Time Travel. A History

Why is it so difficult—so degradingly difficult—to bring the notion of Time into mental focus and keep it there for inspection? What an effort, what fumbling, what irritating fatigue!

—Vladimir Nabokov (1969)

PEOPLE KEEP ASKING what time is, as if the right combination of words could slip the lock and let in the light. We want a fortune-cookie definition, a perfect epigram. Time is “the landscape of experience,” says Daniel Boorstin. “Time is but memory in the making,” says Nabokov. “Time is what happens when nothing else does”—Dick Feynman. “Time is nature’s way to keep everything from happening all at once,” says Johnny Wheeler or Woody Allen. Martin Heidegger says, “There is no time.”^*1

What is time? Time is a word. The word refers to something, or some things, but surprisingly often the conversation goes off track when people forget whether they’re arguing about the word or the thing(s). Five hundred years of dictionaries have created the assumption that every word must have a definition, so what is time? “A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future” (American Heritage Dictionary of the English Language, fifth edition). A committee of lexicographers labored over those twenty words and must have debated almost every one. Nonspatial? That word is not to be found in this very dictionary, but all right, time is not space. Continuum? Presumably time is a continuum—but is that known for sure? “Apparently irreversible” seems a hedge. You sense they’re trying to tell us something they hope we already know. The challenge is not so much to inform us as to offer some discipline and care.

Other authorities offer entirely different constructions. Not one of them is wrong. What is time? “The general term for the experience of duration,” according to the Encyclopaedia Britannica (many editions). The very first English dictionary, Robert Cawdrey’s in 1604, avoided the problem and skipped right from thwite (“shave”) to timerous (“fearefull, abashed”). Samuel Johnson said “the measure of duration.” (And duration? “Continuance, length of time.”) A 1960 children’s book trimmed the definition to a single word: Time Is When.^*2

The people who compose definitions for dictionaries try to avoid the circularity that comes when they use the very word they are defining. With time it’s unavoidable. The lexicographers of the OED throw up their hands. They divide “time” (only the noun, not the interjection^*3 or the obscure conjunction) into thirty-five distinct senses and almost a hundred subsenses, including: a point in time; an extent of time; a specific period of time; time available…; the amount of time taken up by something; and time viewed as a medium through which travel into the past or future is hypothesized or imagined to be possible. (“Cf. time travel.”) They are covering all the bases. Perhaps their best effort is sense number ten: “The fundamental quantity of which periods or intervals of existence are conceived as consisting, and which is used to quantify their duration.” Even that definition merely postpones the circularity. Duration, period, and interval are defined in terms of time. The lexicographers know very well what time is, until they try to define it.

Like all words, time has boundaries, by which I don’t mean hard and impenetrable shells but porous edges. It maps weirdly between languages. A Londoner might say, “He did it fifty times, at the very least,” while in Paris, where the word for time is temps, fifty times is cinquante fois. Meanwhile, when the weather is good, the Parisian says, C’est beau temps. A New Yorker thinks the time and the weather are different things.^*4 And that is just the beginning. Many languages use a separate word to ask “What is the time?” as opposed to “What is time?”

In 1880 the United Kingdom enacted a legislative definition of time, the Statutes (Definition of Time) Act. This declared itself to be “an Act to remove doubts as to the meaning of Expressions relative to Time occurring in Acts of Parliament, deeds, and other legal instruments.” It was enacted “by the Queen’s most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal [Time Lords!], and Commons.” If only these wise men and woman could have solved the problem by fiat. Removing doubts about the meaning of time is an ambitious goal. Alas, it turns out that they were not dealing with What is time? but only What is the time? The time in Great Britain, as defined by the act, is Greenwich mean time.^*5

What is time? At the dawn of the written word, Plato struggled with the question. “A moving image of eternity,” he said. He could name the parts of time: “days and nights and months and years.” Moreover:

When we say that what has become is become and what becomes is becoming, and that what will become is about to become and that the nonexistent is nonexistent—all these are inaccurate modes of expression. But perhaps this whole subject will be more suitably discussed on some other occasion.

