String Theory Introduction
String theory (ST) has been developing in fits and spurts over the past 25 years. Before ST, the two prevailing theories of the workings of the universe were Quantum Theory, which correctly describes events at extremely small scales, and Einstein's Theory of General Relativity, which correctly describes events at extremely large scales. These theories worked harmoniously in each of their respective scales but when combined, cataclysmic rifts between them emerged. ST can bridge the gap between these two incompatible theories. String theory assumes that everything in the universe is made up of the same fundamental matter, one-dimensional strings whose vibration patterns result in the matter we see every day. These strings exist in a world so small, so undetectable, that it is believed they vibrate in a total of 9 dimensions (6 extremely small dimensions in a form of a Calabi-Yau manifold & 3 extended (everyday) dimensions.)
How can there be 6 extra dimensions? Why haven't we seen or detected them in our everyday life? Please refer to Figures 1 & 2, which provide a two-dimensional analogy.
The most beautiful aspect of ST is that there is only one input parameter in the whole equationÐ the tension of the string. In the Standard Model the input parameters are the experimental masses, charges, and spins of the 19 fundamental particles. While the Standard Model explains all of the occurrences we can detect with our current technological knowledge, it does little to explain why particles have their intrinsic properties. For example, why does an electron have a negative charge and a mass of 9.1 x 10^-31 kg? String Theory is like a super-super-computerÉ.by inputting just one number it is conceivable to get our universe and its physical properties as output. The key to these strings and small multi-dimensions is their super-microscopic size.
The six curled-up dimensions and the strings they contain exist at a size known as the Planck size, about 10^-33 cm. Please refer to Figure 3. At this size it would take a particle accelerator the size of the universe to probe these small spaces. Thus we will never "see" strings like we can "see" protons and electrons.
Some other websites that give an introduction to Superstring Theory:
Superstring Theory Basics at "The Official" String Theory Website
Superstring Theory at Ira Flatow's Science Friday Site - Contains a 50-minute RealAudio broadcast that includes Brian Greene, author of The Elegant Universe. HIGHLY RECOMMENDED!
A Superstring Theory Tutorial by a UC Santa Barbara grad student