Gravity and String theory

Among the numerous revolutionary ideas of the twentieth century that changed our vision of the world, the Einstein’s theory of gravity and quantum mechanics are undoubtedly the most innovative and intrepid proposals. Although both imply a radical change of Newtonian physics, quantum mechanics is long far away from our daily intuition.

General Relativity uses crystalline and tangible concepts, like continuous space-time geometry. We are used to see curved surfaces. The property that leads to strange phenomena is the fact that the curvature involves also the time direction. This implies that in General Relativity the rate at which we age depends, not only on our motion –like in the special theory of relativity- but also on where we are standing. These effects are not evident in the weak gravitational field of the earth.

Quantum mechanics deals instead with a random microscopic universe, with different possible realities and with discontinuous changes. The evolution of any process is not determined, we are only given to know the probability that different things can occur; it is a world invaded by the uncertainty. Phenomena do not acquire a real meaning until the moment they are observed. All possible realities are equally valid, latent realities which are materialized when they interact with our measurement devices. The naïve attempt to construct a quantum version of General Relativity with the usual quantum field theory rules leads to absurd results, like e.g. that the mass of the electron is infinity.

Quantum physics is known to govern the world of elementary particles. But the gravitational force among elementary particles is too weak to be detected. This prevents us from setting up experiments that could measure quantum gravitational phenomena in the current laboratories. However, in extreme situations quantum gravitational effects become very important. Examples are the physics near the center of a black hole or the physics in the first moments of the big-bang. In order to understand how the universe was born, it is crucial to know in full detail the theory that incorporates at the same time all the forces, the theory that unifies gravity and quantum mechanics. A candidate for such theory is string theory.
 
String theory has two important properties that today are believed to be indispensable requirements for any physical theory: it preserves special relativity and quantum mechanics. The basic idea is to replace particles by strings, that is, one-dimensional objects whose vibration modes represent the different particles of the universe, in a way that all principles of quantum physics are preserved. The internal consistency of the theory requires that there must be six hidden spatial dimensions. They are believed to be wrapped in a tiny space so that they cannot be directly observed. However, these extra dimensions have important implications. In particular, the effective physical theory that governs our four dimensional world is dictated by this internal space, and understanding its structure is one of the main challenges in order to connect string theory with the real world.