String theory

String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that are the basis of the Standard Model of particle physics. For this reason, string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Study of string theories has revealed that they require not just strings but other objects, variously including points, membranes, and higher-dimensional objects.

History

String theory was originally invented to explain peculiarities of hadron behavior. In particle-accelerator experiments, physicists observed that the angular momentum of a hadron is exactly proportional to the square of its energy. No simple model of the hadron, such as picturing it as a set of smaller particles held together by spring-like forces, was able to explain these relationships. In order to account for these "Regge trajectories," physicists turned to a model where each hadron was in fact a rotating string, moving in accordance with Einstein's special theory of relativity. This led to the development of bosonic string theory, which is still the version first taught to many students. (The original need for a viable theory of hadrons has been fulfilled by quantum chromodynamics, the theory of quarks and their interactions. It is now hoped that string theory or some descendant of it will provide a fundamental understanding of the quarks themselves.)

Related Topics:
Hadron - Angular momentum - Energy - Special theory of relativity - Bosonic string theory - Quantum chromodynamics - Quark

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Bosonic string theory is formulated in terms of the Nambu-Goto action, a mathematical quantity which can be used to predict how strings move through space and time. By applying the ideas of quantum mechanics to the Nambu-Goto action—a procedure known as quantization—one can deduce that each string can vibrate in many different ways, and that each vibrational state appears to be a different particle. The mass the particle has, and the fashion with which it can interact, are determined by the way the string vibrates—in essence, by the "note" which the string sounds. The scale of notes, each corresponding to a different kind of particle, is termed the "spectrum" of the theory.

Related Topics:
Nambu-Goto action - Quantum mechanics - Quantization

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

These early models included both open strings, which have two distinct endpoints, and closed strings, where the endpoints are joined to make a complete loop. The two types of string behave in slightly different ways, yielding two spectra. Not all modern string theories use both types; some incorporate only the closed variety.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

However, the bosonic theory has problems. Most importantly, as the name implies, the spectrum of particles contains only bosons, particles like the photon which obey particular rules of behavior. While bosons are a critical ingredient of the Universe, they are not its only constituents. Investigating how a string theory may include fermions in its spectrum led to supersymmetry, a mathematical relation between bosons and fermions which is now an independent area of study. String theories which include fermionic vibrations are now known as superstring theories; several different kinds have been described.

Related Topics:
Bosons - Photon - Fermions - Supersymmetry - Superstring theories

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of an unknown 11-dimensional theory called M-theory. These discoveries sparked the second superstring revolution. (Several meanings of the "M" have been proposed; physicists joke that the true meaning will only be chosen when the theory is finally understood.)

Related Topics:
1990s - Edward Witten - M-theory - Second superstring revolution

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Many recent developments in the field relate to D-branes, objects which physicists discovered must also be included in any theory which includes open strings of the super string theory.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~