📖 Standard Introduction

String theory is a candidate theory in theoretical physics that attempts to unify quantum mechanics and general relativity. It proposes that the fundamental building blocks of the universe are not point particles, but one-dimensional "strings". Different vibration patterns of these strings correspond to different elementary particles. String theory requires spacetime to have 10 or 11 dimensions (M-theory), with the extra 6 or 7 dimensions compactified at the Planck scale. The theory encompasses five different string theories (Type I, Type IIA, Type IIB, Heterotic SO(32), Heterotic E8×E8), later unified by Witten's M-theory. String theory predicts the existence of gravitons and naturally incorporates gravity, making it a candidate for the "Theory of Everything".

💬 Popular Introduction

String theory says: Everything in the universe is not made of "points", but of extremely tiny "strings", like guitar strings. These strings vibrate, and different vibration patterns produce different particles - just like guitar strings vibrating to produce different notes. Electrons, photons, quarks - all are different "notes" of the same string. More amazingly, string theory says our universe is not 3-dimensional, but 10 or 11-dimensional. It's just that the extra dimensions are too small for us to see. This theory attempts to explain "everything" - from the smallest particles to the entire universe, from quantum mechanics to gravity. Although there is no experimental evidence yet, it is the "ultimate theory" dreamed of by physicists.

💡 Core Concepts

Basic Assumption: The fundamental components of the universe are not point particles, but one-dimensional "strings"

Dimensions: Requires 10 or 11 dimensions to be self-consistent (3 spatial dimensions + 1 time dimension + 6 or 7 curled dimensions)

Historical Development:

• 1968: Veneziano proposed the string model to describe strong interactions
• 1984: First string theory revolution, discovery of anomaly cancellation
• 1995: Second string theory revolution, Witten proposed M-theory
• 1997: Maldacena proposed AdS/CFT correspondence

Five String Theories:

• Type I、Type IIA、Type IIB、Heterotic SO(32)、Heterotic E8×E8
• M-theory unifies these five theories