String Theory (ST) is the theory of everything. Pretty simple, right? A short sentence that encloses such a beauty and elegant theory.
But, first at all, one may ask what ST is. The answer is very simple: a Quantum Gravity Theory. ST provides us a nice way to construct, at Quantum level, a unified theory for all the Interactions.
All the fundamental forces have some similarities in their inner structure. Let's start with Electromagnetism (EM). From the ancient times, mankind has felt fascination by Natural Phenomena as electrical storms. Magnetism has been present in our important ages. Without magnetism, we cannot drive across the seas, or even more fundamental: we couldn't see without it. Our optical cones in our eyes are photoreceptors made to explore the surrounds. Another interesting application of EM is to keep our bodies separate from the others. This is made possible due to the electromagnetic forces between atoms in all the matter. We can see, hear and even taste EM in our modern technology.
But, what does EM so special? R. P. Feynman once said that understanding EM would give us the key to understand the Universe, and he wasn't wrong. EM give us the first insight into Modern Physics. At the beginning of twentieth century, Physics was walking from the classical street (made of Newton's ideas: causality) into the quantum one (reign by probabilities). Newton never could find the physical origin of the force, he put it on mathematical language: force is the result of a change of momentum (awesome!). This problem remained unanswered for at least four centuries until Quantum Mechanics give us the solution: Force (or interaction) is the result of the interchange of mediators (quanta of force) which carry momentum. This mediators are called virtual particles. In the case of EM, the mediator is the Photon, a particle with zero rest mass and spin one (I will talk about this two things later on another article, I promise). Following this ideas, we can say that you don't cross the floor until the Earth's hot center because the atoms in your body interchange photons with the atoms in the floor, warning them to avoid the fusion of both sets of atoms.
Then, if the EM as a force has a mediator, the other force known, Gravity, must have one too. But Gravity and EM are different. Gravity is a (geometrical) perturbation of the Spacetime. EM is a perturbation travelling ON the Spacetime: EM doesn't interact directly with the intrinsic properties of Spacetime. The landscape was growing up with the discovery of the other two fundamental interactions: Weak Force (WF), responsible for the beta decay; and Strong Force (SF) which keeps attached the atomic nuclei. This new forces had many things in common with EM. First, both have quantum description. The WF quanta are the Z and W massive bosons, and the SF quanta are the massless Gluons (some of you could say to me here that the SF quanta are the pions. That's true in some part. Pions are interchanged by nucleons, but aren't fundamental. Pions are combinations of Quarks and Gluons, and since any mediator must be fundamental -i.e. without internal structure-, Pions are rejected). Second, the three theories seems to share the same mathematical structure. SF and WF are examples of Yang - Mills theories. This feature made them "easy" to unify. This remarkable work of unify this three giants was made by S. Weimberg, S, Glashow and A. Salam (awarded with the Nobel Price in the seventy's). And third, the most striking: the mediators of the three forces are bosons of spin one. What this would mean?? It's a total mystery.
And, what about Gravity? Can't be seen as a Yang Mills field. Actually, is more complicated than that. Another problem with Gravity lies in the renormalizability. General Relativity (GR) cannot be renormalized, so it couldn't be a good model for Gravity at high energies and low dimensions. Here the theorists said that maybe GR must be a hidden face of a big theory, a quantum theory of Gravitation. If Gravity has a quantum description, it must have then a mediator. This is called Graviton, which is a particle of spin two. This behavior is unique! All the other mediators are spin one, so here must be something missing. Today this question is still open.
So, when do physicists began to think in Strings as a theory of Quantum Gravity?
