© BCCP, Lawrence Berkeley National Laboratory.
One of the most striking, and difficult to imagine, features of string theories is that they predict the existence of extra dimensions of space. The best-studied string theories have ten spatial dimensions, rather than the three to which we are accustomed. Where are these extra dimensions, and why can't we see them? One answer is that they could be very small, curled up into compact geometries rather than infinite in extent like the spatial dimensions in which we live. This means that above each point in our visible dimensions, a small space of extra dimensions like the ones shown here may be hidden.