under scrutiny or at least pale by comparison to alternatives.
Of course, there’s still the elephant in the room (or the many elephants in many rooms). That idea of “many worlds” or the “multiverse.”
I can’t even come close to presuming to pass judgment on Everettian theory, or on Wallace’s account of it, but here are some areas I’m left puzzling about. I know this review is going to get long, so I’ll try to be brief, especially since I don’t have answers, just questions.
Ontology:
Occam's Razor tells us that "entities should not be multiplied beyond necessity." But Occam’s Razor doesn’t always cut cleanly. Wallace would have it that the Everett universe has the advantage in that it doesn’t require the existence of unrealized probabilities, but it does so, arguably, at the cost of multiplying entities indefinitely, across its “many” worlds. I don’t think there’s a clear winner on the Occam scale.
Maybe more importantly, I’m unsure what to make of the notion of an Everettian “world.” In keeping with his emphasis on realism, Wallace treats the branched worlds of Everettian physics as physical, not logical worlds. The very physical world we live in right now is one of those Everettian worlds. To ask “where” the others are places a pretty severe stress on the word “where” since they aren’t anywhere in our universe.
Wallace asks us though to consider that the Everett universe is not so strange after all. For example, the extension of reality into “many worlds” is not so different from the expansion of our concept of space — once we considered the solar system to be the universe, but now we’ve expanded that concept to encompass countless solar systems in countless galaxies.
But the extension that the Everett universe requires is of a different kind — it’s not an expansion of the concept of physical space, it’s an uncontrolled multiplication of discrete, non-interacting physical spaces.
Personal Identity:
Each of us experiences continuous existence in only one Everettian world — we are, as Wallace says, branch-bound.
But there are indefinitely many worlds in which “I” also exist, alongside the one I experience. And, as time goes along, with more branching, there will be more and more “me’s” populating them. All of those “me’s” will continue a first person experience, in multiple branches of the future, but first person experience itself is singular, not multiple, so only one of those branches will be experienced by (this) me.
Our branch-boundedness guarantees that nothing really changes in terms of our singular first person experience. As Wallace says, “Despite appearances, the branching structure of Everettian quantum mechanics has few or no consequences for our everyday beliefs and actions.”
It is only from a “God’s eye view” that there are multiple me’s in multiple branches, with their own first person experiences.
Some ethical questions struggle to rise up. What are my responsibilities to those branched versions of myself, or the branched versions of other persons? Once branched, each is responsible for its own future, but, in the future branchings, my actions now have consequences for all of my branched selves and the branched selves of everyone I affect in my present world.
Testability:
Since the many worlds of Everettian physics don’t interact and aren’t directly detectable (see exception below), there is no sense in which we can detect the presence of other worlds than our own, or detect effects events in our own world could have on those others. We don’t directly observe branchings, and once branched, other worlds continue separately and independently.
Testability — some way in which we can observe and test whether the branching actually happens — is a core element of scientific method. I wouldn’t be the first to point out this weakness in multiverse theory.
This is where Wallace’s insistence that Everettian theory is a literal interpretation of quantum mechanics needs to bear weight. He, I think, would have it that the burden of proof lies not on Everettian theory but on other interpretations of quantum mechanics that add such features as wavefunction collapse or hidden variables. As he says, “. . . the Everett interpretation just is quantum mechanics. If you’re after some experiment to distinguish Everett-interpreted quantum mechanics from an operational interpretation of quantum mechanics [i.e., a non-realist interpretation], then sure, I haven’t got anything to offer.”
Given that, according to Wallace, Everettian theory is a literal interpretation of quantum mechanics, all tests of quantum mechanics are for him tests of Everettian theory. Therefore, Everettian theory is testable, despite our inability to directly detect the presence of the many worlds it predicts.
The one exception to the indetectability of other Everettian worlds, by the way, is discussed by Wallace in Chapter 10, specifically in a discussion of David Deutsch’s work on neutron interference. I admit I wasn’t fully able to follow that discussion. More homework for me.
