With his theories of relativity and gravity long confirmed and his Nobel Prize ten years old, he was by far the most famous scientist in the world. “Photographers lunged at me like hungry wolves,” he had written in his diary when his ship landed in New York two months earlier.
For years Einstein had insisted, like Aristotle and Newton before him, that the universe was a magnificent and immortal cathedral, fixed for all eternity. In this picture, time runs from the infinite past to the infinite future, and little changes in between. When a prominent Belgian scientist proposed in 1927 that the universe was growing like an expanding balloon, Einstein pronounced the idea “abominable.”
By 1931, however, the great physicist had been confronted with telescopic evidence that distant galaxies were in flight. Perhaps even more convincing, his mathematical model for a static universe had been shown to be like a pencil balanced on its point: give it a tiny nudge and it starts to move. When he arrived in Pasadena, Einstein was ready to acknowledge a cosmos in flux. He told the men gathered in the library in their suits and ties that the observed motion of the galaxies “has smashed my old construction like a hammerblow.” Then he swung down his hand to emphasize the point.
What arose from the shards of that hammerblow was the cosmology of the Big Bang: the idea that the universe is not static and everlasting—that it “began” some fourteen billion years ago in a state of extremely high density and has been expanding and thinning out ever since. According to current data, our universe will keep expanding forever.
Sean Carroll, a professor of physics at the California Institute of Technology, is a Big Bang cosmologist. He is also one of a small platoon of physicists who call themselves quantum cosmologists. He wants to know what happened at the very first moment of the Big Bang, whether time or anything else existed before it, and how we can tell the future from the past. Such bedrock questions in physics, which have been seriously posed only recently, might be likened to Descartes asking for proof of his existence.
Quantum cosmology is speculative work, but Carroll explained its allure: “It’s high risk, high gain.” We do not yet possess a full theory of gravity, space, and time in the quantum era. Nevertheless, some of the sharpest minds in physics, including Stephen Hawking, Andrei Linde, and Alexander Vilenkin, have pondered the subject. It is a tiny field, not for the timid. The first difficulty is that the birth of our universe was a one-performance event, and we weren’t there in the audience. An understanding of the very beginning of the universe also requires an understanding of so-called quantum gravity: gravity at enormously high densities of matter and energy, which are impossible to replicate. Most physicists believe that in this quantum era, the entire observable universe was roughly a million billion billion times smaller than a single atom. The temperature was nearly a million billion billion billion degrees. Time and space churned like boiling water. Of course, such things are unimaginable. But theoretical physicists try to imagine them in mathematical form, with pencil and paper. Somehow, time as we know it emerged in that fantastically dense nugget. Or perhaps time already existed, and what emerged was the “arrow” of time, pointing toward the future.
Physicists hope that within the next fifty years or so, string theory or other new theoretical work will provide a good understanding of quantum gravity, including an explanation of how the universe began. Until then, the quantum cosmologists will debate their hypotheses, each backed up with pages of calculations.
When I reached Carroll on Skype, he was wearing a hoodie and jeans in the comfortable study of his home, in Los Angeles. I was stationed in an uninhabitable guest room of my house, in Concord, Massachusetts: practically next door, in cosmological terms. Carroll is an articulate explicator of science as well as a highly regarded physicist—he’s written scientific papers with titles such as “What If Time Really Exists?”—and he talks about his favorite subject with evident pleasure. He is forty-nine years old and barrel-chested, with puffy cheeks, jowls, a full head of reddish hair, and a mischievous schoolboy glint in his eye.
Carroll is obsessed with the relative smoothness and order of the universe. Order in physics has a concrete meaning. It can be quantified. Furthermore, conditions of disorder are more probable than conditions of order, just as a deck of cards, once shuffled, is more likely to be jumbled up than precisely arranged by number and suit. Applying those considerations to the cosmos at large, physicists have suggested that given the amount of matter that exists we should expect the universe to be far more disordered and lumpy than it is. The observable universe has something like one hundred billion galaxies in it, but when viewed over sufficiently large expanses of space, it looks as uniform as the sand on a beach. Any large volume of space looks about like any other. It would be far more probable, say the physicists, to see that same material concentrated in a much smaller number of ultralarge galaxies, or in large clusters of galaxies, or perhaps even in a single massive black hole—analogous to all the sand on a beach concentrated in a few silicon boulders.
