But I think there was a certain kind of youthful exuberance that took hold when the theory was in its early infancy in the 1980s and early ’90s and so forth, which perhaps was a little bit unfounded, because it was such an immature theory that you really couldn’t make pronouncements about it that you could have any real faith in. I personally—as do, I think, many string theorists—view string theory as a possible next step toward a deeper understanding of the laws of physics. It could be the final step; we can’t judge yet.
But I think the most sober way of looking at it is that we have quantum mechanics, we have general relativity, we have to put them together in some consistent way. String theory is a possible way of doing that, and therefore we should explore it and see where it goes. I think it would be unfortunate if simply by virtue of its being advertised within a certain framework of it being the final theory, one then judges it differently from any other scientific theory, which is on its merits.
STEINHARDT: That’s a stupendous retreat from what many people have claimed.
GREENE: You really think so?
STEINHARDT: Yes, sure. And it’s worse than that. Some people even claim this idea that you have this googol, or perhaps infinite, number of possibilities is something we should come to accept—that it’s now derived from string theory, that string theory should be accepted as true, and since it has led to this multiplicity of possibilites, we should all accept this conclusion as true.
GREENE: Naturally, scientists quite generally, and string theorists in particular, often describe their work without giving all of the associated qualifications all of the time. I, for example, have spoken of string theory as a possible final theory, as the possible theory that would unite all forces and all matter in one consistent framework. And I generally try to say—but perhaps not always—that this is not yet a proven theory; this is our hope for what it will achieve. We aren’t certain that this is where it’s going to lead. We just need to explore and see where we land.
Similarly, I think that if you sat down and spoke to the folks you’re referring to in a more informal setting—who talk about having all of these different universes emerge from string theory and about how it’s a new framework that we have to think about things, and in which we are one of many universes—they would say, “Yes, what we really mean is, this is the place string theory seems to have led us, so we want to explore it.” Is it necessarily the framework? I think most of them would say, “We don’t know; we’re just shooting in the dark, because this is our best approach to unifying general relativity and quantum mechanics, and we’re going to explore where it leads us.” They’re not necessarily saying that this is definitely where it goes, because that’s the nature necessarily of research: You don’t know where it’s going to lead, you just keep on going and see where it takes you.
STEINHARDT: But what angers people is even the idea that you might accept that possibility—that the ultimate theory has this googol of possibilities for the laws of physics. That should not be accepted. That should be regarded as an out-and-out failure, requiring some saving idea. The fact is that everywhere we look in the universe, we see only one set of laws. Also, the universe is smooth and uniform, smoother and more uniform than we need for humans to exist. Yet we are asked to accept the idea that the greater universe, beyond where we can see, is completely different. Is that science or is that metaphysics?
ISAACSON: That’s exactly it. We were talking about why it is that it arouses such passion and then started directly debating string theory. I’d love to take it right back to Einstein. Twice you said something that I find very interesting, which is, we have to find a way to make his two grand pillars of 20th-century physics compatible, general relativity and quantum theory. Of course Einstein totally would believe that, because he loved unification, he loved unity. Secondly, he and Newton agreed on one big thing, which is that nature loves simplicity. But I’ve always wondered about the more metaphysical philosophical question: How do we know that God likes simplicity? How do we know he wants these things to be compatible? How do we know that quantum theory and relativity have to be reconcilable?
GREENE: There are actually some people who suggest that they don’t necessarily have to be compatible. I’ve never really been convinced by their arguments at all. To me, it seems evident that the laws that we’re talking about, and quantum theory in particular, are not meant just to describe small things—that’s where it was developed and that’s where its unusual features manifest themselves more strongly—but quantum theory is meant to be a theory that applies everywhere, on all scales.
General relativity starts as a theory that describes big things because that’s where gravity matters, but when you look at the equations of general relativity, in principle they can apply on arbitrarily small scales. The thing is, when you get to really tiny scales, you notice that there’s a deep incompatibility between the two theories, and moreover you realize that there are realms of the universe that enter those domains. You have, for instance, a black hole, which you can say begins as a star that then exhausts its nuclear fuel, collapses under its own weight, gets smaller and smaller—at some point the star gets so small that quantum mechanics really starts to matter in a significant way. Gravity matters the whole time, because it’s so heavy. If those two theories don’t work together, how do you describe what happens to this collapsing star?
ISAACSON: That’s true of the Big Bang as well?
STEINHARDT: Yes, and that’s why cosmology is the key battleground for trying to sort out how quantum physics and gravity relate. You can’t avoid using them both to understand where the universe emerged from, or whether it had a beginning, or what happened before the Bang.
ISAACSON: And it’s impossible to imagine a cosmos in which those two theories aren’t in some fundamental way totally reconcilable?
STEINHARDT: It would be a mistake. It would be inconsistency.
