Sylvester James Gates, Jr. has a number of "firsts" to his name. His doctoral dissertation at M.I.T. was the first ever at that university on supersymmetry. In 1994, he became the first recipient of the American Physical Society's Edward A. Bouchet Award, given to a minority physicist who has made significant contributions to his field. And when in 1998 he was named the first John S. Toll Professor of Physics at the University of Maryland, he became the first African-American to hold an endowed chair in physics at a major U.S. research university. Here, Gates talks about firsts he hopes to see in string theory, including the first signs of supersymmetry and perhaps of that most elusive beast—a unification of the four forces of nature.
Note: For a definition of unfamiliar terms, see our glossary.
NOVA: Where does your fascination with physics comes from?
Gates: When I was eight years old, my father bought a set of Encyclopedia Britannica. I was bored one day so I started opening the books, and I came across Schroedinger's equation, which is one of the foundations of modern physics. It was like walking on a beach and finding a marvelous shell and being fascinated by it. I thought to myself, "Gee, one day I wonder if I could ever understand that."
Earlier I had found out that stars were not just things you could look at but places you could go. I had the sense that science and technology were the ways that you got there, so there was this resonance with science and technology and mathematics. When I was in high school my physics teacher wrote an equation on the blackboard and then performed a very simple experiment—rolling a ball down an inclined plane. He showed that what I saw there in front of my eyes was described by the mathematical equation he had written on the blackboard.
NOVA: Were you surprised by that?
Gates: I was utterly shocked. It's not obvious from our everyday experience that mathematics has anything to do with nature. For me this was astounding because mathematics is something we do in our heads. It's like a game of Dungeons and Dragons. It's a fantasy. But because mathematics must also adhere to logical precepts, it restricts our imagination. Why it is that mathematics is the only language that our species has found to describe nature is a mystery that will probably never be solved. But it is the way that we have found our deepest understanding of nature. For the theoretical physicist, mathematics is like an extra-sensory-perception organ that we use to see the universe.
From billiard balls to spaghetti
NOVA: What exactly are strings?
Gates: Strings create everything, including space and time and even us. The way that this is done is unfortunately extremely mathematical and detailed, but I can use an analogy. If I took a little piece of spaghetti and plucked it, it would vibrate back and forth. As it vibrates, it makes a note. The note that you get depends on how you pluck it. Roughly speaking, the idea of strings is that when you pluck it a certain way you might be looking at an electron. When you pluck it a different way you might see a particle of light. If you pluck it a third way you might be looking at a quark. All the particles are in fact different vibratory modes of this single object.
NOVA: Why are physicists so excited about strings? How do they advance our understanding of the universe?
Gates: For about 2,000 years essentially all of our physics has been based on billiard balls. The technical name is geometrical point particle, but the idea at the end of the day was that if you can figure out how billiard balls work, then you can actually figure out everything. It got us to the moon, it got us computers, it got us all sorts of marvelous technologies that we now take for granted. But these billiard balls also ran us into a conflict between general relativity and quantum mechanics. String theory seems to get us out of this. I think Einstein would be extremely happy with the way things developed, because he called for a unified field theory long before anyone else imagined that such a thing was possible, and string theory very definitely is a unified field theory.
NOVA: Backtrack for us a bit. What do you mean by a unified field theory?
Gates: We have observed four forces in action in nature. There's the force of gravity. There's the electromagnetic force—we use that in computers, we use that in our modern information technology. There is the strong nuclear force, the force from which the sun derives its energy. We found the weak nuclear force when substances that were naturally radioactive, such as radium, were found to glow in the dark. Could there be more forces? The answer is yes. There could be more, but we have not seen them in laboratory experiments, and physics is always about what we see in the laboratory. A unified field theory has to explain all of this and include gravity.
“Gravity is the odd man out for deep philosophical reasons.”
