The essential idea behind string theory is -- all of the different 'fundamental' particles of the Standard Model are really just different manifestations of one basic object: a string.
The String Theory provides the first way of uniting two major scientific principals, general relativity and quantum mechanics into a single harmonious whole. All the happenings of the universe arise from the vibrations of a single entity or string: microscopically tiny loops of energy that lie deep within the heart of the matter with the potential to unify all the forces of nature providing a master equation governing all known as the 'Theory of Everything'.
General relativity describes gravity in terms of the curvature of space-time by matter/energy and successfully quantifies the very large. Quantum mechanics, on the other hand, explains the behavior of atoms and subatomic particles, characterizing the very small.
Picture an electron, for instance, as a point with no internal structure. A point cannot do anything but move. If string theory is correct, then under an extremely powerful 'microscope' we would realize that the electron is not really a point, but a tiny loop of string. A string can do something aside from moving--- it can oscillate in different ways. If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron. But if it oscillates some other way, well, then we call it a photon, or even a quark.
Perhaps the most remarkable thing about string theory is that such a simple idea works--- it is possible to derive (an extension of) the Standard Model (which has been verified experimentally with incredible precision) from a theory of strings. But it should also be said that, to date, there is no direct experimental evidence that string theory itself is the correct description of Nature. This is mostly due to the fact that string theory is still under development. We know bits and pieces of it, but we do not yet see the whole picture, and we are therefore unable to make definite predictions. In recent years many exciting developments have taken place, radically improving our understanding of what the theory is.
By positing that all matter and all fundamental forces can be described in terms of the vibrations of tiny, one-dimensional strings. (Mathematically, the theory also requires the existence of multiple, extra dimensions, said to be 'curled-up' and as such beyond the realm of our sensory experience.) The idea here is that these minuscule strands of energy vibrating are comprised within 11 dimensions creating every particle and force in the universe known as the M-Theory.
Theoretical Physicist, Brian Greene compares the wiggling strands of string theory to the strings of musical instruments. Not only do different patterns of vibration produce different particles, but the whole universe is 'akin to a string symphony vibrating matter into existence.'
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