What is the difference between 10 and 11?
The simplest string theory is ten dimensional. Strings can interact with each other. If the interaction among strings is large, the theory is hard to describe. It turns out that when strings interact very, very strongly, something surprising happens. A new dimension opens up and we have a theory in eleven dimensions, the ten we started with plus an extra circle. In eleven dimensions we do not have strings, we have membranes. Membranes wrapped along the 11th dimension give rise to strings.
— Who’s Counting? Is it 10 or 11? (dimensions, that is — M Theory is making me Manic!)
— Prof. Juan Maldacena
— Institute for Advanced Study
2013.09.01 Sunday ACHK