Physicists who work with a concept called string theoryenvision our universe as an eerie place with at least nine spatialdimensions, six of them hidden from us, perhaps curled up in some wayso they are undetectable. The big question is why we experience theuniverse in only three spatial dimensions instead of four, or six, ornine.
Two theoretical researchers from the University ofWashington and Harvard University think they might have found theanswer. They believe the way our universe started and then diluted asit expanded -- what they call the relaxation principle -- favoredformation of three- and seven-dimensional realities. The one we happento experience has three dimensions.
"That's what comes out whenyou do the math," said Andreas Karch, a University of Washingtonassistant professor of physics and lead author of a new paper thatdetails the theory.
Karch and his collaborator, Lisa Randall, aphysics professor at Harvard, set out to model how the universe wasarranged right after it began in the big bang, and then watch how thecosmos evolved as it expanded and diluted. The only assumptions werethat it started with a generally smooth configuration, with numerousstructures -- called membranes, or "branes" -- that existed in variousspatial dimensions from one to nine, all of them large and none curledup.
The researchers allowed the cosmos to evolve naturally,without making any additional assumptions. They found that as thebranes diluted, the ones that survived displayed three dimensions orseven dimensions. In our universe, everything we see and experience isstuck to one of those branes, and for it to result in athree-dimensional universe the brane must be three-dimensional.
Other realities, either three- or seven-dimensional, could be hidden from our perception in the universe, Karch said.
"Thereare regions that feel 3D. There are regions that feel 5D. There areregions that feel 9D. These extra dimensions are infinitely large. Wejust happen to be in a place that feels 3D to us," he said.
Inour world, forces such as electromagnetism only recognize threedimensions and behave according to our laws of physics, their strengthdiminishing with distance. Gravity, however, cuts across alldimensions, even those not recognized in our world, Karch and Randallsay. But they theorize that the force of gravity is localized and, withseven branes, gravity would diminish far more quickly with distancethan it does in our three-dimensional world.
"We know there arepeople in our three-brane existence. In this case we will assume thereare people somewhere nearby in a seven-brane existence. The people inthe three-brane would have a far more interesting world, with morecomplex structures," Karch said.
With gravity diminishing rapidlywith distance, a seven-dimensional existence would not have planetswith stable orbits around their sun, Karch said.
"I am notprecisely sure what a universe with such a short-range gravity wouldlook like, mostly because it is always difficult to imagine how lifewould develop under completely different circumstances," he said. "Butin any case, planetary systems as we know them wouldn't form. Thepossibility of stable orbits is what makes the three-dimensional worldmore interesting."
Karch and Randall detail their work in theOctober edition of Physical Review Letters, published by the AmericanPhysical Society. The research was supported by grants from the U.S.Department of Energy and the National Science Foundation.
Karchsaid they hope the work will spark extensive scientific exploration ofmany other questions involving string theory, extra dimensions and theevolution of the cosmos.
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