The improbable smoothness of the observable universe, in turn, points toward unusually tidy conditions near the Big Bang. We don’t understand why. But the order and smoothness, known to physicists as a state of low entropy, is a clue. “I strongly believe that the low entropy of the early universe is a puzzle that the wider cosmology community doesn’t take nearly as seriously as they should,” Carroll told me. “Misunderstandings like that offer opportunities for making new breakthroughs.”
Carroll and other physicists believe that order is intimately connected to the arrow of time. In particular, the forward direction of time is determined by the movement of order to disorder. For example, a movie of a glass goblet falling off a table and shattering on the floor would look normal to us; if we saw a movie of scattered shards of glass jumping off the floor and gathering themselves into a goblet perched on the edge of a table, we would say that the movie was being played backward. Likewise, clean rooms left unattended become dusty with time, not cleaner. What we call the future is the condition of increasing mess; what we call the past is increasing tidiness. Our ability to easily distinguish between the two shows that time in our world has a clear direction. Time also has a clear direction in the cosmos at large. Stars radiate heat and light, slowly spend their nuclear fuel, and finally turn into cold cinders drifting through space. Never does the reverse happen.
Which brings us back to the unexpected orderliness of our universe. Working with Alan Guth, a pioneering cosmologist at the Massachusetts Institute of Technology, Carroll has developed a not-yet-published theory called two-headed time. In this model of the universe, time has existed forever. But unlike the static cosmos imagined by Aristotle and Newton and Einstein, this universe changes as the eons go by. The evolution of the cosmos is symmetric in time, such that the behavior of the universe before the Big Bang is nearly a mirror image of its behavior after. Until fourteen billion years ago, the universe was contracting. It reached a minimum size at the Big Bang (which we call t = 0) and has been expanding ever since. (Other quantum cosmologists have proposed similar models.) It’s like a Slinky that falls to the floor, reaches maximum compression on impact, and then bounces back to larger dimensions. Because of the unavoidable random fluctuations required by quantum physics, the contracting universe would not be an exact mirror image of the expanding universe; a physicist named Alan Guth probably did not exist in the contracting phase of our universe.
It is well known in the science of order and disorder that, other things being equal, larger spaces allow for more disorder, essentially because there are more places to scatter things. Smaller spaces therefore tend to have more order. As a consequence, in the Carroll-Guth picture, the order of the universe was at a maximum at the Big Bang; disorder increased both before and after. Recall that the forward direction of time is determined by the movement of order to disorder. Thus the future points away from the Big Bang in two directions. A person living in the contracting phase of the universe sees the Bi
g Bang in her past, just as we do. When she dies, the universe is larger than when she was born, just as it will be for us. “When I came to understand that the reason I can remember the past but not the future is ultimately related to conditions at the Big Bang, that was a startling epiphany,” said Carroll.
If you think of time as a long road and the Big Bang as a pothole somewhere in that road, then a sign at the pothole telling you the direction to the future would have two arrows pointing in opposite directions. Hence the name “two-headed time.” Near the pothole itself, caught between the two arrows, time would have no clear direction. Time would be confused. In the subatomic version of goblets and houses, shards of glass would jump off the floor to form goblets as often as those goblets would fall and shatter. Unattended houses would become neater as often as they would become more cluttered. Both movies would be equally familiar to any subatomic being living at the Big Bang.