Here Aristotle, too, found himself in difficulties. “To start, then: the following considerations would make one suspect that it either does not exist at all or barely, and in an obscure way. One part of it has been and is not, while the other is going to be and is not yet.” The past has gone out of existence, the future has not yet been born, and time is made up of these “things which do not exist.” On the other hand, he said—looked at differently—time seems to be a consequence of change, or motion. It is “the measure” of change. Earlier and later, faster and slower—these are words that are “defined by” time. Fast is a lot of motion in a little time, slow is a little motion in a lot of time. As for time itself: “time is not defined by time.”

Later, Augustine, like Plato, contrasted time with eternity. Unlike Plato, he could hardly stop thinking about time. It obsessed him. His way of explaining was to say that he understood time very well, until the moment he tried to explain. Let us reverse Augustine’s process: stop trying to explain and instead take stock of what we know. Time is not defined by time—that needn’t paralyze us. When we leave aside the search for epigrams and definitions, it turns out we know a great deal.^*6

—

WE KNOW THAT time is imperceptible. It is immaterial. We cannot see it, hear it, or touch it. If people say they perceive the passing of time, that’s just a figure of speech. They perceive something else—the clock ticking on the mantel, or their own heartbeat, or other manifestations of the many biological rhythms below the level of consciousness—but whatever time is, it lies outside the grasp of our senses. Robert Hooke made this very point to the Royal Society in 1682:

I would query by what Sense it is we come to be informed of Time; for all the Information we have from the senses are momentary, and only last during the Impressions made by the Object. There is therefore yet wanting a Sense to apprehend Time; for such a Notion we have.^*7

Yet we experience time in a way that we do not experience space. Close your eyes, and space disappears: you may be anywhere; you may be big or small. Yet time continues. “I am listening not to Time itself but to the blood current coursing through my brain, and thence through the veins of the neck heartward, back to the seat of private throes which have no relation to Time,” says Nabokov. Cut off from the world, with no sensory perception, we may still count the time. Indeed, we habitually quantify time (“…and yet we conceive of it as a Quantity,” said Hooke). This leads to a plausible definition: Time is what clocks measure. But what is a clock? An instrument for the measurement of time.^*8 The snake swallows its tail again.

Once we conceive of time as a quantity, we can store it up, apparently. We save it, spend it, accumulate it, and bank it. We do all this quite obsessively nowadays, but the notion is at least four hundred years old. Francis Bacon, 1612: “To choose Time, is to save Time.” The corollary of saving time is wasting it. Bacon again: “Prolix and florid Harangues…and other personal Speeches are great Wasters of Time.” No one would have begun thinking about time as a bankable commodity who was not already familiar with money. Time hath, my lord, a wallet at his back, / Wherein he puts alms for oblivion. But is time really a commodity? Or is this just another shabby analogy, along with time the river?

We go back and forth between being time’s master and its victim. Time is ours to use, and then we are at its mercy. I wasted time, and now doth time waste me, says Richard II; For now hath time made me his numbering clock. If you say that an activity wastes time, implying a substance in finite supply, and then you say that it fills time, implying a sort of container, have you contradicted yourself? Are you confused? Are you committing a failure of logic? None of those. On the contrary, you are a clever creature, when it comes to time, and you can keep more than one idea in your head. Language is imperfect; poetry, perfectly imperfect. We can occupy the time and pass the time in the same breath. We can devour time or languish in its slow-chapp’d power.

Newton, who invented the idea of mass, knew that time didn’t have any, that it’s not a substance, yet he said that time “flows.” He wrote this in Latin: tempus fluit. The Romans said tempus fugit, time flees, or at any rate that motto began appearing on English sundials in the Middle Ages. Newton would have seen that. True, the hours speed by and are gone, once we learn to measure them, but how can time flee? It’s another figure of speech. And how can time flow, if it has no substance?

