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. Similarly, the elementary particles we observe in particle accelerators in string theory could be thought of as the musical notes of elementary strings.

In string theory the string must be stretched under tension in order to become excited. The string tension in string theory is denoted by the quantity 1/(2 p a'), where a' is pronounced "alpha prime"and is equal to the square of the string length scale.

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 1.616 x 10-35 centimeters, or 10-20 the diameter of a proton, or 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 (or financing) 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 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. Particle accelerators could be on the verge of finding evidence for high energy supersymmetry in the next decade. Evidence for supersymmetry at high energy would be compelling evidence that string theory was a good mathematical model for Nature at the smallest distance scales.

Planck Length

From Wikipedia:
Because the Planck length is so much smaller than modern technology is capable of measuring accurately, no form of experimentation can discover it's true length. Research on the Planck length is therefore mostly theoretical.

In some forms of quantum gravity, the Planck length is the length scale at which the structure of spacetime becomes dominated by quantum effects, and it would become impossible to determine the difference between two locations less than one Planck length apart. The effects of quantum gravity are unknown; often it is suggested that spacetime might have a discrete or Quantum foamy structure at Planck length scale. The Planck area, equal to the square of the Planck length, plays a role in black hole entrophy.

According to the generalized uncertainty principle, the Planck length is, within a factor of order unity, the shortest measurable length. The search of the laws of physics valid at the Planck length are a part of the search for the Universal Theory.

## String Theory In A Nutshell

## Overview

superstringtheory.com explains it best.

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 modesof that guitar string under tension. Similarly, the elementary particles we observe in particle accelerators in string theory could be thought of as the musical notes of elementary strings.In string theory the string must be stretched under tension in order to become excited. The string tension in string theory is denoted by the quantity 1/(2 p a'), where a' is pronounced "alpha prime"and is equal to the square of the string length scale.

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 1.616 x 10-35 centimeters, or 10-20 the diameter of a proton, or 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 (or financing) 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 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. Particle accelerators could be on the verge of finding evidence for high energy supersymmetry in the next decade. Evidence for supersymmetry at high energy would be compelling evidence that string theory was a good mathematical model for Nature at the smallest distance scales.

## Planck Length

From Wikipedia:Because the Planck length is so much smaller than modern technology is capable of measuring accurately, no form of experimentation can discover it's true length. Research on the Planck length is therefore mostly theoretical.

In some forms of quantum gravity, the Planck length is the length scale at which the structure of spacetime becomes dominated by quantum effects, and it would become impossible to determine the difference between two locations less than one Planck length apart. The effects of quantum gravity are unknown; often it is suggested that spacetime might have a discrete or Quantum foamy structure at Planck length scale. The Planck area, equal to the square of the Planck length, plays a role in black hole entrophy.

According to the generalized uncertainty principle, the Planck length is, within a factor of order unity, the shortest measurable length. The search of the laws of physics valid at the Planck length are a part of the search for the Universal Theory.

## Scale of the Universe

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## Video