String Theory

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  • 1. String Theory
    Basic Properties
  • 2. There are two actions used to describe how the strings move through space and time. They predict where the strings are located on the world sheet which describes the embedding of the string in spacetime.
    Nambu-Goto
    Polyakov
    The Nambu-Goto action is used for 1 x 1 dimensional flat targets
    The Nambu-Goto action is not the primary action that physicists use when they develop quantized versions of string theory.
    The Polyakov action is more easily quantized than the Nambu-Goto action because it is linear and it is therefore used more often.
  • 3. Characteristics of the strings
    The strings oscillate because of tension and kinetic energy
    The quantum mechanics of strings states that these oscillations take on vibrational modes.
    These modes behave differently from particles.
    The String’s mass, spin, and charge is determined by the string’s dynamics.
  • 4. Characteristics of the strings
    In a guitar string, the different notes that sound when the string is plucked are a result of the properties of the string: tension, length, thickness.
    The notes coming out of the guitar are excitation modes of the string.
    The particles that are observed in particle accelerators can be thought of as musical notes, or excitations modes of strings.
    Different excitation modes make the strings appear as electrons, photons, and so on.
    Strings have a miniscule length that is known as the Planck length. The Planck length is about 10-33 centimeters
  • 5. Levels of magnification:1. Macroscopic level - Matter2. Molecular level3. Atomic level - Protons, neutrons, and electrons4. Subatomic level - Electron5. Subatomic level - Quarks6. String level
  • 6. Worldsheets…
    Graphs are used to describe the position of point-like particle’s motion.
    These graphs depict the worldline of the particle
    The worldline represents the particles history in spacetime: where it has been positioned.
    Worldsheets can be used to reflect when strings split or connect.
    • This picture represents worldsheets of closed strings spitting or connecting.
  • How many string theories are there?
    Theorist can build string theories by setting different characteristics.
    Start with a tiny string
    It is either an open string or a closed string
    It can have only bosons (particles that transmit forces) or it can also have fermions (particles that make up matter)
    If there are only bosons, the theorist will have a bosonic string theory
    If there are also fermions, then they will need supersymmetry (equal number of bosons and fermions: equal amount of forces and matter.
    This supersymmetry leaves the theorist with a superstring theory.