by Lee Smolin
theory of principle, 10
Thomson, William (Lord Kelvin), 13, 35
‘t Hooft, Gerard, xv, 278, 313, 317, 318–19
time
background-independent theories and, 44, 54, 82
dimensions of, 39
space-time relational theory, 321
wrong assumptions about, 256–58
See also units of distance and time
Tomonaga, Sin-Itiro, 55
topological conservation laws, 322
topological string theory, 195
topos theory, 258
Townsend, Paul, 134
Trivedi, Sandip, 156
Trivers, Robert, 342–43
twistor theory/space, 244–45
Uhlenbeck, George, 47
uncertainty principle
cosmological constant and, 152
definition/description, 6, 152
inflation theory and, 207
variables and, 55
Unger, Roberto Mangabeira, 258
unification of acceleration and gravity, 39–41
unification of electricity and magnetism
aether and, 34–35, 37
consequences of, 33
Einstein and, 36, 37
“field” idea, 31–33, 32
Newton’s laws, problem with, 33–37
Maxwell and, ix, 11, 31, 32–33, 46, 55, 196, 247
as model for today, 31
relativity and, 35–36
theory of light and, 33, 41
timing of, 11
unification of gravity and electromagnetism, 11–12, 45, 46–47
unification of heat and matter, 19
unification of motion and rest, 21–22, 24, 36, 39, 203, 219
unification of physics
crisis of failure of, 66
need for theory, 4–8
new theory for quantum mechanics, 10–11
Nielsen’s antiunification program, 316
See also specific unifications
unification of planets’ motions, 27–30
unification of particles and forces, 11–13, 66–67
unification proposals
determination of truth of, 20, 24–26, 27–31, 247
of Earth and planets, 20, 21
evolution by natural selection and, 19
explanations of, 22–23
fundamental force and, 11
further unifications with, 24, 30–31
of gravity and electromagnetic field, 11–12
interpreting evidence for, 24
Kepler example, 27–30
of light and sound, 19–20
of motion and rest, 21–22, 36
observers’ perspective and, 22–23
rationality and, 25–27
reactions to, 18–19
single law and, 10
as sometimes false, 19–20
for sun, stars, and moon, 18–19, 20–21, 23, 24
testing of, 20, 23
unexpected consequences and, 19, 20, 33, 40–41
See also specific proposals/theories
unification scale, 70
unified field theory
abandonment of work on, 52–53, 54, 67
Einstein and, 49–50, 51–52, 53
extra dimensions and, 50–51, 52
nuclear forces and, 50
problems with, 50–53
and quantum theory, 51–52
search for, 38–39
unstable theories, 51, 53
See also specific theories
uniform motion, 21–22, 39–40
units of distance and time, 215–16, 228
universe
cosmic circles and, 20–21
cosmological natural selection and, 167–68
deceleration and, 15–16
early discoveries about, x–xi
Earth as center and, 20–21
eternal inflation and, 162
as evolving, 150–52
expansion of, x, 15–16, 44, 150, 152–53
as quantum computer, 317–18
and standard model of cosmology, 16
See also inflation theory
vacuum (ground-state) energy, 70, 152, 158
Vafa, Cumrun, 138, 283
Valentini, Antony, 322–23, 326–27, 353
Van Nieuwenhuizen, Peter, 92–95
variable-speed-of-light (VSL) cosmology, 229–31
Veneziano, Gabriele, 103
Volkov, Dmitri, 68
Volovik, Grigori, 153, 247, 315
VSL (variable-speed-of-light) cosmology, 229–31
waves as particles, 67, 132
weak anthropic principle, 162–67
weak bosons, 58
weak interaction scale, 70
weak nuclear force
description, x, 11
gauge theory and, 58
particles carrying, 61
quantum field theory, 55–56
unification with electromagnetic field, 11
Weinberg, Steven
anthropic principle and string landscape, 165–66, 168–69
as leader in particle physics, 289
Weinberg-Salam model of electroweak force, 61
Yang-Mills theories and, 88
Weinberg-Salam model of electroweak force
discovery/description, 61, 196
noncommutative geometry and, 247
requirements of, 62
Wen, Xiao-Gang, 247, 315
Wess, Julius, 68, 69
Weyl, Herman, 45–46, 49, 58
Wheeler-DeWitt equation, 314–15
Wheeler, John Archibald, 145, 251, 263, 279
Wilson, Kenneth, 110, 249
Wilson lines, 110
WIMPs (weakly interacting massive particles), 175
winding number, 130, 130
Winkler, Oliver, 251
Wise, Mark, xi–xii
Witten, Edward
cosmological constant problem, 154
as one of few “leaders” in string theory, 274–75
spacetime as emergent, 240
string theory and, 113, 115, 122–23, 129
string theory/supergravity and, 135
twistor space, 245
unification of string theory and, 129, 134–36
Wittgenstein, Ludwig, 293
Wolpert, David, 298
women in science, treatment of, 335–37, 343
W particles, 61, 67
Yang-Mills equations, 50
Yang-Mills theories
description/history, 58, 361 n3 (ch. 4)
maximally super theory, 134
quantum gravity theory, 88–89
standard model, 317
twistor theory, 245
Yau, Shing-tung, 122–23
Yoneya, Tamiaki, 106
Zatsepin, Georgiy, 220–21
Z particles, 61, 67
Zumino, Bruno, 68, 69, 92
Zweig, George, 56
Preface
WHAT IS TIME?