GREENE: Although Freeman Dyson seems to have unusual views on this.
STEINHARDT: Yes, and I also would say I don’t understand them—
ISAACSON: And that’s where I got my question, but I tried to avoid Freeman Dyson because I was afraid I would totally misunderstand even his question.
STEINHARDT: But you ask a good question: “And how do we know?” The answer is, we don’t know that things have to be simple. But a couple of interesting things have happened historically as we have followed that line of reasoning. We’ve managed to push the program of understanding the universe to small scales and large scales, by pursuing this approach of looking for simplicity. Particularly when we look at the cosmos. Now that we can see out to the farthest observable edges of the cosmos, we can see that the laws of physics are the same and that the physical conditions are also remarkably similar throughout the observable universe.
Until we have hard evidence to the contrary, I think we push this program of looking for unification and simplicity until it takes us as far as we can go. I have no sort of moral principles about the scientific method; I simply think it’s the most efficient method humans have found yet of taking what we know and adding new knowledge. If we think up some other program of thinking that does better, we should adopt it, but at the moment—
ISAACSON: So it’s a fundamental part of the program of thinking that the laws are unified at some level, and that we’ll eventually get to more simplicity, not more Byzantine complexity, in the laws of nature?
GREENE: So long as you’re willing to adjust the measure of simplicity and complexity as you learn more and more about the universe. If you were to present quantum mechanics to Newton, at first it might seem fairly complicated, because it uses a completely different body of mathematics, different kinds of ideas, invokes concepts that you can’t directly see, and that certainly feels like it’s a layer of complexity. But when you study it for many decades and you become used to its unusual features, you look at it and you see that it is just one simple little equation—Schrödinger’s equation.
; From a pure mathematical standpoint, it’s a linear equation—in technical terms, the simplest kind of equation to analyze—and it describes data. So your sense of what’s complicated and simple now gets shifted by a layer, because you’re judging this framework, which to a 16th- or 17th-century scientist might seem really bizarre, but from a modern perspective it works and you attune your aesthetic sense so that it actually feels pretty elegant, and pretty simple.
ISAACSON: You use the word “elegant” often, which—
GREENE: It’s become hackneyed, but if a theory is so simple that its deep equation can be put across a T-shirt in 20-point type, then we generally view that as fairly simple. Certainly that’s the case with both general relativity and quantum mechanics.
ISAACSON: There’s a wonderful book that Einstein wrote called The Evolution of Physics with Leopold Infeld, in 1938, which is not easy to find. I’ve gone over it again two or three times, because I just love the way it was written. It was written to make money for both of them, because it’s the ’30s, and Hitler, and refugees and stuff. It’s a popular book, but it has a deep philosophical argument, and the publisher is reissuing the book because I was pushing them to get it out there.
The deep philosophical argument is that it will be a field theory approach that will work. It starts with Galileo. It talks about matter and particles and just makes the argument that in the end it’s all going to be reconciled through field theory. It’s about whether there’s going to be a great distinction between a field theory and a theory of matter.
GREENE: You can even take that question one step further, which is: Is there even a distinction between, say, a field theory which has been so successful and a string theory, which appears at first sight to invoke different ideas from what you’d get if you were just doing a purely field-theoretical approach?
One of the big ideas, one of the big results, in the last decade in string theory has been to find a close association, in fact an equivalence, between field theories and string theories. Even though string theory starts with a very different point of view and you can study it without it ever seeming to be a field theory, you realize—
ISAACSON: Isn’t the mathematics different, though?
GREENE: The mathematics appears different at first, but one of the amazing kinds of discoveries in string theory in the last decade has been something called “dualities,” which is, something can look one way and something else can look completely different, but if you study them with adequate intensity and adequate precision you can find that they’re actually the same thing, just described in different languages. Like a book in French and in Sanskrit—they don’t look the same, but if you have a dictionary that relates them, you can say, “Oh, this is the same book.” Similarly, we have string theory framed in the language that’s relevant to string theory—
ISAACSON: A mathematical language that’s non-field theory—
GREENE: A mathematical language that feels non-field theory, that looks non-field theory. And then you have a field theory framed in the language of quantum field theory, and they seem different, they look different, but a lot of work has been done to set up the dictionary that allows you to say, “This element of this is that element of that,” and vice versa, and you realize they’re actually talking about the same theory, just in a different language.
ISAACSON: Is that sort of a reflection of the basic duality at the heart of quantum mechanics?
GREENE: No, this is a different kind of duality—and Paul has a different view of this—because the dualities that you’re referring to sort of are inherent in each of these approaches, irrespective of the fact that they happen to be talking about the same theory. It’s a completely unexpected and deep relationship between them, but fundamentally shows that string theory isn’t that different from field theory; it’s field theory—it’s particular kinds of field theory—just organized in a different way, making it look different at first sight. But it basically confirms what you were saying from that book of seventy years ago—that field theory seems to be the tool that will take us to the next step.
