Einstein's Unfinished Revolution

by Lee Smolin

  Those measurement devices are part of the everyday, familiar world of large objects. One thing we can be sure of is that big everyday things display none of the bizarre behavior quantum mechanics describes. A chair is here or it is there, never in a combination of such states. When we wake up in the middle of the night in a strange hotel room, we may be unsure where the chair is, but we can be sure it is somewhere. And after we collide with it in the dark, our future does not become entangled with its future.

  In the world as we experience it, cats are either alive or dead, even if they are locked in a box. When we open the box, the cat does not suddenly resolve from a combination of dead and alive to dead. If we find it dead it will likely have been so for some time, as we will instantly smell.

  Ordinary objects appear to share none of the quantum weirdnesses of the atoms of which they are made. This seems obvious, but it raises a mystery. Quantum mechanics is the core theory of nature. As such it must be universal. If it applies to an atom it must apply to two atoms, or ten or ninety. And we have excellent experimental evidence that it does. Delicate experiments, in which large molecules are put in quantum superpositions, show us that they are just as quantum weird as electrons. For one thing, they diffract and interfere as waves.

  But then quantum mechanics must apply to the vast collections of atoms that make up you or me or our cat or the chair on which she is perched. But it doesn’t seem to. Nor does quantum mechanics appear to apply to any of the instruments and machines we employ to image the atoms and reveal their quantum weirdnesses.

  How can this be?

  In particular, when we measure a property of an atom, we employ big devices. The atoms may be in superpositions of states and so be several places at once, but the measuring instrument always indicates just one out of the possible answers to the questions we pose. Why is that? Why does quantum mechanics not apply to the very devices we use to measure quantum systems?

  This is called the measurement problem. It has been controversial and unresolved since the 1920s. The fact that, after all this time, we have found no agreement among experts means there is something basic about nature we have yet to understand.

  So there is somewhere a transition between the quantum world, in which an atom can be several places at once, and the ordinary world, in which everything is always somewhere. If a molecule made from ten or ninety atoms can be described by quantum mechanics, but a cat cannot, then somewhere between the two there is a line delineating where the quantum world stops. An answer to the measurement problem would tell us where that line is and explain the transition.

  There are people who are sure they know the answer to the measurement problem. We will meet some of them and their ideas later on. We will want to look out for what price we have to pay to expunge this quantum insanity from our understanding of the world.

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  BROADLY SPEAKING, the people who aim to address the mysteries of quantum mechanics fall into two classes.

  The first group assumes that the theory as it was formulated in the 1920s is essentially correct. They believe the problem is not with quantum theory; it is instead with how we understand it or speak about it. This strategy to mitigate the strangeness of quantum mechanics goes back to some of the founders, beginning with Niels Bohr.

  Niels Bohr was a Danish physicist who, while still in his twenties, was the first to apply quantum theory to atoms. As he grew older he became the de facto leader of the quantum revolution, partly due to the attractiveness of his ideas and partly because he educated and mentored many of the young quantum revolutionaries.

  The second group has concluded that the theory is incomplete. It can’t be made sense of because it is not the whole story. They seek a completion of the theory that will tell us the rest of the story and, by doing so, resolve the mysteries of quantum mechanics. This strategy goes back to Albert Einstein.

  More than anyone else, Einstein was responsible for initiating the quantum revolution. He was the first to articulate the dual nature of light as a particle and a wave. He is by now better known for his theory of relativity, but his Nobel Prize was for his work on quantum theory, and he himself admitted that he spent much more time on quantum theory than on relativity. Yet, even if he initiated the quantum revolution, Einstein did not become one of its leaders, because his realism required that he reject the theory as it was developed in the late 1920s.

  In the language introduced in the preface, those in the first group are mostly anti-realists or magical realists. Realists find themselves in the second group.

  Those who argue for the incompleteness of quantum mechanics point to the fact that in most cases it can only make statistical predictions for the results of experiments. Rather than saying what will happen, it gives probabilities for what might happen. In a letter to his friend Max Born in 1926, Einstein wrote:

  Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the ‘old one’. I, at any rate, am convinced that He is not playing at dice.1

  Einstein was also friends with Niels Bohr, and their divergent responses to quantum mechanics fueled a passionate debate between them that lasted more than forty years, till Einstein’s death. It continues between their intellectual descendants to this day. Einstein was the first person to clearly articulate the need for a revolutionary new theory of atoms and radiation, but he was unable to accept that quantum mechanics was that theory. His first response to quantum mechanics was to argue that it was inconsistent. When that failed, he argued instead that quantum mechanics gives an incomplete description of nature, which leaves out an essential part of the picture.

