There was a strong science vibe in New York City as the World Science Festival rang in its third year. Founded by string theorist and author Brian Greene and journalist Tracy Day, the festival hosted forty events over four days that sold out at various locations throughout the city.
"Time, I think, is a little bit like love," began moderator John Hockenberry. "It's accessible to all of us; it is intuitively experienced by all of us in the same way; yet it retains its mystery at whatever level you weigh in on it. It is a mysterious force that we all can experience and share." And so Hockenberry opened the panel discussion at the World Science Festival titled "Time Since Einstein." The award-winning journalist nearly slaps his audience awake with his booming voice and dramatic inflection, and by the end of the talk you just want to chat with him for hours.
The panel members were enough to make this science nerd drool: physicists George Ellis, Roger Penrose, Sean Carroll, and Fotini Markopoulou-Kalamara, who were joined by the two very physics-savvy philosophers David Albert and Michael Heller. The panel had its work cut out for it, given the task to cover everything we've learned and begun to question about time since the relativity revolution incited by Einstein. By the end, my friend and I wondered, "Would someone with no physics background think these people knew an incredible amount about time, or nothing at all?" And really, both are true. For as much as we've learned since Einstein, we've only found more to wonder about.
What is time? No, really, what is it? A dimension of space-time, says relativity, but is it physical? Or just mental? Why does it seem to move forward and not back? Does the universe acknowledge the arrow of time that we experience? How big is time? Does our notion of "now" have any validity? Is time quantized? And are the answers to these questions within the grasp of physics?
Einstein revolutionized our notion of time, and experimental evidence has since shown that time is in fact relative; it depends on where you are and how you are moving. Time was once the stage on which events occurred, but after Einstein, it became a player on the stage, equally influenced by the other players. "Our notion of time is very physical," says Markopoulou-Kalamara. She's referring to the fact that our concept of time often relies on physical cues, such as something we heard or saw. So is time just a series of events or changes in spatial dimensions? Einstein linked space and time into the dimension of space-time, and now scientists wonder if they can ever untangle the two. The nature of time, like many of physics' most confounding issues, is explored on both the largest and the smallest scales we know of.
The notion of time plays different roles in general relativity and quantum mechanics. Markopoulou-Kalamara is at the forefront of the pursuit to link the two. She has worked in the development of loop quantum gravity, an alternative to string theory, and she is also developing models of space-time that account for the flow of time. While quantum mechanics suggests that the future is not planned, we don't know if it equals a spontaneous present where we can chose our actions. Following this thread, she had this back and forth with Hockenberry:
Hockenberry: Can there be a non-deterministic future that actually exists?
Markopoulou-Kalamara: This is the conflict between the two theories--general relativity and quantum theory. So in general relativity, yes, it is deterministic. You never have to choose anything, everything is chosen for you.
H: Which is depressing. (laughs from the audience)
M-K:...Or freeing. (more laughs and some applause.) But in quantum mechanics that's not true.
H: Which is...liberating or depressing?
Einstein, a member of the elite group that shaped quantum mechanics, took a deterministic view; maybe he thought an intrinsically probabilistic universe was too frightening. Quantum mechanics doesn't guarantee free will, but does it prove that the future is distinct from the past? George Ellis says that it does.
"There are many theoretical physicists who think the flow of time is an illusion," he says. "And I think that's a great mistake...according to quantum physics you don't know the outcome of events until they happen. We know what happened in the past, there's a time called the present when things are happening, and there's a time in the future which is not yet determined. That's my view on it, which is not a very widely supported one."
A professor emeritus of applied mathematics at the University of Capetown, Ellis is the co-author with Stephen Hawking of The Large Scale Structure of Space Time, and investigates the physical foundations of the flow of time. He notes that subatomic particles, in the form of cosmic rays, affected the formation of life on this planet beginning four billion years ago. Even if we knew everything there was to know about the Earth then, we couldn't have predicted the way things would turn out. Doesn't that imply a distinct future?
Time on the quantum scale also challenges our notion of the present. One audience member asked if "now" really exists; can we take a picture of this moment and make it distinct? To totally freeze this moment would mean to collapse the wave functions of many things which would otherwise have been left as probabilites. In some ways, "now" is not definite.
The notion that time moves forward is something we humans can't escape. As Carroll pointed out, if you're out in space you might get confused about up and down, but you wouldn't get confused about the past and the present (unless you got a severe case of space madness). But most of the laws of physics are untouched by that notion--they are time symmetric, meaning if you run them backwards they look the same as they do running forward. A few rare particle experiments, however, done at CERN and Fermilab, have demonstrated a time asymmetry. The second law of thermodynamics is another heavy hitter for the idea of an arrow of time. The second law states that in a closed system things get more random; one can scramble an egg but not unscramble it. Entropy will always increase, and moving backwards in time would violate that.
Carroll, a senior research associate at the California Institute of Technology, is the author of the upcoming From Eternity to Here, a book about cosmology and the arrow of time. Always good for an elegant and enlightening talk despite the great complexity of his own research, he discussed the question "How long does time go?":
"Scientists have gotten used to the idea that when people ask us 'What happened before the big bang?' we give St. Augustine's answer: we say there was no such thing as before the big bang. But in very recent times, beyond Einstein, we're realizing that we have absolutely no justification for saying that that's true. We have to move beyond Einstein to understand what happened at the big bang. And the answer might be that the universe came into existence at the big bang; there's nothing before. Or it might not. There could be something before the big bang....Cosmologists, people who are working on quantum gravity, are very interested in what we've learned since Einstein to answer these questions and go back and answer St. Augustine's question."
Ellis adds that Einstein also hated the idea of a beginning of time. It does seem rather odd that something with a very distinct beginning would simply have no end, or to think that even 14 billion years after the big bang (if the universe is infinite) we are still infinitesimally close to the beginning.
It seems the discussion of time swings quickly between the largest scale and the smallest scale. Discussion of the ultimate length of time begs the question, is time quantized? Can we break it up into packets like photons and quarks? Markopoulou-Kalamara says it depends on your notion of time. In terms of the geometric notion of time, as in the time dimension of a space-time, she says yes; she believes it has to break down. But if you're referring to time as change, something that has happened, "I doubt it," she says. Ellis adds that quantization of time may have to confront something like Zeno's paradox of infinite halves:
"If you look between my fingers, the question is how many points are between there. And in some views of physics there's not just an infinite number of points, there's an uncountable infinity of points in between. I don't believe it. I believe there's got to be a discrete number of points. The same thing happens if I were to say one...two. How many points were between there and there? Was there an uncountable infinity of points? I don't think so. I think there was a discrete number of time events between that point and that point."
So, what is time?
"Change," says Markopoulou-Kalamara.
"I would have to prepare a two-semester course," says Heller.
Ellis: "It's what clocks measure."
Hockenberry closed with "The coming of wisdom with time," by W.B. Yeats:
Though leaves are many, the root is one;
Through all the lying days of my youth
I swayed my leaves and flowers in the sun;
Now I may wither into the truth.