Here's how Amanda Peet describes her career to other physicists: "I began with aspects of the black hole information problem, moved to entropy computations when D-brane physics arrived, developed aspects of holographic gravity/gauge dualities useful to black holes and black branes, and most recently concentrated on the search for string theoretic mechanisms for resolution of classical spacetime singularities." Fortunately, New Zealander Peet is equally conversant speaking in a language that the rest of us can understand. In this engaging interview, the University of Toronto string theorist discusses the implications of string theory for questions of cosmology, everything from what took place 15 billion years ago at the birth of the universe to what takes place today in the heart of black holes.
Note: For a definition of unfamiliar terms, see our glossary.
String theory as unifier
NOVA: Is it possible to describe string theory in a nutshell?
Peet: The central idea of string theory is actually remarkably simple. The details are, of course, complicated, but the fundamental idea behind string theory is really very easy to explain. It is that the fundamental constituents of everything—matter, like electrons, and the forces—are essentially tiny, tiny vibrating strings. So it's as if there's this tiny little rubber band that's inside all of the molecules in your body and so forth, and that explains everything, at least in the string approach.
So why introduce this complexity? What's wrong with just saying that the fundamental constituents of everything are tiny little point particles? That turns out to not be able to handle what happens when you get gravity and quantum mechanics together. If you like, you can think of strings as the aboriginal stuff, the original stuff out of which everything else is made.
One of the central ideas of string theory is that one object, which is this little vibrating string, can give rise to so many different particles (or at least what you see as particles). It's a little bit like a violin string that, when plucked in different ways, can produce different notes. And you can make music with it. The idea is that this aboriginal string can vibrate in different ways and look to you like different particles, both matter and force-carrying particles.
NOVA: Why are physicists so interested in a theory that explains everything, in so-called unification?
Peet: Unifying things can be based on various motivations. I guess one of them is aesthetics, or an attempt to simplify the complexity that we see out there in the universe. It's a top-down motivation. It's as if we want to derive the one thing from which we'll be able to derive all of the forces and all of the different particles.
There are also what you might call some bottom-up motivations for unification, and one of them comes from looking at the strength of forces that we see out there in the universe. One of the things we know about gravity, for example, is that its strength increases at high energies. As you crank up the energies of your accelerator, or look at things back in the big bang, it's clear that gravity is stronger at higher energies. The electromagnetic and weak forces also get stronger at high energies. Yet we also know that the strong nuclear force, the one that holds protons and neutrons together inside the nucleus, gets weaker at high energies. So this is something that we can measure directly in the lab, mostly in particle accelerators.
“What you finally get down to is this little aboriginal string, and that’s it. There are no further layers of the onion to try to take off.”
It goes against something that a lot of people get taught in high school physics, or Physics 101, which is that there are constants that describe the strength of the forces. But a combination of relativity and quantum effects makes these forces vary. If you sit down and draw a graph extrapolating what you know much further out than you can measure, you discover that it looks like the forces might unify together at an extremely high energy.
Approaching the big bang
NOVA: Are those the conditions that existed in the early universe?
Peet: There are two ways to probe high energies. One of them is to try to crank up the energies of your particle accelerators, and another one is the quintessential high-energy experiment, which is the birth of the universe. The energies that we're talking about for unification of the strength of the forces, we can't get anywhere near that in the lab, even with our biggest, most expensive particle accelerators. So it would have been back at the birth of the universe.
NOVA: If we were to wind a cosmic film of the expansion of the universe backwards towards the big bang, we would get back to a point that has no size. What happens in string theory at that point?
Peet: If there is a minimum distance that makes sense in string theory, then any distances that are smaller than that, well, that just isn't even there in the theory. The way I like to put it is, the buck stops at string theory. It's as if you keep peeling off the layers of the onion to find more and more substructure inside your particles, or whatever you're interested in. What you finally get down to is this little aboriginal string, and that's it. There are no further layers of the onion to try to take off to see what's smaller than that. It doesn't make sense to even discuss distances that are smaller than that string scale. [To grasp just how small strings really are, see A Sense of Scale.]
NOVA: What does that do for cosmology?
Peet: From the point of view of cosmology, there are two ideas that might describe the birth of the universe. One of them is the big bang. And what happens in the big bang is that before the big bang, there wasn't anything. And then somehow, there was a very special thing that happened in which time began, and the universe began, and then the universe got bigger. What string theory tells us about that big bang origin of the universe is that it started in a less singular way than we used to think. It's as if there is no pointy, horrible quantum mechanical behavior at the beginning of the universe. It had to come from smoother, softer behavior at those extremely high energy scales. That's what strings do for you that particles can't do for you. So what it would do is it would change the nature of the big bang.
NOVA: Why do point particles give pointy, horrible behavior?
Peet: If you just had point particles as your basic structure of everything, then right at the beginning of the universe, everything would have been compressed down to some incredibly tiny—essentially infinitely tiny—distance scale, and the temperature would have been essentially infinite, and that's one of the reasons why it was difficult to describe the birth of the universe within a theory that has only particles and doesn't have strings as the fundamental quantities. Because it seemed like you would have to deal with infinite things, and the trouble with infinity is that it's very difficult to calculate anything.
