What is time?
This deceptively simple question is the single most important problem facing science as we probe deeper into the fundamentals of the universe. All of the mysteries physicists and cosmologists face—from the Big Bang to the future of the universe, from the puzzles of quantum physics to the unification of forces and particles—come down to the nature of time.
The fact that time is real may seem obvious. You experience it passing every day when you watch clocks tick, bread toast, and children grow. But most physicists, from Newton to Einstein to today’s quantum theorists, have seen things differently. The scientific case for time being an illusion is formidable. That is why the consequences of adopting the view that time is real are revolutionary.
Here, the author of The Trouble with Physics argues that a limited notion of time is holding physics back—and what we need now is a major shift in scientific thought. The true reality of this manmade construct could be the key to the next big breakthrough in theoretical physics—and could hold implications relevant to issues from climate change to the economy.
What if the laws of physics themselves were not ageless? What if they could evolve? Time Reborn offers a radical approach to cosmology that embraces the concept of time and opens up a whole new universe of possibilities.
“With rare conceptual daring, Smolin beckons toward a new perspective for doing cosmological theory . . . A thrilling intellectual ride.” —Booklist, starred review
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INTRODUCTION The scientific case for time being an illusion is formidable. That is why the consequences of adopting the view that time is real are revolutionary.
The core of the physicists’ case against time relies on the way we understand what a law of physics is. According to this dominant view, everything that happens in the universe is determined by a law, which dictates precisely how the future evolves out of the present. The law is absolute and, once present conditions are specified, there is no freedom or uncertainty in how the future will evolve.
As Thomasina, the precocious heroine of Tom Stoppard’s play Arcadia, 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.”
I used to believe that my job as a theoretical physicist was to find that formula; I now see my faith in its existence as more mysticism than science.
Were he writing lines for a modern character, Stoppard would have had Thomasina say that the universe is like a computer. The laws of physics are the program. When you give it an input — the present positions of all the elementary particles in the universe — the computer runs for an appropriate amount of time and gives you the output, which is all the positions of the elementary particles at some future time. Within this view of nature, nothing happens except the rearrangement of particles according to timeless laws, so according to these laws the future is already completely determined by the present, as the present was by the past.
This view diminishes time in several ways.1 There can be no surprises, no truly novel phenomena, because all that happens is rearrangement of the atoms. The properties of the atoms themselves are timeless, as are the laws controlling them; neither ever changes. Any feature of the world at a future time can be computed from the configuration of the present. That is, the passage of time can be replaced by a computation, which means that the future is logically a consequence of the present.
Einstein’s theories of relativity make even stronger arguments that time is inessential to a fundamental description of the world, as I’ll discuss in chapter 6. Relativity strongly suggests that the whole history of the world is a timeless unity; present, past, and future have no meaning apart from human subjectivity. Time is just another dimension of space, and the sense we have of experiencing moments passing is an illusion behind which is a timeless reality.
These assertions may seem horrifying to anyone whose worldview includes a place for free will or human agency. This is not an argument I will engage in here; my case for the reality of time rests purely on science. My job will be to explain why the usual arguments for a predetermined future are wrong scientifically.
In Part I, I will present the case from science for believing that time is an illusion. In Part II, I will demolish those arguments and show why time must be taken to be real if fundamental physics and cosmology are to overcome the crises they currently face.
To frame the argument of Part I, I trace the development of the concepts of time used in physics, from Aristotle and Ptolemy through Galileo, Newton, Einstein, and on to our contemporary quantum cosmologists, and show how our concept of time was diminished, step by step, as physics progressed. Telling the story this way also allows me to gently introduce the material the lay reader needs for an understanding of the argument. Indeed, key points can be introduced by ordinary examples of balls falling and planets orbiting. Part II tells a more contemporary story, since the argument that time must be reinserted into the core of science arose as a result of recent developments.
My argument starts with a simple observation: The success of scientific theories from Newton through the present day is based on their use of a particular framework of explanation invented by Newton. This framework views nature as consisting of nothing but particles with timeless properties, whose motions and interactions are determined by timeless laws. The properties of the particles, such as their masses and electric charges, never change, and neither do the laws that act on them. This framework is ideally suited to describe small parts of the universe, but it falls apart when we attempt to apply it to the universe as a whole.
All the major theories of physics are about parts of the universe — a radio, a ball in flight, a biological cell, the Earth, a galaxy. When we describe a part of the universe, we leave ourselves and our measuring tools outside the system. We leave out our role in selecting or preparing the system we study. We leave out the references that serve to establish where the system is. Most crucially for our concern with the nature of time, we leave out the clocks by which we measure change in the system.
