Theoretical physicists rarely achieve the status of rock stars, but Argentina's Juan Maldacena, 30, may be an exception. Last summer at a banquet for some of the world's leading theoreticians at Santa Barbara, California, a chorus energetically took up the tune of the Macarena with Maldacena as the focus of attention. The lyrics, admittedly understandable only to a select few, were a lighthearted testimony what most physicists see as a stunning achievement. "You start with the brane/and the brane is B.P.S/Then you go near the brane/And the space is A.D.S.3/ Who knows what it means?/I don't, I confess./Ehhhh! Maldacena!/Super Yang-Mills/With Very large N/Gravity on a sphere/Flux without end/ Who says they're the same?/Holographic, he contends./Ehhh Maldacena!"
What Maldacena accomplished in two startlingly imaginative papers currently available on the Los Alamos U.S. National Laboratories web site (http://xxx.lanl.gov/abs/hep-th/9607235, and http://xxx.lanl.gov/abs/hep-th/9711200) is to show a possible approach to achieving physic's Holy Grail, a quest which stumped Einstein for more than 30 years. It is familiarly referred to by physcists as the TOE--the Theory of Everything--a universal theory which will provide consistent mathematical predictions for both galaxies in outer space and for the interactions of sub-atomic particles. A major problem until now has been that quantum mechanics provides an accurate description of three of nature's fundamental forces, but cannot handle the fourth, gravity. (The three forces it does deal with are the electromagnetic force, transmitted by photons, the strong force which holds matter together and which is transmitted by gluons, and the weak force, which is present in radiation and is transmitted by Z particles). Einstein's theories of relativity reliably describe gravity and they make extremely accurate predictions for large objects like planets and galaxies. But when relativity is applied to the boisterous environment of sub atomic particles, it often produces gibberish. Maldacena's papers point to a possible way through the impasse. They elaborate on an esoteric approach to particle physics called "superstring theory." "It's a major breakthrough," says Andrew Strominger, 43, a leading theoretician of superstrings, who recently moved to Harvard from the University of Santa Barbara, and also managed to convince Maldacena to accept a post in Harvard's Physics department. "When I came to Harvard, I wanted to bring him with me because I thought he was quite good. I didn't realize quite how good. He really did something quite astonishing."
David Nelson, the chairman of Harvard's physics department was even more impressed. Although Maldacena only received his PhD from Princeton two years ago, Harvard has already signed him on as an associate professor. "Entering Harvard as an associate professor is very unusual," says Nelson."It happens only once in every ten years." But Nelson thinks that Maldacena's papers earned him the position. "Juan's contribution, in a couple of months, took over the research direction in string theory," says Nelson."You could see it by plotting the number of people who access his paper on the internet. A physicist would be proud to have a thousand citations in which people base research on his paper over a ten year period. Juan was up to 500 after six months. That's a real indicator of the attention and excitement created by his ideas. "
In fact, while most physics papers can count 10 to 20 citations on the average, Juan Maldacena's first paper has already registered 118, and his second paper now counts 589, and is still going. Edward Witten, who is probably the top name in string theory today, says there is no question that Maldacena has made a sensation. "His insight had had a tremendous impact on the direction of research in the last year and a half," says Witten. "I would say that he is a very promising young star."
The University of Chicago's Jeffrey Harvey, 44, the man who actually wrote the lyrics to the Maldacena macarena, thinks that Maldacena "is the most impressive new person that I have seen in the field in the last decade." Says Harvey,"It wasn't just a technical accomplishment. It was more of a conceptual leap. I knew who he was before he wrote this paper. I knew he was very impressive and had done some very nice work. When I saw the paper, I really thought that he had gone a bit off the deep end. It seemed to me that he was making connections and speculating without solid evidence. I thought that here was a young person who had just gotten a little too confident and was trying to push things too hard. I was completely wrong. He just had this fantastic combination of technical things to back it up, and intuition about how things should work. He was absolutely right, and he has really set the agenda for a lot of research in string theory over the last year."
Maldacena's conceptual leap is based on analyzing the frontier where quantum mechanics and general relativity meet. The only time when you actually have a conflict between relativity and quantum mechanics, Maldacena points out, is when you are dealing with a sub-atomic object with enough mass to affect gravity. Maldacena found what he was looking for in a miniature black hole, a phenomenon which British cosmologist Stephen Hawking had speculated in the 1970s was likely to have occurred during the Big Bang. The black holes used in Maldacena's calculations are smaller than a positron (the positive-charged counterpart of an electron) and have a mass of about one kilogram. At the frontier presented by this environment, both the quantum mechanics used to predict the interactions of sub-atomic particles and the formulas used for larger objects start to come together. While conventional physics tends to see particles as tiny dimension-less dots, Maldacena has been working in a esoteric area of physics known as "superstring theory." Superstring theorists contend that matter is ultimately composed of tiny, vibrating strings, less than 10 to the -33 centimeters long (1X10sup -33). These strings can change into a wide variety of particles depending on how they happen to be vibrating within the fabric of space-time. Equally intriguing, they interact in 10 spatial dimensions (plus time, which makes an 11th dimension), rather than just the conventional three spatial dimensions (plus time). A weakness of both string theory and more conventional theories is that they haven't been able to describe the force of gravity at the sub-atomic level.
