We have already stated that since the D-brane is a dynamical object, and couples to gravity, it should have a mass per unit volume. This tension will govern the strength of its response to outside influences which try to make it change its shape, absorb energy, etc. We have already computed a recursion relation (5.11) for the tension, which follows from the underlying T-duality which we used to discovere D-branes in the first place.

In this chapter we shall see in detail just how to compute the value of the tension for the D-brane, and also for the orientifold plane. While the numbers that we will get will not (at face value) be as useful as the analogous quantities for the supersymmetric case, the structure of the computation is extremely important. The computation puts together many of the things that we have learned so far in a very elegant manner which lies at the heart of much of what will follow in more advanced chapters.

Along the way, we will see that D-branes can be constructed and studied in an alternative formalism known as the ‘boundary state’ formalism, which is essentially conformal field theory with certain sorts of boundaries included. For much of what we will do, it will be a clearly equivalent way of formulating things which we also say (or have already said) based on the spacetime picture of D-branes.