Jul 8, 2010
What “time” is it? How much “time” does it take to buy groceries? Time seems so natural: the flow of moments, the knife’s-edge present, the indeterminable past, the unknowable future. Time—the absolute standard.
But what seems so natural and absolute is in reality not so. Einstein shocked the world by showing that time was relative. What does “relative” mean? What new ideas do scientists have about time? What’s real about time?
Gregory Benford, a physicist and science fiction writer, admits, “Time has always puzzled me. One of my first novels was called Timescape because I tried to write about time as a landscape in which you can move instead of being marched through. The plot is about how physicists are trying to study the problem of time because they discovered a way of propagating information back in time. Not people, just information. The whole novel is about what this discovery implies. Usually, science works by making a discovery or inventing something, and then later ascertaining the implications—and that’s a good way to structure a novel. So toward the end of the novel, the physicists discover that they have successfully conveyed information backward in time about a disaster that will happen in their future [which had already happened in the physicists’ world]. But they’re still living in the same world [in which the disaster already happened]. So what’s the answer? How is this possible? How can it be reconciled?”
Benford says, “The answer is that when the information went backward, it caused the universe to split into two threads, and in one of them, the disaster was averted. The physicists saved the world, but it’s the ‘other’ world, and they are still living in their own world in which the disaster occurred. So other people are much happier elsewhere—and that’s the physicists’ reward, even though they themselves are still stuck in the same world.”
Benford explains that “according to the laws of the universe, particularly general relativity, one can, in principle, propagate information backward in time, and yet one never sees it happening. And the answer is quantum mechanics, which splits reality into branching universes.”
He speculates on the philosophical implications. “To me, this is the universe’s way of saying ‘nice try, but no cigar,’” he says. “The universe is set up in such a way that these things are not only conceivable, you can actually do them—but still you cannot change your world. And this is the deep lesson of reality: Even when you become God-like in that you are actually and literally creating a universe, you still don’t get to live there.”
How significant is it that, apparently, we have one dimension of time and three of space? “Are there universes in which there is no time?” Benford responds. “I guess the stand-up answer would be ‘yes, but not much happens there.’ You can conceive of a universe in which there are two axes of time. This would mean that you could make loops in time. You could create paradoxes immediately; it’d be no problem at all. How would such a universe function? You would have to abandon causality. There would be laws, but you could always go back and rig the results. What kind of universe is that? It’s not one where I would want to live—a universe with infinite many plot lines. Scary!”
Thus, it would seem that time being limited to one dimension is what gives the universe its intelligibility.
“Remember, we emerged from the universe,” Benford says. “We evolved in this universe, and the only way we could do so was that it had to make sense. Could creatures evolve in a universe with two axes of time? Well, they couldn’t be like us. This is a good example of something that’s beyond our conceptual horizon. We literally cannot conceive of such a world because it would literally make no sense. And maybe a universe with two time axes doesn’t exist because even God couldn’t figure it out.”
So time is not the static backdrop of events, but rather something active and dynamic. How does time happen? Can time expand and contract?
Kip Thorne, an expert on Einstein’s theory of relativity and the warping of the universe, explains that when time ticks, “what I see is not always what you see.”
How can that be?
“Galileo, Newton, and all of the great scientists before the 20th century thought of space and time as absolute and having no real intimate connection,” Thorne says. “But Einstein taught us otherwise. Einstein’s great insight in 1905 was to recognize that space and time are personal. Your time flows at one rate, my time flows at a different rate; you may see a space I may see as a mixture of space and time. In a very precise sense, it’s hard to grasp.”
Thorne likes to begin with time. “Einstein’s special relativity and general relativity tell us that if you move at a high speed past me and I watch clocks that you carry, those clocks will appear to me to tick more slowly than my clocks tick. But at the same time, you’re going past me, and of course you see me moving relative to you; you look at my clocks, you see my clocks tick slower than yours. So I see your clocks tick slower, and you see my clocks tick slower. It’s crazy!”
It sounds impossible. “It’s crazy,” Thorne repeats, “but it’s not impossible. It is possible because what you regard as two simultaneous events, occurring at the same time but at different locations in space, I don’t see as simultaneous. If you have two firecrackers and you carry them with you and you move at high speed and you set them off simultaneously, measurements that I make will show the firecracker in the back go off first, the firecracker in the front go off afterward. So there are weird things in how time seems to behave in simultaneity.”
