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17 April 2014 12:48 pm ,
Vol. 344 ,
Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Why Only Three Dimensions?
4 October 2005 (All day)
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.