String theorists have come up with some extremely elegant and subtle conjectures as to why, out of the nine spatial dimensions they posit, exactly three of them expanded to enormous size after the big bang while the remaining six got choked off and remained tiny. But there is another sort of explanation, one that is perhaps easier to grasp: in a world where the number of spatial dimensions was anything other than three, beings like us simply could not exist. In a space of more than three dimensions, there would be no stable planetary orbits. (This was proved a century ago by Paul Ehrenfest.)
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