Professor of Physics and Mathematics, Columbia University
Author, The Elegant Universe
Author, The Fabric of the Cosmos
Well, as we go about our daily lives, the world seems very real. Time, gravity, even three dimensions of space seem as obvious as the air we breathe. But, as modern physics has progressed over the past century, the universe has been slowly revealed to be a more exotic place than most of us are capable of imagining.
What is the nature of reality? We explore this issue today with our guest Prof. Brian Greene. Prof. Greene is currently Professor of Physics and Mathematics at Columbia University. He holds degrees from both Harvard and Oxford, and is one of the leading physicists developing mathematical models of string theory, a theory unifying the four fundamental forces. He is the author of two popular books, The Fabric of the Cosmos and The Elegant Universe, which was made into an eponymous television series for Nova on PBS. Besides all of this, he has also been described on superstringtheory.com as String Theory’s answer to John Cusack.
Prof. Brian Greene (BG) talks with Charles Lee (CL) about string theory.
CL: I am curious if you can give us an overview of the types of problems that string theory is trying to solve and why it is an elegant description of the universe.
BG: Well, for the last thirty years of his life, Albert Einstein was seeking what he called a Unified Theory of Physics. And, a unified theory would be one in which all physical phenomenon, at least in principle, could be described using the equations and ideas in the theory. He never found the theory. One of the main reasons is because his own theory of the large, concerning things that are big and subject to gravity, his so-called Theory of General Relativity, turns out to be in conflict with another set of laws called Quantum Mechanics, which are the laws that describe small things, molecules, atoms, etc. And for many years, nobody could figure out a way to put these two theories, the theories of the big and very small, into some harmonious union. That finally is what string theory has accomplished, giving us in principle one theory of the big, the small, and everything in between.
CL: How is it that string theory is able to do this where other theories have failed?
BG: The key idea of string theory is to replace the old idea that the basic elementary constituents of matter are little dot particles of effectively no size at all. We replace that idea with little tiny filaments of energy, little tiny loops of string as we call it. And it turns out that as we pass from an ingredient of no size to an ingredient that has some size, albeit very small size is the key step that allows us to go beyond the problems that all previous attempts have faced at unifying quantum theory and general relativity.
CL: What are these strings? How can we conceptualize it? Is it matter or space itself?
BG: It’s hard to answer that question. I like to think of the strings as little filaments of energy. And from Einstein’s e=mc2 equation, energy is the most convertible currency in physics. Energy can manifest itself as matter or as radiation. Therefore, in a sense, when we talk about strings as being energy, they are one at the same time matter, radiation, and energy. Many of us think they may be the fundamental ingredients in space and time as well.
CL: And one of the outcomes of the theory is that this requires ten or eleven dimensions of space-time.
BG: Yes, it’s a very strange implication of the theory, and for the very first time we do find that string theory does unify quantum mechanics and general relativity. But, when we study that unification, there is a particular cost to that unification. And, as you mention, that cost is we have to admit that the universe has more than three dimensions of space. And, that’s a very strange idea. Depending on the formulation, the theory requires additional six or seven dimensions of space that nobody has yet seen. So, it’s a very unfamiliar idea, but that’s where this unification leads us.
CL: Where are these extra dimensions of space, if they exist?
BG: It’s a very good question. I’ve been working on that for basically fifteen years, since I was a graduate student. Just to set up what the issue is, we all know about three dimensions of space. Those are left-right, back-forth, and up-down, the three dimensions that we’re all immersed within and move through those dimensions freely in everyday life. Literally, string theory is saying that in addition to those, there are others. So, your question is where are they? And, one suggestion that I’ve worked on, as well as other people, is that the extra dimensions are tightly curled up. They may be very, very small. So, they may be all around us, just too tiny for us to see with the naked eye or most powerful magnifying equipment. That is one place that the extra dimensions may be hiding.
CL: And that is one of the criticisms of string theory. There really is no evidence for extra dimensions.
