A long pipe with a wave rolling along it. From far away, the pipe looks like just a line — one dimension has vanished. That's roughly how physicists get from an 11-dimensional theory of gravity (which describes everything: particles and forces alike) down to 10 dimensions. They take one dimension and close it into a tiny ring, invisible at large scales.
In this work, the authors performed this trick with mathematical rigor. They started with the central equation of 11-dimensional supergravity — the condition linking spacetime curvature to field fluxes. It turns out that after folding one dimension into a ring, this condition transforms exactly into the condition for the 10-dimensional type IIA theory, which includes both familiar gravity and fields resembling electromagnetic ones.
The result is not just a technical trick. It shows that the laws for fields in 10 dimensions are not an independent set of rules but rather a shadow of a simpler, more symmetric construction in 11 dimensions. This is like how the shadow of a 3D object on a wall can look more complicated than the object itself, even though it is entirely determined by it.
The authors emphasize that this connection works not only for empty space but also when accounting for flux quantization: that is, when fields can take only certain discrete values, not arbitrary ones. This is a step toward a complete picture where the 10-dimensional theory is not just cut off from above but naturally grows out of the 11-dimensional one when one dimension is curled into a ring.
The number of dimensions in string theory (10 or 11) is not a whim but a requirement of mathematical consistency: with fewer dimensions, the equations develop meaningless infinities.
In Carl Sagan's novel "Contact," the heroine travels through a wormhole woven from extra dimensions — roughly the kind of geometry explored in such works.