David Gross shared the 2004 Nobel Prize in Physics and is the director of the Kavli Institute for Theoretical Physics at the University of California at Santa Barbara. Gross is one of the leading lights of string theory, and yet he is the first to admit that physicists really don't know what string theory is. "It's as if we've stumbled in the dark into what we thought was a two-bedroom apartment and now we're discovering is a 19-room mansion," he says. "At least. Maybe it's got a thousand rooms, and we're just beginning our journey." To continue any distance on that journey, much less complete it, Gross forsees the need for an utter break with conventional physics—a break he thinks will change our very understanding of space and time.
Note: For a definition of unfamiliar terms, see our glossary.
A boost for the big bang
NOVA: So why should anybody care about string theory?
Gross: Well, for one thing, because it's attempting to answer the "why" questions that children ask. Why is the sky blue? Well, we sort of know why the sky is blue, but if we really pursue that question down to the structure of atoms and the quarks, electrons, and laws that hold the atoms together, we end up with theories, including string theory, that attempt to understand those questions.
Another reason that we have to understand what makes up the physical universe at its most fundamental level is that we need that information to figure out the history of the universe. To understand the universe in the state that it began in, the so-called big bang, we need laws of physics that work better than our current set of rules and procedures, which break down when we try to push them back to the beginning.
NOVA: What are the limitations of the big bang theory at present?
Gross: The big bang theory is the idea that if we go back early enough in the history of the universe—and we can do this, of course, by looking at starlight coming to us from billions of years ago—we will see a very hot and dense period where the universe was much smaller, denser, and hotter. And that explosion or hot state left remnants that we can observe today in the microwave background. So we know that that aspect of the theory is true.
If we push back even farther, that hotter or denser state becomes even hotter and denser. And if we extrapolate using Einstein's theory of general relativity, we find total disaster. That is, we find a singularity, in which the forces that act on particles become infinitely strong. Things break down completely, and the theory no longer makes sense.
Our conclusion is not that the universe doesn't make sense, but that the equations are wrong. They're applicable maybe at later times, but they're not applicable at the beginning of the universe. So we desperately need something like string theory appears to be—a theory that is consistent.
NOVA: Basically you need a theory that's going to work under those conditions.
Gross: Yes, and one that can provide an answer to questions that with our present theories we can't dream of answering, such as: How does the universe begin? What starts it off? What is the state of the universe at the beginning? Is that unique or arbitrary? Who fixes the initial conditions? There are questions that can't be answered within the standard theory. We're not sure that string theory, or any theory, could provide answers to the beginning of the universe, but it's a goal that many of us are desperate to try to reach.
NOVA: And string theory can help provide these answers?
Gross: String theory is the best hope at the present. And the questions are some of the most interesting questions that we can ask. All of us would like to know: Is the universe arbitrary? Could it have been otherwise than it is? Could there have been different laws, different constants of nature? Has it begun only once, or is it cyclic? Was there beginning to time? Does time have any meaning before it began? These are wonderful questions, and we're now in a position to address them. We have no guarantee that we will find the answers, of course, but the effort is as important as finding the answer.
Breaking with the past
NOVA: Does string theory constitute a revolution in physics?
Gross: In string theory I think we're in sort of a pre-revolutionary stage. We have hit upon, somewhat accidentally, an incredible theoretical structure, many of whose consequences we've worked out, many of which we're working out, which we can use to explore new questions. But we still haven't made a very radical break with conventional physics. We've replaced particles with strings—that in a sense is the most revolutionary aspect of the theory. But all of the other concepts of physics have been left untouched—a safe thing to do if you're making changes.
“This revolution will likely change the way we think about space and time, maybe even eliminate them completely as a basis for our description of reality.”
On the other hand, many of us believe that that will be insufficient to realize the final goals of string theory, or even to truly understand what the theory is, what its basic principles are. That at some point, a much more drastic revolution or discontinuity in our system of beliefs will be required. And that this revolution will likely change the way we think about space and time, maybe even eliminate them completely as a basis for our description of reality—that is, leave us regarding them rather as emergent approximate concepts that are useful under certain circumstances. That is an extraordinarily difficult change to imagine, especially if we somehow change what we mean by time, and is probably one of the reasons why we're still so far from a true understanding of what string theory is.
NOVA: What would this sea change entail?
Gross: In order to achieve a true understanding of string theory, some new idea will be required, and most likely, some break with the concepts on which we've traditionally based physical theory. This has happened before. In the last century, there were two such revolutions having to do with relativity and with the quantum theory, which was an incredible break with the classical notions of physics. Those revolutions were achieved in the end by discontinuous jumps that broke completely with the past in certain respects. It's not too hard to predict that such a discontinuity is needed in string theory.
What's harder to predict is what kind of discontinuity is needed. Discontinuity jumps like that—revolutions—are impossible to predict. They require some totally new idea. A lot of us are waiting for such a new idea that will give us an alternate to our traditional notion of space and time perhaps—or perhaps some other new idea. Something is missing that is most likely not just another technical development, another improvement here or there, but something that truly breaks with the past. And all the indications are that it has to do with the nature of space and time.
NOVA: When you say physicists don't understand string theory, what do you mean?
