" One of the features of the theory is that the strings can only vibrate in specific dimensions of space-time. Actually, only eleven dimensions. Any more, any less, the theory breaks down mathematically. Of course, in our universe we can only perceive four dimensions so the other seven must be, well, think of them as curled up and inaccessible to our reality, very hard to describe in words, easier to conceptualize in the formulae. "

― Glenn Cooper , The Resurrection Maker