The critical factor here is the absolute, inviolable standardization of the speed of light, which was Einstein’s great insight in his special theory of relativity. “Einstein intuited that the speed of light will be the same as measured by everyone, no matter how they move through the universe,” Thorne explains. “Now, in reality, if you go deeply into the philosophy of science, what’s really going on here, Einstein says, if you define the rate and flow of time in a manner that makes the laws of physics look simple—so that, for example, time is ticked in a regular way by atomic clocks, as atoms vibrate—then, having made that choice, the speed of light is the same as seen by everybody, which means that time is personal and that space is personal. It’s a very deep insight.”
The way this works is that, taken together, time and space are, in a sense, absolute, but each, taken alone, can vary. What this means, in a sense, is that what seems like time to me may seem like space to you, and vice versa.
Thorne says, “When Einstein recognized this weird behavior in space and time, the word he used was ‘relative.’ I prefer ‘personal.’ They are relative to you or relative to me. And that’s where the name ‘relativity’ comes from. Actually, it was one of Einstein’s teachers, Hermann Minkowski, who looked at Einstein’s laws and said, ‘I can write these laws in a much more beautiful mathematical form if I regard space and time as unified into a single thing called “space-time.”’ And this space-time, then, has four dimensions: one dimension of time and three dimensions of space. And I can formulate this unification in such a way that I can understand the personal nature of space, the personal nature of time, by envisioning, in some sense, slicing through space-time, so that each observer takes a different three-dimensional surface in space-time. What you take is different from what I take [though unobservable until relative velocities approach the speed of light]. But, taken together, there is a unified space and time. This means that we have the universe’s entire future history, its entire past history, all of space going throughout the universe, all of space and time—and each point in this four-dimensional structure, one dimension of time and three dimensions of space, is an event.”
Thorne continues: “If I think of time as going upward, the birth of Abraham Lincoln occurred down here; his death occurred up there. During his life, he went through space-time, through this ‘block universe’ where each point represents an event in space-time. With space and time unified, we describe his life in this geometric way by a curve in this four-dimensional space-time block universe.”
However, there is a difference between space directions and time directions, Thorne says. “They are distinguished in that you can always go out in space and come back. You can always go forward in time, but you can never come back. Time has this inexorable flow in the forward direction,” he explains. “So it’s a very rich block universe with these different kinds of behaviors.”
I ask Thorne if the future is really already “there.”
“In the block universe, as I step back and think about it, mathematically and geometrically, it’s already there,” he says. “The entire universe is there: the beginning, the end (if there is an end). It’s a little fuzzy in my mind because I don’t know what’s up there at the end, whereas I think I know what’s at the beginning. We do something like this all the time. In a novel, the whole story is there! It’s in the book; from beginning to end, it’s all there—but as you read it through, you feel the flow, you feel that it’s all in front of you. Everyone who lives in this block universe feels the flow of time from past to future, but I, as a physicist looking in from the outside, just see it as a unified entity.”
So, space and time, taken together, are absolute. But taken separately, space and time are relative. Thorne says “personal”—space and time are personal. We each have our own personal space and time, based on our relative movement (relative to each other).
I hear that what happens in particle physics—at extremely small distances—“dooms” space-time.
Nima Arkani-Hamed, an innovative string theorist, explains that “space-time is doomed as an elementary consequence of the existence of both quantum mechanics and gravity. If you think space-time is something real, then you should be able to meaningfully talk about separations of space and separations of time, of any arbitrary amount that you like. So we can talk about separations of time in seconds and separations of space in meters, but you should also be able to talk about arbitrarily short times and arbitrarily short distances, and we know that that’s impossible because of gravity. Because if we try to probe very, very tiny distances, we have to use a lot of energy (by the uncertainty principle), and when we get to tiny enough distances, we have to use so much energy that we collapse the little region of space-time that we’re trying to look at into a black hole, and that makes it impossible to probe what’s going on in that region.”
Black holes would form due to E=mc2, Einstein’s famous equation, which shows that with the enormous energy comes sufficiently dense mass to collapse the mass-energy in the area and warp space-time. “It becomes impossible to see the region you were trying to look at,” Arkani-Hamed reiterates. “So this is a very simple thought experiment that takes together the two basic ingredients, quantum mechanics and gravity, to discover that it’s simply impossible to make sense of space and time separations that are smaller than a very tiny amount. The distances we’re talking about are the Planck length, 10-33 centimeters, and the Planck time, 10-43 seconds. Just from this simple argument, we know that space and time can’t really make sense because we can’t talk about arbitrary separations in space or time.”