What that stringy minimum distance phenomenon does for you is to soften out that infinite behavior into something that's finite. It would say that everything was crunched down to a very small size, but it wasn't infinitely small. And the temperature would have been very big, but not infinitely big. And so it provides you with a hope of actually calculating what happened at the beginning of the universe.
Another possibility for the birth of the universe is what you might call the big bounce. The idea is that, previous to that cataclysmic event that we call the big bang, there might have been a universe that was bigger. So perhaps what happened is that the universe was bigger, and then it got smaller, and then it went through this sort of fiery intermediate stage, out of which came another expanding universe. So you have a contraction, and then a very quantum mechanical, nasty intermediate stage, and then out of that comes the big bang—that's another possibility. It would be more accurate to call it the big bounce, really.
NOVA: There are things called "branes"—or slices through the higher dimensional world—that give rise to alternate scenarios for the big bang. How?
Peet: Suppose we had the following situation: Our world is just a three-dimensional plane inside a bigger dimensional space. And what happened to create that cataclysmic event that made our universe blow up and expand in this scenario is that there was another brane that was somewhere else in the universe, and it came happily along, and then all of a sudden it crashed into our brane, and it dumped a whole lot of energy onto our brane. This provides a way of explaining where the cataclysm came from. It happened because of a collision of branes.
NOVA: Do you think this is an idea that's viable, that's here to stay?
Peet: It's hard to say whether the colliding branes scenario will turn out to be the one that we'll all be using 40 years from now to explain the origin of the universe to our students. It's fair to say that before the advent of branes, we were pretty unsure about whether we could make string theory match onto the dominant theory at that time of how the universe began, which is called inflation. What inflation explains is how the universe got to be so big and so flat and so forth. And once the notion of branes came along, that gave us more ingredients in our stringy tool box, and it provides us with potential new ideas about explaining the origin of the universe.
Penetrating black holes
NOVA: Are there other cosmological phenomena besides the big bang that string theory can help explain better than other theories?
Peet: One of the most famous predictions of Einstein's theory that seems to be held up when we look with telescopes out into the universe is that there are black holes out there. These are objects that are big and heavy, and they have a surface surrounding them called an event horizon. It's very unfortunate if you fall through this event horizon, although it doesn't feel very unfortunate when you're falling through it. The bad stuff comes later. Because when you fall towards the center of the black hole, you get torn apart and crushed. That's really very disastrous.
“You can never prove that a theory of nature is correct. All you can prove is that it’s the best theory you have.”
At the center of the black hole is, in Einstein's theory at least, a place where there is a singularity. And what is a singularity? It's a place at which, essentially, everything blows up. If you were to ask me, right at the core of a black hole, "What's the probability that I would produce electrons and anti-electrons?", the answer would be infinity. You could ask me, "What is the force on someone who is unfortunate enough to fall through to the middle of the black hole, the very center, the core of the black hole?" And that would also be infinity. So in Einstein's theory, even if you put it together with quantum mechanical point particles, you've still got that problem of not being able to compute anything right in the middle of a black hole.
NOVA: How does string theory address that region?
Peet: String theory addresses problems of black holes in many different ways. One of them is that the extended nature of the string, the fact that it's not pointy—it's sort of a floppy, extended structure—that allows us to say things about what happens in the very center of a black hole. For certain classes of black holes that we can discuss theoretically, it has enabled us to resolve those singularities. What that means is we can provide you with a way of calculating what happens deep inside those special kinds of black holes.
Testing string theory
NOVA: What would it take, do you think, to prove or confirm that string theory is right?
Peet: You can never prove that a theory of nature is correct. All you can prove is that it's the best theory you have that satisfies your theoretical consistency and describes the real world to the accuracy that we can test it. I'm not sure if we'll ever know whether a particular theory is the truth, because physics is an operational science. What we do is experiments, and we check our theoretical predictions against our experimental results. Once we've come up with a theory that agrees with the experimental results, we then try to predict something new that we haven't measured before. That's the process by which we keep refining our theories of nature.
It is always easier to falsify a theory than to prove it's correct. String theory, as yet, can't be falsified, partly because it is such a big structure. It's got so many ideas in it and incorporates so many new concepts, like extra dimensions and supersymmetry and unification, that at the moment, string theory is a flexible enough structure that it cannot be falsified.
NOVA: Then how do you respond to critics who say, "This is just not testable. It's not science."
Peet: String theorists worry a lot about whether our approach to understanding all of the forces and unification is the right track to be following. I think the best justification at present is that it's really, by far, the best approach to trying to understand the quantum theory of gravity. It's certainly better than taking Einstein's general relativity theory and trying to kludge it together with the standard model]. Just taking general relativity and the standard model, which is a quantum theory, doesn't enable you to calculate anything in extreme regimes deep inside black holes or back at the origin of the universe.
So it's the best we've got. And if it turns out that a part of it is not really the right way to be proceeding, what we'll find is that we'll need to add extra ideas or change our attack somewhat. But as scientists, all we can really do is work with the best theory that we've got and keep refining it as the experimental data keep coming in.