According to Carroll and Guth, the two-headed-time theory could become even more elaborate and strange. The point of minimum size and maximum order of the universe might not have been the Big Bang of our universe but the Big Bang of another universe, some kind of grand protouniverse. Our universe, and possibly an infinite number of universes, could have been spawned from this parent universe, and each of the universes could have its own Big Bang. The process of spawning new universes from a parent universe is called eternal inflation. The idea was developed by quantum cosmologists in the early 1980s. In brief, an unusual energy field (but one permitted by physics) in the protouniverse acts like antigravity and causes exponentially fast expansion. This unusual energy field has different strengths in different regions of space. Each such region expands to cosmic proportions, and the energy field becomes ordinary matter, forming a new universe that is closed off and completely out of contact with the protouniverse that sired it.
A second major hypothesis is that the universe, and time, did not exist before the Big Bang. The universe materialized literally out of nothing, at a tiny but finite size, and expanded thereafter. There were no moments before the moment of smallest size because there was no “before.” Likewise, there was no “creation” of the universe, since that concept implies action in time. Even to say that the universe “materialized” is somewhat misleading. As Hawking describes it, the universe “would be neither created nor destroyed. It would just BE.” Such notions as existence and being in the absence of time are not fathomable within our limited human experience. We don’t even have language to describe them. Nearly every sentence we utter has some notion of “before” and “after.”
One of the first quantum cosmologists to suggest that the universe could appear out of nothing was Alexander Vilenkin, a Ukrainian scientist who came to the United States in 1976, when he was in his midtwenties. He is now a professor of physics at Tufts University. When I visited him in his office on a hot day in July, he was wearing sandals and a loose black shirt. His single window looked out on a dull brick building across the street. “The view from my previous office was better,” he said. Boxes of unpacked books littered the floor; on his bookshelf was an Einstein doll given to him by his daughter.
In the Soviet Union, Vilenkin’s acceptance to a graduate program in physics was rescinded, possibly at the instigation of the KGB. Before he emigrated, he worked as a night watchman in a zoo, giving him plenty of time to think cosmological thoughts. In the United States, Vilenkin got his PhD in biophysics. “I was doing cosmology on the side,” he said. “It was not a reputable field of research at that time.” Vilenkin is a serious man who, unlike many physicists, does not much joke around, and he takes his work on the universe at t = 0 extremely seriously. “No cause is required to create a universe from quantum tunneling,” he says, “but the laws of physics should be there.” Briefly, we chat about what “there” means when time and space do not yet exist. On this score, Vilenkin likes to quote Saint Augustine, who was often asked what God was doing before He created the universe. In his Confessions, Augustine replied that since God created time when He created the universe, there was no “before.”
When Vilenkin talks about quantum tunneling, he is referring to a spooky phenomenon in quantum physics, in which objects can perform such magic feats as instantly appearing on the far side of a mountain without traveling over the top. That mystifying ability, which has been verified in the laboratory, follows from the fact that subatomic particles behave as though they can be in many places at once. Quantum tunneling is common in the tiny world of the atom but is highly improbable in our human world. It has never been observed at larger scales—which explains why the phenomenon seems so absurd. But in the quantum era of cosmology, very near t = 0, the entire universe was the size of a subatomic particle. Thus, the entire universe could have “suddenly” appeared from wherever things originate in the impossible-to-fathom quantum haze of probabilities. (I put “suddenly” in quotation marks because time didn’t exist, but I have just now realized that in this very sentence I used the verb “did,” which is the past tense of “do” . . .)
What does it mean to say that the entire universe was like a subatomic particle, existing in the twilight world of the quantum? James Hartle, a leading quantum cosmologist at the University of California, Santa Barbara, has, with Hawking, developed one of the most detailed models of the universe “during” the quantum era near the Big Bang. Time appears nowhere in Hartle and Hawking’s equations. Instead, they use quantum physics to compute the probability of certain snapshots of the universe.
Although an expert in quantum theory, Hartle admits to being baffled by the application of quantum physics to the universe as a whole. “It is a mystery to me,” he said, “why we have quantum mechanics when there is only one state of the universe.” In other words, why should there be probabilities that alternative conditions of the universe exist when we inhabit only one? And do those alternative conditions actually exist somewhere?
