§2.6: String Theory

String theory replaces point-like particles with one-dimensional, extended strings
as the fundamental objects.

So what is string theory, then?

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 are created by the string. These notes could be called the excitation modes of that guitar string under tension.

There are two basic types of string theories: those with closed string loops that break into open strings, and those with closed string loops that don't break into open strings

String theory replaces point-like particles with these one-dimensional, extended strings as the fundamental objects. It began in the late 1960's as an attempt to describe strong nuclear forces, the forces responsible for keeping the nucleus of an atom together. In string theory, the elementary particles could be thought of as the "musical notes" or excitation modes of elementary strings.

There were two main problems that stopped the string theory description of strong nuclear forces. One was that the theory required the existence of a kind of particle that didn't belong in a nucleus -- a particle with no mass and two units of spin. The second problem was that the theory required space-time to have at least ten dimensions (nine space and one time), while the rest of physics deals in three space and one time dimension. To top it all off, a relativistic quantum field theory known as quantum chromodynamics (QCD) emerged at this time as a convincing point-particle theory of the strong nuclear force.