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Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime / - : - (by Sean Carroll, 2019) -

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Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime / -  :     - (by Sean Carroll, 2019) -

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime / - : - (by Sean Carroll, 2019) -

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Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime / - : - (by Sean Carroll, 2019) -
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2019
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Sean Carroll
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Sean Carroll
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upper-intermediate
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10:09:56
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105 kbps
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mp3, pdf, doc

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime / - : - :

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: Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

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PROLOGUE Don_t Be Afraid You don_t need a PhD in theoretical physics to be afraid of quantum mechanics. But it doesn_t hurt. That might seem strange. Quantum mechanics is our best theory of the microscopic world. It describes how atoms and particles interact through the forces of nature, and makes incredibly precise experimental predictions. To be sure, quantum mechanics has something of a reputation for being difficult, mysterious, just this side of magic. But professional physicists, of all people, should be relatively comfortable with a theory like that. They are constantly doing intricate calculations involving quantum phenomena, and building giant machines dedicated to testing the resulting predictions. Surely we_re not suggesting that physicists have been faking it all this time? They haven_t been faking, but they haven_t exactly been honest with themselves either. On the one hand, quantum mechanics is the heart and soul of modern physics. Astrophysicists, particle physicists, atomic physicists, laser physicists_everyone uses quantum mechanics all the time, and they_re very good at it. It_s not just a matter of esoteric research. Quantum mechanics is ubiquitous in modern technology. Semiconductors, transistors, microchips, lasers, and computer memory all rely on quantum mechanics to function. For that matter, quantum mechanics is necessary to make sense of the most basic features of the world around us. Essentially all of chemistry is a matter of applied quantum mechanics. To understand how the sun shines, or why tables are solid, you need quantum mechanics. Imagine closing your eyes. Hopefully things look pretty dark. You might think that makes sense, because no light is coming in. But that_s not quite right; infrared light, with a slightly longer wavelength than visible light, is being emitted all the time by any warm object, and that includes your own body. If our eyes were as sensitive to infrared light as they are to visible light, we would be blinded even when our lids were closed, from all the light emitted by our eyeballs themselves. But the rods and cones that act as light receptors in our eyes are cleverly sensitive to visible light, not infrared. How do they manage that? Ultimately, the answer comes down to quantum mechanics. Quantum mechanics isn_t magic. It is the deepest, most comprehensive view of reality we have. As far as we currently know, quantum mechanics isn_t just an approximation of the truth; it is the truth. That_s subject to change in the face of unexpected experimental results, but we_ve seen no hints of any such surprises thus far. The development of quantum mechanics in the early years of the twentieth century, involving names like Planck, Einstein, Bohr, Heisenberg, Schr?dinger, and Dirac, left us by 1927 with a mature understanding that is surely one of the greatest intellectual accomplishments in human history. We have every reason to be proud. On the other hand, in the memorable words of Richard Feynman, _I think I can safely say that nobody understands quantum mechanics._ We use quantum mechanics to design new technologies and predict the outcomes of experiments. But honest physicists admit that we don_t truly understand quantum mechanics. We have a recipe that we can safely apply in certain prescribed situations, and which returns mind-bogglingly precise predictions that have been triumphantly vindicated by the data. But if you want to dig deeper and ask what is really going on, we simply don_t know. Physicists tend to treat quantum mechanics like a mindless robot they rely on to perform certain tasks, not as a beloved friend they care about on a personal level. This attitude among the professionals seeps into how quantum mechanics gets explained to the wider world. What we would like to do is to present a fully formed picture of Nature, but we can_t quite do that, since physicists don_t agree about what quantum mechanics actually says. Instead, popular treatments tend to emphasize that quantum mechanics is mysterious, baffling, impossible to understand. That message goes against the basic principles that science stands