Maldacena's hypothesis makes it possible to use the world of 4 dimensions to look at what is happening in a sub-atomic world of 10 dimensions and to take into account the force of gravity at the atomic level.
The effect is something like a hologram. "A hologram is a three-dimensional image presented on a two dimensional surface," Maldacena explains. "The idea was that gravity theory could have a similar description. You could describe physics in this ten dimensional state by the physics living on the boundary of this state, which is four dimensional." Andrew Strominger, another leader in superstring theory sums up Maldacena's idea: "His conjecture is that a quantum theory with gravity in space-time is equivalent to a quantum theory without gravity on the boundary of that space time." Jeffrey Harvey adds,"The idea that you can describe aspects of gravity using gauge theories normally used to describe elementary particles, the idea that there should be this kind of connection was remarkable. It is hard to get your mind around it."
That holds for string theory as well. Most non-physicists have a hard time imagining a world with ten or 11 dimensions (A dimension is defined here as an independent axis in space or space-time). Brian Greene, author of the "The Elegant Universe," draws an analogy with an ant walking along a stretched section of garden hose. From a distance the ant appears to be walking on a one-dimensional string, which makes it possible to only move in two directions: backwards and forwards. But if you move closer, you realize that the hose has some thickness to it, and the ant can actually move around the circumference of the hose. That gives the ant an extra dimension. He can move backwards, forwards, or somewhere around the diameter of the hose. If you take the analogy a bit further, mathematicians have designed a class of 6-dimensional shapes which are known as a Calabi-Yau space. From a distance, a Calabi-Yau looks like a seriously tangled garden hose. Theoreticians speculate that strings are affected by dimensions similar to those found in a Calabi-Yau space at every point in the universe, but the distances involved are so small that they are undetectable. The current range of accelerators can detect particles that are 10 to the -18 meters (a billionth of a billionth of a meter). But strings, which roughly match Planck length, are only around 10 to the -35 meters long. An accelerator large enough to focus directly on a string would have to be the size of a galaxy. Further complicating matters, string theory also encompasses "branes,"--essentially extended objects that arise in the theory. A one-brane is a string. A two-brane is a membrane, or two-dimensional object. A three-brane has three extended dimensions. P-branes have p spatial dimensions.
Whether it can be directly proven or not, some theoreticians are convinced that the vibrating string itself is the final elemental particle, and that what we now think of as different particles will turn out to be different manifestations of vibrating strings. In the same vein, what we now take to be four different forces may ultimately turn out to be different manifestations of the same force. (The Electromagnetic Force and the Weak Force are now recognized by some physicists as the Electroweak force).
Maldacena is characteristically modest about his achievement. "I think it has opened up some new areas," he says. "But with any new area, you do not know how far it will go. It could get stuck at some point, and we would have to find a new area. In the last few years, people have been having ideas that unified the ideas that went before. But I think we have a long way to go before we can do experimental predictions. That is the real objective: to do predictions for cosmology and particle physics. The challenge is to develop the theory well enough so that you can do predictions for experiments that are being done now."
Even Maldacena admits that it may never be possible to get a direct look at what is happening. "As a physicist," he says,"you don't normally solve the problem that want to solve. You work on a similar problem, and then you try to see if there is a connection." But there is no question in Maldacena's mind that the relationship of gravity to quantum particles is one of the central questions of physics. "If we really want to understand the beginning of the universe," he says. "We have to understand quantum gravity." Maldacena also thinks that physicists have to be selective when it comes to picking problems to solve. When I asked him if he thought it would ever be possible to travel forwards or backwards in time, he said that it was not a topic that he had any deep feelings about. "One always has to choose where one thinks one can make progress," he says.
Maldacena says his own interest in physics began when he was in high school in Argentina, where his father, an electrical engineer, ran an elevator company. "I was interested in how things work," he says. "I read popular science books explaining electricity, and how a radio and television works. I decided to study physics. My father was an engineer and I knew what it was like to be an engineer, but I didn't know what it was like to be a physicist."
After graduating from high school Maldacena gained admission to the Instituto Balseiro in Barloche, Argentina where he studied from 1988 to 1991. Harvard's David Nelson explains that the Instituto Balseiro, originally established to develop Argentina's nuclear program, has one of the best traditions for training physicists in the world. "It is the equivalent of Los Alamos in the U.S." At Balseiro, Maldacena soon developed an interest in particle physics, and that directed him towards Princeton, where he obtained a PhD, and then went on to do post-doctoral work at Rutgers in 1996-97. Why Rutgers? "I had a number of offers," Maldacena explains,"but I delayed them for a year so that I could continue my research." Maldacena taught a semester at Harvard last fall, but then took a semester without teaching this spring. He has been commuting to the prestigious Center for Advanced Studies in Princeton during the week, and back to Harvard and Cambridge on the weekends. The work at the Institute of Advanced Studies, where Einstein worked for decades trying to develop a Unified theory, now puts Maldacena in close contact with people like Edward Witten, considered to be one of the top string theorists. Maldacena says his parents and two sisters--one is a chemist and the other works in physical education--are delighted with his success.
So is Harvard. Says David Nelson: "He is one of the few theorists I know who has really made a significant mark."
-- School for Advanced Studies, Princeton, New Jersey 1999