In the 1930s some physicists tried to study Einstein's equations of gravitation and combine those with the new laws of quantum theory, the physics of very small objects. When you try to put those two pieces of mathematics together, they do not coexist peacefully. In fact, you get nonsense out of those calculations. And so until string theory came along there was this schism in which we had some laws that we knew worked very well but only for small things. We had other laws that worked very well for large things in gravitation. But when you tried to combine the laws, they broke down. It was only with string theory that we found a way to successfully marry these two very different sets of mathematical equations.
The fourth force
NOVA: Why has gravity resisted being incorporated with the other forces?
Gates: Gravity is the odd man out for deep philosophical reasons. When Newton wrote down his mathematical description of gravity, there was an assumption lurking in the equations; they didn't depend on time. His equations say that something in one location can have an effect at another location far removed at the instant that it occurs. So the instant something happens here, it's automatically and instantaneously known at the second location. This idea of action at distance, which is an almost magical kind of idea, sits conceptually under Newton's description of gravity. Newton was aware of this and in his famous Principia spoke about the philosophical unpleasantness of including such an idea.
As soon as Einstein wrote down his equations, we understood that action at a distance cannot apply in our universe. Einstein's equations say that when an event occurs in one location, it takes some amount of time before it is known in another location; the fastest that the communication between these two separated points can occur is at the speed of light. The notion that there's a time separation between cause and effect leads us to the notion that something actually carries the information back and forth. For the electromagnetic forces and by extension all the other forces, those carriers are things like the photon. When you look at Einstein's equations, since they are so very radically different from Maxwell's equations, the object that carries the information about the disturbance is something drastically different. This carrier is called the graviton. Part of the reason the descriptions could not be coalesced is that the equations of gravity are just so radically different from the equations of all the other forces.
NOVA: What is the nature of those gravitons?
Gates: Einstein's equations describe how space and time are curved. The analogy is that the universe is like a sheet of rubber, and when you put a piece of mass some place, it dents the rubber and causes things to fall in, and that's analogous to how gravity works. Now, if you were to take that mass that you dropped on a sheet of rubber and jiggle it back and forth, what would happen to the sheet of rubber? Very quickly you would build up ripples on the sheet of rubber as you take the mass point and jiggle it back and forth. Those ripples are in fact the graviton. So it's the waves of gravitational energy in spacetime that are responsible for communicating the gravitational force.
Inside Einstein's mind
NOVA: It's become a cliché that Einstein was a genius. What was it about the way he approached problems that made him such a genius?
Gates: Einstein made the statement once that imagination is more important than knowledge. For a long time I was very puzzled by this statement. How could it be that imagination—which I associated with play fantasies and hobbits and such matters—how could it possibly be that that was more important than knowledge? Now, having worked as a physicist for over two decades, I think he was saying that when you try to create new knowledge, the only tool we have as humans is our imagination. The creation of new rational paradigms is itself an irrational process.
The genius of Einstein was such that when he wrote down his marvelously complicated equations of general relativity, he gave a picture for what the equations meant, and why they had to be. Suppose you were in an elevator standing on the surface of the Earth and had a ball in your hand. You let go of the ball and it falls. You wouldn't think very much about that because that's how gravity always acts. But suppose that I take you in this elevator with no windows so you can't see out, and I put a rocket motor under it and I put you out in space. Then I turn the rocket motor on so that it causes the acceleration to occur within the elevator. You're moving faster and faster, and you're standing on the floor of the elevator and you have this same ball in your hand, and you let go.
“Einstein was one of those physicists who really wanted to know the mind of God.”
What happens to the ball? It appears from your point of view to fall to the floor. So for this man in the elevator, he cannot tell whether he is standing on the surface of the Earth, a gravitating body, or whether he is out in free space with a rocket that's driving the elevator to greater and greater speeds. When you translate that story to mathematics, it turns out you get Einstein's equations, and that's what we're missing for string theory. We're still looking for that man in the elevator. [For more on Einstein and his genius, see the NOVA Web site Einstein Revealed.]
