Today, mathematicians regularly think about abstract spaces having arbitrary numbers of dimensions. We speak about n-dimensional space, and we have developed geometry and calculus in any number of dimensions. As we saw in chapter 10, Allan Cormack, the inventor of the theory behind CT scanning, wondered how CT would work in four dimensions, purely out of intellectual curiosity. Great things have come from this spirit of pure adventure. When Einstein needed four-dimensional geometry for curved space and time in general relativity, he was pleased to learn it already existed, thanks to Bernhard
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