String theory is a theory within physics which is under development and which aims at explaining all forces and particles of nature from more basic principles.
Currently one uses two traditional theories which are not 100% correct, but which work well within great but different areas. One is the standard theory wich is based on quantum mechanical principles and that can explain the electromagnetic force, the weak force and the strong force. This theory works very well for very small objects and small distances, but the theory is very clumpsy, with several constants that are set by measurement, and not by the theory itself.
The other is the general theory of relativity that can explain the gravitational force, and the differences measured in length, time, velocity and mass for the same object when one changes from one referance frame to another. This theory works best when the masses and distances are great.
String theory and a more developed version of it, M-theory, is based on the principles of quantum mechanics and relativity and can explain all forces, all particles and most constants based on more fundamental notions, but the theory is still not completed even though it has been worked on for 30 years.
String theory, is based on an axiom that there are more than 3 spatial dimmentions, and hence more that 3 directions that stand in a right angle at each other, actually 10 in the current version, or M-theory, which is described in the following.
Objects or particles can have a shape of all from 0 to 10 dimmentions. These are called N-branes where N is the number of dimmentions the object has. Branes can also be of any sizes from that of tiny elementary particles to the size of an universe. A 1-brane is also called a string.
Our universe is an enormous 3-dimmentional object, a so-called 3-brane or word-brane, that is placed in an even greater super-space with more that 3 dimmentions, and with possibly many more universes formed as branes.
The elementary particles like electrons and quarks are actually strings. These can be open in both ends or be closed to small rings. When a string is open, the open ends are fixed to a 3- brane, and hence confined within the realm of a universe. The ring-shaped strings originate within a universe, but are free to jump over to other universes and interact there too.
Several of the dimmentions are however shrunken to tiny bubles around the strings, but that can happen in many ways, so that the boubles get different shapes and are wrapped around each other in different ways.The strings can vibrate in several ways, and they can be oriented in several ways inside the shrunken dimmentions that are wrapped around them.
All these possibilities of vibration and orientation of a string gives it different combination of properties, and actually the same properties that the differnt known particles have, like electrical charge, spin, charm, colors, mass, energy, boson and fermion properties and the like.
In modern quantum mechanics, forces are explained as emission of particles called bosons, having integer spin, from one particle, and caching of that boson by another particle, so that the energy and other properties in the boson is transfered between the two particles. The different vibrations and orientations also explain the bosons of various kind and thus the various kind of forces.
Ring-formes strings may as already mentioned possibly move out from the word-brane and transverse the superspace over to another word-branes or universe. Gravitons that transfere the gravitational forse is thought to act that way. In this way our universe and other universes may interfere with each other by gravitation. This may explain why the gravitational force is felt so weak and possibly also the hidden mass acting in our universe.
The shrinking of many dimmentions and the confinement to the 3-dimmentional word-brane is the cause that our world looks 3-dimmentional to us.
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