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String Theory and M-Theory

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594 Black holes in string theory<br />

Fig. 11.5. Lines with constant τ of the 3-center solution with identical charges.<br />

Multi-center solutions<br />

There are stationary multi-black holes solutions that are known as multicenter<br />

solutions. The reason these exist is that, when each of the black<br />

holes preserves the same supersymmetry charge, this supersymmetry is an<br />

unbroken symmetry of the multi-black hole system. In this case, the BPS<br />

condition result in a no-force condition, which means that the total force<br />

acting on each of the black holes due to the presence of the others exactly<br />

cancels, so that each of them can remain at rest. The various attractive <strong>and</strong><br />

repulsive forces due to gravity, scalar fields, <strong>and</strong> gauge fields are guaranteed<br />

to cancel out due to supersymmetry. This is true even though the field<br />

configurations are much more complicated than they are for a single black<br />

hole.<br />

The attractor equations can be generalized to the case where are blackhole<br />

horizons, with charges encoded in harmonic three-forms Γp, at different<br />

points xp. In the special case where all of the component black-holes have<br />

the same charges, the flow parameter τ is naturally defined to be<br />

τ = <br />

p<br />

1<br />

. (11.134)<br />

| x − xp |<br />

Surfaces with constant τ in the 3-center case are displayed in Fig. 11.5. In<br />

general, the charges are not identical. In order to describe such a solution,<br />

known as a multi-center solution, one has to consider a slightly generalized<br />

metric of the form<br />

ds 2 = −e 2U (dt + ωidx i ) 2 + e −2U dx · dx. (11.135)

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