Gross: One of the strangest aspects of where we are in string theory after 35 years is that we don't really know what string theory is. There are all these people working on string theory and doing wonderful things, sometimes answering old problems, sometimes coming up with new scenarios. But if you really ask them, "What is string theory?" they'll give you a glib remark, a glib description, and describe certain of its aspects. If you ask them again, "What is string theory?" if they're honest they'll say, "Well, we don't know." We have this incredibly powerful set of tools and methods that describe this intellectual structure, and yet we really don't know what lies at the core of that, what the unifying principles are, what the theory actually is that has all of these different aspects that we can partially describe.
Benefits of string theory
NOVA: If string theory is right, will there be technological or other benefits to society?
Gross: Well, there's a certain advantage to working in a field where all conceivable applications—good or bad—are so far in the future that you don't have to worry about it. At the moment it is inconceivable that any of the advances that we will make in understanding the laws of physics in this domain could have any application.
However, it was inconceivable in the past that the laws of quantum mechanics would have any applications, and now they totally dominate our technology. So while I can't imagine any practical applications, we're notoriously bad at imagining such things.
NOVA: There would be intangible benefits, of course, just in improved knowledge of the workings of the universe.
Gross: Sure. The main reason why people should care about research in fundamental physics is the same reason they care about astronomy and cosmology. People, children, want to know what we're made out of, how it works, and why the universe is the way it is. That has driven the human race forever, that curiosity to know. We can't turn it off. We can't stop people from asking these questions. And what I have found is that everyone, if you can explain string theory to them, is absolutely fascinated by the attempts to answer these questions and even more so by the answers when they come.
NOVA: But it's pure theory.
Gross: It's pure theory in a way that physics has rarely evolved, partly because in the past there were experimenters around who were able to provide important information. It was often a race between the theorists and the experimenters as to who could come up with a clue first, and experimenters usually won that race. Nature is a lot cleverer than we are, and if you can steal her secrets by observation, it's a lot quicker than trying to re-invent them.
In this case, however, when we're probing the structure of the world at extraordinarily high energies, we can't yet build instruments that will do that directly. So we don't really have any choice but to rely on theory. And in my opinion we haven't by any means yet exhausted the bag of new ideas that can emerge from this theory, which is incredibly rich and has told us already that there are things that nobody would have ever imagined are conceivable, such as extra-big dimensions of space. Yet it's possible.
NOVA: What would it mean to you if you heard that they had found experimental evidence for supersymmetry, one of the key predictions of string theory?
Gross: Well, I'd win a lot of bets—I'd collect a few cases of wine! No, it would be an enormously important thing. From the point of view of string theory alone, it would be a real affirmation that nature has taken advantage of a beautiful structure. You know, Einstein was once asked about some experiments that contradicted his theory of relativity. Someone asked him, "So Professor Einstein, what would you say if these turned out to be true and your theory of relativity was wrong?" And Einstein said, "Oh, I would have been very disappointed that God didn't take advantage of this beautiful idea."
“Finding evidence of supersymmetry would be one of the most important discoveries of all time.”
I think we would be very disappointed if nature didn't take advantage of this gorgeous property, this gorgeous symmetry. But to a string theorist it would be enormously re-affirming because supersymmetry is really at the heart of string theory in ways that we're still exploring. It's also very useful for the more traditional particle physics. It solves many of the problems that were encountered in the standard model. It explains why gravity is so weak, why stars have so many protons. It explains this incredible hierarchy of scales in the universe.
If it were discovered, it would be important for ordinary particle physics, to confirm the explanation of these particles. It would give us, perhaps, the missing matter in the universe, the dark matter that constitutes 30 percent of the universe. One of the best possible candidates for this dark matter is supersymmetric particles. So we need to find them in the lab. Then we can see if they are the same ones that are throughout the universe. For all these reasons, finding evidence of supersymmetry would be one of the most important discoveries of all time. And we expect it to be discovered, of course.
NOVA: How far are we along the road to fully understanding string theory?
Gross: String theory could be right and totally incomplete. It's as if we've stumbled in the dark into what we thought was a two-bedroom apartment and now we're discovering is a 19-room mansion. At least. Maybe it's got a thousand rooms, and we're just beginning our journey. It's very hard to tell. Often in physics, you have an experiment, which is a great crutch and a great help because it at least tells you what the problem is and where it ends. All you have to do is get there.
But when you're exploring a theory of this type and its implications are most evident in the early history of the universe, about which we have very little direct evidence, we're not sure where the story ends and how much farther we have to go. It is quite unknown.
NOVA: With a total lack of experimental evidence, what gives you faith that you are heading in the right direction?
Gross: Why do we still have faith in this theory, which we can't yet truly test and which hasn't yet succeeded in calculating anything? Well, partly, it is its incredible intellectual structure, which continues to develop in a consistent and increasingly powerful way. The fact that it generates interesting mathematics is very exciting to some. That doesn't have that much of an impact on me, but the fact that it has this conceptual structure does.
The fact that it has begun to address some questions that have been around for 70 or 80 years and reconciles relativity and quantum mechanics convinces me that it is on the right track. Unless you have some faith, you're not going to stay in this kind of speculative field.
NOVA: And you've got the faith.
Gross: Yes. Six out of seven days of the week!
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