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String Theory Demystified

String Theory Demystified

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CHAPTER 13<br />

D-Branes<br />

One of the most interesting developments in string theory over the last decade or so<br />

was the realization that the theory could incorporate higher-dimensional extended<br />

objects—that is, objects beyond one-dimensional strings. When these objects are<br />

associated with Dirichlet boundary conditions we call these extended objects<br />

Dp-branes, where p indicates the number of spatial dimensions it has. The word<br />

“brane” comes about by analogy. In our everyday world of three spatial dimensions,<br />

we are familiar with the notion of a membrane, which is a two-dimensional surface<br />

that can separate two regions. The idea of a Dp-brane is to generalize this concept<br />

to consider an extended object of p dimensions.<br />

If the number of spatial dimensions of the D-brane is equal to the total number<br />

of spatial dimensions in the entire space-time, we say that we have a space-fi lling<br />

brane. There are three pertinent examples we can think of immediately to illustrate<br />

space-fi lling branes:<br />

• If space-time is just the three spatial dimensions and one time dimension<br />

we are used to from everyday life and special/general relativity, then a<br />

D3-brane would be a space-fi lling brane.<br />

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