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 spacetime, they aren't tied down to a guitar. Nonetheless, they have tension. 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 10-33 centimeters, 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.
Srimad-Bhagavatam 3.26.32 P Fundamental Principles of Material Nature
When egoism in ignorance is agitated by the sex energy of the Supreme Personality of Godhead, the subtle element sound is manifested, and from sound come the ethereal sky and the sense of hearing.
It appears from this verse that all the objects of our sense gratification are the products of egoism in ignorance. It is understood from this verse that by agitation of the element of egoism in ignorance, the first thing produced was sound, which is the subtle form of ether. It is stated also in the Vedanta-sutra that sound is the origin of all objects of material possession and that by sound one can also dissolve this material existence. Anavrttih sabdat means "liberation by sound." The entire material manifestation began from sound, and sound can also end material entanglement, if it has a particular potency. The particular sound capable of doing this is the transcendental vibration Hare Krsna. Our entanglement in material affairs has begun from material sound. Now we must purify that sound in spiritual understanding. There is sound in the spiritual world also. If we approach that sound, then our spiritual life begins, and the other requirements for spiritual advancement can be supplied. We have to understand very clearly that sound is the beginning of the creation of all material objects for our sense gratification. Similarly, if sound is purified, our spiritual necessities also are produced from sound.
Here it is said that from sound the ether became manifested and that the air became manifested from ether. How the ethereal sky comes from sound, how the air comes from sky and how fire comes from air will be explained later on. Sound is the cause of the sky, and sky is the cause of srotram, the ear. The ear is the first sense for receiving knowledge. One must give aural reception to any knowledge one wants to receive, either material or spiritual. Therefore srotram is very important. The Vedic knowledge is called sruti; knowledge has to be received by hearing. By hearing only can we have access to either material or spiritual enjoyment.
In the material world, we manufacture many things for our material comfort simply by hearing. They are already there, but just by hearing, one can transform them. If we want to build a very high skyscraper, this does not mean that we have to create it. The materials for the skyscraper--wood, metal, earth, etc.--are already there, but we make our intimate relationship with those already created material elements by hearing how to utilize them. Modern economic advancement for creation is also a product of hearing, and similarly one can create a favorable field of spiritual activities by hearing from the right source. Arjuna was a gross materialist in the bodily conception of life and was suffering from the bodily concept very acutely. But simply by hearing, Arjuna became a spiritualized, Krsna conscious person. Hearing is very important, and that hearing is produced from the sky. By hearing only can we make proper use of that which already exists. The principle of hearing to properly utilize preconceived materials is applicable to spiritual paraphernalia as well. We must hear from the proper spiritual source.