by Lee Smolin
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THERE ARE DIFFERENT KINDS of anti-realists, which leads to different views on quantum mechanics.
Some anti-realists believe that the properties we ascribe to atoms and elementary particles are not inherent in those objects, but are created only by our interactions with them, and exist only at the time when we measure them. We can call these radical anti-realists. The most influential of these was Niels Bohr. He was the first to apply quantum theory to the atom, after which he became the leader and mentor to the next generation of quantum revolutionaries. His radical anti-realism colored much of how quantum theory came to be understood.
Another group of anti-realists believes that science, as a whole, does not deal in or talk about what is real in nature, but rather only ever talks about our knowledge of the world. In their view, the properties physics ascribes to an atom are not about that atom; they are instead only about the knowledge we have of the atom. These scientists can be called quantum epistemologists.
And then there are the operationalists, a group of anti-realists who are agnostic about whether there is a fundamental reality independent of us or not. Quantum mechanics, they argue, is not in any case about reality; it is rather a set of procedures for interrogating atoms. It is not about the atoms themselves; it is about what happens when atoms come into contact with the big devices we use to measure them. Heisenberg, the best of Bohr’s protégés, who invented the equations of quantum mechanics, was, at least partly, an operationalist.
In contrast to the disputes between radical anti-realists, quantum epistemologists, and operationalists, all realists share a similar perspective—we agree about the answer to both questions I posed above. But we differ on how we answer a third question: Does the natural world consist mainly of the kinds of objects that we see when we look around ourselves, and the things that constitute them? In other words, is what we see when we look around typical of the universe as a whole?
Those of us who say yes to this question can call ourselves simple or naive realists. I should alert the reader that I use the adjective “naive” to mean strong, fresh, uncomplicated. For me, a view is naive if it is not in need of sophisticated arguments or convoluted justifications. I would argue that a naive realism is, whenever possible, to be preferred.
There are realists who are not naive in this sense. They believe that reality is vastly different from the world we perceive and measure.
An example of such a view is the Many Worlds Interpretation, which teaches that the world we perceive is only one of a vast and ever-growing number of parallel worlds. Its proponents call themselves realists, and they have some claim to that designation by virtue of their answering yes to the first two questions. But, in my opinion, they are realists only in the most technical, academic sense. They may perhaps be called magical realists, for they believe that what is real is far beyond the world we perceive. Magical realism in this sense is almost a form of mysticism, for it implies that the true world is hidden from our perception.
Is it possible to formulate a theory of atoms that is realist in the most general and naive sense, and so answers yes to all three questions? It is, and that is the story I want to tell in this book. But that theory is not quantum mechanics, and if it is right, then quantum mechanics is wrong, in the sense that quantum mechanics must then give a very incomplete description of nature.
Part of the story I want to tell here is how this naively realistic theory of nature was pushed aside, while a theory that required us to embrace either anti-realism or mysticism thrived. But I will end on a hopeful note, by sketching a way we may progress to a realist view of nature that encompasses the quantum.
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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 mecha
nics 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.
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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,1 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.
The story I tell in this book is shaped like a play in three acts. Part 1 teaches the basic concepts of quantum mechanics we will need while tracing the story of its invention. The main theme here is the triumph of the anti-realists, led by Bohr and Heisenberg, over the realists, whose champion was Einstein. Please note that the story I tell here is just a sketch; the real history is far more complex. Part 2 traces the revival of realist approaches to quantum mechanics, beginning in the 1950s, and explains their strong and weak points. The heroes here are an American physicist named David Bohm and an Irish theorist, John Bell.
The conclusion of part 2 will be that realist approaches are possible, and work well enough to undermine the claims that quantum physics requires us all to become anti-realists. Still, for me, none of these approaches have the ring of truth. I believe we can do better; indeed, for reasons I will explain, I would venture that the correct completion of quantum mechanics will also solve the problem of quantum gravity, as well as give us a good cosmological theory. Part 3 introduces contemporary efforts to construct this realist theory of everything, some mine, some by others.
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WELCOME 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.
PART 1
AN ORTHODOXY OF THE UNREAL
ONE
Nature Loves to Hide
Reality is the business of physics.
—ALBERT EINSTEIN
Quantum mechanics has been the core of our understanding of nature for nine decades. It is ubiquitous, but it is also deeply mysterious. Little of modern science would make sense without it. But experts have a hard time agreeing what it asserts about nature.
Quantum mechanics explains why there are atoms, and why those atoms are stable and have distinct chemical properties. Quantum mechanics also explains how atoms combine into diverse molecules. As a result, it is the basis for how we understand the shapes and interactions of those molecules. Life would be incomprehensible without the quantum. From the behavior of water to the shapes of proteins to the fidelity and transmittal of information by DNA and RNA, everything in biology depends on the quantum.
Quantum mechanics explains the properties of materials, such as what makes a metal a conductor of electricity, while another is an insulator. It explains light and radioactivity, and is the basis of nuclear physics. Without it we wouldn’t understand how the stars shine. Nor could we have invented the chips or the lasers on which so much of our technology is based. Quantum mechanics is the language that we use to write the standard model of particle physics, which contains all we know about the elementary particles and the fundamental forces by which they interact.
According to our best theory of the early universe, all matter, along with the patterns that eventually coalesced into the galaxies, was yanked into existence from the quantum randomness of the vacuum of empty space by the rapid expansion of the universe. I don’t expect the reader to understand precisely what this means, but perhaps the words evoke an image. In any case, if this is right, then without quantum physics there would literally be nothing except empty spacetime.
Yet for all its success, there is a stubborn puzzle at the heart of quantum mechanics. The quantum world behaves in ways that challenge our intuition. It is often said that in quantum physics an atom can be in two places at once, but that is only the start; the full story is far weirder than that. If an atom can be here or there, we must speak of states in which
it is, somehow, simultaneously both here and there. This is called a superposition.
If you are new to the quantum world, you are undoubtedly wondering what it means for an atom to be somehow both here and there. Don’t be discouraged if you find this confusing. You are absolutely right to wonder what it means. This is one of the central mysteries of quantum mechanics. It is enough, for now, if you just accept this as a mystery, to which we attach the term “superposition.” Later we will be able to demystify it.
Here is a first step. When we say that a quantum particle is in a “superposition of being here and there,” this is related to the wavelike nature of matter, for a wave is a disturbance that is spread out, and so it can be both here and there.
We speak of elementary particles, but everything quantum, including atoms and molecules, is both a particle and a wave. Here is a taste of what that means. If we do an experiment that asks where an atom is, the result will be that it is somewhere definite. But between measurements, when we are not looking for it, it turns out to be impossible to project where it might be. It is as if the likelihood or propensity of finding the particle spreads as a wave when we are not looking. But as soon as we look again, it is always somewhere.
Imagine playing a game of hide-and-seek with an atom. We open our eyes, or turn on a detector, and we see it somewhere. But when we close our eyes it dissolves into a wave of potentiality. Open our eyes again and it is always somewhere.
Another feature unique to the quantum world is called entanglement. If two particles interact, and then move apart, they remain intertwined in the sense that they seem to share properties which cannot be broken down to properties each enjoys individually.
We can stretch our imagination to apply these new concepts to atoms and molecules which are too small to see directly. We must study them indirectly, and to do that we employ large and complex measurement devices.