From the Prologue What is at stake in the argument over quantum mechanics? Why does it matter if our fundamental theory of the natural world is mysterious and paradoxical? Behind the century-long argument over quantum mechanics is a fundamental disagreement about the nature of reality--a disagreement which, unresolved, escalates into an argument about the nature of science. Two questions underlie the schism. First off, does the natural world exist independently of our minds? More precisely, does matter have a stable set of properties in and of itself, without regard to our perceptions and knowledge? Second, can those properties be comprehended and described by us? Can we understand enough about the laws of nature to explain the history of our universe and predict its future? The answers we give to these two questions have implications for larger questions about the nature and aim of science, and the role of science in the larger human project. These are, indeed, questions about the boundary between reality and fantasy. People who answer yes to these two questions are called realists. Einstein was a realist. I am also a realist. We realists believe that there is a real world out there, whose properties in no way depend on our knowledge or perception of it. This is nature--as it would be, and mostly is, in our absence. We also believe that the world may be understood and described precisely enough to explain how any system in the natural world behaves. If you are a realist, you believe that science is the systematic search for that explanation. This is based on a naive notion of truth. Assertions about objects or systems in nature are true to the extent that they correspond to genuine properties of nature. If you answer no to one or both of these questions, you are an anti-realist. Most scientists are realists about everyday objects on the human scale. Things we can see, pick up, and throw around have simple and easily comprehended properties. They exist at each moment somewhere in space. When they move, they follow a trajectory, and that trajectory has, relative to someone describing them, a definite speed. They have mass and weight. When we tell our partner that the red notebook they are looking for is on the table, we expect that this is simply true or false, absolutely independent of our knowledge or perception. The description of matter at this level, from the smallest scales we can see with our eyes up to stars and planets, is called classical physics. It was invented by Galileo, Kepler, and Newton. Einstein's theories of relativity are its crowning achievements. But it is not easy, or obvious, for us to be realists about matter on the scale of individual atoms. This is because of quantum mechanics. Quantum mechanics is presently our best theory of nature at the atomic scale. That theory has, as I have alluded to, certain very puzzling features. It is widely believed that those features preclude realism. That is, quantum mechanics requires that we say no to one or both of the two questions I asked above. To the extent that quantum mechanics is the correct description of nature, we are forced to give up realism. Most physicists are not realists about atoms, radiation, and elementary particles. Their belief, for the most part, does not stem from a desire to reject realism on the basis of radical philosophical positions. Instead, it is because they are convinced quantum mechanics is correct and they believe, as they have been taught, that quantum mechanics precludes realism. If it is true that quantum mechanics requires that we give up realism, then, if you are a realist, you must believe that quantum mechanics is false. It may be temporarily successful, but it cannot be the fully correct description of nature at an atomic scale. This led Einstein to reject quantum mechanics as anything more than a temporary expedient. Einstein and other realists believe that quantum mechanics gives us an incomplete description of nature, which is missing features necessary for a full understanding of the world. Einstein sometimes imagined that there were "hidden variables" which would complete the description of the world given by quantum theory. He believed that the full description, including those missing features, would be consistent with realism. Thus, if you are a realist and a physicist, there is one overriding imperative, which is to go beyond quantum mechanics to discover those missing features and use that knowledge to construct a true theory of the atoms. This was Einstein's unfinished mission, and it is mine. [...] This all matters because science is under attack in the early twenty- first century. Science is under attack, and with it the belief in a real world in which facts are either true or false. Quite literally, parts of our society appear to be losing their grip on the boundary between reality and fantasy. Science is under attack from those who find its conclusions inconvenient for their political and business objectives. Climate change should not be a political issue; it is not a matter of ideology, but an issue of national security, and should be treated as such. It is a real problem, which will require evidence- based solutions. Science is also under attack from religious fundamentalists who insist ancient texts are the teachings of unchanging truths by God. In my view, there is little reason for conflict between most religions and science. Many religions accept-- and even celebrate-- science as the way to knowledge about the natural world. Beyond that, there is mystery enough in the existence and meaning of the world, which both science and religion can inspire us to discuss, but neither can resolve. All that is required is that religions not attack or seek to undermine those scientific discoveries which are considered to be established knowledge because they are supported by overwhelming evidence, as judged by those educated sufficiently to evaluate their validity. This is indeed the view of many religious leaders from all faiths. In return, scientists should view these enlightened leaders as allies in the work for a better world. In addition, science is under attack from a fashion among some humanist academics-- who should know better--who hold that science is no more than a social construction that yields only one of an array of equally valid perspectives. For science to respond clearly and strongly to these challenges, it must itself be uncorrupted by its own practitioners' mystical yearnings and metaphysical agendas. Individual scientists may be--and, let's face it, sometimes are--motivated by mystical feelings and metaphysical preconceptions. This doesn't hurt science as long as the narrow criteria that distinguish hypothesis and hunch from established truth are universally understood and adhered to. But when fundamental physics itself gets hijacked by an anti- realist philosophy, we are in danger. We risk giving up on the centuries- old project of realism, which is nothing less than the continual adjustment, bit by bit as knowledge progresses, of the boundary between our knowledge of reality