Idea on the edgeSpeculative · Universe

Extra dimensions

Beyond the three familiar directions of space and one of time, additional dimensions may be curled up so small that they hide inside every point.

Speculative · not yet proven

Idea: More than four space-time dimensions
Origin: String theory, Kaluza-Klein theory
Size: Compact, below experimental reach
Tests: Collider searches, gravity at short range

A hidden six-dimensional shape

Lunch / Wikimedia Commons, CC BY-SA 2.5

A Calabi-Yau manifold, a compact shape that could hide extra dimensions — A Calabi-Yau manifold, the kind of compact shape in which string theory's extra dimensions could hide. Image: Lunch / Wikimedia Commons, CC BY-SA 2.5.

Hidden directions

The idea that space has extra directions goes back to Kaluza and Klein, who unified gravity and electromagnetism by adding a curled-up fifth dimension. Modern versions add more, all compact and too small to walk through.

We would not notice them at everyday scales, the way a thread looks one-dimensional from far away but has a small circumference up close.

Why they matter

String theory needs six or seven extra dimensions to be consistent. In some brane models the extra directions also explain why gravity is so weak compared with the other forces, because it spreads into them.

Searches

The Large Hadron Collider looks for missing energy that could escape into extra dimensions, and tabletop experiments test gravity at short distances. Nothing has shown up so far, which only pushes the dimensions smaller.

Key publications

Year	Paper	Venue	Link
1998	Arkani-Hamed, Dimopoulos & Dvali, The hierarchy problem and new dimensions at a millimeter	Phys. Lett. B 429, 263	DOI
1999	Randall & Sundrum, Large Mass Hierarchy from a Small Extra Dimension	PRL 83, 3370	DOI
1999	Randall & Sundrum, An Alternative to Compactification	PRL 83, 4690	DOI

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

Worth knowing

An extra dimension was first proposed over a century ago.
They could be tiny loops or more complicated shapes.
We would not notice them at everyday scales.