for, which include the idea that the world is fundamentally intelligible. We have something of a mental block when it comes to quantum mechanics, and we need a bit of quantum therapy to help get past it. When we teach quantum mechanics to students, they are taught a list of rules. Some of the rules are of a familiar type: there_s a mathematical description of quantum systems, plus an explanation of how such systems evolve over time. But then there are a bunch of extra rules that have no analogue in any other theory of physics. These extra rules tell us what happens when we observe a quantum system, and that behavior is completely different from how the system behaves when we_re not observing it. What in the world is going on with that? There are basically two options. One is that the story we_ve been telling our students is woefully incomplete, and in order for quantum mechanics to qualify as a sensible theory we need to understand what a _measurement_ or _observation_ is, and why it seems so different from what the system does otherwise. The other option is that quantum mechanics represents a violent break from the way we have always thought about physics before, shifting from a view where the world exists objectively and independently of how we perceive it, to one where the act of observation is somehow fundamental to the nature of reality. In either case, the textbooks should by all rights spend time exploring these options, and admit that even though quantum mechanics is extremely successful, we can_t claim to be finished developing it just yet. They don_t. For the most part, they pass over this issue in silence, preferring to stay in the physicist_s comfort zone of writing down equations and challenging students to solve them. That_s embarrassing. And it gets worse. You might think, given this situation, that the quest to understand quantum mechanics would be the single biggest goal in all of physics. Millions of dollars of grant money would flow to researchers in quantum foundations, the brightest minds would flock to the problem, and the most important insights would be rewarded with prizes and prestige. Universities would compete to hire the leading figures in the area, dangling superstar salaries to lure them away from rival institutions. Sadly, no. Not only is the quest to make sense of quantum mechanics not considered a high-status specialty within modern physics; in many quarters it_s considered barely respectable at all, if not actively disparaged. Most physics departments have nobody working on the problem, and those who choose to do so are looked upon with suspicion. (Recently while writing a grant proposal, I was advised to concentrate on describing my work in gravitation and cosmology, which is considered legitimate, and remain silent about my work on the foundations of quantum mechanics, as that would make me appear less serious.) There have been important steps forward over the last ninety years, but they have typically been made by headstrong individuals who thought the problems were important despite what all of their colleagues told them, or by young students who didn_t know any better and later left the field entirely. In one of Aesop_s fables, a fox sees a juicy bunch of grapes and leaps to reach it, but can_t quite jump high enough. In frustration he declares that the grapes were probably sour, and he never really wanted them anyway. The fox represents _physicists,_ and the grapes are _understanding quantum mechanics._ Many researchers have decided that understanding how nature really works was never really important; all that matters is the ability to make particular predictions. Scientists are trained to value tangible results, whether they are exciting experimental findings or quantitative theoretical models. The idea of working to understand a theory we already have, even if that effort might not lead to any specific new technologies or predictions, can be a tough sell. The underlying tension was illustrated in the TV show The Wire, where a group of hardworking detectives labored for months to meticulously gather evidence that would build a case against a powerful drug ring. Their bosses, meanwhile, had no patience for such incremental frivolity. They just wanted to see drugs on the table for their next press conference, and encouraged the police to bang heads and make splashy arrests. Funding agencies and hiring committees are like those bosses. In a world where all the incentives push us toward concrete, quantifiable outcomes, less pressing big-picture concerns can be pushed aside as we race toward the next immediate goal. This book has three main messages. The first is that quantum mechanics should be understandable_even if we_re not there yet_and achieving such understanding should be a high-priority goal of modern science. Quantum mechanics is unique among physical theories in drawing an apparent distinction between what we see and what really is. That poses a special challenge to the minds of scientists (and everyone else), who are used to thinking about what we see as unproblematically _real,_ and working to explain things accordingly. But this challenge isn_t insuperable, and if we free our minds from certain old-fashioned and intuitive ways of thinking, we find that quantum mechanics isn_t hopelessly mystical or