Newton took pains to distinguish two kinds of time. We might call them physical time and psychological time, but he lacked those words, so he had to struggle a bit. The first kind he called, with a flurry of adjectives, “time absolute true and mathematical” (tempus absolutum verum & Mathematicum). The other was time as conceived by the common people—the vulgus—and this he called “relative” and “apparent.” True time—mathematical time—he inferred from a technological feature of his world, the consistency of clocks. He and the clockmakers both leaned on Galileo here—it was Galileo who established that a swinging pendulum of a given length divides time into regular pieces. He measured time by using his pulse. Shortly thereafter, doctors began using clocks to time pulses. The ancients looked to the heavens for measuring time: the sun, the stars, the moon—those were reliable. They gave us our days, months, and years. (When Joshua needed more time to smite the Amorites, he asked God to halt the sun and moon in their tracks—“Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon.” Who among us has not wanted to stop time?) Now machinery takes over the reckoning.

Another circularity creeps in—a chicken-and-egg problem. Time is how we measure motion. Motion is how we measure time. Newton tried to escape that by fiat. He made Absolute Time axiomatic. He needed a reliable backbone for his laws of motion. The first law: an object moves at a constant velocity, unless acted upon by some external force. But what is velocity? Distance per unit time. When Newton declared that time flows equably, aequabiliter fluit, he meant that we can count on unit time. Hours, days, months, years: they are the same everywhere and always. In effect, he imagined the universe as its own clock, the cosmic clock, perfect and mathematical. He wanted to say that when two of our earthly clocks differ, it’s because of some fault in the clocks, not because the universe speeds up and slows down hither and yon.

—

NOW IT IS fashionable among physicists and philosophers to ask whether time is even “real”—whether it “exists.” The question is debated at conferences and symposia and analyzed in books. I have put quotation marks around those words because they are so problematic in themselves. The nature of reality hasn’t been settled either. We know what it means to say that unicorns are not real. Likewise Santa Claus. But when scholars say time is not real, they mean something different. They haven’t lost faith in their wristwatches or their calendars. They use “real” as code for something else: absolute, special, or fundamental.

Not everyone would agree that physicists like to debate the reality of time. Sean Carroll writes, “Perhaps surprisingly, physicists are not overly concerned with adjudicating which particular concepts are ‘real’ or not.” Leave that to philosophers, I think he means. “For concepts like ‘time,’ which are unambiguously part of a useful vocabulary we have for describing the world, talking about ‘reality’ is just a bit of harmless gassing.” The business of physicists is to construct theoretical models and test them against empirical data. The models are effective and powerful but remain artificial. They themselves are a kind of language. Still, physicists do get caught up in debating the nature of reality. How could they not? “The nature of time” was the subject of an international essay contest organized in 2008 by FQXi, an institute devoted to foundational questions of physics and cosmology. One winning essay, chosen from more than a hundred, was Carroll’s own: “What If Time Really Exists?” This was a deliberately contrarian exercise. “There is a venerable strain of intellectual history that proclaims that time does not exist,” he noted. “There is a strong temptation to throw up one’s hands and proclaim the whole thing is an illusion.”

A landmark on that road is an essay published in 1908 by the journal Mind, “The Unreality of Time,” by John McTaggart Ellis McTaggart. He was an English philosopher, by then a fixture at Trinity College, Cambridge.^*9 McTaggart was said (by Norbert Wiener) to have made a cameo appearance in Alice’s Adventures in Wonderland as the Dormouse, “with his pudgy hands, his sleepy air, and his sidelong walk.” He had been arguing for years that our common view of time is an illusion, and now he made his case. “It doubtless seems highly paradoxical to assert that Time is unreal,” he began. But consider…

He contrasts two different ways of talking about “positions in time” (or “events”). We may talk about them relative to the present—the speaker’s present. The death of Queen Anne (his example) is in the past, for us, but at one time it lay in the future and then came round to the present. “Each position is either Past, Present, or Future,” writes McTaggart. This he labels, for later convenience, the A series.