NOVA: How was this theory received at the time? Did people accept it, or was it too radical an idea?
Gates: A prediction of this bending and flexing of the spacetime grid is that light from stars can be bent. At the end of the First World War an expedition returned from South America and said we've seen the bending of starlight precisely as the theory of general relativity predicts. At that point the second great crowning of Einstein occurred. He was already a world-famous scientist. This one broke out into popular media. Einstein was like a rock star in his day.
NOVA: Why did Einstein pursue the idea of unification?
Gates: Einstein clearly had a large spiritual dimension to his thinking. He talked about God, and he clearly believed that the universe has an overall grand and beautiful pattern to the way it works. With his achievement of special relativity, more of that apparent pattern was revealed to mankind. But he also understood that it could not be the full pattern. Einstein was one of those physicists who really wanted to know the mind of God, which more prosaically means the entire picture.
NOVA: But he failed in that endeavor?
Gates: By the time of Einstein's death in the '50s almost no serious physicists were engaged in the quest for unification. He was thought to be this doddering, sympathetic old figure who led an earlier revolution but somehow fell out of it. There's a sadness about the latter part of his career. But now, 50 years after his death, it looks quite different, because we can see unification coming back in a major way and having profound impacts. The idea of unification is such a potent one that even though the scientific community apparently dropped it, the structure of the universe seems to demand it, and the community is driven back to the idea of unification.
NOVA: Why should unification matter to most people? Does it have any practical applications in the real world?
Gates: Unification has driven science for about 150 years, and every time we've used the idea it has brought us a broader and deeper understanding of science, which ultimately advances our technology. The unification of magnets and electricity was carried out by a British scientist named James Clerk Maxwell in 1864. He wrote four equations that are the code for modern developments such as computers, cell phones, and beepers. In this sense physics provides us with the DNA for advanced technology.
NOVA: Is the elegance of that something that appeals to you?
Gates: The drive for unity is certainly to me one of the deepest elegances of physics. It's a tradition that we've been following as a community since the work of Newton. One way to view Newton's explanation of gravity with the famous story of the dropping apple is that he unified the heavens with Earth. He said that something that happens here on Earth—namely an apple dropping off a tree—is directly connected with the paths of objects we see in the heavens. That's a kind of reconnection of, in some sense, Earth and heaven. We've been doing this for a long time, so my own personal feeling is that there's a rightness about this.
NOVA: How do you respond to critics of string theory who say it can't be proved experimentally, so it's not really science?
Gates: String theory is often criticized as having had no experimental input or output, so the analogy to a religion has been noted by a number of people. In a sense that's right; it is kind of a church to which I belong. We have our own popes and House of Cardinals. But ultimately science is also an act of faith—faith that we will be capable of understanding the way the universe is put together.
NOVA: But what are the reasons for believing that string theory is correct?
Gates: The well-known physicist and Nobel laureate Sheldon Glashow once supposedly described string theory as the theory of everything that predicts nothing. At the end of the day, if string theory does not provide us with a testable prediction—whether it be in the context of elementary particle physics or cosmology and black hole physics—then nobody should believe it.
The power of science is an acceptance and openness to the notion that we are fallible and must therefore be corrected by nature herself. Many other human belief systems start off with the assumption that the answer is already known. In science, it's precisely the opposite; we start out admitting to not knowing the answer. So as we struggle with our marriages of space and time, our addition of extra dimensions, our paradigm shifts from little billiard balls to little pieces of spaghetti, these exercises are all subjected to a single question: Is it there in the laboratory? Can you find its evidence? Until that happens, I am of the opinion that you should be skeptical about string theory.
On the other hand, there is a kind of elegance to string theory, and given the history of how theoretical physics has evolved thus far, it is totally conceivable that some if not all of these ideas will turn out to be correct.
NOVA: Is there any conceivable way, then, to look for evidence that string theory is correct?
