Certain theoretical physicists have a way of describing the most incomprehensible subjects with great articulateness. Joe Lykken epitomizes that kind of physicist. Hearing him describe things like dark matter and superparticles and extra dimensions leaves you feeling that you almost understand them. But Lykken, who works at the Fermi National Accelerator Laboratory in Batavia, Illinois, doesn't stop there. Listen in as he explains the importance of each of these subjects to string theory.
Note: For a definition of unfamiliar terms, see our glossary.
Of strings, branes, and other dimensions
NOVA: What is so compelling about string theory to make you want to devote your career to it?
Lykken: I think when a lot of people go into theoretical physics, they're looking for the big answers to the big questions. String theory holds out the promise that we can really understand questions that you might not even have thought were scientific questions—questions about how the universe began, questions of why the universe is the way it is at the most fundamental level. The idea that a scientific theory that we already have in our hands could answer the most basic questions is extremely seductive. Of course, for this to really happen, string theory first of all has to be right, which we don't know, and then we have to be able to test it and understand it eventually in experiments.
String theory itself probably won't be understood even in my lifetime at the deeper level. But I do think that there are ideas coming out of string theory that we will test and we will confirm hopefully in experiments, and that's what I'm really hoping for. I want to see during my career that at least some of these big ideas coming out of string theory we'll actually get our hands on and see that they do happen in the real world.
NOVA: Your work hinges on one of those really big ideas—on so-called "branes." What are they?
Lykken: String theory is not just a theory of strings. It's also a theory of membrane-like objects that we call branes. You can think of a brane as being a slice through the higher dimensional world that string theory says exists. One thing you can ask is: Is it possible that perhaps the particles that we're made out of and the forces that hold us together might actually be living on one of these branes? That in a sense we might be trapped on one of these walls on a slice in the higher dimensional world of string theory? This idea is known as the brane-world hypothesis. It's not something that we know how to predict rigorously from string theory, but it seems to be consistent with the ideas that are contained in string theory.
“It’s a very old idea that extra dimensions might exist. It goes back way before string theory.”
It's a very powerful idea, because if it's right it means that our whole picture of the universe is clouded by the fact that we're trapped on just a tiny slice of the higher dimensional universe. It's sort of like what Copernicus did when he said the Earth is not a special place in the universe; we just happen to live in some weird little corner. If the brane world idea is right, the same thing happened to us. It just happens that the kind of stuff we're made out of is trapped on a brane, trapped on a slice in a much bigger, higher dimensional universe. There may be many of these extra dimensions. String theory tells us that there should be six or seven extra dimensions. They could be very large, they could even be infinitely large, and the reason we don't see them is because we can't move off of our brane, off of our wall in the extra dimensional space. [To understand why it's hard to envision additional dimensions, see Imagining Other Dimensions.]
NOVA: For a long time we thought that extra dimensions had to be tiny and curled up, but that's changed. Can you tell me about that?
Lykken: It's a very old idea that extra dimensions might exist. It goes back way before string theory. The explanation that people offered for why those extra dimensions were not seen is that they were very small; they were curled up into tiny circles or other kinds of small spaces. And that idea persisted until about five years ago, when it was realized that that was not the only way to hide an extra dimension. In addition to making it small and thus hard to see, you could also hide it by just having the brane world idea that says that ordinary matter, for reasons that we might understand from string theory, just can't move in extra dimensions. Other kinds of matter can move in extra dimensions. So extra dimensions aren't completely hidden—gravity in particular might be able to "see" into them.
The special force
NOVA: What's special about gravity and gravitons in string theory that allows them to "see" into this extra dimension or even move into it?
Lykken: Even before string theory came along Einstein told us that gravity is a very special force. It's not like the other forces. Gravity has to do fundamentally with fluctuations of space and time itself. So when we talk about extra dimensions of space, gravity will know about those extra dimensions at a fundamental level, because gravity itself is connected with space and time. The other forces of nature that hold us together may not see the extra dimensions, and the particles we are made of may not see the extra dimensions. But gravity, and in particular the graviton—the quantum of gravity that carries the gravitational force—should always be able to move off into extra dimensions. String theory puts this on a much more concrete basis and tells you exactly why a graviton can move in the higher dimensional space, whereas the stuff we're made out of may not be able to move in those extra dimensions.
NOVA: What makes gravitons different from all other particles? Why can they move around?
Lykken: In string theory the difference between gravity and the stuff that we're made out of is that gravity is made of closed loops of strings, whereas the stuff that we're made out of may actually come from strings that have ends, what are called open strings, and those ends would actually be tied to a brane that lives in this higher dimensional space.
NOVA: But gravitons, because they're closed, can just drift around?
Lykken: You can think of gravitons as closed loops of string that are floating around in the higher dimensional space, whereas you and I are the ends of open strings that are tied to branes and cannot move around in the higher dimensional space.
NOVA: When you first thought of this idea that extra dimensions could be other than tiny and curled up, did you think that was a pretty far-out idea?