This deceptively simple question is the single most important problem facing science as we probe more deeply 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 the forces and particles—come down to the nature of time.
The progress of science has been marked by the dismissal of illusions. Matter appears to be smooth but turns out to be made of atoms. Atoms seem indivisible but turn out to be built of protons, neutrons, and electrons, the first two of which are made of still more elementary particles called quarks. The sun appears to go around the Earth, but it’s the other way around—and when you get right down to it, it turns out that everything moves relative to everything else.
Time is the most pervasive aspect of our everyday experience. Everything we think, feel, or do reminds us of its existence. We perceive the world as a flow of moments that make up our life. But physicists and philosophers alike have long tol
d us (and many people think) that time is the ultimate illusion.
When I ask my nonscientific friends what they think time is, they often answer that its passage is deceptive and whatever is actually real—truth, justice, the divine, scientific laws—lies outside it. The idea that time is an illusion is a philosophical and religious commonplace. For millennia, people have reconciled themselves to life’s hardships and our mortality by believing in the possibility of an eventual escape to a timeless and more real world.
Some of our most illustrious thinkers assert the unreality of time. Plato, the greatest philosopher of the ancient world, and Einstein, the greatest physicist of the modern world, both taught a view of nature in which the real is timeless. They saw our experience of time as an accident of our circumstance as human beings—an accident that hides the truth from us. Both believed that the illusion of time must be transcended to perceive the real and the true.
I used to believe in the essential unreality of time. Indeed, I went into physics because as an adolescent I yearned to exchange the time-bound, human world, which I saw as ugly and inhospitable, for a world of pure, timeless truth. Later in life, I discovered that it was pretty nice to be human and the need for transcendent escape faded.
More to the point, I no longer believe that time is unreal. In fact, I have swung to the opposite view: Not only is time real, but nothing we know or experience gets closer to the heart of nature than the reality of time.
My reasons for this volte-face lie in science—and, in particular, in contemporary developments in physics and cosmology. I’ve come to believe that time is the key to the meaning of quantum theory and its eventual unification with space, time, gravity, and cosmology. Most important, I believe that to make sense of the picture of the universe that cosmological observations are bringing to us, we must embrace the reality of time in a new way. This is what I mean by the rebirth of time.
Much of this book sets out the scientific argument for believing in the reality of time. If you are one of the many who believe that time is an illusion, I aim to change your mind. If you already believe that time is real, I hope to give you better reasons for your belief.
This is a book for everyone, because there is no one whose thinking about the world is not shaped by how they see time. Even if you have never pondered its meaning, your thinking—the very language with which you express your thoughts—is colored by ancient metaphysical ideas about time.
When we adopt the revolutionary view that time is real, how we think about everything else will change. In particular, we will tend to see the future in a new way, one that vividly highlights both the opportunities and the dangers confronting the human species.
A small part of the story of this book is the personal journey that led me to rediscover time. My initial motivation might best be described in the language not of science but of fatherhood, through the conversations I have had with my young son, especially when I put him to bed at the end of the day. “Daddy,” he asked once as I read to him, “did you have my name when you were my age?” Here was a child awakening to the knowledge that there was a time before him and seeking to connect the short story of his life so far to a longer epic.
Every journey has a lesson to teach, and mine has been to realize just how radical an idea is contained in the simple statement that time is real. Having begun my life in science searching for the equation beyond time, I now believe that the deepest secret of the universe is that its essence rests in how it unfolds moment by moment in time.