Alternatively, we may talk about the positions in time relative to one another. “Each position is Earlier than some, and Later than some, of the other positions.” The death of Queen Anne is later than the death of the last dinosaur but earlier than the publication of “The Unreality of Time.” This is the B series. The B series is fixed. It is permanent. The order can never change. The A series is changeable: “an event, which is now present, was future and will be past.”

Many people found this A series and B series distinction persuasive, and it lives on robustly in the philosophical literature. By a chain of reasoning McTaggart uses it to prove that time does not exist. The A series is essential to time, because time depends on change, and only the A series allows for change. On the other hand, the A series contradicts its own premises, because the same events possess the properties of pastness and futureness. “Neither time as a whole, nor the A series and B series, really exist” is his apparently inevitable conclusion. (I could say “was” because the paper appeared in 1908. But I can also say “is” because the paper exists in libraries and online and, more abstractly still, in the fast-expanding tapestry of interwoven ideas and facts that we call our culture.)

You may have noticed—and if so, you’re more observant than most of his readers—that McTaggart began by assuming the thing he is trying to prove. He considered all positions in time, all possible events, as if they were already laid out in a sequence, points on a geometer’s line, M, N, O, P, arranged from the point of view of God or the logician. Call this the eternal point of view, or eternalism. The future is just like the past: you can see it in the mind’s eye, neatly diagrammed. Our experience to the contrary is merely a product of mental states: memories, perceptions, and anticipations, which we experience as “pastness,” “presentness,” and “futurity.” An eternalist says that reality is timeless. So time is unreal.

In fact this is a mainstream view of modern physics. I won’t say the mainstream view—in these tempestuous days no one can say for sure what that is. Many of the most respected and established physicists espouse the following:

• The equations of physics contain no evidence for a flow of time.

• The laws of science do not distinguish between the past and the future.

• Therefore—do we have a syllogism?—

• Time is not real.

The observer—physicist or philosopher—stands outside and looks in. The human experience of time is suspended for abstract observation. Past, present, and future are bounded in a nutshell.

And what of our persistent impressions to the contrary? We experience time in our bones. We remember the past, we await the future. But the physicist notes that we are fallible organisms, easily fooled and not to be trusted. Our prescientific ancestors experienced the flat earth and traveling sun. Could our experience of time be equally naïve? Perhaps—but scientists have to come back to the evidence of our senses in the end. They must test their models against experience.

“People like us, who believe in physics,” Einstein said, “know that the distinction between past, present, and future is only a stubbornly persistent illusion.” Who believe in physics—I detect something wistful in that. “In physics,” repeats Freeman Dyson, “the division of space-time into past, present, and future is an illusion.” These formulations retain a bit of humility that is sometimes lost in the quoting. Einstein was consoling a bereaved sister and son, and perhaps thinking as well of his own pending mortality. Dyson was expressing hopeful bonds of kinship with people of the past and people of the future: “They are our neighbors in the universe.” These are beautiful thoughts, but they were not intended as final statements about the nature of reality. As Einstein himself said on an earlier occasion, “Time and space are modes by which we think, and not conditions in which we live.”

There is something perverse about a scientist’s believing that the future is already complete—locked down tight, no different from the past. The first motivation for the scientific enterprise, the prime directive, is to gain some control over our headlong tumble into an unknown future. For ancient astronomers to forecast the movements of heavenly bodies was vindication and triumph; to predict an eclipse was to rob it of its terror; medical science has labored for centuries to eradicate diseases and extend the lifetimes that fatalists call fixed; in the first powerful application of Newton’s laws to earthly mechanics, students of gunnery computed the parabolic trajectories of cannonballs, the better to send them to their targets; twentieth-century physicists not only managed to change the course of warfare but then dreamt of using their new computing machines to forecast and even control the earth’s weather. Because, why not? We are pattern-recognition machines, and the project of science is to formalize our intuitions, do the math, in hopes not just of understanding—a passive, academic pleasure—but of bending nature, to the limited extent possible, to our will.

