New answers tagged branes
In the presence of orientifold planes, every object (away from the orientifold fixed points) including a D-brane has "two copies". So it's a matter of convention whether this pair of copies is counted as "one D-brane" or "two D-branes". Because the whole half-space is a "copy" of the other one, it makes sense to only consider one-half of the space as "the ...
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