inexplicable. It_s just physics. The second message is that we have made real progress toward understanding. I will focus on the approach I feel is clearly the most promising route, the Everett or Many-Worlds formulation of quantum mechanics. Many-Worlds has been enthusiastically embraced by many physicists, but it has a sketchy reputation among people who are put off by a proliferation of other realities containing copies of themselves. If you are one of those people, I want to at least convince you that Many-Worlds is the purest way of making sense of quantum mechanics_it_s where we end up if we just follow the path of least resistance in taking quantum phenomena seriously. In particular, the multiple worlds are predictions of the formalism that is already in place, not something added in by hand. But Many-Worlds isn_t the only respectable approach, and we will mention some of its main competitors. (I will endeavor to be fair, if not necessarily balanced.) The important thing is that the various approaches are all well-constructed scientific theories, with potentially different experimental ramifications, not just woolly-headed _interpretations_ to be debated over cognac and cigars after we_re finished doing real work. The third message is that all this matters, and not just for the integrity of science. The success to date of the existing adequate-but-not-perfectly-coherent framework of quantum mechanics shouldn_t blind us to the fact that there are circumstances under which such an approach simply isn_t up to the task. In particular, when we turn to understanding the nature of spacetime itself, and the origin and ultimate fate of the entire universe, the foundations of quantum mechanics are absolutely crucial. I_ll introduce some new, exciting, and admittedly tentative proposals that draw provocative connections between quantum entanglement and how spacetime bends and curves_the phenomenon you and I know as _gravity._ For many years now, the search for a complete and compelling quantum theory of gravity has been recognized as an important scientific goal (prestige, prizes, stealing away faculty, and all that). It may be that the secret is not to start with gravity and _quantize_ it, but to dig deeply into quantum mechanics itself, and find that gravity was lurking there all along. We don_t know for sure. That_s the excitement and anxiety of cutting-edge research. But the time has come to take the fundamental nature of reality seriously, and that means confronting quantum mechanics head-on. 1 What_s Going On Looking at the Quantum World Albert Einstein, who had a way with words as well as with equations, was the one who stuck quantum mechanics with the label it has been unable to shake ever since: spukhaft, usually translated from German to English as _spooky._ If nothing else, that_s the impression we get from most public discussions of quantum mechanics. We_re told that it_s a part of physics that is unavoidably mystifying, weird, bizarre, unknowable, strange, baffling. Spooky. Inscrutability can be alluring. Like a mysterious, sexy stranger, quantum mechanics tempts us into projecting all sorts of qualities and capacities onto it, whether they are there or not. A brief search for books with _quantum_ in the title reveals the following list of purported applications: Quantum Success Quantum Leadership Quantum Consciousness Quantum Touch Quantum Yoga Quantum Eating Quantum Psychology Quantum Mind Quantum Glory Quantum Forgiveness Quantum Theology Quantum Happiness Quantum Poetry Quantum Teaching Quantum Faith Quantum Love For a branch of physics that is often described as only being relevant to microscopic processes involving subatomic particles, that_s a pretty impressive r?sum?. To be fair, quantum mechanics_or _quantum physics,_ or _quantum theory,_ the labels are all interchangeable_is not only relevant to microscopic processes. It describes the whole world, from you and me to stars and galaxies, from the centers of black holes to the beginning of the universe. But it is only when we look at the world in extreme close-up that the apparent weirdness of quantum phenomena becomes unavoidable. One of the themes in this book is that quantum mechanics doesn_t deserve the connotation of spookiness, in the sense of some ineffable mystery that it is beyond the human mind to comprehend. Quantum mechanics is amazing; it is novel, profound, mind-stretching, and a very different view of reality from what we_re used to. Science is like that sometimes. But if the subject seems difficult or puzzling, the scientific response is to solve the puzzle, not to pretend it_s not there. There_s every reason to think we can do that for quantum mechanics just like any other physical theory. Many presentations of quantum mechanics follow a typical pattern. First, they point to some counterintuitive quantum phenomenon. Next, they express bafflement that the world can possibly be that way, and despair of it making sense. Finally (if you_re lucky), they attempt some sort of explanation. Our theme is prizing clarity over mystery, so I don_t want to adopt that strategy. I want to present quantum mechanics in a way that will make it maximally understandable right from the start. It will still seem strange, but that_s the nature of the beast. What it won_t seem, hopefully, is