TheEmergentMultiverse

• Awesome! Changed my mind.
• The Everett interpretation is QM when interpreted as a literaly theory of the world.
• Worlds are an emergent, local, macroscopic phenomena
• This book, by contrast, takes an extremely conservative approach to quantum mechanics. It supposes that neither the mathematical formalism of quantum mechanics nor the standard conception of science is in any need at all of modification. Rather: the unmodified quantum theory can be taken as representing the structure of the world just as surely as any other theory of physics. EG: quantum mechanics can be taken literally. The only catch is that, when we do take it literally, the world turns out to be rather larger than we had anticipated: indeed, it turns out that our classical ‘world’ is only a small part of a much larger reality.
• One of the notable things about discussing the interpretation of quantum mechanics with physicists and with philosophers is that it is the physicists who propose philosophically radical ways of interpreting a theory, and the philosophers who propose changing the physics.
• Macroscopic superpositions do not describe indefiniteness, they describe multiplicity.
• One might still be tempted to object: very well, but we don’t observe the Universe as being in superpositions of containing live cats and containing dead cats, any more than we observe cats as being in superpositions of alive and dead. But it is not at all clear that we don’t observe the Universe in such superpositions. After all, cats are the sort of perfectly ordinary objects that we seem to see around us all the time—a theory that claims that they are normally in macroscopically indefinite states seems to make a nonsense of our everyday lives. But the Universe is a very big place, as physics has continually reminded us, and we inhabit only a very small part of it, and it will not do to claim that it is just ‘obvious’ that it is not in a superposition.

• When a system is prepared in a superposition of a live-cat and a dead-cat state. The answer is that the system’s surroundings will rapidly become entangled with it, so that we do not just have a superposition of live and dead cat, but a superposition of extended quasi-classical regions—‘worlds’, if you like—some of which contain live cats and some of which contain dead cats. If the correct way to understand such superpositions is as some sort of multiplicity, then our failure to observe that multiplicity is explained quite simply by the fact that we live in one of the ‘worlds’ and the other ones don’t interact with ours strongly enough for us to detect them.

  This, in short, is the Everett interpretation. It consists of two very different parts: a contingent physical postulate, that the state of the Universe is faithfully represented by a unitarily evolving quantum state; and an a priori claim about that quantum state, that if it is interpreted realistically it must be understood as describing a multiplicity of approximately classical, approximately non-interacting regions which look very much like the ‘classical world’.

  And this is all that the Everett interpretation consists of. There are no additional physical postulates introduced to describe the division into ‘worlds’, there is just unitary quantum mechanics. For this reason, it makes sense to talk about the Everett interpretation, whereas it does not to talk about the hidden-variables interpretation or the quantum-logical interpretation. The ‘Everett interpretation of quantum mechanics’ is just quantum mechanics itself, ‘interpreted’ the same way we have always interpreted scientific theories in the past: as modelling the world.

• The main challenge for the Everett interpretation, then, is to show exactly how our classical world, which seems to be so well described by a single quasi-classical universe in which quantum processes occur at random, can be incorporated in the macroscopically indefinite structure of deterministic, unitarily evolving quantum mechanics
• The question of whether the quantum state is actually a collection of quasi-classical worlds is then the question of whether the quantum state has the structure of a collection of quasi-classical worlds—a question which, I shall argue, decoherence theory has now fairly conclusively answered in the affirmative.
• Some of the most intractable problems in the philosophy of probability appear to have answers in Everettian quantum mechanics that are simply not available in single-universe theories.
• Notice that this is a lot of what makes studying the Everett interpretation important. Anyone who accepts that scientific theories are supposed to explain and describe the world, but whoever believes that the Everett interpretation fails to solve the measurement problem should recognize that there is a profound problem with pretty much all of twentieth-century physics. In a way, it would actually be quite exciting to believe this—by contrast, the Everett interpretation is actually rather conservative.