Remember Laplace’s perfect intelligence, vast enough to comprehend all the forces and the positions and to submit them to analysis. “To it nothing would be uncertain, and the future as the past would be present to its eyes.” This is how the future becomes indistinguishable from the past. Tom Stoppard joins the parade of philosophers wittily paraphrasing him: “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.” It bears asking—because so many modern physicists still believe something like this—why? If no intelligence can be so comprehensive, no computer can do so much computing, why must we treat the future as though it were predictable?

The implicit answer, sometimes explicit, is that the universe is its own computer. It computes its own destiny, step by step, bit by bit (or qubit by qubit). The computers we know, in the early twenty-first century, not counting the tantalizing quantum variety, operate deterministically. A given input always leads to the same output. Our input, again, is the totality of initial conditions and our program is the laws of nature. These are the whole kit and caboodle: the entire future is already there. No information needs to be added, nothing remains to be discovered. There shall be no novelty, no surprise. Only the clanking of the logical gears remains—a mere formality.

Yet we have learned that in the real world things are always a little messy. Measurements are approximate. Knowledge is imperfect. “The parts have a certain loose play upon one another,” said William James, “so that the laying down of one of them does not necessarily determine what the others shall be.” James might have been pleasantly surprised by the revelations of quantum physics: the exact states of particles can never be perfectly known; uncertainty reigns; probability distributions replace the perfect clockwork dreamt of by Laplace. “It admits that possibilities may be in excess of actualities,” James might have said—that is, he did say it, but in advance of the actual science—“and that things not yet revealed to our knowledge may really in themselves be ambiguous.” Just so. A physicist with a Geiger counter can never guess when the next click will come. You might think that our modern quantum theorists would join James in cheering indeterminism.

The computers in our thought experiments, if not always the computers we own, are deterministic because people have designed them that way. Likewise, the laws of science are deterministic because people have written them that way. They have an ideal perfection that can be attained in the mind or in the Platonic realm but not in the real world. The Schrödinger equation, the screwdriver of modern physics, manages the uncertainties by bundling up the probabilities into a unit, a wave function. It’s a ghostly abstract object, this wave function. A physicist can write it as ψ and not worry too much about the contents. “Where did we get that from?” said Richard Feynman. “Nowhere. It’s not possible to derive it from anything you know. It came out of the mind of Schrödinger.” It just was, and is, astoundingly effective. And once you have it, the Schrödinger equation returns determinism to the process. Calculations are deterministic. Given proper input, good quantum physicists can compute the output with certainty and keep on computing. The only trouble comes in the act of returning from the idealized equations to the real world they are meant to describe. Finally we have to parachute in from the Platonic abstract mathematics to the sublunary stuff on laboratory benches. At that point, when an act of measurement is required, the wave function “collapses,” as physicists say. Schrödinger’s cat is either alive or dead. According to a limerick:

It comes as a total surprise

That what we learn from the ψ’s

Not the fate of the cat

But related to that:

The best we can ever surmise.

This collapse of the wave function is the trigger for a special kind of argumentation in quantum physics, not about the mathematics but about the philosophical underpinnings. What can this possibly mean? is the basic problem, and the various approaches are called interpretations. There is the Copenhagen interpretation, first among many. The Copenhagen approach is to treat the collapse of the wave function as an awkward necessity—just a kludge to live with.^*10 The slogan for this interpretation is “Shut up and calculate.” There are the Bohmian interpretation, the quantum Bayesian, the objective collapse, and—last but definitely not least—the many worlds. “Go to any meeting, and it is like being in a holy city in great tumult,” says the physicist Christopher Fuchs. “You will find all the religions with all their priests pitted in holy war.”