inexplicable or unintelligible. We will make no effort to follow historical order. In this chapter we_ll look at the basic experimental facts that force quantum mechanics upon us, and in the next we_ll quickly sketch the Many-Worlds approach to making sense of those observations. Only in the chapter after that will we offer a semi-historical account of the discoveries that led people to contemplate such a dramatically new kind of physics in the first place. Then we_ll hammer home exactly how dramatic some of the implications of quantum mechanics really are. With all that in place, over the rest of the book we can set about the fun task of seeing where all this leads, demystifying the most striking features of quantum reality. Physics is one of the most basic sciences, indeed one of the most basic human endeavors. We look around the world, we see it is full of stuff. What is that stuff, and how does it behave? These are questions that have been asked ever since people started asking questions. In ancient Greece, physics was thought of as the general study of change and motion, of both living and nonliving matter. Aristotle spoke a vocabulary of tendencies, purposes, and causes. How an entity moves and changes can be explained by reference to its inner nature and to external powers acting upon it. Typical objects, for example, might by nature be at rest; in order for them to move, it is necessary that something be causing that motion. All of this changed thanks to a clever chap named Isaac Newton. In 1687 he published Principia Mathematica, the most important work in the history of physics. It was there that he laid the groundwork for what we now call _classical_ or simply _Newtonian_ mechanics. Newton blew away any dusty talk of natures and purposes, revealing what lay underneath: a crisp, rigorous mathematical formalism with which teachers continue to torment students to this very day. Whatever memory you may have of high-school or college homework assignments dealing with pendulums and inclined planes, the basic ideas of classical mechanics are pretty simple. Consider an object such as a rock. Ignore everything about the rock that a geologist might consider interesting, such as its color and composition. Put aside the possibility that the basic structure of the rock might change, for example, if you smashed it to pieces with a hammer. Reduce your mental image of the rock down to its most abstract form: the rock is an object, and that object has a location in space, and that location changes with time. Classical mechanics tells us precisely how the position of the rock changes with time. We_re very used to that by now, so it_s worth reflecting on how impressive this is. Newton doesn_t hand us some vague platitudes about the general tendency of rocks to move more or less in this or that fashion. He gives us exact, unbreakable rules for how everything in the universe moves in response to everything else_rules that can be used to catch baseballs or land rovers on Mars. Here_s how it works. At any one moment, the rock will have a position and also a velocity, a rate at which it_s moving. According to Newton, if no forces act on the rock, it will continue to move in a straight line at constant velocity, for all time. (Already this is a major departure from Aristotle, who would have told you that objects need to be constantly pushed if they are to be kept in motion.) If a force does act on the rock, it will cause acceleration_some change in the velocity of the rock, which might make it go faster, or slower, or merely alter its direction_in direct proportion to how much force is applied. That_s basically it. To figure out the entire trajectory of the rock, you need to tell me its position, its velocity, and what forces are acting on it. Newton_s equations tell you the rest. Forces might include the force of gravity, or the force of your hand if you pick up the rock and throw it, or the force from the ground when the rock comes to land. The idea works just as well for billiard balls or rocket ships or planets. The project of physics, within this classical paradigm, consists essentially of figuring out what makes up the stuff of the universe (rocks and so forth) and what forces act on them. Classical physics provides a straightforward picture of the world, but a number of crucial moves were made along the way to setting it up. Notice that we had to be very specific about what information we required to figure out what would happen to the rock: its position, its velocity, and the forces acting on it. We can think of those forces as being part of the outside world, and the important information about the rock itself as consisting of just its position and velocity. The acceleration of the rock at any moment in time, by contrast, is not something we need to specify; that_s exactly what Newton_s laws allow us to calculate from the position and the velocity. Together, the position and velocity make up the state of any object in classical mechanics. If we have a system with multiple moving parts, the classical state of that entire system is just a list of the states of each of the individual parts. The air in a normal-sized room will have perhaps 1027 molecules of different types, and the state of that air would be a list of the position and velocity of every one of