Extra dimensions could be huge, even a millimeter wide. Gravity leaks into them.
ADD Large Extra Dimensions
Up to seven flat extra dimensions, potentially as large as a millimeter.
Looping ambient scene for Braneworld & the Bulk. String-theory-inspired models where our universe is a brane in a higher-dimensional bulk. The bulk pre-exists and persists. Variants differ in the bulk geometry: warped (RS), flat with large dimensions (ADD), or with parallel branes (Ekpyrotic).
§1 · The claim, in one sentence
The hierarchy problem is solved by allowing gravity to dilute into up to seven large extra dimensions.
§2 · Why it might be true
Arkani-Hamed, Dimopoulos, and Dvali (ADD) proposed in 1998 that the hierarchy problem can be solved by allowing the extra dimensions to be very large, up to a millimeter, rather than warped or compactified at the Planck scale.
In ADD, our 3+1 dimensions are a 3-brane embedded in a higher-dimensional flat Minkowski bulk with up to 7 extra dimensions. The Standard Model is confined to our brane, but gravity propagates into the full higher-dimensional space. The fundamental gravity scale can be as low as TeV; gravity appears weak only because it is diluted into the extra-dimensional volume.
The family stance
A higher-dimensional spacetime, the bulk, with its own geometry. Different variants propose different bulk structures.
§2.5 · Evidence
- Elegant solution to hierarchy if extra dimensions are large
- Direct experimental tests via sub-millimeter gravity experiments
§3 · What you'd need to test it
- Deviations from Newton's law at the size of the extra dimensions
- Production of microscopic black holes at TeV colliders
- Kaluza-Klein graviton emission in scattering experiments
§4 · Where it breaks
- Sub-millimeter gravity tests have not seen deviations
- LHC has not produced microscopic black holes
- Simplest ADD scenarios excluded by collider searches
- Astrophysical constraints (supernova energy loss) further constrain it
Go deeper
In ADD, the hierarchy emerges from volume dilution: M_Pl² ~ M*^(2+d) · R^d, where M_Pl is the 4D Planck mass, M* is the higher-dimensional fundamental scale, R is the size of the extra dimensions, and d is the number of large extra dimensions.
For M* ~ TeV and d = 2, R ~ 1mm. For d = 7, R is much smaller. The original "large extra dimensions" referred to R far larger than the Planck length.
Direct gravity tests probe R via measurements of the inverse-square law at sub-millimeter distances. Current bounds: R < ~30 micrometers for d = 2.
▸§5 · Who built it, and when(2 sources, 2 established)
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