So, what exactly is time?
Interesting article in Quanta by Dan Falk. I was particularly intrigued by the idea of time as an emergent phenomenon, which rang true. (For those interested in emergence, I highly recommend The Emergence of Everything: How the World Became Complex. To put it somewhat cryptically, emergence is how one gets from the Big Bang to the phenomenon of a person appreciating, for example, a sonnet.) The article begins:
Einstein once described his friend Michele Besso as “the best sounding board in Europe” for scientific ideas. They attended university together in Zurich; later they were colleagues at the patent office in Bern. When Besso died in the spring of 1955, Einstein — knowing that his own time was also running out — wrote a now-famous letter to Besso’s family. “Now he has departed this strange world a little ahead of me,” Einstein wrote of his friend’s passing. “That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”
Einstein’s statement was not merely an attempt at consolation. Many physicists argue that Einstein’s position is implied by the two pillars of modern physics: Einstein’s masterpiece, the general theory of relativity, and the Standard Model of particle physics. The laws that underlie these theories are time-symmetric — that is, the physics they describe is the same, regardless of whether the variable called “time” increases or decreases. Moreover, they say nothing at all about the point we call “now” — a special moment (or so it appears) for us, but seemingly undefined when we talk about the universe at large. The resulting timeless cosmos is sometimes called a “block universe” — a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.
Many physicists have made peace with the idea of a block universe, arguing that the task of the physicist is to describe how the universe appears from the point of view of individual observers. To understand the distinction between past, present and future, you have to “plunge into this block universe and ask: ‘How is an observer perceiving time?’” said Andreas Albrecht, a physicist at the University of California, Davis, and one of the founders of the theory ofcosmic inflation.
Others vehemently disagree, arguing that the task of physics is to explain not just how time appears to pass, but why. For them, the universe is not static. The passage of time is physical. “I’m sick and tired of this block universe,” said Avshalom Elitzur, a physicist and philosopher formerly of Bar-Ilan University. “I don’t think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it’s not there.”
Last month, about 60 physicists, along with a handful of philosophers and researchers from other branches of science, gathered at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, to debate this question at the Time in Cosmology conference. The conference was co-organized by the physicist Lee Smolin, an outspoken critic of the block-universe idea (among other topics). His position is spelled out for a lay audience in Time Reborn and in a more technical work, The Singular Universe and the Reality of Time, co-authored with the philosopher Roberto Mangabeira Unger, who was also a co-organizer of the conference. In the latter work, mirroring Elitzur’s sentiments about the future’s lack of concreteness, Smolin wrote: “The future is not now real and there can be no definite facts of the matter about the future.” What is real is “the process by which future events are generated out of present events,” he said at the conference.
Those in attendance wrestled with several questions: the distinction between past, present and future; why time appears to move in only one direction; and whether time is fundamental or emergent. Most of those issues, not surprisingly, remained unresolved. But for four days, participants listened attentively to the latest proposals for tackling these questions — and, especially, to the ways in which we might reconcile our perception of time’s passage with a static, seemingly timeless universe.
Time Swept Under the Rug
There are a few things that everyone agrees on. The directionality that we observe in the macroscopic world is very real: Teacups shatter but do not spontaneously reassemble; eggs can be scrambled but not unscrambled. Entropy — a measure of the disorder in a system — always increases, a fact encoded in the second law of thermodynamics. As the Austrian physicist Ludwig Boltzmann understood in the 19th century, the second law explains why events are more likely to evolve in one direction rather than another. It accounts for the arrow of time.
But things get trickier when we step back and ask why we happen to live in a universe where such a law holds. “What Boltzmann truly explained is why the entropy of the universe will be larger tomorrow than it is today,” said Sean Carroll, a physicist at the California Institute of Technology, as we sat in a hotel bar after the second day of presentations. “But if that was all you knew, you’d also say that the entropy of the universe was probably larger yesterday than today — because all the underlying dynamics are completely symmetric with respect to time.” That is, if entropy is ultimately based on the underlying laws of the universe, and those laws are the same going forward and backward, then entropy is just as likely to increase going backward in time. But no one believes that entropy actually works that way. Scrambled eggs always come after whole eggs, never the other way around.
To make sense of this, physicists have proposed that the universe began in a very special low-entropy state. In this view, which the Columbia University philosopher of physics David Albert named the “past hypothesis,” entropy increases because the Big Bang happened to produce an exceptionally low-entropy universe. There was nowhere to go but up. The past hypothesis implies that every time we cook an egg, we’re taking advantage of events that happened nearly 14 billion years ago. “What you need the Big Bang to explain is: ‘Why were there ever unbroken eggs?’” Carroll said.
Some physicists are more troubled than others by the past hypothesis. Taking things we don’t understand about the physics of today’s universe and saying the answer can be found in the Big Bang could be seen, perhaps, as passing the buck — or as sweeping our problems under the carpet. Every time we invoke initial conditions, “the pile of things under the rug gets bigger,” said Marina Cortes, a cosmologist at the Royal Observatory in Edinburgh and a co-organizer of the conference.
To Smolin, the past hypothesis feels more like an admission of failure than a useful step forward. As he puts it in The Singular Universe: “The fact to be explained is why the universe, even 13.8 billion years after the Big Bang, has not reached equilibrium, which is by definition the most probable state, and it hardly suffices to explain this by asserting that the universe started in an even less probable state than the present one.”
Other physicists, however, point out that it’s normal to develop theories that can describe a system given certain initial conditions. A theory needn’t strive to explain those conditions.
Another set of physicists think that the past hypothesis, while better than nothing, is more likely to be a placeholder than a final answer. Perhaps, if we’re lucky, it will point the way to something deeper. “Many people say that the past hypothesis is just a fact, and there isn’t any underlying way to explain it. I don’t rule out that possibility,” Carroll said. “To me, the past hypothesis is a clue to help us develop a more comprehensive view of the universe.”
The Alternative Origins of Time
Can the arrow of time be understood without invoking the past hypothesis? Some physicists argue that gravity — not thermodynamics — aims time’s arrow. In this view, gravity causes matter to clump together, defining an arrow of time that aligns itself with growth of complexity, said Tim Koslowski, a physicist at the National Autonomous University of Mexico (he described the idea in a 2014paper co-authored by the British physicistJulian Barbour and Flavio Mercati, a physicist at Perimeter). Koslowski and his colleagues developed simple models of universes made up of 1,000 pointlike particles, subject only to Newton’s law of gravitation, and found that there will always be a moment of maximum density and minimum complexity. As one moves away from that point, in either direction, complexity increases. Naturally, we — complex creatures capable of making observations — can only evolve at some distance from the minimum. Still, wherever we happen to find ourselves in the history of the universe, we can point to an era of less complexity and call it the past, Koslowski said. The models are globally time-symmetric, but every observer will experience a local arrow of time. It’s significant that the low-entropy starting point isn’t an add-on to the model. Rather, it emerges naturally from it. “Gravity essentially eliminates the need for a past hypothesis,” Koslowski said.
The idea that time moves in more than one direction, and that we just happen to inhabit a section of the cosmos with a single, locally defined arrow of time, isn’t new. Back in 2004,