BG: I would criticize string theory in that way myself, because none of us, myself included, will ever believe that string theory is correct until it does make contact with physical reality through some experimental prediction that is tested. The very nice recent development is that it is possible to test in the next five years using a new particle accelerator being built in Geneva, Switzerland. It may be the case that the particle accelerator will have the capacity to look for the extra dimensions. Basically, the accelerator will send particles going around a tunnel in opposite directions near the speed of light. And, every so often, the particle will be directed to slam into each other in a big collision. The hope is that in that collision some debris will be created that will be ejected out of our dimensions and into the other dimensions. And, how would we recognize that some stuff had been ejected. We’ll measure the energy after the collision, and we’ll compare it with the energy before the collision. If there’s a little bit of energy deficit at the end, and if it’s missing in just the right pattern, that would give us indirect evidence that the energy had drifted off into the other dimensions, and that is why we don’t see it.
CL: But, aren’t there other theories that could explain the loss of energy besides string theory?
BG: You can always come up with alternate explanations in physics. The explanations that you ultimately believe are the ones that make contact with experimental observation, and also resolve a whole host of other theoretical issues that no other approach has been able to give insight into. So, string theory unites general relativity with quantum mechanics, which are both experimentally confirmed. And, if a prediction of this unified framework is borne out by experiment, will it prove string theory? No. But, it will be pretty darn good circumstantial evidence that we’re going in the right direction. Of course, then we’ll try to come up with other predictions. And, every prediction that’s confirmed is another reason why we believe that the theory may be correct. That’s basically how science progresses, and that’s basically the track that we’ll follow.
CL: Is the allure of string theory it’s able to collapse other theories into one unified framework?
BG: It’s certainly part of it. It’s quite impressive the developments in physics over the past fifty years, such as the structure of the nucleus, the nuclear forces, and the basic structure of matter. All of these previous developments that occurred before string theory, which have survived experimental testing, naturally fit within string theory. So, here’s this theory that comes along and not only embraces general relativity and quantum mechanics, but pretty much embraces all of the successful discoveries in the last fifty years, and that is very elegant.
CL: Of all the fields in physics, how did you become interested in string theory?
BG: Well, I was a graduate student at Oxford in the mid-1980s. And, 1984 was a big year for string theory. Almost nobody was working on string theory in 1983. But, in 1984, two physicists, John Schwarz at Caltech and Michael Green in London, had a breakthrough. They made that breakthrough showing that the math in string theory worked, even though it was unclear before then that it actually would. That really took the physics world by storm. And, anybody who was a high-energy theorist took note, and many dropped the work that they had been pursuing and started to work on string theory. So, as a beginning graduate student, it was a natural thing to jump in on the ground floor, because it was a rare moment in the history of physics where the older physicists weren’t really at a great advantage. They hadn’t been thinking about string theory, so we all jumped into it together. And, it was a very productive time.
CL: Have you looked back on it and thought of other fields that you might have gone into?
BG: Well, when I went into Oxford, I though that I might study gravity, which was always mysterious to me. Although, I thought that I might study it in more prosaic contexts, the structures of galaxies and black holes, things of that sort. And, I did start to work on that. But, when the foundation of the theory describing gravity was up for grabs and there was a potential of making progress, there was really a very strong allure.
CL: One of the motivations you mention in your book is that your interests stem from how these theories apply to our daily lives. How can string theory apply to our daily lives?
BG: The physics that we’re developing certainly doesn’t directly talk about how we live our daily lives, because the environments in which the physics that we’re developing is relevant is very far from human experience. The environments are those that are extremely massive, like those in black holes or the entire universe. Or, they are environments that are very small like, like particles, and sub-atomic structures inside of particles. So, you might think it would have no bearing on everyday life. And, my feeling is that it doesn’t have any direct bearing on everyday life. It doesn’t give you some new piece of technology that makes life easier. But, when you understand the laws of physics better. When you can look at a flower, or teacup, or anything and understand its structure more deeply. I feel that you have a very enriched way of living your life, and it helps you understand your own place in the universe when you understand the laws that gave rise to the universe and allowed it to evolve into the form that we currently witness. So, it doesn’t help us in any concrete, direct way, but it does help us change the way we think about the universe. And, that affects how we live.
CL: Some might not see physics in this way, rather as more cerebral and not emotional.