This would mean that neither space nor time is continuous. There’s some sort of a separation or discreteness. But does this mean that space and time become quantized with discrete elements, just like electromagnetic radiation with its photons?
“That is the most naïve sort of idea for what might be happening,” Arkani-Hamed responds, “but there are approaches to quantum gravity which take that naïve idea very seriously. It’s very hard to lay down some kind of discrete latticelike structure on space in a way that’s compatible with the laws of relativity. So it’s not that. It isn’t that there are some particlelike elements of space and time; there’s something much more subtle and interesting going on.”
One “amazing idea,” he says, is how “holography can begin to give us a more radical idea of how this discreteness happens.” Holography, he explains, is where three-dimensional information is encoded in a two-dimensional hologram (so a drastically smaller number of bits of information are needed). This means that the required information to specify a space, he says, is related to “the boundary surface of the space, not to the volume of the space. It’s completely counterintuitive.”
Arkani-Hamed concludes that, “Without understanding emergent space-time, we don’t know how to address any of the really fundamental questions that quantum mechanics and gravity were supposed to address: What happens at the center of black holes? What is the origin of the universe? Emergent time is a clear intellectual bottleneck.”
That space and time are emergent means that they are not fundamentally existent, but form out of something deeper. Something deeper than space and time? What could that be? How far can this go?
Lee Smolin, an innovative and controversial physicist, suggests that reimagining time is key for explaining everything. “Here’s the big question,” he says: “Is space an arena in which things happen, in which things come on and off? Or is there no arena; is space just a network of relationships? If the whole universe entirely were moved 10 meters to the left, would it matter? Does it even mean something to say that because no relationships would change? Newton said, ‘Yes, it would mean something.’ Leibniz said, ‘No, it’s even wrong to ask the question because there’s no discernible difference between the universe here and the universe 10 meters to the left.’ This very question of whether space is an absolute background or whether space is an aspect of reality that grows out of a network of relationships of causality is the fundamental question that has been around for 400 years.”
Smolin focuses on causal relationships, which can only involve events whose relative velocities are slower than the speed of light. He says, “The interesting thing is that if you write down a list of all the causal relations between all the events in the universe, you describe the geometry of space-time almost completely. There’s still some information that you have to put in, such as counting how many events take place, but then you have everything.”
Smolin believes that events themselves create the concept of space. “And that’s not just metaphor,” he says. “It’s the best way of understanding Einstein’s theory of general relativity. If you follow this point of view, then causality is the fundamental aspect of time; our experience with the flow of time is a product of that, a derivative of that.”
He continues: “For me, this is one of the profound issues: Is time really fundamental, so that it’s the one thing that’s not emergent? Or is time somehow emergent or a consequence of other things. Personally, I’m thinking more and more in the direction that time is fundamental, that it’s not emergent.”
Is Smolin differentiating time from space, teasing them apart?
Yes, he says.
Sounds quite radical.
“Sure it is,” he replies, “but maybe it’s also the most conservative solution to a set of conundrums that we face when we try to bring together quantum theory and gravity because the other alternative is that time goes away completely, that time is emergent, which would mean that the world is nothing but a vast collection of disconnected moments, with not necessarily any relation among them. The relationship between time and causality would dissolve. There comes a moment where you say, ‘Do we really have a technical problem, or do we just have a conceptual misunderstanding?’ and that’s what I think about time.”
Everyone feels time. No one knows time. The hidden metronome of happenings that seems to beat out the intervals of the cosmos and life belies the cryptic complexity of time. What’s real about time?
Einstein showed that time and space are not absolute. Separately, each is relative, personal. Collectively, they form a fundamental unity. Time energizes the core of reality, marks events, constructs the cosmos.
Some say that time is not real; it’s an illusion of events. Others say that time, not space, is most fundamental. Is time continuous or discrete? Or emergent in some strange new way?
As British biologist J.B.S. Haldane famously said, “The universe is not only queerer than we suppose, but queerer than we can suppose.” It’s time to get closer to truth.
Robert Lawrence Kuhn speaks with Gregory Benford, Kip Thorne, Nima Arkani-Hamed, and Lee Smolin in “What’s Real About Time?”—the 23rd episode in the new season of the Closer To Truth: Cosmos, Consciousness, God TV series (62nd in total).
The series airs on PBS World (often Thursdays, twice) and many other PBS and noncommercial stations. Every Thursday, participants will discuss the current episode.
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