# How do D-branes fit into closed string theories?

• Demystifier
In summary, D-branes are branes on which open strings end, but it is unclear how they can also exist in closed string theories IIA and IIB. Some suggest that closed strings interact with branes by connecting to them in a timelike manner, while others propose a connection through Ramond-Ramond fields. The exact relationship between D-branes and closed string theories is still not completely understood.
Demystifier
Gold Member
By definition, D(irichle)-branes are branes on which open strings end. Then how closed string theories IIA and IIB may contain D-branes? Or more precisely, why the branes appearing in closed string theories are still called D-branes?

Come on, is it possible that nobody knows the answer?
If so, it would also help if someone would say that he/she is also puzzled with this.

Hmm, here it goes a guess. The only way to connect a D brane with a closed string is by attaching all the closed circle with the D-Brane. Since this is the time-like dimension, or the endline of worldsheet 'pant', you can see that closed strings will only connect branes in a time-like manner, you can say that they will be moving and connecting branes to branes. So, closed strings will be the particles with which the branes interacts with each other. This is why when we are talking about the Type IIB version of string theory, string theory peopole frequently talk about ads/cft, because IIB is a superstring picture that clearly shows gravitational relations, or, interactions between strings.

Thinking about 'pants', or cobordism, the open strings would be 'ortogonal' to this, so, and exchange of closed strings would be more or less equivalent to stretching open strings.When people talk about N coincident branes, a more precise picture is achieved by adding a B anticomutative to the open strings connecting the branes. This field reduces the minimum energy stable energy along the aligning dimension, this is like the branes were being geometricaly deformed along this dimension. This is a sign of some kind of gravitational attraction between branes. And gravity is more clearly present in the IIB version.

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Note that this is just a guess, I really don't understand these things at all...

So essentially, is this because a closed string (without a loop) is dual to an open string (with a loop)?

Well, I guess so... Not sure though.

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The way D-ranes are usually related to type II is by means of the Ramond-Ramond fields that appear in the perturbative spectrum of stthat string theories. You need a source for that fields and it sis simple to see that point particles or on-dimensional strings can´t source them. Dp-branes, of apropiate dimensions.

Certainly this doesn't too much correspond they way the D-branes are incorporated in the bosonic string picture. If you read the original papers of Polchinsky you will se that he mentions an open string sector in the hilbert space of the type-II strings.

B.T.W. I made the same question some time ago in this phorum, try to search and see the answers there.

I see, so the strings couple to branes through the perturbative spectrum, like, in the perturbative spectrum, depending on the order and symmetry or anti-symmetry of the p form, it can couple to a variety of p branges.

But I would like to know if the antisymmetric modes are related to the type I with non commutative background B field.

I had the same question a while ago but never really got to the bottom of it (since these things tend to be quite deep).

My understanding is that D-branes in IIB string theory are identified with black p-branes which occur as solitonic solutions of IIB supergravity. The basic idea is that the ends of open strings ending on the D-brane can meet, forming a closed string which can leave the brane and vice-versa. Thus, open strings in IIB are a non-perturbative effect whereas they are already there perturbatively in e.g. Type I ST.

## 1. What are D-branes and closed strings?

D-branes and closed strings are two fundamental objects in string theory. D-branes are extended objects that can have different dimensions, while closed strings are one-dimensional objects that are closed loops. They both play important roles in understanding the behavior of strings and the nature of the universe.

## 2. What is the difference between D-branes and closed strings?

The main difference between D-branes and closed strings is their dimensionality. D-branes can have different dimensions, such as 0, 1, 2, or more, while closed strings are always one-dimensional. Additionally, D-branes have endpoints that are attached to them, while closed strings do not.

## 3. How do D-branes and closed strings interact?

D-branes and closed strings can interact through the exchange of closed strings between them. This interaction is governed by the laws of string theory, and it plays a crucial role in understanding the dynamics of D-branes and their relationship to closed strings.

## 4. What is the significance of D-branes and closed strings in string theory?

D-branes and closed strings are crucial in understanding the fundamental principles of string theory and how it relates to other theories in physics. They also play a key role in the study of black holes and the behavior of matter at the subatomic level.

## 5. How are D-branes and closed strings related to the concept of extra dimensions?

In string theory, it is proposed that there are extra dimensions beyond the three spatial dimensions we experience in our daily lives. D-branes and closed strings provide a way to understand and visualize these extra dimensions, as D-branes can exist in different dimensions and closed strings can wrap around these dimensions.

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