String Theory

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String Theory For Dummies From String Theory For Dummies by Andrew Zimmerman Jones String theory, often called the “theory of everything,” is a relatively young science that includes such unusual concepts as superstrings, branes, and extra dimensions. Scientists are hopeful that string theory will unlock one of the biggest mysteries of the universe, namely how gravity and quantum physics fit together.

String Theory Features String theory is a work in progress, so trying to pin down exactly what the science is, or what its fundamental elements are, can be kind of tricky. The key string theory features include:

• All objects in our universe are composed of vibrating filaments (strings) and membranes (branes) of energy.
  
  
• String theory attempts to reconcile general relativity (gravity) with quantum physics.
  
  
• A new connection (called supersymmetry) exists between two fundamentally different types of particles, bosons and fermions.
  
  
• Several extra (usually unobservable) dimensions to the universe must exist.
  
  

There are also other possible string theory features, depending on what theories prove to have merit in the future. Possibilities include:

• A landscape of string theory solutions, allowing for possible parallel universes.
  
  
• The holographic principle, which states how information in a space can relate to information on the surface of that space.
  
  
• The anthropic principle, which states that scientists can use the fact that humanity exists as an explanation for certain physical properties of our universe.
  
  
• Our universe could be “stuck” on a brane, allowing for new interpretations of string theory.
  
  
• Other principles or features, waiting to be discovered.
  
  

Superpartners in String Theory String theory’s concept of supersymmetry is a fancy way of saying that each particle has a related particle called a superpartner. Keeping track of the names of these superpartners can be tricky, so here are the rules in a nutshell.

• The superpartner of a fermion begins with an “s,” so the superpartner of an “electron” is the “selectron” and the superpartner of the “quark” is the “squark.”
  
  
• The superpartner of a boson ends in “–ino,” so the superpartner of a “photon” is the “photino” and of the “graviton” is the “gravitino.”
  
  

Use the following table to see some examples of the superpartner names.

Some Superpartner Names Standard Particle Superpartner Higgs boson Higgsino Neutrino Sneutrino Lepton Slepton Z boson Zino W boson Wino Gluon Gluino Muon Smuon Top quark Stop squark Keeping Track of String Theory’s Many Names String theory has gone through many name changes over the years. This list provides an at-a-glance look at some of the major names for different types of string theory. Some versions have more specific variations, which are shown as subentries. (These different variants are related in complex ways and sometimes overlap, so this breakdown into subentries is based on the order in which the theories developed.) Now if you hear these names, you’ll know they’re talking about string theory!

• Bosonic string theory
  
  
• Superstring theory (or Supersymmetric string theory)
  
  
  • Type I, Type IIA, Type IIB, Heterotic string theories (Type HE, Type HO)
    
    
• M-theory
  
  
  • Matrix theory
    
    
• Brane world scenarios
  
  
  • Randall-Sundrum models (or RS1 and RS2)
    
    
• F-theory
  
  

Key Events in String Theory History Although string theory is a young science, it has had many notable achievements. What follows are some landmark events in the history of string theory:

1968: Gabriele Veneziano originally proposes the dual resonance model.

1970: String theory is created when physicists interpret Veneziano’s model as describing a universe of vibrating strings.

1971: Supersymmetry is incorporated, creating superstring theory.

1974: String theories are shown to require extra dimensions. An object similar to the graviton is found in superstring theories.

1984: The first superstring revolution begins when it’s shown that anomalies are absent in superstring theory.

1985: Heterotic string theory is developed. Calabi-Yau manifolds are shown to compactify the extra dimensions.

1995: Edward Witten proposes M-theory as unification of superstring theories, starting the second superstring revolution. Joe Polchinski shows branes are necessarily included in string theory.

1996: String theory is used to analyze black hole thermodynamics, matching earlier predictions from other methods.