• Solar system analogy: For consider the theory that the Solar System is alone in an otherwise empty void. This theory is extraordinarily successful at accounting for virtually all the phenomena which we have observed, throughout human history. The only thing it cannot handle are some tiny fluctuations in the pattern of radiation which falls upon the Earth, the only humanly observable part of which is a scattering of rather faint points of light in the night sky. How absurd, then, for physicists to take seriously their best theory of the origins of those fluctuations.

  No matter (surely) that the theory explains the fluctuations with extraordinary accuracy; no matter (surely) that it has made countless novel predictions about them which experiment has confirmed; fundamentally (surely) it is just unbelievable, for it claims the existence of a Universe beyond the Solar System that is absurd, staggeringly so, compared to what we believed before: not just one Sun, not just a few thousand, but trillions of trillions, incomprehensibly far away, almost of them invisible to the naked eye and ‘detectable’ (says the theory), at most, by the most advanced instruments.

  And to be sure, the ability of this absurd theory to explain (and not merely predict) much about our solar system—the distribution of the elements, the creation of the Sun—is a neat trick. But to take advantage of that trick, we do not need (surely) to believe this theory, for even our best scientific theory surely should be disbelieved if it claims that what we hitherto regarded as the whole Universe is relegated to the smallest part of an unthinkably greater reality.

• The Everett ‘interpretation’ of quantum mechanics is just quantum mechanics itself, taken literally as a description of the Universe. As such, it is the only solution of the quantum measurement problem which is not committed either to a sea change in the goals of the scientific enterprise or to the wholesale rewriting of our current best physics.
• A basic claim of this book is that worlds, in the Everett interpretation, are emergent entities —and that this is actually rather a mundane claim, and that it puts Everettian worlds on a par with all manner of unmysterious, scientifically respectable entities.
• macroscopically definite quantum states represent classical states of affairs in just the way that they are usually taken to.

• In general adding together two states with certain structures might cause those structures to overlap and cancel out, so that the structure of the resultant state cannot just be read off from the structures of the components. Indeed, in both electromagnetism and quantum theory, the technical term for this ‘cancelling out’ is the same: interference.

  In general, however—and in particular in the case of the cat—there is no interference between the live-cat and dead-cat states, and so both lots of structure continue to be present The reason we can be confident of this is because of decoherence, which in general prevents the macroscopic degrees of freedom of quantum systems from interfering, and so guarantees that structures instantiated by the macroscopic degrees of freedom of quantum systems are not erased when those systems are in superpositions of macroscopically definite states.

• And so unitary quantum mechanics, interpreted realistically, is a many-worlds theory—not because the ‘worlds’ are present in some microphysically fundamental sense but because the quantum state instantiates many different macroscopic systems.
• Certain quantum-mechanical histories of certain systems instantiate—simulate, if you like—a quasi-classical history. Superpositions of those histories then instantiate multiple quasi-classical histories—always assuming that interference between histories can be neglected.
• The interactions w/ the environment gives the QM universe an ~classical branching structure.
• the real reason that we can regard the quantum state as approximately classical is that it is dynamically isomorphic, very nearly, to a system of one classical point particle.
• Notice that it is not merely the linearity of quantum mechanics which allows us to interpret superpositions as instantiating multiple structures. Rather, it is the disappearance of interference terms between the relevant terms in those superpositions. Basis preservation is a sufficient condition for this to occur; as we will shortly see, it is not a necessary condition.
• To conclude: chaotic, isolated, unitarily evolving quantum systems cannot approximate classical ones on long enough timescales to reproduce classical phenomenology.
• So: prepare a heavy particle in a macroscopic superposition and expose it to a scattering environment, and that environment will become entangled with the particle, causing the coherence between the terms in the superposition to decay. it is by now well known that in realistic situations, coherence is lost very, very quickly
• Decoherence, alone, does not solve the measurement problem. Systems prepared in superpositions of macroscopically different positions will decohere on timescales much more swift than their characteristic dynamical timescales. Systems prepared in superpositions of macroscopically different momentums will quickly evolve into states with macroscopically different positions, and these too will swiftly decohere.

