 # Theory of relativity

Special theory of relativity   |   General theory of relativity   |   Tensor calculus

# Quantum mechanics

Schrodinger's equation   |   Matrix mechanics

### String theory explained

In physics, string theory is a theoretical framework in which the point-like objects of particle physics are replaced by one-dimensional entity called strings. It describes how these strings propagate through space and interact with themselves. On distance scales larger than the string scale, a string looks just like an simple particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational pattern of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.

What are strings? It is a one dimensional entity. But when it moves or vibrates it sweeps out a two dimensional area. These strings came in two forms — closed strings and open strings. An open string has ends that don’t touch each other, while a closed string is a loop with no open end. It was eventually found that these early strings, called Type I strings, could go through five basic types of interactions, as shown this figure. The first equation is the equation of relativistic closed string which motion is not periodic. The equation of relativistic open string is periodic and its motion can be described by the center of the mass motion of the string and oscillations about the center of mass. The first two terms on the right-hand-side describe the position of the center-of-mass of the string and the velocity of the center-of-mass of the string through the momentum in the I'th direction and the time coordinate &tao; The sum over oscillatory terms describes the oscillation of the string about its center of mass. In the classical description of the string, the coefficients α and essentially correspond to Fourier coefficients, as the sum correlates to a Fourier series giving the contribution of each mode to the string vibration. When the relativistic string is quantized, and represent annihilation and creation operators that destroy and create excitations of each mode, in perfect analogy with the quantum harmonic oscillator.     