Road to acceptance
NOVA: Is it an exciting time to be a string theorist?
Brian Greene: It's an amazing time to be a string theorist. The last few years have witnessed a tremendous amount of progress, so much so that I think no one in their wildest dreams would have imagined that we'd have gotten as far as we have.
Do you think string theory will ever be accepted as widely as, say, the theory of general relativity? What would it take for that to happen?
Well, the real reason why general relativity is widely accepted is because it made predictions that were borne out by experimental observations. The primary one that put general relativity on the map was its prediction of the bending of starlight by the sun, which in 1919 was confirmed by observation during a solar eclipse. That was the moment when general relativity emerged from the realm of theory and entered the realm of being a piece of reality as we know it.
For string theory to have the kind of acceptance of general relativity, it's got to do the same thing. It's got to make a prediction that is borne out by some experiment. And as yet, we haven't quite gotten to the stage where we can make definitive predictions which, if they're found, the theory was right, and if they're not found, the theory was wrong.
But we have gotten to the stage where we can make some rough predictions for things that might happen at the future accelerators that are now being built, in particular one in Geneva, Switzerland, called the Large Hadron Collider, which should be ready about 2007 or 2008. If some of the predictions that string theory says might happen are borne out through experiment at that accelerator, then I think it's quite possible that string theory would be as accepted as general relativity.
Can you give an example of a prediction that might be experimentally verified in coming years?
Sure. One of the strangest features of string theory is that it requires more than the three spatial dimensions that we see directly in the world around us. That sounds like science fiction, but it is an indisputable outcome of the mathematics of string theory. So the question is, where are these extra dimensions? One suggestion is that they're all around us, but they're small relative to the dimensions that we directly see and therefore are more difficult to detect.
What the theory also predicts—not necessarily but possibly—is that energy can escape from our known dimensions and leak into these extra dimensions under appropriate circumstances. Those appropriate circumstances might be generated in high-energy collisions that will take place at the new atom smasher, the Large Hadron Collider.
"I've spent something like 17 years working on a theory for which there is essentially no direct experimental support."
So it's possible that through these high-energy collisions we will find that there is less energy at the end of the collision than there was at the start. If the energy loss is of just the right sort, it could be very strong evidence that the energy has seeped off into these extra dimensions. If that were true, if that were the best explanation we could find, that would be strong evidence that the extra dimensions are real, and that in turn would be strong evidence that the framework of string theory is correct.
A super theory
What sort of experimental results that support string theory do you think would make even die-hard detractors like Sheldon Glashow stand up and take notice?
Well, one would be the one I just mentioned. The next, in fact, more likely experimental result in many people's minds to be found is something called supersymmetry. The full name of string theory is really superstring theory. The "super" stands for this feature called supersymmetry, which, without getting into any details, predicts that for every known particle in the world, there should be a partner particle, the so-called supersymmetric partner. So for the electron, there should be the supersymmetric partner electron, or the selectron, as people call it. For quarks, there should be supersymmetric quarks, or squarks. For neutrinos, sneutrinos, and so forth.
So there's this whole spectrum of sparticles, if you will, that the theory predicts should exist, but as yet nobody has ever seen any one of these partner particles. We believe the reason is because they're much heavier than the known particles, and the heavier something is, the more energy, the more oomph it takes to produce it in a collision. We hope that these new atom smashers, the Large Hadron Collider in particular, will have enough energy to start producing these particles that the theory predicts.
Now, if those particles are found, it will not prove string theory correct. It will prove this supersymmetric aspect of the theory to be correct. And it turns out that string theory is not the only theory that allows for supersymmetry, so this isn't necessarily a smoking gun. But it would be a strong piece of circumstantial evidence that string theory is on the right track. So in many ways, I consider that to be the most likely experiment to have positive outcomes and to at least take us this particular distance down the path toward confirming string theory.
Have you ever had doubts about string theory?
All the time! I mean, it is a very strange research career, in a way. So far I've spent something like 17 years working on a theory for which there is essentially no direct experimental support. It's a very precarious way to live and to work.
The funny thing is, I sometimes get the impression that some people outside of the field think that there's some element of security that we have in working on a theory that hasn't made any predictions that can be proven false. In a sense, we're working on something unfalsifiable. And there sometimes is a sense that we're happy about that. But let me state categorically, if the theory is wrong, I'd like to know it today so I wouldn't waste my time on it any longer.
