# Matrix geometry for F-strings

A stack of N D-branes has the strange property that the traverse D-brane coordinates are matrix-valued. When the matrices commute, they can be interpreted as ordinary coordinates for N indistinguishable objects. But in general they correspond to something classical geometry doesn't describe. In particular this leads to a non-perturbative description of string theory on asymptotically Minkowski spacetime: Matrix Theory.

S-Duality exchanges F-strings and D1-strings. This means this odd "matrix geometry" should happen for F-strings as well. The question is, how can we see it directly, without invoking S-duality?

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I apologize for posting additional questions before carefully reading and replying to the discussion around the answers to my previous questions. This is not out of disrespect to the effort put into writing these answers, for which I am very grateful. This is merely because I suspect the site will be closed in 2 days for which reason I'm shooting all the questions I got. –  Squark Dec 11 '11 at 21:44

is indeed an exact description of fundamental type IIA strings (and similarly $E_8\times E_8$ heterotic strings) at any (e.g. weak) coupling where you can explicitly see the off-diagonal degrees of freedom. You could say that this description is was obtained by dualities from the low-energy dynamics of D1-branes and you would be right. However, when properly interpreted etc., it's a description of fundamental strings, too.
However, if the objects we want to connect are fundamental strings, which are light, the uncertainty principle guarantees that they will not be sitting in a fixed position determined with the accuracy better than $L_{\rm string}$ which is why the description of the perturbations in terms of off-diagonal open strings is impossible.