In a number of the previous chapters, we probed various systems while remaining largely in the limit where D-branes are pointlike in their transverse directions. However, we learned in chapter 10 that D-branes have an intrinsic geometry of their own, which can be seen when we place a lot of them together to produce a large back-reaction on the spacetime geometry, or if we were to turn up the string coupling (for fixed string tension) such that Newton's constant is strong. Both sorts of situation can and will be forced upon us later, so it is worthwhile trying to understand what we can learn by probing the supergravity geometry with different types of branes (we have already probed extremal p-branes with Dp-branes in section 10.3). If we choose things such that there is some supersymmetry preserved, we can use it to help us learn many useful things.

Probingpwith D(p - 4)

Let us probe the geometry of the extremal p-branes with a D(p - 4)-brane. From our analysis of chapter 11, we know that this system is supersymmetric. Therefore, we expect that there should still be a trivial potential for the result of the probe computation, but there is not enough supersymmetry to force the metric to be flat. There are actually two sectors within which the probe brane can move transversely.