The attempt to extend physics to cosmology brings new challenges that require fresh thinking. A cosmological theory cannot leave anything out. To be complete, it must take into account everything in the universe, including ourselves as observers. It must account for our measuring instruments and clocks. When we do cosmology, we confront a novel circumstance: It is impossible to get outside the system we’re studying when that system is the entire universe.
Moreover, a cosmological theory must do without two important aspects of the methodology of science. A basic rule of science is that an experiment must be done many times to be sure of the result. But we cannot do this with the universe as a whole — the universe only happens once. Nor can we prepare the system in different ways and study the consequences. These are very real handicaps, which make it much harder to do science at the level of the universe as a whole.
Nonetheless, we want to extend physics to a science of cosmology. Our first instinct is to take the theories that worked so well when applied to small parts of the universe and scale them up describe the universe as a whole. As I’ll show in chapters 8 and 9, this cannot work. The Newtonian framework of timeless laws acting on particles with timeless properties is unsuited to the task of describing the entire universe.
Indeed, as I will show in detail, the very features that make these kinds of theories so successful when applied to small parts of the universe cause them to fail when we attempt to apply them to the universe as a whole.
I realize that this assertion goes counter to the practice and hopes of many colleagues, but I ask only that the reader pay close attention to the case I make for it in Part II. There I will show in general, and illustrate by specific example, that when we attempt to scale up our standard theories to a cosmological theory, we are rewarded with dilemmas, paradoxes, and unanswerable questions. Among these are the failure of any standard theory to account for the choices made in the early universe — choices of initial conditions and choices of the laws of nature themselves.
Some of the literature of contemporary cosmology consists of the efforts of very smart people to wrestle with these dilemmas, paradoxes, and unanswerable questions. The notion that our universe is part of a vast or infinite multiverse is popular — and understandably so, because it is based on a methodological error that is easy to fall into. Our current theories can work at the level of the universe only if our universe is a subsystem of a larger system. So we invent a fictional environment and fill it with other universes. This cannot lead to any real scientific progress, because we cannot confirm or falsify any hypothesis about universes causally disconnected from our own.
The purpose of this book is to suggest that there is another way. We need to make a clean break and embark on a search for a new kind of theory that can be applied to the whole universe — a theory that avoids the confusions and paradoxes, answers the unanswerable questions, and generates genuine physical predictions for cosmological observations.
I do not have such a theory, but what I can offer is a set of principles to guide the search for it. These are presented in chapter 10. In the chapters that follow it, I will illustrate how the principles can inspire new hypotheses and models of the universe that point the way to a true cosmological theory. The central principle is that time must be real and physical laws must evolve in that real time.
The idea of evolving laws is not new, nor is the idea that a cosmological science will require them.2 The American philosopher Charles Sanders Peirce wrote in 1891:
To suppose universal laws of nature capable of being apprehended by the mind and yet having no reason for their special forms, but standing inexplicable and irrational, is hardly a justifiable position. Uniformities are precisely the sort of facts that need to be accounted for. . . . Law is par excellence the thing that wants a reason.
Now the only possible way of accounting for the laws of nature and for uniformity in general is to suppose them results of evolution.”3
The contemporary philosopher Roberto Mangabeira Unger has more recently proclaimed:
You can trace the properties of the present universe back to properties it must have had at its beginning. But you cannot show that these are the only properties that any universe might have had. . . . Earlier or later universes might have had entirely different laws. . . . To state the laws of nature is not to describe or to explain all possible histories of all possible universes. Only a relative distinction exists between lawlike explanation and the narration of a one-time historical sequence.”4
Paul Dirac, who ranks with Einstein and Niels Bohr as one of the most consequential physicists of the 20th century, speculated: “At the beginning of time the laws of Nature were probably very different from what they are now. Thus, we should consider the laws of Nature as continually changing with the epoch, instead of as holding uniformly throughout space-time.”5 John Archibald Wheeler, one of the great American physicists, also imagined that laws evolved. He proposed that the Big Bang was one of a series of events within which the laws of physics were reprocessed. He also wrote, “There is no law except the law that there is no law.”6 Even Richard Feynman, another of the great American physicists and Wheeler’s student, once mused in an interview: “The only field which has not admitted any evolutionary question is physics. Here are the laws, we say, . . . but how did they get that way, in time? . . . So, it might turn out that they are not the same [laws] all the time and that there is a historical, evolutionary, question.”