The many-worlds interpretation—MWI, to those in the know—is a fantastic piece of make-believe championed by some of the smartest physicists of our time. They are the intellectual heirs of Hugh Everett, if not Borges. “The MWI is the one with all the glamour and publicity,” wrote Philip Ball, the English science writer (ex-physicist), in 2015. “It tells us that we have multiple selves, living other lives in other universes, quite possibly doing all the things that we dream of but will never achieve (or never dare). Who could resist such an idea?” (He can, for one.) The many-worlds champions are like hoarders, unable to throw anything away. There is no such thing as a path not taken. Everything that can happen does happen. All possibilities are realized, if not here, then in another universe. In cosmology universes also abound. Brian Greene has named nine different types of parallel universes: “quilted,” “inflationary,” “brane,” “cyclic,” “landscape,” “quantum,” “holographic,” “simulated,” and “ultimate.” The MWI cannot be demolished by means of logic. It’s too appealing: any argument you can make against it has already been considered and (in their minds) refuted by its distinguished advocates.

To me, the most effective physicists are the ones who retain a degree of modesty about their program. Bohr said, “In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, so far as it is possible, relations between the manifold aspects of our experience.” Feynman said, “I have approximate answers and possible beliefs and different degrees of certainty about different things, but I’m not absolutely sure of anything.” Physicists make mathematical models, which are generalizations and simplifications—by definition incomplete, stripped down from the cornucopia of reality. The models expose patterns in the messiness and capitalize upon them. The models themselves are timeless; they exist unchanging. A Cartesian graph plotting time and distance contains its own past and future. The Minkowskian spacetime picture is timeless. The wave function is timeless. These models are ideal, and they are frozen. We can comprehend them within our minds or our computers. The world, on the other hand, remains full of surprises.

William Faulkner said, “The aim of every artist is to arrest motion, which is life, by artificial means and hold it fixed.” Scientists do that, too, and sometimes they forget they are using artificial means. You can say Einstein discovered that the universe is a four-dimensional space-time continuum. But it’s better to say, more modestly, Einstein discovered that we can describe the universe as a four-dimensional space-time continuum and that such a model enables physicists to calculate almost everything, with astounding exactitude, in certain limited domains. Call it spacetime for the convenience of reasoning. Add spacetime to the arsenal of metaphors.

You can say the equations of physics make no distinction between past and future, between forward and backward in time. But if you do, you are averting your gaze from the phenomena dearest to our hearts.^*11 You leave for another day or another department the puzzles of evolution, memory, consciousness, life itself. Elementary processes may be reversible; complex processes are not. In the world of things, time’s arrow is always flying.

One twenty-first-century theorist who began to challenge the mainstream block-universe view was Lee Smolin, born in New York in 1955, an expert on quantum gravity and a founder of the Perimeter Institute for Theoretical Physics in Canada. For much of his career Smolin held conventional views of time (for a physicist) before, as he saw it, recanting. “I no longer believe that time is unreal,” he declared in 2013. “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.” The rejection of time is itself a conceit. It is a trick that physicists have played on themselves.

“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,” Smolin wrote. Timelessness, eternity, the four-dimensional spacetime loaf—these are the illusions. Timeless laws of nature are like perfect equilateral triangles. They exist, undeniably, but only in our minds.

Everything we experience, every thought, impression, intention, is part of a moment. The world is presented to us as a series of moments. We have no choice about this. No choice about which moment we inhabit now, no choice about whether to go forward or back in time. No choice to jump ahead. No choice about the rate of flow of the moments. In this way, time is completely unlike space. One might object by saying that all events also take place in a particular location. But we have a choice about where we move in space. This is not a small distinction; it shapes the whole of our experience.

Determinists, of course, believe that the choice is an illusion. Smolin was willing to treat the persistence of the illusion as a piece of evidence, not to be dismissed glibly, requiring explanation.

For Smolin, the key to salvaging time turns out to be rethinking the very idea of space. Where does that come from? In a universe empty of matter, would space exist? He argues that time is a fundamental property of nature but space is an emergent property. In other words, it is the same kind of abstraction as “temperature”: apparent, measurable, but actually a consequence of something deeper and invisible. In the case of temperature, the foundation is the microscopic motion of ensembles of molecules. What we feel as temperature is an average of the energy of these moving molecules. So it is with space: “Space, at the quantum-mechanical level, is not fundamental at all but emergent from a deeper order.” (He likewise believes that quantum mechanics itself, with all its puzzles and paradoxes—“cats that are both alive and dead, an infinitude of simultaneously existing universes”—will turn out to be an approximation of a deeper theory.)

