Why do we need string theory?

The goal of theoretical physics is to find a theory that can unite all fundamental forces. In 1970s, the standard model based on quantum field theory has been able to unite electromagnetic force, strong and weak nuclear forces. However, to incorporate the gravitational force into the quantum field theory was very difficult. The string theory, which is dramatically different from quantum field theory, turns out to be quite successful in uniting all fundamental forces. 

What is string theory?

In this theory, all fundamental particles are considered as the vibrational modes (patterns) of a fundamental string. The "particles" are analogous to the "notes" in music. Different vibrational frequencies of a violin string produce different notes. Similarly, different vibrational modes of a fundamental string give rise to different particles.

The most important "prediction" of the string theory is that the space must NOT be three dimensional (length, width and height). If the fundamental string can vibrate only in three dimensional space, no meaningful results can be obtained. To derive the observed properties of all fundamental particles, the fundamental string must vibrate in a space with 9 or 10 dimensions.

How come we do not see extra dimensions?

Currently, there are two different models that may explain why we do not see the 6 or 7 extra dimensions: compactification and braneworld. The compactification model assumes that the extra dimensions are simply too small to be observed. This model was proposed first because similar idea has been suggested as early as in 1920s.

In the braneworld model proposed by Randall and Sundrum, the reason why we cannot see extra dimensions is because we are confined in a braneworld (with 3 dimensions of space) that is embedded in the bulk universe (with 9 or 10 dimensions of space). This model is based on the study of D-branes, a feature arising from the boundary condition on the ends of an open string. According to this model, photons and all other particles (except gravitons) cannot escape from our braneworld. Therefore, we cannot see anything outside our braneworld.

Can string theory unite science with religion?

For religious people, the braneworld model should be much more exciting than the compactification model. If the compactification model were true, there would be no place for God. The braneworld model opens the possibility for the existence of other braneworlds or even some intelligent life in the bulk universe. Since the bulk universe has more dimensions than our braneworld, it has more degrees of freedom. The intelligent life in the bulk universe should be much more powerful than us. More discussions on God and science ....

Web Links:

Stretching your brane

Brane theory alters physics big picture

D-brane technology

The physics of extra dimensions

The elegant universe - from NOVA

Beyond String Theory

Official String Theory Web Site