Lykken: Not too many years ago people who talked about large extra dimensions would have been considered crackpots, to put it lightly. What's really changed is that people have realized that there are very hard-core, rigorous concepts coming out of string theory that allow you to talk about the possibility of visible extra dimensions, extra dimensions that you can see with experiments in a rigorous way. So we've turned something that used to be fantasy into hard science that we can go out and do experiments on right now. That's the real excitement about extra dimensions. It's not just something that we talk about, it's something we can do real experiments on right now.
Detecting other dimensions
Lykken: The key to testing extra dimensions in experiments has to do with gravity. The simplest way that you can see evidence for an extra dimension is to try to produce a high-energy graviton, the quantum of gravity, which could then move off into the extra dimensions. In a particle accelerator we try to collide very high-energy particles and hope occasionally to produce a high-energy graviton that moves off into extra dimensions and disappears.
This is something we don't see directly—we don't see the graviton that disappears—but we notice that energy and momentum were carried off by some invisible particle and from that we deduce that something strange happened. In this case a high-energy particle, a graviton, moved off into extra dimensions. So this is the simplest kind of experiment you can do, and if you can eliminate other kinds of possibilities for things that carry off energy invisibly, you would then be able to claim that you've seen evidence for extra dimensions of space.
NOVA: What are you doing at Fermi Lab to tackle this question?
Lykken: The experiments that we're doing right now are colliding protons with anti-protons at the highest energies at which particles have ever been collided. We're hoping that occasionally in these very high-energy collisions we will make a very high-energy graviton that can move off into the extra dimensions, and then we will see the disappearance of a large amount of energy and momentum and deduce from that, we hope, the presence of extra dimensions.
NOVA: What would that actually look like?
Lykken: In the particle detector what you will see from this collision will look like a jet of high-energy particles, and then you'll see that there is nothing balancing the energy and momentum of that very high-energy jet. There was something there, a high-energy graviton, but it disappeared into the extra dimensions. We see a very high-energy jet of particles going in one direction and nothing balancing it off in the other direction. That's what we call a missing energy signature. That's the kind of thing where we jump up and down and say, "This could be extra dimensions."
“You really are looking for those few golden, rare events where something miraculous happens.”
NOVA: How close do you think you might be to actually seeing this?
Lykken: It is not the case that you can turn on your particle accelerator and all of a sudden you're producing lots of gravitons and you say, "Okay, there's extra dimensions, let's go home." This is a very long and complicated process. You sift through literally billions and billions of these collisions and most of the time what you see is relatively uninteresting ordinary physics. So you really are looking for those few golden, rare events where something miraculous happens, like you produce a graviton that goes into extra dimensions.
The tough job for the experimentalist is to design an experiment that can actually pull out the particular kinds of things you're interested in. The tough job for a theorist like myself who's interested in the experiments is to give them a detailed enough prediction for what they should be looking for that they can actually put it into a computer program, put it into their data analysis, and use that to sort out what they're looking at.
Seeking evidence of supersymmetry
NOVA: What else are you looking for experimentally besides extra dimensions?
Lykken: One of the other major predictions of string theory is that there should be supersymmetry. Supersymmetry predicts that for every elementary particle that we've already seen there is a superpartner particle, a heavier particle that is similar to the kinds of elementary particles we already see. That's a very strong prediction, and it's something we can look for in particle colliders where we can produce new kinds of heavy particles. In fact, we're doing that all the time when we run these experiments. What we don't know is exactly what the masses of these superpartner particles will be, so we don't know exactly when we will find them.
NOVA: What would it mean if supersymmetric particles were found?
Lykken: If supersymmetry is discovered by finding superpartner particles, this would mean many very interesting things. First of all, since it is a generic prediction of string theory, it's evidence that string theory is on the right track. Secondly, it relates the kind of particles that matter is made out of to the kinds of particles that carry forces, the particles like the photon that carry the forces of nature. So it would give us a much deeper understanding as to how the different kinds of elementary particles are related to one another.
In addition, it would really open up an entirely new world, because it would be telling us that all of the particles that we've been studying for the past 50 years are just a little bit of all of the particles that are really there. We would probably need decades to map out this whole other world of superpartner particles and understand what they were.
Even better than that we have evidence that leads us to believe that it may be that most of the dark matter that makes up most of our universe may be composed of the lightest of the superpartner particles. If that is true, then when we discover superpartner particles we will actually be producing in the laboratory the mysterious dark matter that makes up most of our universe, and that would be a fantastic discovery.
NOVA: What is dark matter?
Lykken: When we look out into the universe there is luminous matter made of stars and galaxies, and then there is dark matter, which we don't see but we can infer is there from its gravitational influence on the stars and galaxies that we do see. From those sorts of observations, we know that most of the matter of the universe doesn't shine in stars, it's some kind of dark matter. Furthermore, we have evidence that this dark matter almost doesn't interact at all with ordinary matter. Whatever it is, it's some kind of massive strange particle that doesn't interact with normal matter very much. This is exactly the kind of thing that is predicted in supersymmetry. In supersymmetric theories it is usually the case that the lightest superpartner particle has exactly the characteristics that dark matter has.
NOVA: What do you think the chances of success are for finding either supersymmetry or gravitons disappearing into extra dimensions?