Probability:
Remember that this is an attempt to construct a “realist” interpretation of quantum theory
Part II of the book is an extensive construction of probability in an Everett universe.
I won’t (and couldn’t reliably) go into the detail. The gist is that Wallace believes that Everettian quantum theory can not only make proper sense of the probabilities of events, but do so in a way that is in some respects superior to classical physics or non-Everettian quantum theory.
Intuitively, I can see it arguable that, since all non-zero probabilities are realized in the Everettian universe, there is a superior sense to the ‘reality” of those probabilities than for a universe in which some of those non-zero probabilities are not realized and simply vanish.
The suspicion I had about probability going into Wallace’s discussion was this: Schrödinger’s wavefunction contains weights assigned to potential attributes of a particle, awaiting realization by measurement. If we interpret those weights as probabilities (e.g., that the spin of a particle will be measured with one value rather than another), as is normally done (and as Wallace does under Everettian quantum theory), what effect do those probabilities have on branching? For example, do higher probability outcomes produce more worlds with that outcome that lower probability ones?
But Wallace has ruled out, for reasons I won’t go into and don’t fully understand, counting worlds — thus there is no sense in which there are “more” worlds with higher probability outcomes than lower probability ones.
So the problem, phrased in the way that I just did to anticipate some such resolution, is a pseudo-problem.
What becomes of probability in an Everettian universe is exactly what one would want to become of it — for any observer, the weights in the wavefunction can be interpreted as the likelihood that that observer will obtain the relevant value for the particle’s attribute when measured. The existence of other branched observers subsequent to the measurement is irrelevant to that observer’s own experience.
Measurement:
The dissolution of the measurement problem is one of the chief advantages that Wallace sees with Everettian theory. No wavefunction collapse, no magic happening when a measurement takes place, because all possible values of the measurement are realized, just in branched worlds.
Fine. But I’m still left with a question. Does the branching take place when measurement takes place? If so, although the measurement doesn’t generate a wavefunction collapse, it does seem to generate branching.
I may be misunderstanding the relationship between measurement and branching, so the gap here may be my own.
I’ve gone on for a long time. This is obviously a fascinating, provocative book. I’d dare anyone to read it and not be challenged by it.
I can’t say Im convinced — after all, my understanding of what I would be convinced of has a lot of gaps. But I won’t stop thinking about this.
I should mention that there are more accessible treatments of Everettian theory out there. See Sean Carroll’s Something Deeply Hidden for example. Wallace’s book is challenging not only conceptually but technically — his treatment is formal and mathematical.
August 13, 2018
This is also a great book on Everett's Many Worlds theory and whenever you are done reading those primers by Kaku. The author has been working on the MWT for more than 20 years.
February 5, 2022
The Emergent Multiverse: Quantum Theory According to the Everett Interpretation is an effort on the part of David Wallace to vindicate Hugh Everett’s idea of many worlds.
The concept of multiverse was initially taken seriously in philosophy than in physics. May be because in physics the idea came much later, that is, with the emergence of quantum mechanics.
Quantum mechanics talks about the theory of particles. According to Everett a particle system is like a tree, where there are many paths/branches to get to the top of a tree. And each branch is a world, a system in itself.
In macro-world, everything behaves as per the “expected” trajectory of classical physics. For instance, things fall down towards the gravity, entropy keeps on increasing, time moves linearly in forward direction etc.
At the micro level, however, the classical physics breaks down. Particles behave in a weird manner. Cloud of possibilities define their trajectories. Consequently, a particle is here and there at the same time.
I had to take a lot of notes while I was going through The Emergent Multiverse. It’s not an easy book to read. I’m sure I might have skipped some points however, one of the most interesting Everett’s ideas that I came across is to treat the measurement apparatus as part of the quantum system.
Although Nobel laureate Roger Penrose does not support Everett’s ideas. As per Penrose, the problem is with Schrödinger equation. Many worlds theory for him is reductive and absurd, since we are not able to access the other types of worlds.
According to the supporters of many worlds’ theory (including myself) complex of reality can be seen as different perspective from different people. In other words, single world with many internal perspectives.