and the realm of fantasy. One danger of anti- realism is to the practice of physics itself. Anti- realism lowers our ambition for a totally clear understanding of nature, and hence weakens our standards as to what constitutes an understanding of a physical system. In the wake of the triumph of anti-realism about the atomic world, we have had to contend with anti- realist speculations about nature on the largest possible scale. A vocal minority of cosmologists proclaims that the universe we see around us is only a bubble in a vast ocean called the multiverse that contains an infinity of other bubbles. And, whereas it is safe to hypothesize that the galaxies we can see are typical of the rest of our universe, one must regard the other invisible bubbles as governed by diverse and randomly assigned laws, so our universe is far from typical of the whole. This, together with the fact that all, or almost all, of the other bubbles are forever out of range of our observations, means the multiverse hypothesis can never be tested or falsified. This puts this fantasy outside the bounds of science. Nonetheless, this idea is championed by not a few highly regarded physicists and mathematicians. It would be a mistake to confuse this multiverse fantasy for the Many Worlds Interpretation of quantum mechanics. They are distinct ideas. Nonetheless, they share a magical- realist subversion of the aim of science to explain the world we see around us in terms of only itself. I would suggest that the harm done to clarity about the aim and purpose of science by the enthusiastic proponents of the multiverse would not have been possible had not the majority of physicists uncritically adopted anti- realist versions of quantum physics. Certainly, quantum mechanics explains many aspects of nature, and it does so with supreme elegance. Physicists have developed a very powerful tool kit for explaining diverse phenomena in terms of quantum mechanics, so when you master quantum mechanics you control a lot about nature. At the same time, physicists are always dancing around the gaping holes that quantum mechanics leaves in our understanding of nature. The theory fails to provide a picture of what is going on in individual processes, and it often fails to explain why an experiment turns out one way rather than another. These gaps and failures matter because they underlie the fact that we have gotten only partway toward solving the central problems in science before seeming to run out of steam. I believe that we have not yet succeeded in unifying quantum theory with gravity and spacetime (which is what we mean by quantizing gravity), or in unifying the interactions, because we have been working with an incomplete and incorrect quantum theory. But I suspect that the implications of building science on incorrect foundations go further and deeper. The trust in science as a method to resolve disagreements and locate truth is undermined when a radical strand of anti-realism flourishes at the foundations of science. When those who set the standard for what constitutes explanation are seduced by a virulent mysticism, the resulting confusion is felt throughout the culture. I was privileged to meet a few of the second generation of the founders of twentieth- century physics. One of the most contradictory was John Archibald Wheeler. A nuclear theorist and a mystic, he transmitted the legacies of Albert Einstein and Niels Bohr to my generation through the stories he told us of his friendships with them. Wheeler was a committed cold warrior who worked on the hydrogen bomb even as he pioneered the study of quantum universes and black holes. He was also a great mentor who counted among his students Richard Feynman, Hugh Everett, and several of the pioneers of quantum gravity. And he might have been my mentor, had I had better judgment. A true student of Bohr, Wheeler spoke in riddles and paradoxes. His blackboard was unlike any I'd ever encountered. It had no equations, and only a few elegantly written aphorisms, each set out in a box, distilling a lifetime of seeking the reason why our world is a quantum universe. A typical example was "It from bit." (Yes, read it again-- slowly! Wheeler was an early adopter of the current fashion to regard the world as constituted of information, so that information is more fundamental than what it describes. This is a form of anti- realism we will discuss later.) Here is another: "No phenomenon is a real phenomenon until it is an observed phenomenon." Here is the kind of conversation one had with Wheeler: He asked me, "Suppose when you die and go up before Saint Peter for your final, final exam, he asks you just one question: 'Why the quantum? ' " (I.e., why do we live in a world described by quantum mechanics?) "What will you say to him?" Much of my life has been spent searching for a satisfying answer to that question. As I write these pages, I find myself vividly recalling my first encounters with quantum physics. When I was a seventeen- year- old high school dropout, I used to browse the shelves at the University of Cincinnati Physics Library. There I came upon a book with a chapter by Louis de Broglie (we will meet him in chapter 7), who was the first to propose that electrons are waves as well as particles. That chapter introduced his pilot wave theory, which was the first realist formulation of quantum mechanics. It was in French, a language I read fitfully after two years of high school study, but I recall well my excitement as I understood the basics. I still can close my eyes and see a page of the book, displaying the equation that relates wavelength to momentum. My first actual course in quantum mechanics was the next spring at Hampshire College. That course, taught by Herbert Bernstein, ended with a presentation of the fundamental theorem of John Bell, which, in brief, demonstrates that the quantum world fits uneasily into space. I vividly recall that when I understood the proof of the theorem, I went outside in the warm afternoon and sat on the steps of the college library, stunned. I pulled out a notebook and immediately wrote a poem to a girl I had a crush on, in which I told her that each time we touched there were electrons in our hands which from then on would be entangled with each other. I no longer recall who she was or what she made of my poem, or if I even showed it to her. But my obsession with penetrating the mystery of nonlocal entanglement, which began that day, has never left me; nor has my urgency to make better sense of the quantum diminished over the decades since. In my career, the puzzles of quantum physics have been the central mystery to which I've returned again and again. I hope in these pages to inspire in you a similar fascination. [...] W elcome to the quantum world . Feel at home, for it is our world, and it is our good fortune that its mysteries remain for us to solve. Excerpted from Einstein's Unfinished Revolution: The Search for What Lies Beyond the Quantum by Lee Smolin All rights reserved by the original copyright owners. 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