Einstein's Unfinished Revolution

by Lee Smolin

  The next step is to marry these two models, giving us a relational hidden variable theory that is also background independent, and which realizes the hypothesis that space and locality are emergent.

  These two models started as separate research programs, but they shared a motif, which is the central role played by the similarities and differences among events. Both models take these as fundamental, while locality is demoted to an accidental and emergent aspect of nature. It slowly dawned on me that these were different perspectives on a single picture, and so one summer day I sat down and opened a fresh notebook to see if I could tell the full story.

  It was immediately clear that the protagonist of this new story is the view. That is, the basic variables are nothing but the views of the universe seen from each event. So I began by fashioning an approach to physics in which these views are fundamental, rather than derived from a more basic structure. In this new perspective, the fundamental laws involve directly only the views and their differences. I call this theory the causal theory of views.6

  The view of an event is nothing but the information available to it from its causal past. The view into the past of an event is like the sky; it is what you see when you look around. Because the speed of light is finite, looking around means looking back, into your past.

  The view of an event, as I use the word here, is entirely real and has nothing to do with opinion.* In the theory I am describing, what is real and objective in the world is the information available at each event making up the history of the world, coming to that event from its causal past.

  Look up! Your view of the world is like a film projected on a two-dimensional sphere, which we call the sky. The view of an event in a model with three (emergent) dimensions of space will then be represented as a two-dimensional sphere that we call the event’s sky. What an event sees on its sky is the events directly in its causal past. More precisely, it sees the energy and momentum coming from each of its parent events. Each parent shows up as a colored point on that event’s sky. Each of these points represents a quantum of energy and momentum that has arrived from a past event. The location of each point on the sky records the direction of the momentum, while the color indicates the magnitude of the energy received.

  The next step is simple: hypothesize that all that the universe consists of is these skies—each one the view of some event. Rather than construct the views from the causal relations, reverse things and derive the causal relations and everything else from the views. This can work because information contained in the totality of views is enough to reconstruct the causal relations and hence the full history.

  As in the real ensemble theory, the laws include the requirement that the variety of all these views is maximized. This has a similar effect of leading to the quantum force. Using this, one can derive quantum mechanics as an approximation to the theory.

  Here is a one-sentence summary of this theory: the universe consists of nothing but views of itself, each from an event in its history, and the laws act to make these views as diverse as possible.

  From here the story unfolds very much like that of the real ensemble theory. Similar views interact with each other, as a result of the mandate to evolve in the direction of ever more diversity. This leads to the emergence of space and of locality in that space. Nonlocality also emerges as interactions which are distant in the emergent space but nearby in terms of similarity of views. Finally, as in the real ensemble formulation, quantum mechanics arises from these nonlocal interactions as an approximate description of the dynamics of views.

  The causal theory of views is then a route to a completion of quantum mechanics. It is a realist completion, because it is a theory of beables, which are the views themselves. Most important, it demonstrates that a single fundamental theory can be at the same time a completion of quantum mechanics and an atomic model of spacetime. It can explain the emergence of both locality and nonlocality, of both spacetime and quantum mechanics.

  This theory is still only part of the story, and there is still much to learn about it, but it is a way the world might be.

  * * *

  —

  FOR US REALISTS, quantum mechanics cannot be the final story. There is still much to discover. Nonetheless, I remain confident that nature is comprehensible. I am optimistic that the universal power of reasoning that each of us has, together with our vast powers of imagination and our ability to invent novel ideas, will suffice to comprehend the universe. I am especially hopeful about a future in which our individual powers are combined and disciplined by our participation in the scientific community. While I find myself at times deeply frustrated by our lack of definite progress on fundamental physics during this last half century, I am optimistic about the long run. I am confident that in the future our descendants will know vastly more about nature than we do.

  I am also sure that the answer to the questions that have bedeviled us for nearly a century will be simple, and expressed in terms of elegant hypotheses and principles of the kind I have put forward here. It would be fortunate indeed if we already have among our library of ideas the answer to how to complete Einstein’s twin revolutions. But if we don’t, I have no doubt our descendants will, so long as we keep the great adventure of science alive.

  EPILOGUE/REVOLUTIONS

  Note to Self

  The truth is out there.

  —THE X-FILES

  Never, never, never, never, never give up.

  —DAVID GROSS

  Einstein told us that we scientists are opportunists who are willing to break the rules and bend the scientific method to our purpose of discovering how nature works. Each scientist is like an entrepreneur, who has a certain amount of capital to invest; for a theoretical physicist that capital consists mainly of time and attention. The most important decisions we make are what problems we work on and which approaches we choose. Which new paper do we study, to which conferences do we travel, and, once there, to which talks do we listen? The rewards come in different forms: the thrill of discovery, the admiration of one’s peers and students, and also one’s career, job opportunities, and salary.