We will have no certainty that it's right until the experiments show that it's right. However, I should say that in my mind there is a strong circumstantial case already that it's correct, because it puts together general relativity and quantum mechanics, and each of those theories has already received a fantastic amount of experimental support. String theory is the most developed theory with the capacity to unite general relativity and quantum mechanics in a consistent manner. I do believe the universe is consistent, and therefore I do believe that general relativity and quantum mechanics should be put together in a manner that makes sense. That's what string theory does, and to me, that's pretty convincing.
Limits to understanding
Is there any way you can make people who know little about mathematics understand the supreme elegance of string theory?
I think so. You know, when we talk about theories of physics being elegant, what we often mean is that a theory is able to explain a wide range of phenomena using a very small number of powerful ideas. The elegance comes from the tremendous reach of these few simple ideas.
"No matter how hard you try to teach your cat general relativity, you're going to fail."
And that really is a core characteristic of string theory. We have this idea that the basic constituents of nature are these vibrating strings, that their vibrational patterns dictate the properties of particles, and they dictate the kinds of forces at work in the world. If the theory is right, that simple notion will perhaps be able to explain, in principle, every physical phenomenon. That powerful reach is where the elegance resides.
Do you think there are limits to how much we can know about the universe?
I don't know. I'd like to think that there aren't, but I suspect that's a little optimistic. An analogy that's used in the NOVA program that I'm quite fond of is: We are certainly aware of intelligent beings on this planet whose capacity to understand the deep laws of the universe is limited. No matter how hard you try to teach your cat general relativity, you're going to fail. There we have an example of an intelligent living being that will never know this kind of truth about the way the world is put together. Why in the world should we be any different? We can certainly go further than cats, but why should it be that our brains are somehow so suited to the universe that our brains will be able to understand the deepest workings?
Well, for example, most people have trouble envisioning a fourth spatial dimension. Can you?
No. I cannot envision anything beyond three dimensions. What I can do is I can make use of mathematics that describe those extra dimensions, and then I can try to translate what the mathematics tells me into lower dimensional analogies that help me gain a picture of what the math has told me. But the picture is certainly inadequate to the task of fully describing what's going on, because it's in lower dimensions, and in higher dimensions, things are definitely different.
To tell you the truth, I've never met anybody who can envision more than three dimensions. There are some who claim they can, and maybe they can; it's hard to say. But it's very hard, when your brain is involved in a world that appears to have three dimensions and is well suited to envisioning that world, to go beyond that and imagine more dimensions. [To give it a try, see Imagining Other Dimensions.]
I'm glad to know it wasn't just me. Getting back to strings, Ed Witten said in his interview that it's conceivable that the big bang could have produced a string so large that it would be present in today's universe and could be visible in telescopes. Do you agree, and if so, where would we look for it, and what might it look like?
I do agree that it's a possibility. But as I think Ed Witten would agree, it's a fairly unlikely scenario. It's not, in anybody's mind, the likely way that we'll confirm the theory. But it is possible that the huge energies present at the time of the big bang could have produced a big string. The bigger a string is, the more energy it takes to create it. And you could have produced one that somehow hung around.
How would you see it? Well, if it floated by your telescope, or if it floated in the fields of vision of your satellite that's observing the heavens, it could create, for instance, a disturbance to the microwave background radiation. It could change the temperature of that radiation along a linear defect, sort of like if you look at your reflection in a cracked mirror. You look fine on one side of the crack. You look fine on the other side of the crack. But at the crack itself, things will be kind of dislocated. A strong wafting by a telescope would kind of do a subtle version of that to the image.
How is it that fundamental strings, which are by definition extremely tiny, could come in a size many orders of magnitude larger than normal?
When we say strings are tiny, we generally mean strings that haven't had a huge amount of energy pumped into them. But the more energy you pump into a string, the more widely it will vibrate and, at some point, the longer it will grow. So it's really a question of how much energy the string embodies. Typically they don't embody that much, and therefore typically they are very small.
How does it feel to make a major discovery, like the time you and two colleagues at the Institute for Advanced Study realized that the fabric of space can tear and reform in a new way?
Well, there's nothing like it really. For a brief moment, you feel like you have seen the universe in a way that nobody previously has. You feel like you are privy to a deep truth of nature that no one has been aware of before. It's a wonderful and remarkable feeling. It's not one that I think one can expect to have occur many times in one's career. You do often discover things that are useful and important, but I think it's a rare privilege to find something fairly fundamental and really get a sense of connection to the universe that's hard to achieve in any other way.