• three problems with extracting quasi-classical behaviour from macroscopic quantum systems: (i) What justifies our treating the macroscopic degrees of freedom as dynamically isolated from the remainder of the system? (ii) Why do chaotic systems behave quasi-classically given that in isolation they evolve into non-quasi-classical states? (iii) Why even when the dynamics of a system is not even approximately classical—such as in the case of quantum measurement—do macroscopic systems still seem to stay in quasi-classical states?

  The answer to all three is decoherence:

  the main effect of the environment is to measure the system in the position basis, and this has no effect on the system if it is already in a reasonably localized state. decoherence constantly transforms delocalized states into mixtures of localized states, and so prevents the system ever ending up in a state so delocalized that the dynamics ceases to be approximately classical..

• Chaos, in particular, is everywhere, and where there is chaos, there is branching (the weather, for instance, is chaotic, so there will be different weather in different branches).

• the question ‘How many branches are there?’ does not, ultimately, make sense. (Branches are emergent) compare to eg: How many feelings did you have?

  Decoherence causes the Universe to develop an emergent branching structure. The existence of this branching is a robust (albeit emergent) feature of reality; so is the mod-squared amplitude for any macroscopically described history. But there is no non-arbitrary decomposition of macroscopically-described histories into ‘finest-grained’ histories, and no non-arbitrary way of counting those histories.

• If we apply to quantum mechanics the same principles as we apply right across science, we find that a multiplicity of quasi-classical worlds are emergent from the underlying quantum physics. These worlds are structures instantiated within the quantum state, but they are no less real for all that.

• Sceptic: But it’s just intuitively obvious that the world isn’t like this! Author: Now I’d like to hear an argument as to why intuitive obviousness is a good guide to the truth in fundamental physics.

  The mainstream position in philosophy of science, these days, is some kind of realism: to understand what scientific theories are telling us about the world, we basically should take them literally and believe what they say. And there have been lots of principled attempts—from positivism to constructive empiricism—to take a different view of science, to say that scientific theories shouldn’t be taken literally, or that we shouldn’t believe their claims about unobservable objects.

But to make your position work, you’d have to defend an entirely new philosophy of science, one that no one has taken seriously thus far—a philosophy of science according to which we should believe our best theories until they cross some kind of weirdness threshold, and then stop believing them. Now, if you can argue—and I mean argue, not thump the table or appeal to intuitions—for that kind of philosophy of science, more power to you. But I doubt it can be done.

• Probability is by general consensus the most serious difficulty in making sense of the Everett interpretation. Yet it is difficult and controversial to say what exactly the problem is, or even why there is a problem at all
• if all analogies were perfect, life would be boring.

• for advocates of Everettian quantum mechanics, there is an alternative to frequentism available. Rather than attempting to extract single-case probabilities from long-run relative frequencies, they can declare those probabilities to be equal to the relative weights of branches.

  The Everett interpretation supplies a objective physical correlate for probabilities, but one which—unlike long-run frequencies—is associated with the outcomes of individual experiments.

  once the single-outcome assumption is dropped, arguments based on symmetry suffice to prove that the objective probabilities can be given only by the Born probability rule.

• in Everettian quantum mechanics there is no need for anything to break the symmetry of a particular process (a problem for dealing with probability in non-branching physics) in order to give one outcome rather than another—for all outcomes occur. Or, put another way, in Everettian quantum mechanics not just the laws, but the actual microstate of the system are invariant under a symmetry transformation, as could not be the case if only one outcome was to occur.
• Purely by considerations of the symmetries of the quantum state, we derive that equally weighted superpositions are equally likely on measurement to give each outcome

• Although branch weights are objective, what makes it true that branch weight=probability is the way in which branch weights figure in the actions of (ideally) rational agents.