  If you are interested only in applying the known laws of physics to broaden our appreciation of how nature works, this is a great period to be a physicist. Beautiful discoveries light the way in condensed matter theory, and we are doing real astronomy using gravitational waves to see the universe. These paradigms are working. Steady progress in mathematics drives advances in mathematical physics, with truly brilliant people leading the way to a better understanding of the mathematical structures of our established and nascent theories. Advances in experimental technique are equally impressive, with Moore’s law paying off in exponentially increasing range and accuracy of astronomical observations. There is nothing wrong with any of this except that little of it addresses the big foundational puzzles. It is only when we try to advance the project of discovering the fundamental laws and principles that we seem to be spinning our wheels.

  At the present moment in fundamental physics and cosmology, there are basically only two ways to bet. We either bet that we know all the fundamental principles, or we bet there are basic ideas and principles missing. The major research programs, such as inflation, string theory, and loop quantum gravity, are all ways of betting that we know the basic principles of fundamental physics. With notable exceptions, workers in these fields take for granted that the basic principles of quantum theory and relativity are sound and apply to the new theory. Many of the people who work outside these programs are doing so because they bet there is much more to be discovered. People like me, who do some of both, are hedging our bets.

  When it comes to quantum mechanics we face the same choice. Either we bet that we have the complete theory in our hands and just need to understand it better, or we bet the theory is incomplete in important ways. The Copenhagen interpretation, the operational interpretations, Everett quantum mechan
ics, and so on are all ways of betting we know everything important about quantum phenomena. Anyone who focuses exclusively on one of the realist proposals such as pilot wave theory or spontaneous collapse is betting their favorite theory will turn out to be the correct completion of quantum mechanics. In either case, the bet assumes that we know all the principles needed to understand nature.

  What about those of us who are convinced that a completion is needed, but are not convinced any of the well-studied ones have the ring of truth? How are we to bet?

  Up to now, my own bets have fallen on both sides of these divides. My most successful bets employed ideas and technical tools from particle physics to solve problems in quantum gravity. This was one of the routes that led to loop quantum gravity. But from time to time I wrote papers reporting my efforts to invent relational hidden variable theories. And the very best of my early papers was an attempt to connect the principle of inertia to quantum foundations. As the years went on I extended my foundational efforts to the landscape issue, which led to my work on the nature of time. But my bread-and-butter work remains quantum gravity, both the phenomenology of the theory and loop quantum gravity.

  A book project is a kind of mental therapy, which forces you to examine your confused thoughts and intuitions and develop them to their logical conclusions. So now that I have written a book which argues that a radically new theory is needed to solve the foundational issues in physics and cosmology, what am I going to do about it? Do I keep to the same safe, hedged program, or go all out on an attempt to solve the real problems?

  To bet that the truth requires something as yet undiscovered, we must spend our time searching for that unknown completion. We can’t just sail down one shoreline and up another. We head west: out of sight of land, following our own compass, or the best facsimile thereof that we can cobble together from the clues we take seriously.

  There is no more reasonable bet than that our current knowledge is incomplete. In every era of the past our knowledge was incomplete; why should our period be any different? Certainly the puzzles we face are at least as formidable as any in the past. But almost no one bets this way. This puzzles me.

  I suspect it’s hard for many physicists to imagine that we are not near the end of our search for the ultimate laws of nature. We have been raised in a culture in which it’s all about having the right answer, and we owe our careers to having been the scientists who had them. But I’ve always had in my head an image of how much more people in the future will know, and how silly our claims to knowledge will look to them. This has probably made me a less effective advocate of my own ideas.

  So what do we do with the partly successful inventions, such as loop quantum gravity? At first, the discovery of a new possible direction, incomplete and without experimental confirmation (in other words, highly vulnerable to criticism, as most new theories are at birth), is very worth our time and focus. That X, however incompletely formulated, is something that just might be true, or be part of the truth, even without positive evidence, is certainly good for a decade of examination. But after a third or more of a century, during which many career-long efforts have failed to budge might be true any closer to must be true, isn’t it time to move on? You might think I’m repeating polemics from the string wars, but I’m thinking, with a great deal of affection, of all of us whose years of hard work have failed to yield the breakthroughs we fantasized about. Including myself; especially myself.

  Why do we write more and more papers on approaches whose deficiencies have been obvious for decades, and almost no papers proposing new completions of quantum mechanics? It is not for lack of caring, for everyone I know who works on quantum foundations has chosen that risky path because they care passionately about how nature resolves the measurement problem and the other puzzles.

  I, for one, am tired of arguing over the ins and outs and relative merits of the existing approaches, and the clever fixes invented to save an idea that is pretty obviously collapsing from insufficiency. So I have a decision to make: I either keep on the present path, which will end up on the top of that low hill just past the next village, or head down into the swamps to stumble along unknown paths in search of undiscovered mountains. If I take the swamp trail, I will almost certainly fail, but I hope to send back reports to interest and inspire those few others who feel in their bones the cost of our ignorance, of giving up the search too soon.