Do you have a most exciting moment from your work with string theory, a sort of "eureka" moment?
That certainly was one. Another one was when I was working with a Harvard professor, Ronen Plesser—he's now at Duke—and we came upon something called mirror symmetry, which was somewhat unexpected. Using Einstein's general relativity, we found basically that the shape of the universe could differ in rather dramatic ways, but yet the physics that we would see wouldn't bear any imprint of that different geometry.
"If the theory turns out to be right, that will be tremendously thick and tasty icing on the cake."
This is a very unusual thing, because Einstein taught us that geometry and physics are very tightly woven together. We found that sometimes they're not as tightly woven as he might have thought. That was another eureka moment, in the sense of really feeling that we had uncovered something unexpected.
What advice would you have for an aspiring string theorist? Go for it, or for God's sake stay away?
I think ultimately you've got to follow your heart in these matters, and if these are the kinds of ideas and questions that are burning within you, and you just can't imagine not having them at the forefront of what you do in day-to-day work, then yeah, you've got to go for it. On the other hand, this is a very speculative field, and it could turn out to all be wrong. And if that's the case, and you would feel, after putting years of research into the subject, that those years were wasted because the theory was wrong, then it's probably not the right field for you.
I and many others, however, would not feel it had been a waste of time if the theory turns out to be wrong, because we've developed a lot of important mathematics. We've developed connections to other, more well established areas of physics, which I think will be important in their own right. We will have done some very valuable work. To me, if the theory turns out to be right, that will be tremendously thick and tasty icing on the cake, but without that icing, to me the work will still have been incredibly interesting and useful.
Getting the word out
What was it like making a documentary based on your book?
It was a lot of fun in many ways, and it was very hard in others. It takes a great deal of work to distill the essence of the ideas and find the right way to communicate them visually that's both entertaining and accurate. Sometimes there can be a tension between wanting something to be more entertaining yet having to be well aware of what that might be doing to the accuracy of the science. For me, it was constantly keeping a watch out to make sure that the science ultimately was dictating what we could and couldn't do.
You've published a bestselling book. You've acted in musicals. You've won judo tournaments. You've even appeared on Late Night with Conan O'Brien. Do such pursuits bring new perspective to your work as a theoretical physicist, or are they just welcome relief from the mental grind?
Well, certainly they're welcome relief. But my feeling is that the creative impulse, and the creative juices, are affected by everything in the life that you lead, sometimes in fairly intangible ways. When writing the book, for instance, I found myself—again, similar to working on the NOVA program—forcing myself to try to see to the heart of the science, so that I could communicate it in a way that would strip away the technical details that people don't really want to hear much about, especially if one isn't trained in these ideas. That helped me to get a clearer overview of where the field was and where it was going and what the important questions are. In that way, it did help set me on a particular course of scientific research that I'm now engaged in. I'm not sure if I'd be doing exactly what I'm doing right now were it not for writing The Elegant Universe.
You're just finishing up a new book. What's that about?
That book is about space and time. The Elegant Universe was about the search for the unified theory, and space and time were supporting characters in that story. In this new book, space and time are the main characters. It's really a discussion of our ever-changing grasp of what these seemingly simple notions of space and time actually are.
"A unified theory would put us at the doorstep of a vast universe of things that we could finally explore with precision."
You mentioned unified field theory. If string theory does lead to the so-called "theory of everything"—I know you don't like that term much—where would theoretical physicists go from there?
Well, I think an analogy that I believe Richard Feynman once used is probably the best one to explain where we would be. If you are learning the game of chess, the first thing you have to do is learn the rules. But after you've learned the rules, the game of chess for you is not over. It's just beginning, because now you can apply those rules to play all sorts of wonderful games that involve all kinds of strategy and allow you to explore the richness of that universe.
Similarly, if we did finally have the unified theory, if we did finally have the deep laws of the universe in hand, that in a very real sense would also be a beginning. It would be the beginning of our quest to use that deep understanding to fully explore this universe, to fully understand black holes and stars and galaxies and even the big bang, to fully understand how things got to be the way they are. So in many ways, it would just be the start. A unified theory would put us at the doorstep of a vast universe of things that we could finally explore with precision.
This feature originally appeared on the site for the NOVA program The Elegant Universe.