  By analogy, a dollar bill is a perfectly physical object. The economy of the world could cease to exist—all life could cease to exist—without needing to affect that physical object. But what makes that physical object deserve the name ‘dollar bill’ nonetheless depends on how that object plugs into the behaviour of human agents. (The analogy isn’t perfect: there’s an aspect of pure convention about our use of dollars as currency, whereas the Born Rule theorem tells us that any rational agent has to treat branch weight as probability.)

• I will argue that the ways in which we use talk of uncertainty, identity, the future and so forth will remain fully justified in the event that we come to accept Everettian quantum mechanics as true. We will have to abandon not our actual use of these concepts, but only (some of) our metaphysical and semantic theories as to how those concepts work. We shall find that, in this arena as well as that of physics, Everettian quantum mechanics is much more conservative than meets the eye.

• Despite appearences, the branching structure of Everettian quantum mechanics has few or no consequences for our everyday beliefs and actions. Ordinary talk of the future is in general neither false nor meaningless if Everettian quantum mechanics is true.
• it is not as if we really have an intuitive grasp of what an electric or a magnetic field is, other than indirectly and by means of instrumental considerations
• From the perspective of a given experimenter, of course, her experiment does have a unique, definite outcome, even in the Everett interpretation.
• Even if we were able to calculate (say) the expansion of a gas in terms of the motions of all its myriad constituents, we would have missed important generalizations about the gas by peering too myopically at its microscopic state. We would, that is, have missed important, robust higher-level generalizations about the gas. And in quantum mechanics, the emergent behaviour is frequently the only one that physically realistic observers can have epistemic access to: decoherence strongly constrains our ability to see genuinely unitary dynamical processes, because it’s too difficult to avoid getting entangled with those same processes.
• This means that the reason I cannot predict the weather in a year’s time is not that I cannot in practice determine the current state of the atmosphere accurately, nor that I cannot in practice do the appropriate calculations. The reason, instead, is that there is no fact of the matter as to what the weather is in a year’s time. From a God’s-eye perspective, the weather is perfectly predictable, but the prediction is that it varies widely from branch to branch. All that the branch-bound observer can do, though, is predict with what probability each outcome will occur—this, though, he can do without an exponentially expensive requirement for computational power.

• This book has presented the case for believing in a multiverse—a reality of many non-interacting quasi-classical worlds—by analysis of our extant physical theories. The argument goes: we have excellent evidence for quantum mechanics, therefore we have good reason to believe quantum mechanics is approximately true; quantum mechanics entails that there are many worlds; so there are.

  For Deutsch, both interference phenomena and the (albeit not yet empirically realized) behaviour of quantum computers can only be made sense of by the existence of parallel universes, independent of our detailed quantum-mechanical analysis of how they work.

• Re: Quantum Computing: there is no logical contradication, although it goes against everything we have learned of science. in the suppostion that the laws of physics contain primitive factorization-implementing processes.

  But to object thus is to miss the point: which is not that there could be no other explanation for the factorization, but that we actually have a very good, in principle thoroughly testable explanation.

  Namely, it involves simple, well-understood algorithms operating in a massively parallel way, within a single computer. It presumes that each computation happens independently, the empirical prediction is that everything will happen as if each computation is occurring independently, and there is no way of explaining the actual computational process taking place which does not assume the computations are happening independently. By Deutsch’s criterion, then, there is no way of so explaining the algorithm which does not accept the reality of all of the independent computations. At least within the quantum computer, there would be many worlds.

• True, when quantum mechanics was new, some physicists thought that it put humans back into the picture, because the principles of quantum mechanics tell us how to calculate the probabilities of various results that might be found by a human observer. But, starting with the work of Hugh Everett forty years ago, the tendency of physicists who think deeply about these things has been to reformulate quantum mechanics in an entirely objective way, with observers treated just like everything else. I don’t know if this program has been completely successful yet, but I think it will be.
  • Steven Weinberg.