…
There’s a paradox inherent in how we think about time. We perceive ourselves as living in time, yet we often imagine that the better aspects of our world and ourselves transcend it. What makes something really true, we believe, is not that it is true now but that it always was and always will be true. What makes a principle of morality absolute is that it holds in every time and every circumstance. We seem to have an ingrained idea that if something is valuable, it exists outside time. We yearn for “eternal love.” We speak of “truth” and “justice” as timeless. Whatever we most admire and look up to—God, the truths of mathematics, the laws of nature—is endowed with an existence that transcends time. We act inside time but judge our actions by timeless standards.
As a result of this paradox, we live in a state of alienation from what we most value. This alienation affects every one of our aspirations. In science, experiments and their analysis are time-bound, as are all our observations of nature, yet we imagine that we uncover evidence for timeless natural laws. The paradox also affects our actions as individuals, family members, and citizens, because how we understand time determines how we think about the future.
In this book, I hope to resolve in a new way the paradox of living in time and believing in the timeless. I will propose that time and its passage are fundamental and real and the hopes and beliefs about timeless truths and timeless realms are mythology.
Embracing time means believing that reality consists only of what’s real in each moment of time. This is a radical idea, for it denies any kind of timeless existence or truth—whether in the realm of science, morality, mathematics, or government. All those must be reconceptualized, to frame their truths within time.
Embracing time also means that our basic assumptions about how the universe works at the most fundamental level are incomplete. When, in the pages that follow, I assert that time is real, what I’m saying is that:
Whatever is real in our universe is real in a moment of time, which is one of a succession of moments.
The past was real but is no longer real. We can, however, interpret and analyze the past, because we find evidence of past processes in the present.
The future does not yet exist and is therefore open. We can reasonably infer some predictions, but we cannot predict the future completely. Indeed, the future can produce phenomena that are genuinely novel, in the sense that no knowledge of the past could have anticipated them.
Nothing transcends time, not even the laws of nature. Laws are not timeless. Like everything else, they are features of the present, and they can evolve over time.
In the course of this book, we will see that these hypotheses point to a new direction for fundamental physics—one that I argue is the only way out of the present conundrums of theoretical physics and cosmology. They also have implications for how we should understand our own lives and deal with the challenges humankind faces.
To explain why the reality of time is so consequential, both for science and for matters beyond science, I like to contrast thinking in time with thinking outside time. The idea that truth is timeless and somehow outside the universe is so pervasive that the Brazilian philosopher Roberto Mangabeira Unger refers to it as “the perennial philosophy.” It was the essence of Plato’s thought, exemplified in the parable, in Meno, of the slave boy and the geometry of a square, in which Socrates argues that all discovery is merely recollection.
We think outside time when we imagine that the answer to whatever question we’re pondering is out there in some eternal domain of timeless truth. Whether the issue is how to be a better parent or spouse or citizen, or what the optimal organization of society might be, we believe there’s something unalterably true out there for us to discover.
Scientists think in time when we conceive of our task as the invention of novel ideas to describe newly discovered phenomena, and of novel mathematical structures to express them. If we think outside time, we believe these ideas somehow existed before we invented them. If we think in time, we see no reason to presume that.
The contrast between thinking in time and outside time is apparent in many arenas of human thought and action. We are thinking outside time when, faced with a technological or social problem, we assume that the possible approaches are already determined, as a set of absolute, pre-existing categories. Anyone who thinks that the correct theory of economics or politics was written down in the century before last is thinking outside time. When we instead see the aim
of politics as the invention of novel solutions to novel problems that arise as society evolves, we are thinking in time. We’re also thinking in time when we understand that progress in technology, society, and science consists in inventing genuinely new ideas, strategies, and forms of social organization—and trust our ability to do so.
When we unquestioningly accept the strictures, habits, and bureaucracies of our various communities and organizations as if they had an absolute reason to be there, we’re trapped outside time. We reenter time when we realize that every feature of a human organization is a result of a history, so that everything about them is negotiable and subject to improvement by the invention of new ways of doing things.
If we believe that the task of physics is the discovery of a timeless mathematical equation that captures every aspect of the universe, then we believe that the truth about the universe lies outside the universe. This is such a familiar habit of thought that we fail to see its absurdity: If the universe is all that exists, then how can something exist outside it for it to be described by? But if we take the reality of time as evident, then there can be no mathematical equation that perfectly captures every aspect of the world, because one property of the real world not shared by any mathematical equation is that it is always some moment.
