Time Travel: A History (30 page)

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).