• Although it began with dinosaurs and ended with time machines, this has been a very conservative book. It has been concerned, from beginning to end, with quantum mechanics: with deterministic, Lorentz-covariant quantum theory, interpreted in a straightforwardly realist way.

  And when we interpret it that way, we find:

  1. That applying the same analysis of emergence and high-level structure we use all over the special sciences tells us that the unitarily evolving state has an emergent branching structure, with each branch giving approximately definite positions and momenta to macroscopic objects and with those objects evolving in an approximately classical way within their branch. (Chapters 2 and 3)
  2. That the emergent branching structure has a measure on it, defined in terms of the underlying physics, which satisfies the probability calculus and which, if we allow ourselves no more license than is anyway needed in classical physics to interpret a physical magnitude as probability, can indeed be so interpreted. (Chapter 4)
  3. That even if we don’t allow ourselves this license, we can still justify why that physical magnitude is probability, because probability actually makes more sense in quantum than in classical physics. (Chapters 5 and 6)
  4. That although the theoretical basis for claims about our uncertainty about the future, and about personal identity more generally, may be different in Everettian quantum mechanics as compared to classical mechanics, nonetheless the role that these concepts actually play in our lives will be unchanged. (Chapter 7)
  5. That relativistic versions of quantum theory are fully Lorentz-covariant, and comply with both the letter and the spirit of relativity theory. Quantum systems can have nonlocal states, but only in the same sense that gauge theories can have nonlocal states; they do not undergo nonlocal interactions. (Chapter 8)
  6. That the decoherence process defines an arrow of time whose explanation is essentially the same as the explanation for the arrows of time defined by other in-practice-irreversible processes in physics. (Chapter 9)

• I conclude that there is no measurement problem in quantum mechanics.

  In actuality, there is no clash between unitary, realist-interpreted quantum mechanics and our observations or our experimental data. The Everett interpretation does not solve the measurement problem: it dissolves it.

• There is no scientific defect in quantum theory which gives us any reason to think that some such theory must be true. In one sense, this is actually a bit of a disappointment. After all, we know that quantum theory (or at least, our best current quantum theory) is going to have to fail somewhere, because of the clash with general relativity; but the experimental regime associated with that failure is horrifically hard to probe empirically. How convenient it would have been if we could have been led to an improved version of quantum mechanics by the measurement problem, and then found that improved version to be naturally compatible with general relativity! Alas, it was not to be.

• In another sense, it is a huge relief. The last eighty years of quantum mechanics have, frankly, been a bit embarrassing for physicists and philosophers of science alike: physicists because they had no fully coherent story to tell about the physical world; philosophers because despite this, the physicists made astonishing progress.

  According to the Everett interpretation, the reason this all worked out is because all along the theory the physicists were using was correct, and straightforwardly realist—but it took a long time to understand it sufficiently well that we really grasped the connection between the theory and our own corner of empirical reality, and in the interim we got by with a series of ad hoc rules that basically worked but that fostered much confusion and forced much obfuscation. Now we basically get the big picture, and the obfuscation is disappearing.

• "I am not ready to take lessons in ontology from quantum physics as it now is. First I must see how it looks when it is purified of instrumentalist frivolity, and dares to say something not just about pointer readings but about the constitution of the world; and when it is purified of doublethinking deviant logic, and—most of all—when it is purified of supernatural tales about the power of the observant mind to make things jump." (Lewis 1986c: xi)

  Well, the Everett interpretation delivers on these demands.