Lykken: I would be extremely surprised if we do not make a major discovery with the experiments currently running at the Tevatron at Fermi Lab. I think there is a good chance to see supersymmetry, and I think there is an excellent chance we will discover something surprising, something that the theorists have not been smart enough to predict. I think if we're very lucky, we can see something very unusual and strange like extra dimensions or maybe something even stranger than that. Because we're looking at the energy frontier where nobody has ever looked before, because we're using the most powerful microscope that anybody has ever seen, we don't know what's there. We're explorers. We are expecting big discoveries.
NOVA: What's at stake for the lab that finds extra dimensions or supersymmetry? Is this Nobel Prize work?
Lykken: It's hard to underestimate the significance of a discovery either of supersymmetry or particularly of extra dimensions. Certainly either one of these discoveries would completely change our view of how the universe works, and that will change the future of particle physics. This fact has not been lost on people that do experiments or on theorists. We are really expecting a watershed in the history of physics through this kind of discovery. It will change all of our thinking about the universe and thus all of our thinking about experiments that we do after that.
NOVA: How do stakes that high affect the nature of competition?
Lykken: We are now at a point where the competition becomes even more intense and it also in a way becomes friendlier, because when you're looking for a really big discovery, my God, I would be so happy if somebody discovered extra dimensions. I don't care if it's me, I don't care if it's at Fermi Lab, I would just like to see these discovered. So it does intensify the competition, but you become so excited about the possibility that these discoveries may actually be there that you also want everybody to win. What's important is that we make the discovery and that we get the physics and that's all we really care about—getting our hands on this new physics. Sure, I would like to be there at the place where it happens, but what I really want is for it to happen.
Is string theory science?
NOVA: How do you respond to the critics who say that string theory isn't testable and therefore isn't really even science?
Lykken: String theory and string theorists do have a real problem. How do you actually test string theory? If you can't test it in the way that we test normal theories, it's not science, it's philosophy, and that's a real problem. We think now with our current understanding of string theory that it's very hard for string theory to make the kind of detailed scientific predictions that you normally make with a theory. We just don't understand how to do that with string theory yet.
“Nature has somehow allowed us to unlock the keys to many fundamental mysteries already. How far can we push that? We won't know until we try.”
What we are doing instead is trying to take ideas from string theory for new kinds of physics, new kinds of phenomena never seen, and look for those in a more general way. If we find those, if we find, say, supersymmetry, it doesn't prove string theory is right, but it does give us a hint that we're moving in the right direction. That's the process for trying to make string theory into real science, but there's no guarantee we will succeed. It could be that string theory is right, but it will turn out to be very difficult or even impossible to do the kinds of detailed scientific tests that you normally require for a scientific theory.
NOVA: Is it possible that string theory is just dead wrong?
Lykken: It's certainly possible that string theory is wrong. We will need experiments to tell us that. To paraphrase Richard Feynman, it doesn't matter how elegant your theory is, it doesn't matter how smart you are, if the experiment says it's wrong, it's wrong. This could happen with string theory, and it wouldn't be the first time. It has already happened in the history of physics several times that there have been periods of many years where all of the smart people, all of the cool people, were working on one kind of theory, moving in one kind of direction, and even though they thought it was wonderful, it turned out to be a dead end. This could happen to string theory. It doesn't matter how enthusiastic we are about it, eventually it has to work out as a physical theory with testable consequences or it will again be a dead end of physics. I don't think that will happen with string theory, but we don't know yet.
NOVA: Is it realistic to hope that we can ever fully understand the universe, or is that arrogant in some way?
Lykken: A lot of people wonder how far we can push our deep understanding of the universe. The way I look at it is that we have already been incredibly lucky. The fact that we can already understand what is happening in the universe down to tiny, tiny distances inside the atom is amazing to me. The fact that we can look out into the bigger universe and understand things that are happening unimaginable distances away. The fact that we have a theory, the big bang theory, for how the universe began, that has been verified by a lot of experimental data and observations. We have been incredibly lucky. Nature has somehow allowed us to unlock the keys to many fundamental mysteries already. How far can we push that? We won't know until we try. We have to keep pushing, and I think that's something we'll never give up on. Any question that is interesting to ask about the universe we will keep asking until we find a way to understand it.
NOVA: What would it mean to find a final theory?
Lykken: I frankly don't believe in the idea of a final theory. To me, when you say a final theory, it suggests that you've stopped asking questions, and I don't think that's in the nature of science. Science is a process of always asking questions, and when you look at the history of science, what you find is every time you get an answer, it suggests new questions. Every time you solve a mystery, it suggests new mysteries, things that you wonder about. So I don't think this process will ever end as long as there are people around to ask questions. I don't think there is any such thing as a final theory. I think string theory may be a fundamental description of all the matter and forces that we see today, but I don't think we will ever stop asking fundamental questions about what all of that means, even if string theory is the right theory and even if we understood it at a much deeper level.
NOVA: Do you think there is a limit to what we can know?
Lykken: I think in the end there should not be any limit; science can always
ask new questions, and as long as we have funding to do experiments, we will
always proceed to gather more knowledge, do
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