# what exactly is the string theory?

Discussion in 'Physics & Math' started by nanok, Feb 2, 2002.

1. ### nanokRegistered Senior Member

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Also if anyone knows any good websites that explain it in detail that would be great too......thanks

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5. ### flamethrowerJuniorRegistered Senior Member

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In String Theory, the myriad of particle types is replaced by a single fundamental building block, a string'. These strings can be closed, like loops, or open, like a hair. As the string moves through time it traces out a tube or a sheet, according to whether it is closed or open. Furthermore, the string is free to vibrate, and different vibration modes of the string represent the different particle types, since different modes are seen as different masses or spins.
Think of a guitar string that has been tuned by stretching the string under tension across the guitar. Depending on how the string is plucked and how much tension is in the string, different musical notes will be created by the string. These musical notes could be said to be excitation modes of that guitar string under tension. In a similar manner, in string theory, the elementary particles we observe in particle accelerators could be thought of as the "musical notes" or excitation modes of elementary strings.

In string theory, as in guitar playing, the string must be stretched under tension in order to become excited. However, the strings in string theory are floating in space-time, they aren't tied down to a guitar. Nonetheless, they have tension. One mode of vibration, or note', makes the string appear as an electron, another as a photon. There is even a mode describing the graviton, the particle carrying the force of gravity.

If string theory is to be a theory of quantum gravity, then the average size of a string should be somewhere near the length scale of quantum gravity, called the Planck length, which is about a millionth of a billionth of a billionth of a billionth of a centimeter. Unfortunately, this means that strings are way too small to see by current or expected particle physics technology and so string theorists must devise more clever methods to test the theory than just looking for little strings in particle experiments.

String theories are classified according to whether or not the strings are required to be closed loops, and whether or not the particle spectrum includes fermions. In order to include fermions in string theory, there must be a special kind of symmetry called supersymmetry, which means for every boson (particle that transmits a force) there is a corresponding fermion (particle that makes up matter). So supersymmetry relates the particles that transmit forces to the particles that make up matter.
Supersymmetric partners to currently known particles have not been observed in particle experiments, but theorists believe this is because supersymmetric particles are too massive to be detected at current accelerators.

Hope this helps a little.