  Even if I’m convinced that something very new is needed, I have little idea how to search for scientific truth except by building on an existing research program, using a well-honed tool kit and methodology. This is research as it is taught, recognized, funded, and rewarded by the academic community. A community, I should mention, that it is necessary to be an active part of to get your work taken seriously by people who know enough to evaluate it. What would I put in my research proposals, if my ideas are not expressible in the language of an already existing and widely followed research program? What problems do I set for my PhD students, if they are not to calculate something using tools developed within a given framework? Do I tell my students to wake up in the morning, make coffee, open a blank notebook, and stare at it until a disheveled angel arrives with a revelation? Is that what I should do myself? How many days, weeks, months, years, how many incoherent scribbled pages, do I tolerate before giving up?

  It is not just that to try to invent a whole new physics is risky for my career and damaging to my emotional stability. I don’t even know how to begin. Almost no one alive has done that, in the way that the revolutionaries of a century ago did. In my experience there is little as terrifying as putting aside the basic principles that form the foundation of our understanding of how we fit into nature—isn’t that why it feels comforting to know them?

  It certainly is easier to work within an existing framework, to test the limits of what we know from the inside, so to speak. We can do this while keeping an open mind about the basic principles and looking out for opportunities to modify those principles or even introduce new ones. Even more important is to keep on the lookout for new opportunities to test theories against experiments and observations. This is what I have done for most of my career, and I venture to say this is true as well of many who work on the main approaches, such as string theory and loop quantum gravity. What we have to show for this is a collection of beautiful results, which may or may not lead to the true story, and, especially precious, a few proposals for new principles, including the holographic principle and the principle of relative locality. But, with all due respect to those of us who invested most of our time in reasonable approaches to the development of reasonable theories, it does not seem to have been enough, this time.

  I say to myself, I’ll take such risks after I get my PhD, after I get my postdoc, after my faculty position, after tenure. But even tenured, senior, famous professors must apply for research grants, and there is always that fancy career-culminating prize, or that comfortable and prestigious chair. So we’ll just wait till retirement. Then we’ll be free to take the big risks. Well, as someone closing in on that, I can report that the only thing you learn is certain, as your fifties and sixties rush by, every day busy with a schedule full of seminars, faculty meetings, working with students, classes, review panels, airplanes, hotels, and conference talks, is that you are not immortal.

  So maybe it’s all up to a brilliant student somewhere, impossibly arrogant, as the young Einstein was, but blindingly talented enough to absorb the essentials of all we have done, before putting them to one side and confidently starting over.

  A friend once told me that the academic world was modeled on monasteries, which were designed to perpetuate old knowledge while resisting the new. Even after decades in the system I am amazed at how the fine mechanics of this work. There is no arguing with the logic of academic fame, which rewards every scientific success with distractions that make it harder to do more science, while imposing enormous disincentives to putting aside
polishing your legacy to take on new challenges.

  The academic world is very well suited to support what Thomas Kuhn called normal science. That is great until it becomes long past due to complete a revolution.

  To my knowledge, few have stumbled on a major discovery by accident; most true breakthroughs were found after years and years of hard, unrewarding work. Feynman said to discover something new you have to take the time to make every mistake possible along the way. And he surely knew.

  So I have no better answer than to face the blank notebook. We do have role models. Einstein did it. Bohr did it. De Broglie, Schrödinger, and Heisenberg did it, as did Bohm and Bell. They each found a path from that blank page to a foundational discovery that enlarged our understanding of how nature works. Start by writing down what you are confident we know for sure. Ask yourself which of the fundamental principles of the present canon must survive the coming revolution. That’s the first page. Then turn again to a blank page and start thinking.

  ACKNOWLEDGMENTS

  This book represents a lifetime of wrestling with the puzzles of quantum foundations, and I have to thank, first of all, Herbert Bernstein, for his revolutionary freshman quantum mechanics course, for making me the grader in the course to make sure I learned how to solve the problems, and for many years of friendship since then. In graduate school I was fortunate to be able to study with Abner Shimony, who has been a role model for all of us wishing to bring the rigor and depth of philosophy to the examination of foundational problems in physics. For that I have to thank Hilary Putnam, who told me that Abner would be able to answer my questions about quantum theory that he wasn’t able to.

  In graduate school and in the years since, I have been fortunate to meet and converse with some of the truly deep thinkers who inspire us still: Steve Adler, Yakir Aharonov, Bryce DeWitt, Cécile DeWitt-Morette, Freeman Dyson, Paul Feyerabend, Richard Feynman, Jim Hartle, Gerard ’t Hooft, Chris Isham, Edward Nelson, Roger Penrose, Leonard Susskind, John Archibald Wheeler, and Eugene Wigner.