In my 1997 book, The Life of the Cosmos, I proposed a mechanism for laws to evolve, which I modeled on biological evolution.8 I imagined that universes could reproduce by forming baby universes inside black holes, and I posited that whenever this happens, the laws of physics change slightly. In this theory, the laws played the role of genes in biology; a universe was seen as an expression of a choice of laws made at its formation, just as an organism is an expression of its genes. Like the genes, the laws could mutate randomly from generation to generation. Inspired by then-recent results of string theory, I imagined that the search for a fundamental unified theory would lead not to a single Theory of Everything but to a vast space of possible laws. I called this the landscape of theories, taking the language from population genetics, whose practitioners work with fitness landscapes. I will not say more about this here, as it is the subject of chapter 11, except to say that this theory, cosmological natural selection, makes several predictions that, remarkably, have held up despite several opportunities to falsify them in the years since.
Over the last decade, many string theorists have embraced the concept of a landscape of theories. As a result, the question of how the universe chooses which laws to follow has become especially urgent. This, I will argue, is one of the questions that can be answered only within a new framework for cosmology in which time is real and laws evolve.
Laws, then, are not imposed on the universe from outside it. No external entity, whether divine or mathematical, specifies in advance what the laws of nature are to be. Nor do the laws of nature wait, mute, outside of time for the universe to begin. Rather the laws of nature emerge from inside the universe and evolve in time with the universe they describe. It is even possible that, just as in biology, novel laws of physics may arise as regularities of new phenomena that emerge during the universe’s history.
Some might see the disavowal of eternal laws as a retreat from the goals of science. But I see it as the jettisoning of excess metaphysical baggage that weighs down our search for truth. In the coming chapters, I will provide examples illustrating how the idea of laws evolving in time leads to a more scientific cosmology — by which I mean one more generative of predictions subject to experimental test. To my knowledge, the first scientist since the dawn of the Scientific Revolution to think really hard about how to make a theory of a whole universe was Gottfried Wilhelm Leibniz, who, among other things, was Newton’s rival, famously in the matter of which of them was the first to invent the calculus. He also anticipated modern logic, developed a system of binary numbers, and much else. He has been called the smartest person who ever lived. Leibniz formulated a principle to frame cosmological theories called the principle of sufficient reason, which states that there must be a rational reason for every apparent choice made in the construction of the universe. Every query of the form, “Why is the universe like X rather than Y?” must have an answer. So if a God made the world, He could not have had any choice in the blueprint. Leibniz’s principle has had a profound effect on the development of physics so far, and, as we will see, it continues to be reliable as a guide in our efforts to devise a cosmological theory.
Leibniz had a vision of a world in which everything lives not in space but immersed in a network of relationships. These relationships define space, not the reverse. Today the idea of a universe of connected, networked entities pervades modern physics, as well as biology and computer science.
In a relational world (which is what we call a world where relationships precede space), there are no spaces without things. Newton’s concept of space was absolute: He saw atoms defined by where they are in space but space in no way affected by the motion of atoms. In a relational world, there are no such asymmetries. Things are defined by their relationships. Individuals exist, and they may be partly autonomous, but their possibilities are determined by the network of relationships. Individuals encounter and perceive one another through the links that connect them within the network, and the networks are dynamic and ever evolving.
As I will explain in chapter 3, it follows from Leibniz’s great principle that there can be no absolute time that ticks on blindly whatever happens in the world. Time must be a consequence of change; without alteration in the world, there can be no time. Philosophers say that time is relational — it is an aspect of relations, such as causality, that govern change. Similarly, space must be relational; indeed, every property of an object in nature must be a reflection of dynamical relations between it and other things in the world.
Leibniz’s principles contradicted the basic ideas of Newtonian physics, so it took some time for them to be fully appreciated by working scientists. It was Einstein who embraced Leibniz’s legacy and used his principles as major motivation for his overthrow of Newtonian physics and its replacement by general relativity, a theory of space, time, and gravity that goes far to instantiate Leibniz’s relational view of space and time. Leibniz’s principles are also realized in a different way in the parallel quantum revolution. I call the 20th-century revolution in physics the relational revolution.
The problem of unifying physics and, in particular, bringing together quantum theory with general relativity into one framework is largely the task of completing the relational revolution in physics. The main message of this book is that this requires embracing the ideas that time is real and laws evolve.
The relational revolution is already in full swing in the rest of science. Darwin’s revolution in biology is one front, manifested both in the notion of a species being defined by its relation to all the other organisms in its environment and in the concept that a gene’s action is defined only in the context of the network of genes regulating its action. As we are quickly coming to realize, biology is about information, and there is no more relational concept than information, relying as it does on a relationship between the sender and receiver at each end of a communications channel.