them. (Strictly speaking, physicists like to use the momentum of each particle, rather than its velocity, but as far as Newtonian mechanics is concerned the momentum is simply the particle_s mass times its velocity.) The set of all possible states that a system could have is known as the phase space of the system. The French mathematician Pierre-Simon Laplace pointed out a profound implication of the classical mechanics way of thinking. In principle, a vast intellect could know the state of literally every object in the universe, from which it could deduce everything that would happen in the future, as well as everything that had happened in the past. Laplace_s demon is a thought experiment, not a realistic project for an ambitious computer scientist, but the implications of the thought experiment are profound. Newtonian mechanics describes a deterministic, clockwork universe. The machinery of classical physics is so beautiful and compelling that it seems almost inescapable once you grasp it. Many great minds who came after Newton were convinced that the basic superstructure of physics had been solved, and future progress lay in figuring out exactly what realization of classical physics (which particles, which forces) was the right one to describe the universe as a whole. Even relativity, which was world-transforming in its own way, is a variety of classical mechanics rather than a replacement for it. Then along came quantum mechanics, and everything changed. Alongside Newton_s formulation of classical mechanics, the invention of quantum mechanics represents the other great revolution in the history of physics. Unlike anything that had come before, quantum theory didn_t propose a particular physical model within the basic classical framework; it discarded that framework entirely, replacing it with something profoundly different. The fundamental new element of quantum mechanics, the thing that makes it unequivocally distinct from its classical predecessor, centers on the question of what it means to measure something about a quantum system. What exactly a measurement is, and what happens when we measure something, and what this all tells us about what_s really happening behind the scenes: together, these questions constitute what_s called the measurement problem of quantum mechanics. There is absolutely no consensus within physics or philosophy on how to solve the measurement problem, although there are a number of promising ideas. Attempts to address the measurement problem have led to the emergence of a field known as the interpretation of quantum mechanics, although the label isn_t very accurate. _Interpretations_ are things that we might apply to a work of literature or art, where people might have different ways of thinking about the same basic object. What_s going on in quantum mechanics is something else: a competition between truly distinct scientific theories, incompatible ways of making sense of the physical world. For this reason, modern workers in this field prefer to call it _foundations of quantum mechanics._ The subject of quantum foundations is part of science, not literary criticism. Nobody ever felt the need to talk about _interpretations of classical mechanics__classical mechanics is perfectly transparent. There is a mathematical formalism that speaks of positions and velocities and trajectories, and oh, look: there is a rock whose actual motion in the world obeys the predictions of that formalism. There is, in particular, no such thing as a measurement problem in classical mechanics. The state of the system is given by its position and its velocity, and if we want to measure those quantities, we simply do so. Of course, we can measure the system sloppily or crudely, thereby obtaining imprecise results or altering the system itself. But we don_t have to; just by being careful, we can precisely measure everything there is to know about the system without altering it in any noticeable way. Classical mechanics offers a clear and unambiguous relationship between what we see and what the theory describes. Quantum mechanics, for all its successes, offers no such thing. The enigma at the heart of quantum reality can be summed up in a simple motto: what we see when we look at the world seems to be fundamentally different from what actually is. Think about electrons, the elementary particles orbiting atomic nuclei, whose interactions are responsible for all of chemistry and hence almost everything interesting around you right now. As we did with the rock, we can ignore some of the electron_s specific properties, like its spin and the fact that it has an electric field. (Really we could just stick with the rock as our example_rocks are quantum systems just as much as electrons are_but switching to a subatomic particle helps us remember that the features distinguishing quantum mechanics only become evident when we consider very tiny objects indeed.) Unlike in classical mechanics, where the state of a system is described by its position and velocity, the nature of a quantum system is something a bit less concrete. Consider an electron in its natural habitat, orbiting the nucleus of an atom. You might think, from the word _orbit_ as well as from the numerous cartoon depictions of atoms you have doubtless been exposed to over the years, that the