In string theory, the fundamental objects are one-dimensional extended entities called "strings." These strings are incredibly tiny and can be thought of as vibrating loops. Unlike point particles in traditional particle physics, which have zero size, strings have length.
The properties and behavior of particles in string theory arise from the various ways in which the strings can vibrate. Different vibration patterns correspond to different particle types. Just as different musical notes are produced by the vibrations of different strings on a musical instrument, the different particles in string theory emerge from the vibrational modes of the string.
The vibrational modes of a string can be understood as the different ways the string can oscillate and deform. The specific pattern of vibrations determines the particle's mass, charge, and other properties. For example, a string vibrating in one mode might represent a particle with the properties of an electron, while a different vibration mode might correspond to a photon or a quark.
In string theory, there are two types of strings based on their shape and topology: closed strings and open strings.
Closed Strings: A closed string is a string that forms a complete loop. It has no endpoints. Closed strings can vibrate in different modes, producing various particles. The vibrations of closed strings correspond to particles that are inherently interacting with gravity, and they naturally include gravitons, which are hypothetical particles that mediate the gravitational force.
Open Strings: An open string is a string with two distinct endpoints. Open strings can vibrate as well, but they have some distinct characteristics. The vibrations of open strings correspond to particles that do not directly interact with gravity. These particles include photons (particles of light) and other gauge bosons that mediate the fundamental forces other than gravity, such as the electromagnetic force or the strong nuclear force.
The difference between closed strings and open strings lies in their boundary conditions. Closed strings have no endpoints, while open strings have endpoints that can interact with other objects (like D-branes, which are important in string theory). These different boundary conditions give rise to different types of vibrational modes and, consequently, different types of particles with varying properties.
It's important to note that string theory describes a unified framework that incorporates both gravity and the other fundamental forces of nature. By treating particles as different vibrational modes of strings, string theory attempts to provide a consistent and mathematically elegant description of the fundamental constituents of the universe.