  I believe that Einstein was unable to accept quantum mechanics as a definitive theory because he had exceedingly high aspirations for science. He was driven by the hope of transcending subjective opinion and discovering a true mirror of nature that exhibits the essence of reality in a few timeless mathematical laws. For him, science aimed to capture the true essence of the world, and that essence is independent of us and can have nothing to do with what we believe or know about it.

  Einstein, of all people, must have felt he had the right to demand this because he had achieved it in his discoveries of special and general relativity. Having laid the groundwork for quantum physics, he sought to capture the essence of the atomic world in a complete description of atoms, electrons, and light.

  Bohr replied that atomic physics required a revolutionary revision in how we understand what science is, as well as in our conception of the relationship between reality and our knowledge of it. This stemmed from the fact that we are a part of the world, so we must interact with the atoms we seek to describe.

  Bohr asserted that once we absorbed this revolutionary change in our thinking, the completeness of quantum mechanics would be unavoidable, because it was built into our being participants in the world we seek to describe. From Bohr’s perspective, quantum theory is complete in the sense that there is no more-complete description of the world to be had.

  If we refuse these philosophical revolutions and insist on maintaining an old-fashioned, commonsense view of reality and its relation to our observations and knowledge, we have to pay a different kind of price. We have to contemplate that we are wrong about some aspect of nature. We have to find out which common assumption is wrong and replace it with a radically new physical hypothesis that opens the way to a new theory that will complete quantum mechanics.

  Thanks to a combination of theory and experiment, starting with a paper by Einstein and two collaborators in 1935, we know one aspect of this completion. The new theory must violate the commonplace assumption that things interact only with other things that are near them in space.

  This assumption is called locality. A big part of the story I will be telling in later chapters is how this commonsense i
dea must be transcended in the theory which will replace quantum mechanics.

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  THIS BOOK HAS THREE PURPOSES. First, I want to explain to laypeople just what the puzzles at the heart of quantum mechanics are. After more than a century of studying quantum physics, it is remarkable that there continues to this day to be no agreement on the solution of these puzzles.

  But having explained the reasons for the debate in a way that is fair to both sides, I will not stay impartial. In the great debate about whether quantum mechanics is the last word or not, I side with Einstein. I believe that there is a layer of reality deeper than that described by Bohr, which can be understood without compromising old-fashioned notions of reality and our ability to comprehend and describe it.

  Thus, my second purpose is to advocate a point of view about the puzzles of quantum mechanics. This is that the problems can be resolved only by progress in science which will uncover a world beyond quantum mechanics. Where quantum mechanics is mysterious and confusing, this deeper theory will be entirely comprehensible.

  I can make this claim because we have known since the invention of quantum mechanics how to present the theory in a way that dissolves the mysteries and resolves the puzzles. In this approach, there is no challenge to our usual beliefs in an objective reality, a reality unaffected by what we know or do about it, and about which it is possible to have complete knowledge. In this reality, there is just one universe, and when we observe something about it, it is because it is true. This can justly be called a realist approach to the quantum world.

  An anti-realist approach ascribes the mysteries of quantum mechanics to subtleties having to do with how we gain knowledge about nature. Such approaches have radical proposals to make about epistemology, which is the branch of philosophy concerned with how we know things. Realist approaches assume we are able to arrive sooner or later at a true representation of the world and so are deliberately naive about epistemology. Instead, realists are interested in ontology, which is the study of what exists. By contrast, anti-realists believe we cannot know what really exists, apart from our representation of the knowledge we have of the world, which is gained only through interacting with it.

  So I will endeavor to reassure readers that quantum mechanics can be understood completely within a realist perspective in which the external world can be completely comprehended as independent from us. There is no mysterious effect of the observer on the observed. Reality is out there, recalcitrant to our will and the choices we make. That reality is fully comprehensible. And that reality consists of a single world.

  The existence of these realist approaches to quantum mechanics does not by itself mean that the philosophically more extravagant proposals are wrong. But it does mean that there is no strong scientific reason to believe in them, because realism is always to be preferred in science, when it can be achieved.

  Why, then, is so much of the talk about quantum theory inspired by the weirder ideas in which reality depends on our knowledge of it or there are multiple realities? This is a problem for historians of ideas. One such historian, Paul Forman, has tied the dominance of Bohr and Heisenberg’s anti-realist philosophy within the scientific community in the 1920s and 1930s to the embrace of chaos and irrationality advocated by Spengler and others in the wake of the First World War.

  That history is fascinating, but it is for scholars to do justice to it. I am not a scholar, I am a scientist, and this brings me to my third purpose in writing this book.