For instance, radio signals. We are always surrounded by radio signals, that could also be with different worlds. Yet each is not interfering with other. Rather all independent from each other.
A thought-provoking book on nested and coexisting worlds. I highly recommend to those who wants to explore the history of emergent multiverse starting from Erwin Schrödinger.
More from my blog post: The Emergent Multiverse by David Wallace
The concept of multiverse was initially taken seriously in philosophy than in physics. May be because in physics the idea came much later, that is, with the emergence of quantum mechanics.
Quantum mechanics talks about the theory of particles. According to Everett a particle system is like a tree, where there are many paths/branches to get to the top of a tree. And each branch is a world, a system in itself.
In macro-world, everything behaves as per the “expected” trajectory of classical physics. For instance, things fall down towards the gravity, entropy keeps on increasing, time moves linearly in forward direction etc.
At the micro level, however, the classical physics breaks down. Particles behave in a weird manner. Cloud of possibilities define their trajectories. Consequently, a particle is here and there at the same time.
I had to take a lot of notes while I was going through The Emergent Multiverse. It’s not an easy book to read. I’m sure I might have skipped some points however, one of the most interesting Everett’s ideas that I came across is to treat the measurement apparatus as part of the quantum system.
Although Nobel laureate Roger Penrose does not support Everett’s ideas. As per Penrose, the problem is with Schrödinger equation. Many worlds theory for him is reductive and absurd, since we are not able to access the other types of worlds.
According to the supporters of many worlds’ theory (including myself) complex of reality can be seen as different perspective from different people. In other words, single world with many internal perspectives.
For instance, radio signals. We are always surrounded by radio signals, that could also be with different worlds. Yet each is not interfering with other. Rather all independent from each other.
A thought-provoking book on nested and coexisting worlds. I highly recommend to those who wants to explore the history of emergent multiverse starting from Erwin Schrödinger.
More from my blog post: The Emergent Multiverse by David Wallace
May 20, 2023
This is a very detailed, technical and philosophical account of the Everrett multi-worlds or multi-branching interpretation of quantum mechanics. Though the author writing around 2010 stresses it is not an interpretation at all rather it is just taking the main rule ( unitary deterministic time-evolution) seriously and deriving measurement ( and the probabilities ) from that by taking into account macroscopic branching. The resulting world view is breath-taking to say the least. He seems to think that it must be true because there are no credible alternatives. Well ... we're going to need more than that.
It is not an easy read, requiring far more than basic quantum mechanics as claimed in the intro. And the author freely dives into deeper areas without explaining the required background rendering sections of the book barely comprehensible. Turning it into a book by an expert for experts and not for a more general audience as promised.
Despite that the basic ideas are clear enough. His emphasis on an ontological approach and almost casual dismissal of epistemic/ operational approaches is always refreshing. Though by 2010 had long become fairly standard. The always silly Copenhagen Interpretation has been consigned to the dustbin of history where it belongs. Remaining only as a good historical example of shared/enforced lunacy by multiple generations of physicists. It is always worth noting that its dogmatic proponents, still in power at the time trashed Everett's promising academic career and pushed him out into the dry wilderness of government work.
The idea of multiple branches has become a standard science fiction theme. And who knows maybe there really are multiple ( infinite in fact) versions of myself in different branches ( histories ). And if this is the case I can only complain why oh why did I get stuck in this one. But I suppose by definition one of us had to be. And I guess it is far from the worst. I wonder how that guy is doing.
It is not an easy read, requiring far more than basic quantum mechanics as claimed in the intro. And the author freely dives into deeper areas without explaining the required background rendering sections of the book barely comprehensible. Turning it into a book by an expert for experts and not for a more general audience as promised.
Despite that the basic ideas are clear enough. His emphasis on an ontological approach and almost casual dismissal of epistemic/ operational approaches is always refreshing. Though by 2010 had long become fairly standard. The always silly Copenhagen Interpretation has been consigned to the dustbin of history where it belongs. Remaining only as a good historical example of shared/enforced lunacy by multiple generations of physicists. It is always worth noting that its dogmatic proponents, still in power at the time trashed Everett's promising academic career and pushed him out into the dry wilderness of government work.
