## String theory

A brief introduction

String theory is a mathematical theoretical framework in physics in which the point-like particles of particle physics are replaced by one-dimensional objects called strings.

String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string.

left shows a point like particle interaction, right shows string loop interaction. image: wikipedia/ Kurochka

Extra dimensions

An important feature of  string theory is that extra dimensions are required for the many properties observed. These extra dimensions are often dealt with via the mathematical concept of compactification.

a single string appears two dimensional but could contain more within it. This is also known as a manifold  image: Alex Dunkel (Maky) /wikipedia

a two-dimensional hypersurface of the quintic Calabi-Yau three-fold manifold. This is a way of representing 2D strings wrapped up on itself. image: wikipedia

Gravity and quantum mechanics

In string theory, one of the vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.

String theory is a broad and varied subject that attempts to address a number of deep questions of fundamental physics. String theory has been applied to a variety of problems in black hole physics, early universe cosmology, nuclear physics, and condensed matter physics, and it has stimulated a number of major developments in pure mathematics.

Because string theory potentially provides a unified description of gravity and particle physics, it is a candidate for a theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter.

Despite much work on these problems, it is not known to what extent string theory describes the real world or how much freedom the theory allows to choose the details.

Attempts to resolved black hole singularities

In relativistic physics a black hole contains a singularity(infinite density etc) but string theory suggests that blackholes don’t contain singularity rather a ‘fuzzball’ of strings with finite density.  A typical 6.8 M☉ fuzzball would have a mean density of 4.0×1017 kg/m3. A bit of such a fuzzball the size of a drop of water (0.05 mL, 5.0×10−8 m3) would have a mass of twenty million metric tons, which is the mass of a granite ball 240 meters in diameter.

How string theory developed

String theory was first studied in the late 1960s as a theory of the strong nuclear force, before being abandoned in favor of quantum chromodynamics. Subsequently, it was realized that the very properties that made string theory unsuitable as a theory of nuclear physics made it a promising candidate for a quantum theory of gravity.

The earliest version of string theory, bosonic string theory, incorporated only the class of particles known as bosons. It later developed into superstring theory, which posits a connection called supersymmetry between bosons and the class of particles called fermions.

Five consistent versions of superstring theory were developed before it was conjectured in the mid-1990s that they were all different limiting cases of a single theory in eleven dimensions known as M-theory. In late 1997, theorists discovered an important relationship called the AdS/CFT correspondence, which relates string theory to another type of physical theory called a quantum field theory.

Challenges of string theory

One of the challenges of string theory is that the full theory does not yet have a satisfactory definition in all circumstances.

Another issue is that the theory is thought to describe an enormous landscape of possible universes, and this has complicated efforts to develop theories of particle physics based on string theory.

These issues have led some in the community to criticize these approaches to physics and question the value of continued research on string theory unification.

Commentary
String Theory is still an active area of research. Here is a recent Q&A about string theory

Source adapted from: String theory. (2015, July 7). In Wikipedia, The Free Encyclopedia. Retrieved 09:55, July 8, 2015, from https://en.wikipedia.org/w/index.php?title=String_theory&oldid=670289441

Wikipedia contributors. (2021, June 2). Fuzzball (string theory). In Wikipedia, The Free Encyclopedia.
Retrieved 01:50, June 28, 2021, from https://en.wikipedia.org/w/index.php?title=Fuzzball_(string_theory)&oldid=1026474149