Why Only Three Dimensions?

Length, width, and depth: Those three dimensions seem plenty for you and me, but string theorists claim there are at least six others hidden from view. So then why is our world not blatantly 9-dimensional or 1-dimensional or anything other than vivid 3D? New research has shown that of all the possible dimensional realities, only those of three or seven dimensions would survive in an expanding universe. We may have ended up being 3D because it was the most probable.

In its basic form, string theory describes subatomic particles as bits of vibrating string. It is purely a theoretical model, as the strings are far too small to be probed by current particle accelerators (ScienceNOW, 26 February 2003) . One of the remarkable predictions of the theory is that there are extra spatial dimensions, which are inaccessible because they are tightly curled up or because physics as we know it is trapped on a 3-dimensional surface, called a 3-brane. Either way, there is nothing that requires the number of accessible dimensions to be three.

So why did we end up with three? Perhaps because a 3D reality has an evolutionary advantage over other dimensions, report Andreas Karch of the University of Washington in Seattle and Lisa Randall of Harvard University in the October edition of Physical Review Letters. The researchers assumed that the universe began as a 9-dimensional volume filled with branes of every dimensionality possible: 1D strings, 2D sheets, 3D cubes, and higher dimensional branes that are harder to imagine. As this universe expanded, these branes destroyed one another if they came too close, analogous to the way matter annihilates antimatter. But 3-branes may have survived because they have a harder time "finding" each other, says Randall. The only alternative reality would be on a 7-brane, where--despite the extra elbow room--life would be hard to imagine, since "seven-dimensional gravity is too weak to allow planets to have stable orbits around stars," Karch says.

Theoretical physicist David Tong of Cambridge University in the United Kingdom finds this work "very speculative," as it leaves open how gravity--the one force that can radiate into extra dimensions--becomes confined to the 3-brane. The researchers say resolving that question is their next step. If it can be worked out, Tong thinks brane evolution could be a fruitful new line of research. "It gives a history to the universe that arrives at the ingredients that others had just assumed," he says.

Related sites
The paper on the arxiv preprint server
Randall's home page
The Official String Theory Web Site