Gates: All of this activity in which we engage will be for naught unless at some point there is experimental verification of these ideas. Interestingly enough, we have some tantalizing hints already that this may be occurring. Evidence for superpartners may have begun to emerge in the laboratory. Superpartners come from supersymmetry, and supersymmetry is part of superstrings, so we may be on the cusp of beginning to see experimental verification. [For more on experimental support for supersymmetry, see Smashing Pictures.]
NOVA: What is supersymmetry?
Gates: In our world as we observe it in the laboratory, the universe breaks into, roughly speaking, two pieces. One of these pieces is matter, such as electrons, quarks, protons, neutrons—what things are made of. The other half of our universe are the things that cause the matter to clump together—the forces of nature—and each force has carriers. The carrier of the gravitational force is the graviton; the carrier of the electromagnetic force is the photon; the carrier of the strong force are things called gluons. Well, gee, what kind of unification is that? The answer is, it's no unification at all.
So let's try one more marriage. Let's look for a marriage where the stuff of our universe and the things that cause the stuff to clump together are intertwined. We know this occurs in string theory, but it also occurs in a smaller theory called supersymmetry. That's the first place we saw this marriage occur, and that's in fact what brought me into theoretical physics. In a supersymmetrical theory, there are new forms of matter and energy called "superpartners" that are required for this unification.
NOVA: What will evidence for supersymmetry mean for string theory?
Gates: Supersymmetry will be one of the signposts pointing to the correctness of superstrings. Superstrings are in some sense very far removed from our reality. When you take most superstring theories apart they contain superpartners. So if a laboratory experiment such as the one being conducted at Brookhaven reveals the presence of superpartners, that will point to the increased possibility that superstring theory is correct.
Slow to take hold
NOVA: String theory was introduced in the '60s, and it is only in the last decade or two that it has become widely studied. Why was string theory ignored at first?
Gates: For a number of reasons. Theoretical physics is of course done by humans, which means that there is a culture, a societal norm, among physicists. That societal norm has a lot of inertia—it's very difficult to change the opinion of that community. And so unless very strong evidence is presented, the community tends not to want to change ideas. That is true for physicists just as it is for other people. So when this radical notion that there were more dimensions than we have traditionally accepted came into physics with essentially no experimental support, the built-in conservatism of the community took over.
“There’s a saying that old physicists accept new ideas when they die.”
There's a saying that old physicists accept new ideas when they die. It's the next generation that brings new ideas to their full fruition. When you get to be an old physicist like me, you know a lot of stuff, and it acts like a ballast on a ship; it pulls you down. You have all the weight of these other things that you know. And sometimes an idea, like a small fairy or a sprite, passes by and you say, "Ah, I don't know what that is, but it can't be very important." Well, sometimes it is.
NOVA: What will it mean if string theory is correct?
Gates: In a general sense it means that a generation of physicists will possess a set of tools and some images about the universe that will allow them to probe the universe and understand the laws of the universe well beyond what we can do today.
Answering the question of what the practical benefits of that understanding will be is beyond my capabilities. But I can look backward to 1864 and Maxwell and his equations. And one can imagine saying, "Professor Maxwell, what do your equations mean?" He would struggle for answers. He would say, "Well, you know, the electric and magnetic phenomena are not separate, they're part of a unity. They make a prediction that waves of electromagnetic energy travel in space and time with the velocity of light." But beyond that I think he would be rather hard-pressed to tell you what it means. One hundred and fifty years later we can answer this question very easily. A large fraction of our technological basis rests on his work.
So if string theory is correct, what does it mean? Well, one can imagine 150 or 200 years from now some marvelous piece of technology that's beyond my imagining. Maybe it's a transporter from "Star Trek," perhaps it's warp drive, maybe our species finally is released from this prison of being contained in a single solar system.
NOVA: Aside from possible technologies 200 years from now, how else is your work meaningful to the general public?
Gates: The kind of physics that my community engages in—trying to understand
the most fundamental structure and issues for our
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