String theory proposes that the fundamental building blocks of the universe are not point-like particles, but tiny, vibrating strings. These strings are incredibly small - about 10⁻³⁵ meters long - and their different vibration patterns correspond to different particles we observe in nature.
What makes string theory special is that it naturally includes gravity. When physicists first developed string theory in the 1970s, they were trying to understand the strong nuclear force. But they discovered something remarkable: one of the vibration patterns of strings behaves exactly like the graviton, the hypothetical particle that carries the gravitational force.
String theory also requires extra dimensions beyond the familiar three dimensions of space and one of time. These extra dimensions are "compactified" - curled up so small that we can't detect them directly. The geometry of these hidden dimensions determines the properties of particles and forces we observe in our four-dimensional world.
Different string vibrations correspond to different particles
Key Concepts in String Theory
The 2D surface swept out by a string as it moves through spacetime
Complex 6-dimensional spaces that compactify the extra dimensions
Extended objects where open strings can end, crucial for gauge theories
Information in a volume can be encoded on its boundary surface
The Five String Theories
Type I
Open and closed strings with SO(32) gauge group
Type IIA
Non-chiral closed strings, even-dimensional branes
Type IIB
Chiral closed strings, odd-dimensional branes
Heterotic SO(32)
Left-moving bosons, right-moving fermions
Heterotic E₈×E₈
Exceptional gauge groups, phenomenologically promising
M-Theory
11D unification of all string theories