NOVA: How do you examine string theory experimentally?
Peet: Since string theory is a theory of extremely high-energy physics, one of the concerns one might have as a string theorist is: how will we really be able to give this theory a good run for its money and really test it out? The traditional way that high-energy physicists have tested theories is to make predictions about what happens in particle accelerators, because that was the most direct way that we could really crank up to very high energies and test our theories. So we can expect string theory and its predictions for low-energy physics to be tested inside accelerators. That's one of the reasons why string theorists are waiting keenly for what's going to come out of Run II at Fermilab and what happens at the Large Hadron Collider when it's finally up and running. [Editor's note: Run II at Fermilab began in March 2001. The Large Hadron Collider is now under construction at CERN on the Franco-Swiss border near Geneva.]
But there is another way of trying to test predictions of string theory, or whatever else might be the final theory describing the highest-energy physics that we can imagine. What that takes advantage of is the fact that the highest energies ever were in the big bang. So it's natural to try to take advantage of that cosmic experiment, even though there was only one experiment.
One of the ways that you can test theories of cosmology is to look at the background radiation that's left behind from the big bang. This is called the cosmic microwave background. It's currently very cold radiation, but back when the big bang was happening, it was extremely hot. It's thought that what was happening back at the big bang, or somewhat later, could imprint on that radiation that then came to us, eventually, as this very cold radiation.
There have been some experiments done already in cosmology that are really producing fantastic data, providing very precise measurements of not only the background radiation itself, but the fluctuations in that background radiation. For example, maybe the temperature in that region of the sky is a little bit colder than the temperature over there. People have been very carefully analyzing those little differences in the background radiation all over the sky to try to tell us about what was happening back much closer to the big bang.
This is really the decade of cosmological data. There are fantastic experiments that have already happened, and there are even more precise ones that are going to be bringing in data very soon. We've already got, from cosmological data, some very interesting information about the universe and the stuff that's in it that has really shaken up the string theory community, in terms of the ways that we try to build models of the real world.
NOVA: What is it about string theory that is so captivating?
Peet: String theory is seductive for many reasons. One of the reasons is that it incorporates a number of really beautiful ideas in mathematics. That wasn't something that started with string theory. Einstein's theory, for example, incorporates some beautiful ideas of mathematics known as Riemannian geometry, and other ideas in particle theory have incorporated other beautiful ideas. But that's not the only reason why we work on string theory with such fervor. It's a theory that provides a way of understanding unification of forces and all the matter, and of trying to describe everything all in one go. What we're really about is trying to explain where we came from. Not from the point of view of biology, which is also an extremely interesting subject, but we're interested in how the universe was born, and why we ended up with all the particles that we have in nature—what explains all of the patterns that we see of different subatomic particles. And it's just a really sexy theory.
“We’re like excited little two-year-olds that are never satisfied to be told, ‘That's just because I say it is.’ We want to know why.”
Another part of it is that it incorporates so many of the different ideas that have gone before. String theory gives you back Einstein's theory of gravity at really low energies. It also incorporates quantum mechanics. It also incorporates some of the beautiful ideas from the theories of the weak and strong interactions. We can describe black holes. We hope that we can do cosmology of the early universe. So part of the reason that it is so sexy is that it's got all of these pieces in it. Our tool kit is big. We're not restricted to working with just a very small number of tools or Lego blocks. We've got a great variety of things that we can try to use, and that provides us with a lot of space in which to work—theory space, if you will.
NOVA: Why is beauty, or elegance, so important to physicists?
Peet: Oh, gosh. It's hard to say what elegance really means. Does the theory look beautiful? What do you mean by beautiful? Well, it could be that you've got the fewest number of ingredients that you have to put into your theory to explain nature. Or it could embody some beautiful mathematics, or a principle that explains something out there in the universe. And I suppose there's a lot of complexity in the particles and the forces. For example, we have what we call three generations of particles. There's an electron, and it has a heavier sister called the muon, and a heavier yet sister called the tau, and so forth.
If the theory that we have to use to try to explain all of these things is as complicated as the reality itself, then that doesn't give too much satisfaction, if you like. What we're looking for is a smaller set of principles that we can use to explain a broader set of phenomena.
NOVA: If string theory is right, what would it mean to the average person?
Peet: If string theory turns out to be the right way to describe the real world—and by right, we mean it's the best model we've got, and it fits all the data—that would provide great intellectual satisfaction. We've been wondering about the origin of the stuff that makes up nature since Greek times and earlier. We've always been on this quest to understand the fundamentals of where things came from. I think it's unlikely that there will be technological applications that will come out of string theory, just because it has to do with such extraordinarily high energies, or the equivalent, such tiny little distance scales, that we can't make in the lab. There will be an enormous intellectual satisfaction that we will have worked out how the universe was born, and how all the particles came about, and why we see the patterns that we see.
String theorists have been on a quest, and still are, to provide answers to the "why" questions of nature. We want to know why the muon particle is a bit over 200 times as heavy as the electron. We're like excited little two-year-olds that are never satisfied to be told, "That's just because I say it is." We want to know why.
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