Pulling the Strings

 

You'd think physicists would be satisfied. It may have taken them several dozen years, but they did succeed in formulating the Standard Model -  the widely accepted and so far most substan- tiated theory on the structure of matter in the universe. This theory neatly divides particles into several "families,"affected by three forces.

Yet physicists are now striving to show that all forces in nature are different aspects of a single, basic component. If they succeed, they hope to have reconciled the Standard Model with another highly acclaimed theory -  Einstein's General Theory of Relativity, which explains gravity. Currently these two landmark achievements don't get along too well.

 

The proposed component underpinning all matter and gravity in the universe is called a "string."It's different from the particles described by the Standard Model. For starters, it's much smaller. In fact, a string compares to an atom in the same way that an atom compares to the entire solar system. Second, all the "regular"particles are usually viewed by physicists as points, lacking dimensions such as length, width, or depth. In contrast, a string is believed to have a dimension -  length. It is able to vibrate in different ways, creating a multitude of particles.

Most of these particles, however, can be revealed only under exceedingly high-energy conditions, such as those that existed in the first few moments after the Big Bang. Since it is unlikely that such conditions can be reproduced on Earth, all physicists working on string theory are, unsurprisingly, theoreticians -  like Dr. Ofer Aharony and Dr. Micha Berkooz, who recently joined the Weizmann Institute of Science's Particle Physics Department.

Aharony and Berkooz hope to prove that all particles making up the universe, including a sought-after particle underlying gravity (the graviton), are created by the vibration of strings as they move, unite, and separate. In other words, material reality is determined by the "music"played by these string elements.

In the beginning it seemed that string theory could hold true only in a universe possessing a whopping 26 dimensions. The number of dimensions then dropped to 10, and eventually the theory turned out to require 11 dimensions. (The number depends somewhat on one's definition of "dimension.") This leaves two options: It is either impossible to apply string theory to our four-dimensional universe (three spatial dimensions and one of time) or our universe may indeed consist of 11 dimensions, with seven of them beyond our perception.

Physicists working on string theory have shown that this second option is in fact conceivable as long as one is willing to accept that the extra sevendimensions exist in "folded"form next to the familiar four. According to their calculations, if these additional, folded dimensions are very small, their existence will not contradict the observed picture of material reality.

To understand what is meant by "folded dimensions,"think of an ant crawling along a pipe. When crawling in a straight line along the pipe, the ant is aware of one dimension -  length. If it were to crawl around the pipe's diameter, it would discover an additional dimension. But should the pipe's diameter be very small, much smaller than the ant itself, it would no longer be able to crawl around the pipe and, in effect, would not even be aware of its existence.

String theory physicists propose a picture of the world in which the remaining seven dimensions are so "folded"or "shrunken"that we are unable perceive them. In trying to present our four-dimensional world as a partial view of the string theory universe, scientists are examining how familiar phenomena would appear within the string theory framework.

One such phenomenon isholography, which can be relatively easily shown to occur near black holes and involves the compression of three-dimensional information into the two-dimensional surface of the black hole. Physicists have managed to explain this phenomenon in certain black holes using stringtheory. Aharony and Berkooz are studying additional problems within this system, including the information problem: Can radiation created near a black hole be used to reveal the nature of the matter swallowed up by the hole? For example, if we throw a chair into a black hole, will we be able to describe this chair on the basis of the radiation produced at the site?

Another phenomenon that may occur near black holes is described by the so-called "little string theory."Despite its friendly name, this theory is far more complex than the original string theory: It describes the joint behavior of numerous intertwined strings. Little string theory does not require the existence of a graviton. Aharony and Berkooz are examining whether the strong combination of several strings eliminates the need for this particle.

The "hottest"topic in their research, however, deals with the relationships among strings themselves. What is the relationship between two strings moving in parallel? According to the picture currently painted by string theorists, strings can join or separate after touching each other even for a split second, creating a tiny tunnel that connects them and immediately disappears. Aharony and Berkooz, together with Prof. Eva Silverstein of Stanford University, are now exploring the possibility of an entirely different, less tangible connection between strings, in which strings influence one another without physically converging. The aim, they explain, is to grasp the true nature of such "long-distance relationships."