orbit of an electron is more or less like the orbit of a planet in the solar system. The electron (so you might think) has a location, and a velocity, and as time passes it zips around the central nucleus in a circle or maybe an ellipse. Quantum mechanics suggests something different. We can measure values of the location or velocity (though not at the same time), and if we are sufficiently careful and talented experimenters we will obtain some answer. But what we_re seeing through such a measurement is not the actual, complete, unvarnished state of the electron. Indeed, the particular measurement outcome we will obtain cannot be predicted with perfect confidence, in a profound departure from the ideas of classical mechanics. The best we can do is to predict the probability of seeing the electron in any particular location or with any particular velocity. The classical notion of the state of a particle, _its location and its velocity,_ is therefore replaced in quantum mechanics by something utterly alien to our everyday experience: a cloud of probability. For an electron in an atom, this cloud is more dense toward the center and thins out as we get farther away. Where the cloud is thickest, the probability of seeing the electron is highest; where it is diluted almost to imperceptibility, the probability of seeing the electron is vanishingly small. This cloud is often called a wave function, because it can oscillate like a wave, as the most probable measurement outcome changes over time. We usually denote a wave function by ?, the Greek letter Psi. For every possible measurement outcome, such as the position of the particle, the wave function assigns a specific number, called the amplitude associated with that outcome. The amplitude that a particle is at some position x0, for example, would be written ?(x0). The probability of getting that outcome when we perform a measurement is given by the amplitude squared. Probability of a particular outcome = |Amplitude for that outcome|2 This simple relation is called the Born rule, after physicist Max Born.* Part of our task will be to figure out where in the world such a rule came from. We_re most definitely not saying that there is an electron with some position and velocity, and we just don_t know what those are, so the wave function encapsulates our ignorance about those quantities. In this chapter we_re not saying anything at all about what _is,_ only what we observe. In chapters to come, I will pound the table and insist that the wave function is the sum total of reality, and ideas such as the position or the velocity of the electron are merely things we can measure. But not everyone sees things that way, and for the moment we are choosing to don a mask of impartiality. Let_s place the rules of classical and quantum mechanics side by side to compare them. The state of a classical system is given by the position and velocity of each of its moving parts. To follow its evolution, we imagine something like the following procedure: Rules of Classical Mechanics 1. Set up the system by fixing a specific position and velocity for each part. 2. Evolve the system using Newton_s laws of motion. That_s it. The devil is in the details, of course. Some classical systems can have a lot of moving pieces. In contrast, the rules of standard textbook quantum mechanics come in two parts. In the first part, we have a structure that exactly parallels that of the classical case. Quantum systems are described by wave functions rather than by positions and velocities. Just as Newton_s laws of motion govern the evolution of the state of a system in classical mechanics, there is an equation that governs how wave functions evolve, called Schr?dinger_s equation. We can express Schr?dinger_s equation in words as: _The rate of change of a wave function is proportional to the energy of the quantum system._ Slightly more specifically, a wave function can represent a number of different possible energies, and the Schr?dinger equation says that high-energy parts of the wave function evolve rapidly, while low-energy parts evolve very slowly. Which makes sense, when we think about it. What matters for our purposes is simply that there is such an equation, one that predicts how wave functions evolve smoothly through time. That evolution is as predictable and inevitable as the way objects move according to Newton_s laws in classical mechanics. Nothing weird is happening yet. The beginning of the quantum recipe reads something like this: Rules of Quantum Mechanics (Part One) 1. Set up the system by fixing a specific wave function ?. 2. Evolve the system using Schr?dinger_s equation. So far, so good_these parts of quantum mechanics exactly parallel their classical predecessors. But whereas the rules of classical mechanics stop there, the rules of quantum mechanics keep going. All the extra rules deal with measurement. When you perform a measurement, such as the position or spin of a particle, quantum mechanics says there are only certain possible results you will ever get. You can_t predict which of the results it will be, but you can calculate the probability for each allowed outcome. And after your measurement is done, the wave function collapses to a completely different function, with all of the new probability concentrated on whatever result you just got. So if you measure a quantum system, in general the best you can do is predict probabilities for various outcomes, but if you were to immediately measure the same quantity again, you will always get the same answer_the wave function has collapsed onto that outcome. Let_s write this out in gory detail. Rules of Quantum Mechanics (Part Two) 3. There are certain observable quantities we can choose to measure, such as position, and when we do measure them, we obtain definite results. 