Alternative theoryCosmology · Quantum gravity

String theory / M-theory

Particles become tiny vibrating strings, the mathematics needs extra dimensions, and the separate versions may all be faces of one deeper framework called M-theory.

Alternative theory · unproven

Core idea
Fundamental objects are one-dimensional strings
M-theory
Unifies five string theories in 11 dimensions
Extra dimensions
Six or seven compact, hidden dimensions
Goal
A single theory of gravity and quantum physics
Status
No direct experimental test yet

Vibrating strings

MissMJ / Wikimedia Commons, CC BY 3.0
Diagram of strings, branes and compact extra dimensions from string theory
A diagram of strings, branes and compact extra dimensions from string theory. Image: MissMJ / Wikimedia Commons, CC BY 3.0.

Strings instead of points

Point particles are replaced by tiny one-dimensional strings. Each vibration mode of a string acts like a different particle, so one object can in principle stand in for the whole particle zoo. Gravity appears naturally, which is the main attraction.

The strings are far smaller than anything current experiments can resolve, so the idea stays out of direct reach.

Extra dimensions

The mathematics only closes with more than four space-time dimensions. The extra directions are compact, curled up so small that they hide inside every point of ordinary space. Their shapes and sizes would set the physics we observe.

M-theory

In the mid 1990s the five known string theories were shown to be limits of one deeper framework, M-theory. It lives in eleven dimensions and includes membranes, not just strings. No experiment has confirmed any of it, and the theory has no single, settled formulation.

Key publications

YearPaperVenueLink
1984Green & Schwarz, Anomaly cancellations in supersymmetric D=10 gauge theory and superstring theoryPhys. Lett. B 149, 117DOI
1995Polchinski, Dirichlet Branes and Ramond-Ramond ChargesPRL 75, 4724DOI
1996Duff, M-theory (the theory formerly known as strings)Int. J. Mod. Phys. A 11, 5623DOI

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

  • Different vibration modes of a string act like different particles.

  • The extra dimensions are hidden in every point of space.

  • M-theory includes membranes as well as strings.

Recommended videos

  • String Theory Explained Kurzgesagt
  • String Theory ScienceClic English

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