Another surprising revelation was that superstring theories are not
just theories of one-dimensional objects. There are higher
dimensional objects in string theory with dimensions from zero
(points) to nine, called (p-branes). In terms of branes, what we
usually call a membrane would be a two-brane, a string is called a
one-brane and a point is called a zero-brane.
So what makes a p-brane? A p-brane is a spacetime object that is a
solution to the Einstein equation in the low energy limit of
superstring theory, with the energy density of the nongravitational
fields confined to some p-dimensional subspace of the nine space
dimensions in the theory. (Remember, superstring theory lives in ten
spacetime dimensions, which means one time dimension plus nine space
dimensions.) For example, in a solution with electric charge, if the
energy density in the electromagnetic field was distributed along a
line in spacetime, this one-dimensional line would be considered a p-
brane with p=1.
A special class of p-branes in string theory are called D branes.
Roughly speaking, a D brane is a p-brane where the ends of open
strings are localized on the brane. A D brane is like a collective
excitation of strings.
These objects took a long time to be discovered in string theory,
because they are buried deep in the mathematics of T-duality. D
branes are important in understanding black holes in string theory,
especially in counting the quantum states that lead to black hole
entropy, which was a very big accomplishment for string theory.
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