"The boundaries of physics have been changing. Now scientists ask not only how the world works (a question the Standard Model answers) but why it works that way (a question the Standard Model cannot answer). Einstein asked "why" earlier in the century, but only in the past decade or so have the "why" questions become normal scientific research in particle physics, rather than philosophical afterthoughts. One ambitious approach to "why" is known as string theory, which is formulated in an eleven-dimensional world. Work on string theory has proceeded so far by study of the theory itself, rather than via the historical fruitful interplay of experiment and theory. As Edward Witten remarks... string theory predicts that nature should be supersymmetric. Supersymmetry is a surprising and subtle idea—the idea that the equations representing basic laws of nature don't change if certain particles in the equations are interchanged with one another."