4. The probability of getting any one particular result can be calculated from the wave function. The wave function associates an amplitude with every possible measurement outcome; the probability for any outcome is the square of that amplitude. 5. Upon measurement, the wave function collapses. However spread out it may have been pre-measurement, afterward it is concentrated on the result we obtained. In a modern university curriculum, when physics students are first exposed to quantum mechanics, they are taught some version of these five rules. The ideology associated with this presentation_treat measurements as fundamental, wave functions collapse when they are observed, don_t ask questions about what_s going on behind the scenes_is sometimes called the Copenhagen interpretation of quantum mechanics. But people, including the physicists from Copenhagen who purportedly invented this interpretation, disagree on precisely what that label should be taken to describe. We can just refer to it as _standard textbook quantum mechanics._ The idea that these rules represent how reality actually works is, needless to say, outrageous. What precisely do you mean by a _measurement_? How quickly does it happen? What exactly constitutes a measuring apparatus? Does it need to be human, or have some amount of consciousness, or perhaps the ability to encode information? Or maybe it just has to be macroscopic, and if so how macroscopic does it have to be? When exactly does the measurement occur, and how quickly? How in the world does the wave function collapse so dramatically? If the wave function were very spread out, does the collapse happen faster than the speed of light? And what happens to all the possibilities that were seemingly allowed by the wave function but which we didn_t observe? Were they never really there? Do they just vanish into nothingness? To put things most pointedly: Why do quantum systems evolve smoothly and deterministically according to the Schr?dinger equation as long as we aren_t looking at them, but then dramatically collapse when we do look? How do they know, and why do they care? (Don_t worry, we_re going to answer all these questions.) Science, most people think, seeks to understand the natural world. We observe things happening, and science hopes to provide an explanation for what is going on. In its current textbook formulation, quantum mechanics has failed in this ambition. We don_t know what_s really going on, or at least the community of professional physicists cannot agree on what it is. What we have instead is a recipe that we enshrine in textbooks and teach to our students. Isaac Newton could tell you, starting with the position and velocity of a rock that you have thrown into the air in the Earth_s gravitational field, just what the subsequent trajectory of that rock was going to be. Analogously, starting with a quantum system prepared in some particular way, the rules of quantum mechanics can tell you how the wave function will change over time, and what the probability of various possible measurement outcomes will be should you choose to observe it. The fact that the quantum recipe provides us with probabilities rather that certainties might be annoying, but we could learn to live with it. What bugs us, or should, is our lack of understanding about what is actually happening. Imagine that some devious genius figured out all the laws of physics, but rather than revealing them to the rest of the world, they programmed a computer to answer questions concerning specific physics problems, and put an interface to the program on a web page. Anyone who was interested could just surf over to that site, type in a well-posed physics question, and get the correct answer. Such a program would obviously be of great use to scientists and engineers. But having access to the site wouldn_t qualify as understanding the laws of physics. We would have an oracle that was in the business of providing answers to specific questions, but we ourselves would be completely lacking in any intuitive idea of the underlying rules of the game. The rest of the world_s scientists, presented with such an oracle, wouldn_t be moved to declare victory; they would continue with their work of figuring out what the laws of nature actually were. Quantum mechanics, in the form in which it is currently presented in physics textbooks, represents an oracle, not a true understanding. We can set up specific problems and answer them, but we can_t honestly explain what_s happening behind the scenes. What we do have are a number of good ideas about what that could be, and it_s past time that the physics community started taking these ideas seriously. * There_s a slight technicality, which we_ll mention here and then pretty much forget

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