For space, the deeper reality is the network of relationships among all the entities that fill it. Things are related to other things; they are connected, and it is the relationships that define space rather than the other way around. This is not a new perspective. It goes back at least to Newton’s great rival Leibniz, who refused to accept the view of time and space as containers in which everything is situated—an absolute background for the universe. He preferred to treat them as relations between objects: “Space is nothing else, but That Order or Relation; and is nothing at all without Bodies, but the Possibility of placing them.” Empty space is not space at all, Leibniz would say, nor would time exist in an empty universe, because time is the measure of change. “I hold space to be something merely relative, as time is,” wrote Leibniz. “Instants, considered without the things, are nothing at all.” With the triumph of the Newtonian program, Leibniz’s view almost faded from view.

To appreciate the network-centered, relational view of space, we need look no further than the connected, digital world. The internet, like the telegraph a century before, is commonly said to “annihilate” space. It does this by making neighbors of the most distant nodes in a network that transcends physical dimension. Instead of six degrees of separation we have billions of degrees of connectedness. As Smolin put it:

We live in a world in which technology has trumped the limitations inherent in living in a low-dimensional space….From a cell-phone perspective, we live in a 2.5 billion–dimensional space, in which very nearly all our fellow humans are our nearest neighbors. The Internet, of course has done the same thing. The space separating us has been dissolved by a network of connections.

So maybe it’s easier now for us to see how things really are. This is what Smolin believes: that time is fundamental but space an illusion; “that the real relationships that form the world are a dynamical network”; and that the network itself, along with everything in it, can and must evolve over time.

He presents a program for further study, based on a notion of “preferred global time” that extends throughout the universe and defines a boundary between past and future. It imagines a family of observers, spread throughout the universe, and a preferred state of rest, against which motion can be measured. Even if “now” need not be the same to different observers, it retains its meaning for the cosmos. These observers, with their persistent sense of a present moment, are a problem to be investigated, rather than set aside.

The universe does what it does. We perceive change, perceive motion, and try to make sense of the teeming, blooming confusion. The hard problem, in other words, is consciousness. We’re back where we started, with Wells’s Time Traveller, insisting that the only difference between time and space is that “our consciousness moves along it,” just before Einstein and Minkowski said the same. Physicists have developed a love-hate relationship with the problem of the self. On the one hand it’s none of their business—leave it to the (mere) psychologists. On the other hand, trying to extricate the observer—the measurer, the accumulator of information—from the cool description of nature has turned out to be impossible. Our consciousness is not some magical onlooker; it is a part of the universe it tries to contemplate.

The mind is what we experience most immediately and what does the experiencing. It is subject to the arrow of time. It creates memories as it goes. It models the world and continually compares these models with their predecessors. Whatever consciousness will turn out to be, it’s not a moving flashlight illuminating successive slices of the four-dimensional space-time continuum. It is a dynamical system, occurring in time, evolving in time, able to absorb bits of information from the past and process them, and able as well to create anticipation for the future.

Augustine was right all along. The modern philosopher J. R. Lucas, in his Treatise on Time and Space, comes back around: “We cannot say what time is, because we know already, and our saying could never match up to all that we already know.” So was the Buddha (as translated via Borges): “The man of a past moment has lived, but he does not live nor will he live; the man of a future moment will live, but he has not lived nor does he now live; the man of the present moment lives, but he has not lived nor will he live.” We know that the past is gone—it is finished, done, signed, sealed, and delivered. Our access to it is compromised, limited by memories and physical evidence—fossils, paintings in attics, mummies, and old ledgers. We know that eyewitnesses are unreliable and records can be tampered with or misread. The unrecorded past no longer exists. Still, experience persuades us that the past happened and keeps happening. The future is different. The future is yet to come; it is open; not everything can happen but many things can. The world is still under construction.