The quantum cosmologists are aware of the vast philosophical and theological reverberations of their work. As Hawking says in A Brief History of Time, many people believe that God, while permitting the universe to evolve according to fixed laws of nature, was uniquely responsible for winding up the clock at the beginning and choosing how to set it in motion. Hawking’s own theory provides an explanation for how the universe might have wound itself up—his method of calculating the early snapshots of the universe has no dependence on initial conditions or boundaries or anything outside the universe itself. The icy rules of quantum physics are completely sufficient. “What place, then, for a creator?” asks Hawking. Lawrence Krauss, a physicist, reaches a similar conclusion in his book A Universe from Nothing, in which he argues that advances in quantum cosmology show that God is irrelevant at best.
One would expect most quantum cosmologists to be atheists, like the majority of scientists. But Don Page, a leading quantum cosmologist at the University of Alberta, is also an evangelical Christian. Page is a master computationalist. When he and I were fellow graduate students in physics at Caltech, he used to quietly take out a fine-point pen whenever confronted with a difficult physics problem. Without flinching or pausing, he scribbled one equation after another in a dense tangle of mathematics until he arrived at the answer. Although he has collaborated with Hawking on major papers, Page parts ways with him on the subject of God. He recently told me, “As a Christian, I think there is a being outside the universe that created the universe and caused all things. God is the true creator. All of the universe is caused by God.” In a guest column on Carroll’s blog (which is called The Preposterous Universe), Page sounds simultaneously like a scientist and a theist:
One might think that adding the hypothesis that the world (all that exists) includes God would make the theory for the entire world more complex, but it is not obvious that is the case, since it might be that God is even simpler than the universe, so that one would get a simpler explanation starting with God than starting with just the universe.
Significantly, most
quantum cosmologists do not believe that anything caused the creation of the universe. As Vilenkin said to me, quantum physics can hypothesize a universe without cause—just as quantum physics can show how electrons can change orbits in an atom without cause. There are no definite cause-and-effect relationships in the quantum world, only probabilities. Carroll put it this way: “In everyday life we talk about cause and effect. But there is no reason to apply that thinking to the universe as a whole. I do not feel in any way unsatisfied by just saying, ‘That’s the way it is.’”
The notion of an event or state of being without cause drives hard against the grain of science. For centuries, scientists have attempted to explain all events as the logical consequence of prior events. Page argues that at the origin of our universe—whether in the two-headed-time model or in the universe-out-of-nothing model—there was no clear distinction between cause and effect. If causality can dissolve in the quantum haze of the origin of the universe, Page and other physicists note, there is reason to question its solidity even in the world that we live in, long after the Big Bang, which is surely part of the same reality. “Causality within the universe is not fundamental,” said Page. “It is an approximate concept derived from our experience with the world.” Strict causality could be an illusion, a way for our brains, and our science, to make sense of the world. But without strict causality, how can we take responsibility for our actions? A crack in the marble foundation of causality could send tremors into philosophy, religion, and ethics.
Quantum cosmology has led us to questions about the fundamental aspects of existence and being, questions that most of us rarely ask. In our short century or less, we generally aim to create a comfortable existence within the tiny rooms of our lives. We eat, we sleep, we get jobs, we pay the bills, we have lovers and children. Some of us build cities or make art. But if we have the luxury of true mental freedom, there are larger concerns to be found. Look at the sky. Does space go on forever, to infinity? Or is it finite but without boundary or edge, like the surface of a sphere? Either answer is disturbing, and unfathomable. Where did we come from? We can follow the lives of our parents and grandparents and their parents backward in time, back and back through the generations, until we come to some ancestor ten thousand years in the past whose DNA remains in our body. We can follow the chain of being even further back in time to the first humans, and the first primates, and the one-celled amoebas swimming about in the primordial seas, and the formation of the atmosphere, and the slow condensation of gases to create Earth. It all happened, whether we think about it or not. We quickly realize how limited we are in our experience of the world. What we see and feel with our bodies, caught midway between atoms and galaxies, is but a small swath of the spectrum, a sliver of reality.
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