• To paraphrase John Bell (1987: 166), all measurements of the system are ultimately measurements of the macroscopic degrees of freedom (any measurement whose results are not ultimately represented in these degrees of freedom will not actually be usable by human experimentalists) so it suffices to give a theory of measurement for these degrees of freedom—which we can do by specifying a phase-space POVM on H macro, or (somewhat less elegantly) by specifying a position measurement.
• the purpose of scientific theories is not to predict the results of experiments: it is to describe, explain, and understand the world. And quantum mechanics—as described in the previous sections—fails to do this.
• In quantum physics as in the rest of science, what motivates physicists is the explanation of phenomena, microscopic or otherwise.
• the consensus view in philosophy is now that instrumentalism is incoherent as a way to make sense of science. Not all serious philosophers of science are realists in the sense that they think that our best scientific theories are approximately true, but there is a consensus that the only way to understand a scientific theory is to understand it as offering a description of the world.
• Quantum cosmology motivated Everett’s original work in 1957; quantum cosmology remains a powerful argument for the Everett interpretation even amongst ‘hard-nosed’ physicists unmoved by more philosophical considerations.
• As David Deutsch has acerbically remarked (2010), if palaeontologists informed us that dinosaurs were such weird animals that the laws of logic don’t apply to them, or that ‘palaentology’ wasn’t really about dinosaurs at all but only about fossils, we’d have a hard time taking them seriously.
• One of the notable things about discussing the interpretation of quantum mechanics with physicists and with philosophers is that it is the physicists who propose philosophically radical ways of interpreting a theory, and the philosophers who propose changing the physics.
• this, in a nutshell, is what the Everett interpretation claims about macroscopic quantum superpositions: they are just states of the world in which more than one macroscopically definite thing is happening at once. Macroscopic superpositions do not describe indefiniteness, they describe multiplicity.
• as physics has continually reminded us, and we inhabit only a very small part of it, and it will not do to claim that it is just ‘obvious’ that it is not in a superposition.
• The ‘Everett interpretation of quantum mechanics’ is just quantum mechanics itself, ‘interpreted’ the same way we have always interpreted scientific theories in the past: as modelling the world.
• The fact remains that, at present, there are no known ways of explaining the quantitative predictions of relativistic quantum theory other than Everett’s. The Everett interpretation is the only game in town.
• The moral is clear: our intuitions as to what is ‘unreasonable’ or ‘absurd’ were formed to aid our ancestors scratching a living on the savannahs of Africa, and the Universe is not obliged to conform to them.
• In general, even in a theory with linear equations, like electromagnetism or quantum theory, adding together two states with certain structures might cause those structures to overlap and cancel out, so that the structure of the resultant state cannot just be read off from the structures of the components. Indeed, in both electromagnetism and quantum theory, the technical term for this ‘cancelling out’ is the same: interference. In general, however—and in particular in the case of the cat—there is no interference between the live-cat and dead-cat states, and so both lots of structure continue to be present. The reason we can be confident of this is because of decoherence, which in general prevents the macroscopic degrees of freedom of quantum systems from interfering, and so guarantees that structures instantiated by the macroscopic degrees of freedom of quantum systems are not erased when those systems are in superpositions of macroscopically definite states.
• I don’t know of an experiment that distinguishes the theory that dinosaurs existed from the ‘theory’ that dinosaurs didn’t exist but the fossils are exactly the way they would have been if dinosaurs had existed.

• In any case, we see this sort of thing a lot in science. We can’t directly observe a dinosaur, or a quark, or a quasar, or the interior of the sun, but that doesn’t stop us taking them seriously.

  Sceptic: In most of those cases, it’s just happenstance that we can’t make the observation. If we were properly situated in space and time, we’d be able to. Author: Well, if we were properly situated in the multiverse, we’d be able to see other worlds. Sceptic: How can you know that? Author: My best theory of physics tells me so. How do you know that you’d be able to see dinosaurs or quasars if you were appropriately situated?

• Why do you keep on insisting that this is just ‘quantum mechanics, taken literally’? quantum-logic theories, the Copenhagen interpretation, operationalism, and the like dont count as ‘taking quantum mechanics literally’ in the sense of accepting that the mathematical structure of the quantum state directly represents the structure of physical reality, just as in classical physics.