What is a probe D-brane in string theory?

[Creighton Hogg]

Stupid Question #2 for today:

I started trying to read hep-th/0403247, QED and String Theory, last night
but I really don't understand this concept of probe D-branes.
When they say that they're taking a probe Dp-brane in the background of a
D9-antiD9 brane system, is the Dp-brane part of the background too or is
it just a mathematical construct, kindof a reverse method of images, meant
to help analyze the physics of the system. If it's part of the
background, I don't understand why it's being called a probe, but if it's
just a construct I don't understand why they're describing the physics on
its worldvolume. Sorry, but I'm rather confused at this point.

========================
[moderator note: a probe is an object - a real object, not just a virtual
gedanken experiment - that is meant to measure and see the properties of
the background, and it is probe because it should be light or invisible
enough so that it does not affect the background, or at least we can
neglect such a "backreaction". Nevertheless, in string theory you are only
allowed to consider probes that are allowed by the theory - not some
fictitious angels. For example, string theory predicts that there are no
objects that are more localized than some concrete (sub)planckian
distance. This really means that shorter distances don't exist in the
geometrical sense. Everyone: please feel free to give a different answer. lm]

 

[R0man]

Thank you for your question. D-brane probes are an important concept in string theory and can be confusing for beginners. Let me try to clarify it for you.

A D-brane is a type of object in string theory that can be thought of as a hypersurface, or a higher-dimensional membrane, on which strings can end. These D-branes can have different dimensions, labeled by a number p, and are often represented as Dp-branes.

Now, when we talk about a probe Dp-brane in the background of a D9-antiD9 brane system, we are essentially talking about placing a Dp-brane in the vicinity of this D9-antiD9 system. The Dp-brane is a real object and is indeed part of the background. However, it is called a "probe" because it is much lighter and less massive than the D9-antiD9 system, and therefore does not significantly affect the dynamics of the system. It is like using a small and delicate instrument to measure the properties of a larger and more complex object.

The reason why we describe the physics on the Dp-brane's worldvolume is because that is where the strings can end and interact with the D-brane. This allows us to study the behavior of strings in the presence of the D9-antiD9 system by analyzing the interactions between the strings and the probe Dp-brane.

I hope this explanation helps to clear up your confusion. Remember, it is always important to ask questions and seek clarification when learning about complex concepts like string theory. Keep reading and exploring, and you will gain a deeper understanding of D-brane probes and other concepts in string theory.