Time Travel

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  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 e
very 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.)