Science › Black holes & quantum gravity
Extra dimensions
1921· Idea
SpeculativeA serious idea with no experimental proof yet.
The speculative idea that space has more than three dimensions, with the extra ones hidden from everyday view.
See it on the Science map →How it works
In 1921 Theodor Kaluza showed that adding a fifth dimension to Einstein's equations produces electromagnetism alongside gravity; Oskar Klein suggested in 1926 that the extra dimension is curled into a tiny circle. String theory needs six or seven extra dimensions. In 1998 physicists proposed some could be as large as a millimeter, if only gravity can travel into them.
Why it matters
If real, they could unify the forces and explain why gravity is so weak. They are the main way string theory connects to our world.
Think of it like
A garden hose looks like a line from far away; up close it has a circular dimension an ant can walk around.
How we know
None found. Precision torsion-balance tests show gravity behaves normally down to about 50 micrometers, and the Large Hadron Collider has seen no particles leaking into extra dimensions.
Still a mystery
If they exist, how many, what shape, and how small? Current experiments only rule out the largest versions.
Connections (2)
▸String theory requires Extra dimensionsSpeculative
The relationship
String theory's math only works if space-time has ten dimensions (eleven in M-theory). Otherwise the quantum version of a vibrating string produces impossible results, like probabilities that do not add up. Since we see three of space and one of time, the other six must be curled into shapes far too small to notice, which then set the particles and forces we observe.
How the link was found
Claud Lovelace (1971) found the original string needs 26 dimensions; superstrings need 10. Candelas, Horowitz, Strominger and Witten (1985) proposed the six curled into "Calabi–Yau" shapes.
Is it proven?
The requirement is a mathematical result. Experiments show no sign of extra dimensions: gravity behaves normally down to about 50 micrometers and colliders see no leakage.
Open questions and mysteries
Which of countless possible shapes describes our universe, and why it stays stable, are unknown.
▸Extra dimensions might explain why gravity is weakest of The four forces of natureSpeculative
The relationship
Gravity is roughly 10^36 times weaker than electromagnetism between two protons. One idea: the other forces are confined to our three-dimensional space, but gravity spreads into extra dimensions too, so most of its strength leaks away. Then gravity would only look weak to us, while being comparable to the other forces at short distances.
How the link was found
Arkani-Hamed, Dimopoulos and Dvali proposed large extra dimensions in 1998; Lisa Randall and Raman Sundrum offered a warped version in 1999.
Is it proven?
Not supported so far. Torsion-balance tests find gravity normal down to about 50 micrometers, and the Large Hadron Collider found no missing energy or tiny black holes predicted by these models.
Open questions and mysteries
Why gravity is so weak, called the hierarchy problem, remains one of physics' main unsolved puzzles.