What is time? Things change, and time is how we keep track.

*1 Die Zeit ist nicht. But he adds, Es gibt Zeit. Time is given.

*2 Beth Gleick, Time Is When (Chicago: Rand McNally, 1960). The present author’s mother.

*3 “Time!”

*4 “By a curious caprice,” wrote the astronomer Charles Nordmann in 1924, “the French language, different from others, designates by a single word, the word temps, two very different things: the time which goes by and the weather, or state of the atmosphere. This is one of the peculiarities which give to our language its hermetic elegance, its concentrated sobriety, its elliptic charm.”

*5 Even this attempt at definition proved tricky. A test case came on August 19, 1898, at 8:15 p.m. (Greenwich mean time), when a man named Gordon was nicked by the police in Bristol for riding his bicycle without a lamp. The local law clearly stated that every person riding a bicycle (which fell under the definition of “carriage”) shall carry a lamp, so lighted as to afford adequate means of signaling the approach of the bicycle, during the period between one hour after sunset and one hour before sunrise. On the evening in question, sunset in Greenwich had occurred at 7:13 p.m., so Gordon was caught riding lampless a full hour and two minutes after sunset.

This did not sit well with the accused man, because the sun set ten minutes later in Bristol than in Greenwich: 7:23, not 7:13. Nonetheless, the justices of the city of Bristol, relying on the Statutes (Definition of Time) Act, found him guilty. After all, they reasoned, everyone would benefit by having “a readily ascertained time of lighting up.”

With the help of his solicitors, Darley & Cumberland, poor Gordon appealed. The question before the Court of Appeals was described as “an astronomical one.” The appellate court saw it his way. They ruled that sunset is not a “period of time” but a physical fact. Justice Channell was insistent: “According to the decision of the Justices, as it stands, a man on an unlighted bicycle may be looking at the sun in the heavens, and yet be liable to be convicted of the offence of not having his lamp lighted an hour after sunset.”

*6 “If you stop, in dealing with such words, with their definition, thinking that to be an intellectual finality, where are you? Stupidly staring at a pretentious sham! ‘Deus est Ens, a se, extra et supra omne genus, necessarium, unum, infinite perfectum, simplex, immutabile, immensum, aeternum, intelligens,’ etc.,—wherein is such a definition really instructive? It means less than nothing, in its pompous robe of adjectives.”—William James

*7 Hooke proceeded to dig himself into a hole. “I say, we shall find a necessity of supposing some other Organ to apprehend the Impression that is made by Time.” What organ? “That which we generally call Memory, which Memory I suppose to be as much an Organ as the Eye, Ear or Nose.” Where is this organ, then? “Somewhere near the Place where the Nerves from the other Senses meet.”

*8 Lee Smolin tries to escape the circularity in Time Reborn by redefining “clock”: “For our purposes, a clock is any device that reads out a sequence of increasing numbers.” Then again, a person counting to one hundred is not a clock.

*9 McTaggart’s name bears explaining. He was christened (by his parents, the Ellises of Wiltshire) John McTaggart Ellis, after his father’s uncle, Sir John McTaggart, a childless Scottish baronet. Sir John then bequeathed a considerable fortune to the Ellises on the condition that they take his surname. In the case of young John, this led to a redundancy. The double dose of “McTaggart” never seems to have bothered him, and he, not the baronet, is the McTaggart most remembered today.

*10 Where did this come from, this idea of a “Copenhagen interpretation”? First, “Copenhagen” is cool kids’ shorthand for Niels Bohr. For several decades, Copenhagen was to quantum theory what the Vatican is to Catholicism. As for “interpretation,” it seems to have started out in German, only the word was Geist, as in Kopenhagener Geist der Quantentheorie (Werner Heisenberg, 1930).

*11 “That there is a place for the present moment in physics becomes obvious when I take my experience of it as the reality it clearly is to me and recognize that space-time is an abstraction that I construct to organize such experiences,” says David Mermin.