String theory is the most recent attempt to reconcile quantum mechanics and general relativity. It's the first candidate for the theory of everything, a manner of describing the known fundamental forces and matter in a mathematically complete system.
We learned in school that matter is made of atoms, which are in turn made of just three basic components: electrons [spinning] around a nucleus composed of neutrons and protons. In chemistry, some of us also learned that generally, there are as many neutrons as are protons in any nucleus of an atom. There are however isotopes where their nucleus contains an uneven mix of protons and neutrons.
It was quite comfortable for us to think of the atom in this way – somewhat reminiscent of planets revolving around the sun. The students that had a special interest in physics went on to discover that the electron is a truly a fundamental particle (it is one of a family of particles known as leptons), but neutrons and protons are made of smaller particles, known as quarks. Quarks are, as far as we now know, also elementary particles.
The universe is made up of atoms and forces through which atoms interact.
Our current knowledge about the subatomic composition of the universe is summarized in what is known as the Standard Model of particle physics. It describes both the fundamental building blocks out of which the universe is made, and the forces through which these blocks interact. There are twelve basic building blocks:-
Six of these are quarks which go by the interesting names of up, down, charm, strange, bottom and top. A proton incidentally, is made of two up quarks and one down.
The other six are Leptons and include the electron and its two heavier siblings, the Muon and the Tauon, as well as three neutrinos.
There are four fundamental forces in the universe:
§ the Weak and
§ Strong nuclear forces also known as the colour force.
Each of these is produced by fundamental particles that act as carriers of the force. The most familiar of these is the photon, a particle of light, which is the mediator of electromagnetic forces. (This means that, for instance, a magnet attracts a nail because both objects exchange photons.) The graviton is the particle associated with gravity. The strong force is carried by eight particles known as gluons. Finally, the weak force is transmitted by three particles, the W+, the W-, and the Z.
We use what we call the Standard Model to describe the interaction of sub-particles and forces with great success. This is, however, with one notable exception - gravity. The gravitational force has proven very difficult to describe microscopically. This has been for many years one of the most important problems in theoretical physics. That is to formulate a single model that describes both the micro and macro elements of the universe. Einstein attempted to unify the general theory of relativity (macro) with electromagnetism using a single field, hoping to recover an approximation for quantum theory. A "theory of everything" is closely related to unified field theory, and also attempts to explain all physical constants of nature.
In the last 20 or so years string theory has emerged as a promising model in attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe, another “Theory of Everything”.
The basic idea is that all of the components of the Standard Model are just different manifestations of one basic element - a string. One way to think about this is by imagining an electron to be a tiny loop of string rather than a single zero-dimensional point. The loop (string) can as well as moving, oscillate in different ways. It is the way strings oscillate that determine the type of subatomic building blocks we recognize. So one kind of oscillation may be an electron whilst another oscillation may be regarded as a photon. This means, if true
The entire universe is made of strings
In recent years many developments have taken place, radically improving our understanding of what the theory is. String theories also require the existence of several extra, unobservable, dimensions to the universe, in addition to the usual four space-time dimensions.
Five major string theories have been formulated with the main differences between them, being the number of dimensions in which the strings are developed within and their characteristics. In the mid 1990s a unification of all previous superstring theories, called M-theory, has been proposed, which asserted that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space.