BG: It’s hard to say. In a very indirect way, you can imagine that if you have a cosmic sense of the universe, issues of everyday life may be important to you, but they may not eat at your soul in the same way if you didn’t have the cosmic view of the universe. But, even in more concrete ways, if you would have asked those who had developed quantum mechanics in the early part of the twentieth century, ‘will this affect our lives?’ I think most of them would have said, ‘no,’ because they were dealing with the structure of atoms, and that again seems very far removed from everyday life. But, today, seventy or eighty years later, those developments in quantum physics have give rise to the cell phone, the personal computer, lasers, medical technology. There’s so much in the world around us that actually owes its existence to the discovery of quantum physics. So perhaps 100 or 500 years in the future, the work we’re doing may have a direct impact in everyday life. It’s possible.
CL: You’re in one of those fields of science that’s still discovering fundamental principles. But, many fields of science have moved beyond fundamental issues and are more technical in nature. Are there any other fields of science that you feel are involved in fundamental issues?
BG: If you want to ask the big questions, like how did the universe begin, a field of physics called cosmology, than you need to understand the fundamental laws. We have attempted to understand how the universe begin by using the known laws of physics, but as any cosmologist will tell you, those laws break down if you try to apply them in the extreme environment that we believe existed near the origin of the universe. So, if you ever want to answer that question, you have to have a theory like string theory, which if it’s correct, is a theory that doesn’t break down under any circumstances, similarly if you really want to understand what happens deep inside a black hole. Stars, when they end their lives as nuclear furnaces, collapse under their own weight giving rise to very dense regions of space that we call black holes. Nobody has been able to figure out what happens deep inside a black hole at its so-called center. If you have a theory that won’t break down in extreme circumstances, you stand a chance of finally understanding these objects. And, we believe these objects exist. There’s observational evidence that they’re really out there. So, we would really like to understand them. So, extreme astrophysics and cosmology, these are the kind of fields where string theory, if it’s right, will have a significant impact.
CL: Prof. Greene, thank you for joining us today for a fascinating discussion.
BG: My pleasure. Thank you.
Grokotron 5000: The Number of Dimensions
CL: Prof. Brian Greene has graciously decided to stick around and play our game the Grokotron 5000, which is our supercomputer formerly known as Deep Blue. Today, the Grokotron 5000 has chosen the game “Number of Dimenision”. So, for the following five items, the Grokotron 5000 would like to know how many dimensions are needed to describe them. Are you ready to play our game, the Grokotron 5000.
CL: Item number one, how many dimensions are needed to describe fast food super-sized meals?
BG: Oh, I would say one, just the dimension of unhealthiness. That’s the only dimension that matters.
CL: Okay, number two, Homer Simpson?
BG: Well, I would say two dimensions. One for how he treats his family, young Bart and the rest of them, and I think that’s a good family dimension. And, also how he does at work, trying to save us all from nuclear disaster. So, I would always want to judge Homer on those two particular scales. And, he seems to being okay. That show has been around 10 or 15 years, and nothing too disastrous has happened.
CL: And, number three, Michael Jackson.
BG: Oh, that’s a complex one. Michael Jackson has so many facets to him that I would be reluctant to limit the number of dimensions. So, I would say that you should start with a large number of dimensions, maybe 100, but allow yourself to increase the number of dimensions as new features and aspects of his character come to light.
CL: I don’t think they can all be comprehended at once.
BG: Well, we certainly haven’t been able to do it with string theory yet, but who know, that may just be a calculation impasse.
CL: Number four, the iPod.
BG: I see these iPods, but I’ve never used one, never played with one. I do know that they’re suppose to store a lot of music, but beyond that I don’t know anything about them.
CL: And, number five, one we’re all interested in, how many dimensions are needed to describe the President of the United States, George Bush.
BG: I think zero dimensions. There really isn’t much one can say, and anything that you do say is perhaps fleshing out the character more than you would want to. So, I’ll just leave it at zero dimensions.
CL: I was expecting negative dimension, but at least we have null. Prof. Greene I want to thank you again for joining us today, playing our game the Grokotron 5000, and discussing all of the fascinating issues in string theory.
BG: My pleasure. Thank you.
- Roger Penrose: Secrets of the Old One