String Theory Explained: A Basic Guide to String Theory

String theory provides a theoretical framework in which all particles, from photons to quarks, are one-dimensional strings as opposed to zero-dimensional points. If a version of string theory were found that held up in all contexts, it would serve as a single mathematical model for describing the nature of the universe—a “theory of everything” that would replace the Standard Model of physics, which does not explain gravity.

Comprehending the ins and outs of string theory requires extensive study, but familiarizing yourself with the main elements of string theory will give you a basic understanding of its core concepts.

1. 1. Strings and branes: Strings are one-dimensional filaments that come in two forms: open strings and closed strings. An open string has ends that don't connect, while a closed string forms a closed loop. Branes (derived from the word "membrane") are sheet-like objects that strings can attach to at either end. Branes are capable of moving through spacetime according to the rules of quantum mechanics.
2. 2. Additional spatial dimensions: Physicists accept that our universe contains three spatial dimensions, but string theorists argue for a model that describes extra dimensions of space. In string theory, at least six additional dimensions go undetected because they are tightly compactified into a complex folded shape called a Calabi-Yau manifold.
3. 3. Quantum gravity: String theory is a theory of quantum gravity because it attempts to merge quantum physics with the theory of general relativity. Quantum physics studies the smallest objects in the universe—like atoms and subatomic particles—while general relativity typically focuses on larger-scale objects in the universe.
4. 4. Supersymmetry: Also known as superstring theory, supersymmetry describes the relationship between two types of particles, bosons, and fermions. In supersymmetry string theory, a boson (or force particle) always has a counterpart fermion (or matter particle), and vice versa. The concept of supersymmetry is still theoretical, as scientists have not yet seen any of these particles. Some physicists speculate that this is because it would take incredibly high energy levels to generate bosons and fermions. These particles may have existed in the early universe before the big bang but then were broken down into the lower-energy particles seen today. The Large Hadron Collider (the world's highest-energy particle collider) may at some point generate enough energy to support this theory—though as of yet, it has not turned up evidence of supersymmetry.
5. 5. Unified forces: String theorists believe they can use interacting strings to explain how the four fundamental forces of nature—the force of gravity, electromagnetic force, strong nuclear force, and weak nuclear force—create a unified theory of everything.

The following timeline marks significant accomplishments in the field of string theory.

• 1968: Gabriele Veneziano, an Italian theoretical physicist working at the European Organization for Nuclear Research (CERN), used data collected from various particle accelerators to formulate the foundations of string theory. He constructed the dual-resonance model after realizing he could use the 200-year-old Euler beta function formula to explain the physical characteristics of strongly interacting particles.
• 1970: The name "string theory" is coined when three physicists—Leonard Susskind, Holger Nielsen, and Yoichiro Nambu—who used Veneziano's model to suggest the universe is made up of tiny vibrating strings.
• 1971: Theoretical physics professor Pierre Ramond initiated the development of superstring theory by formulating the concept of supersymmetry.
• 1974: Japanese physicist Tamiaki Yoneya discovered that string theory contains a particle with the properties of a graviton—a quantum particle that bears gravitational force—and realized that aspects of string theory may also be a theory of gravity.
• 1984: English physicist Michael Green and American physicist John Schwarz discovered the anomaly cancellation in type I string theory, which became known as the Green-Schwarz mechanism. This event further connected string theory ideas with supersymmetry and launched the first superstring revolution.
• 1985: The "Princeton String Quartet"—David Goss, Jeffrey Harvey, Emil Martinec, and Ryan Rohm—discovered heterotic strings, which are closed strings that are hybrids of a superstring and bosonic string.
• 1995: Edward Witten, a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey, suggested that the five different accepted versions of string theory aren’t actually separate theories. Witten proposed that they are just varying limits of one single theory that Witten called M-theory. The idea of M-theory launched the second superstring revolution.

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