In the social sphere, the liberal concept of a world of autonomous individuals (conceived by the philosopher John Locke as analogous to the physics of his friend Isaac Newton) is being challenged by a view of society as composed of interdependent individuals, only partly autonomous, whose lives are meaningful only within a skein of relationships. The new informational halo within which we are so recently enmeshed expresses the relational idea through the metaphor of the network. As social beings, we see ourselves as nodes in a network whose connections define us. Today the idea of a social system made up of connected, networked entities increasingly crops up in social theories formulated by everyone from feminist political philosophers to management gurus. How many users of Facebook are aware that their social lives are now organized by a potent scientific idea?
The relational revolution is already far along. At the same time, it is clearly in crisis. On some fronts, it’s stuck. Wherever it is in crisis, we find three kinds of questions under hot debate. What is an individual? How do novel kinds of systems and entities emerge? How are we to usefully understand the universe as a whole?
The key to these puzzles is that neither individuals, systems, nor the universe as a whole can be thought of as things that simply are. They are all compounded by processes that take place in time. The missing element, without which we cannot answer these questions, is to see them as processes developing in time. I will argue that to succeed, the relational revolution must embrace the notion of time and the present moment as a fundamental aspect of reality.
In the old way of thinking, individuals were just the smallest units in a system, and if you wanted to understand how a system worked you took it apart and studied how its parts behaved. But how are we to understand the properties of the most fundamental entities? They have no parts, so reductionism (as this method is called) gets us no further. The atomic viewpoint has no place to go here; it, too, is truly stuck. This is a great opportunity for the nascent relational program, for it can — and indeed must — seek the explanation for properties of elementary particles in the network of their relations.
This is already happening in the unified theories we have so far. In the Standard Model of Particle Physics, which is the best theory we have so far of the elementary particles, the properties of an electron, such as its mass, are dynamically determined by the interactions in which it participates. The most basic property a particle can have is its mass, which determines how much force is needed to change its motion. In the Standard Model, all the particles’ masses arise from their interactions with other particles and are determined primarily by one — the Higgs particle. No longer are there absolutely “elementary” particles; everything that behaves like a particle is, to some extent, an emergent consequence of a network of interactions.
Emergence is an important term in a relational world. A property of something made of parts is emergent if it would not make sense when attributed to any of the parts. Rocks are hard, and water flows, but the atoms they’re made of are neither solid nor wet. An emergent property will often hold approximately, because it denotes an averaged or high-level description that leaves out much detail.
As science progresses, aspects of nature once considered fundamental are revealed as emergent and approximate. We once thought that solids, liquids, and gases were fundamental states; now we know that these are emergent properties, which can be understood as different ways to arrange the atoms that make up everything. Most of the laws of nature once thought of as fundamental are now understood as emergent and approximate. Temperature is just the average energy of atoms in random motion, so the laws of thermodynamics that refer to temperature are emergent and approximate.
I’m inclined to believe that just about everything we now think is fundamental will also eventually be understood as approximate and emergent: gravity and the laws of Newton and Einstein that govern it, the laws of quantum mechanics, even space itself.
The fundamental physical theory we seek will not be about things moving in space. It will not have gravity or electricity or magnetism as fundamental forces. It will not be quantum mechanics. All these will emerge as approximate notions when our universe grows large enough.
If space is emergent, does that mean that time is also emergent? If we go deep enough into the fundamentals of nature, does time disappear? In the last century, we have progressed to the point where many of my colleagues consider time to be emergent from a more fundamental description of nature in which time does not appear.
I believe — as strongly as one can believe anything in science — that they’re wrong. Time will turn out to be the only aspect of our everyday experience that is fundamental. The fact that it is always some moment in our perception, and that we experience that moment as one of a flow of moments, is not an illusion. It is the best clue we have to fundamental reality.
Table of ContentsPreface xi
Weight: The Expulsion of Time
1. Falling 3
2. The Disappearance of Time 12
3. A Game of Catch 25
4. Doing Physics in a Box 37
5. The Expulsion of Novelty and Surprise 46
6. Relativity and Timelessness 54
7. Quantum Cosmology and the End of Time 76
Light: Time Reborn
Interlude: Einstein’s Discontent 93
8. The Cosmological Fallacy 97
9. The Cosmological Challenge 105
10. Principles for a New Cosmology 116
11. The Evolution of Laws 125
12. Quantum Mechanics and the Liberation of the Atom 142
13. The Battle Between Relativity and the Quantum 156
14. Time Reborn from Relativity 166
15. The Emergence of Space 174
16. The Life and Death of the Universe 188
17. Time Reborn from Heat and Light 195
18. Infinite Space or Infinite Time? 215
19. The Future of Time 229
Epilogue: Thinking in Time 254