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AH, BUT does it? For anyone versed in modern theoretical physics, that鈥檚 not such a silly question. 鈥淚 don鈥檛 know any mathematical reason why three-dimensional space is more consistent than any other number,鈥� says of Stanford University in California.

Susskind is one of the founders of string theory, which is our best stab yet at a unified understanding of physics 鈥� and perhaps the best-known model in which extra dimensions are found. One of string theory鈥檚 peculiar features is that when applied to fewer than nine spatial dimensions, the mathematics goes wild, predicting violent fluctuations that rip apart the very fabric of the universe.

But extra dimensions do more than just save string theory鈥檚 blushes. In a wider set of theories, gravity leaking into a higher space could explain why in our three dimensions it is so weak compared with the other fundamental forces, and why the expansion of the universe is apparently accelerating.

So perhaps the question becomes: why does space have three visible dimensions?

One idea from string theory is that . Or perhaps our 3D universe exists on one of many 鈥渂ranes鈥�, membrane-like entities that float around in a larger, higher-dimensional space.

of Harvard University and Andreas Karch of the University of Washington in Seattle have shown that branes colliding in such a space would tend to form 3D branes 鈥� an explanation of sorts for why we see a universe with three dimensions. If large other dimensions do exist, particles escaping into them might take energy from those we see. 鈥� so far without success.

Such failures rile of the University of Aix-Marseille in France. 鈥淭he idea that space could have more than three dimensions was proposed more than half a century ago and has delivered more disappointments than results,鈥� he says.

So we鈥檙e back to the more basic question: why only three dimensions? Perhaps because some aspect of our universe wouldn鈥檛 work otherwise. The precise degree of quantum weirdness we observe (see 鈥�Where does quantum weirdness end?鈥�) seems only possible in 3D space, for instance. And Susskind points out that in any other number of dimensions the electromagnetic and gravitational forces would have very different strengths. With fewer than three spatial dimensions, for example, gravity does not lead to attractive forces. Atoms, planets and stars would not form properly 鈥� and humans would not be around to ponder the existence of dimensions.

鈥淎toms, planets and stars may not form properly with fewer than three dimensions鈥�

Did such a peculiarly curiosity-friendly 3D universe arise as part of a 鈥渕耻濒迟颈惫别谤蝉别鈥� in which every possible type of universe exists, a possibility espoused by string theory? Or is it just one of a kind? That鈥檚 a toughie, but either way, we鈥檙e still a distance away from explaining the essential three-ness of space, says Randall. 鈥淚 think it would be lovely to have an answer, but as of now we don鈥檛.鈥�

Read more: 鈥�10 mysteries that physics can鈥檛 answer鈥� yet鈥�

This article appeared in print under the headline 鈥淲hy does space have three dimensions?鈥