The idea of multiple branches has become a standard science fiction theme. And who knows maybe there really are multiple ( infinite in fact) versions of myself in different branches ( histories ). And if this is the case I can only complain why oh why did I get stuck in this one. But I suppose by definition one of us had to be. And I guess it is far from the worst. I wonder how that guy is doing.
March 27, 2021
Ok someone's got to do the gritty grunt work so academics don't write off Everettian philosophers as flighty cranks but damn a book like this is going to have a hard time finding an audience with normies outside the ivory tower. Good and rigorous. I will link to Sean Carroll and Jim Baggot who do good popularization on this stuff. When Wallace does one I will promote it no doubt if it is good.
https://www.goodreads.com/book/show/4...
https://www.youtube.com/watch?v=p7XId...
https://www.goodreads.com/book/show/5...
And the author himself.
https://www.youtube.com/watch?v=bK9JE...
https://www.goodreads.com/book/show/4...
https://www.youtube.com/watch?v=p7XId...
https://www.goodreads.com/book/show/5...
And the author himself.
https://www.youtube.com/watch?v=bK9JE...
July 4, 2023
It’s a very well structured book for different kind of readers: from the introduction is clear which chapters should be read as consecutive chapters and others that the reader can jump according to his/her interests. The statement of the Everettian interpretation without modifying the math of quantum mechanics is clearly defined and from there the discussions begins. Is interesting how the author explain the specifics from the physics point of view and then goes to the philosophical aspects giving a close discussion about the different topics around the Many Worlds interpretation. I’m not giving 5 stars because some chapters have a lot of math, although it would be OK for those who are used to them, it also can cause lost of interest in a more general reader.
April 4, 2023
I had to take a lot of notes while I was going through The Emergent Multiverse. Its not an easy book to read. I’m sure I might have skipped some points however, one of the most interesting Everett’s ideas that I came across is to treat the measurement apparatus as part of the quantum system.
Although Nobel laureate Roger Penrose does not support Everett’s ideas. As per Penrose, the problem is with Schrödinger equation. Many worlds theory for him is reductive and absurd, since we are not able to access the other types of worlds.
According to the supporters of many worlds’ theory (including myself) complex of reality can be seen as different perspective from different people. In other words, single world with many internal perspectives.
For instance, radio signals. We are always surrounded by radio signals, that could also be with different worlds. Yet each is not interfering with other. Rather all independent from each other.
A thought-provoking book on nested and coexisting worlds. I highly recommend to those who wants to explore the history of emergent multiverse starting from Erwin Schrödinger.
More from my blog post: The Emergent Multiverse by David Wallace
Although Nobel laureate Roger Penrose does not support Everett’s ideas. As per Penrose, the problem is with Schrödinger equation. Many worlds theory for him is reductive and absurd, since we are not able to access the other types of worlds.
According to the supporters of many worlds’ theory (including myself) complex of reality can be seen as different perspective from different people. In other words, single world with many internal perspectives.
For instance, radio signals. We are always surrounded by radio signals, that could also be with different worlds. Yet each is not interfering with other. Rather all independent from each other.
A thought-provoking book on nested and coexisting worlds. I highly recommend to those who wants to explore the history of emergent multiverse starting from Erwin Schrödinger.
More from my blog post: The Emergent Multiverse by David Wallace
November 13, 2016
This book discusses the "natural" interpretation of quantum mechanics. Wallace does a fabulous job of addressing all the issues with the Everett Interpretation, including showing how the Born rule can be arrived at through a decision theoretic approach. Some parts of the book can be very technical and these can be skipped if you are not interested. The last chapter was one of my favorites, discussing various implications of the multiverse interpretation.
January 30, 2017
I get lost in much of the mathematics so it's not really a fair review. The arguments in prose are compelling but this just isn't a book for the layperson and it'll be a while before I can consider myself otherwise.
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