  I have been on Einstein’s side in the search for a deeper but simpler reality behind quantum mechanics since first reading him on the subject as a high school dropout. My journey in physics began with reading Einstein’s autobiographical notes, where, in the last few years of his life, in the 1950s, he reflected on the two main tasks he felt were left incomplete in physics. These were to make sense of quantum physics and, after that, to unify the new understanding of the quantum with gravity, by which he meant his general theory of relativity. I recall thinking that maybe I could try to help. I was unlikely to succeed, but perhaps here was something worth striving for.

  After, as it were, getting my mission from reading Einstein’s autobiographical notes, I found that book by de Broglie, talked my way into a good college, found great teachers, and got lucky several times in my applications for graduate school and beyond. I’m having a wonderful life, and as a scientist on the frontier, I’ve had many chances to take a shot on goal, aimed at solving Einstein’s two big questions.

  I haven’t succeeded, at least so far. Very unfortunately, neither has anyone else. At the same time, over the past several decades there has at least been progress toward understanding the problem. That is not nearly as good as it would be to solve the problem, but neither is it nothing. We know much better than Einstein did the obstacles that a theory that transcends the limits of quantum mechanics must overcome. And because of that, some very interesting proposals and hypotheses have been put forward, which may frame the deeper theory for which we search.*

  I have been thinking about the question of how to go beyond quantum mechanics since the mid-1970s, and I’ve never been more excited and optimistic about the prospects for success. So this is my third reason for writing this book, which is to bring to a wider audience a report from the front in our search for the world beyond the quantum.

  TWO

  Quanta

  If we break quantum mechanics down to its most essential principle, it is this:

  We can only know half of what we would need to know if we wanted to completely control, or precisely predict, the future.

  This disrupts the basic ambition of physics, which is to be able to predict the future. It was hoped that this power would follow if only we could give the physical world a complete description. By describing fully the motion of every particle and the action of every force, we would be able to work out exactly what would happen in the future. Before quantum mechanics was formulated in the 1920s, we physicists were confident that if we could learn the laws that govern the fundamental particles, we would be able to predict and explain everything that happened in the world.

  The hypothesis that the future is completely determined by the laws of physics acting on the present configuration of the world is called determinism. This is an extraordinarily powerful idea, whose influence can be seen in diverse fields. If you appreciate the extent to which determinism dominated thought in the nineteenth century, you can begin to understand the revolutionary impact of quantum mechanics across all fields, because quantum mechanics precludes determinism.

  To emphasize this point, I like to quote from Tom Stoppard’s play Arcadia, in which his precocious heroine, Thomasina, explains to her tutor:

  If you could stop every atom in its position and direction, and if your mind could comprehend all the actions thus suspended, then if you were really, really good at algebra you could write the formula for all the future; and although nobody can be so clever as to do it, the formula must exist just as if one could.1

  A complete description of nature, at a given time, is called a state. For example, if we think of the world as composed of particles whizzing around, the state tells us where each of them is, and how fast and in what direction each is moving, at that moment.

  The power of physics comes from its laws, which dictate how nature changes in time. They do this by transforming the state of the world as it is now to the state at any future time. A law of physics functions in some ways like a computer program: it reads in input and puts out output. The input is the state at a given time; the output is the state at some future time.*

  Along with the computation comes an explanation of how the world changes in time. The law acting on the present state causes the future states. A successful prediction of the future state is taken as a validation of that explanation. The prediction is deterministic, in that a precise input leads to a precise output. This confirms
a belief that the information that went into describing the state is in fact a complete description of the world at one moment of time.

  This concept of a law is basic to a realist conception of nature and, as such, transcends any one theory. Newtonian mechanics and Einstein’s two theories of relativity all work the same way. One applies the law to the state at an initial time, and it transforms that state to the state at some future time. This schema for explaining nature was invented by Newton, so we call it the Newtonian paradigm.

  It is also worth mentioning that in almost all cases so far known, the laws are reversible. One can input the state at some future time and run the law backward to output the state at an earlier time. (The issue of the reversibility of time and of the fundamental laws is a central concern of chapters 14 and 15.)

  It is often the case that the information needed to completely describe the state of a physical system comes in pairs. Position and momentum.* Volume and pressure. Electric field and magnetic field. We need both to predict the future. Quantum mechanics says we can know only one.

  This means we can’t precisely predict the future. That is just the first of the blows to our comfortable intuitions that we will have to absorb from quantum theory.

  Which member of each pair is the one that can be known? Quantum mechanics says you choose! This is the basis of its challenge to realism.

  There is more to say about the impossibility of predicting the future. To get there, let’s take advantage of the great generality quantum mechanics claims, and speak a bit abstractly. We want to describe some physical system in terms of a pair of variables—we will call them A and B. Quantum physics asserts a two-part principle.