Allahabad: String theory is based on the idea that the elementary constituents of matter are not point particles, but one dimensional objects i.e. strings. This theory automatically combines quantum mechanics, and general relativity -- Einstein's theory of gravity.
It also has the potential for explaining the other known forces of nature -- gravitational, strong, weak and electromagnetic forces as the elementary particles interact via various kinds of forces. These forces give rise to complicated interaction between elementary particles. These interactions can be measured by studying how the elementary particles scatter off each other.
In the theoretical study of elementary particles we must remember two important points:
In typical experiments elementary particles move very fast. Hence we need to use special theory of relativity.
The elementary particles are very small. Hence we need to describe them using quantum mechanics.
There is a mathematical theory, known as the standard model, which describes all the elementary particles and their forces if we leave out gravity. This theory is based on the principles of quantum mechanics and special theory of relativity. In principle this theory can be used to calculate the result of any experiment that we wish to perform involving elementary particles.
So far the standard model has been extremely successful in explaining almost all observed experimental data.
There is, however, a serious reason why standard model is not complete. It does not contain one important force that we observe in nature, namely, the gravitational force.
In all present day experiments gravitational force between elementary particles is extremely small and beyond measurement. But any complete theory must account for all forces, however small.
That is where the string theory steps in. It is a mathematical model that contends that different elementary particles are different vibrational states of a string.
We need to formulate a theory of strings consistent with the principles of quantum mechanics and special theory of relativity.
It turns out that as a consequence of these two requirements strings automatically exert gravitational force on each other. Furthermore, the laws governing this gravitational force are automatically consistent with the principles of quantum mechanics and general theory of relativity.
However, string theory is so tightly constrained that we cannot adjust it to suit our needs. In particular we cannot demand that the dimension of space is what we observe in nature (ı.e. 3), or that different vibrational states of the string behave like the elementary particles we observe in nature.
We have to take what string theory gives us. First of all one finds that there are five consistent string theories. They differ from each other in the way the string vibrates.
Furthermore one finds that in each of these five string theories the dimension of space is 9! It requires 9 coordinates to describe a point in space instead of the usual 3 coordinates.
This does not describe what we see in nature. This, however, is not the end of the story. The intrinsically 9 dimensional theory can be made to appear as a 3 dimensional theory by using an old idea known as compactification.
I have worked on various aspects of string theory, but the contribution that was cited in the prize is strong weak coupling duality. This refers to certain symmetries of string theory which are hidden and not easy to discover. In the mid 90's I devised specific strategies for discovering and finding evidence for such symmetries. This was later used by others to discover many other duality symmetries, and eventually led to the realization that the five consistent string theories known at that time are all related by various duality transformations, and hence different limits of a single underlying theory.
(Dr Ashoke Sen is Professor at Harishchandra Research Institute in Allahabad and is one of the foremost string theorists in the world. He received the Fundamental Physics Prize, that carries a cash reward of $3 million, in 2012. Dr Sen will be chatting with IBNLive readers who are interested in knowing about this theory and Indian research in fundamental sciences. Click here to post your questions.)
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