STEINHARDT: I’m not sure how much faith I put in claims like that, because that’s basically talking about what mathematics you use. It would be equivalent to saying the explanation is going to involve calculus. And while it’s likely that field theory will be among the useful mathematical tools, it’s likely that we’re going to have to discover some new mathematics along the way to get to a final answer. I think most of us, Einstein included, tend to focus less on the tools and more on the underlying physical concepts.
I want to come back to one of the issues you raised—you asked about simplicity. I should have emphasized the following: It could very well have been that when we began to look out at the universe, we discovered different laws of physics, different gravitational forces, different electromagnetic forces, other bizarre differences from what we see nearby—curiously, the very things that some people believe string theory predicts. Then we would be convinced experimentally that we don’t live in a simple universe and that something like the stringy landscape picture is correct. We would also live in a universe which, due to its nonuniformity and our limited vantage point here on Earth, affords us no hope of understanding the universe in its entirety. Then the picture of a random universe would be compelling. The moment we begin to look into space, things would look so different from what we observe here—
ISAACSON: Einstein felt a little bit that way, I think, as quantum mechanics progressed in the late ’40s; he was an old guy, but he kept discovering more and more particles and more and more forces that he was not even willing to accommodate, as he stood there at his blackboard with Valentine Bargmann and Peter Bergmann and all of his assistants. But he was vaguely depressed by the fact that nature seemed to like more and more forces and particles to be discovered that were not reconcilable.
STEINHARDT: Although I would guess that Einstein would love the concept of string theory. Not all string theorists feel the same way, but I view it as the fulfillment of Einstein’s program of geometrizing the laws of physics. Einstein took gravity and turned it into wiggling Jell-O-like space, and now string theory turns everything in the universe, all forces, all constituents, into geometrical, vibrating, wiggling entities. String theory also uses the idea of higher dimensions, which is also something Einstein found appealing.
What I was commenting on earlier was where the string program has gone recently, which I described as a crash. I can’t say for sure how Einstein would view it, but I strongly suspect he would reject the idea. I read an interesting quote from Einstein—I think in the ’50s—in which he said that he had failed at constructing a unified theory and expressed his concern that it would be a very long time before there was any success. The reason, he claimed, which I thought was interesting, was because physicists no longer know about logic and philosophy. He didn’t mention mathematics but, rather, logic and philosophy.
ISAACSON: What he really felt was that he had become more and more of a confirmed realist, or a scientific realist, and he felt that it was a philosophical question—that there was an underlying physical reality independent of our observations of it and that’s what science was supposed to discover. And because that became so out of fashion with—he called them neo-positivists. You can call them whatever you want, but I assume most people in the forefront of quantum mechanics would not subscribe to the theory that there’s underlying physical reality independent of any observations of it, which is a philosophical pillar on which you build science. And that was his philosophical problem.
STEINHARDT: I interpret it differently. By that time, something that might be called an American attitude toward physics had taken over. It was an attitude where the connection between physics and philosophy was broken. You were supposed to focus on what was calculable: Take your theory, make predictions with it, calculate with it, test if your calculations are right using experiments, and just stay away from philosophical questions. With this ap
proach, the idea was that we can systematically inch our way forward in science.
ISAACSON: Yes, that’s a good point. He could certainly have meant that, because he believed that, too.
STEINHARDT: So that in fact the reason a unified theory from that point of view would be beyond our reach is because if you didn’t have deep philosophical principles to guide you, you just would never find your way. I thought that was an interesting quote, because it reflects some of my concerns about where both cosmology and fundamental physics are going—that maybe they’ve lost their way. Of course, it’s unfashionable to appeal to philosophical viewpoints. But maybe that would be a healthy thing.
GREENE: Hey, we’ve got a research group with a philosopher as part of our group, so—
ISAACSON: But you’ve got art, you’ve got music—
GREENE: No, I’m not referring to those undertakings at all. We have a group that’s trying to address questions such as the arrow of time. Where did the arrow of time come from? Why does time seem to unfold in one direction but not in reverse? This is a question that philosophers have studied intently for a long time, and they have refined the question in such a way that when we talk about various possible solutions, they’re able to see the solutions and say, “Well, wait a second, you’re actually assuming the answer in the solution in some hidden way that we’ve long since parsed out, and let me explain to you how that goes.”
We’ve found it very useful to talk to philosophers who perhaps haven’t studied quantum field theory with the kind of technical intensity a graduate student or a researcher in physics would have, but they’ve taken a step back and sort of looked at big questions and really thought them through at a fundamental level. And that’s extraordinarily helpful.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 25