the D0-branes in pairs.”

It is interesting to remark that a ${\bf Z}_2$ quotient of the above solution leads to the charge 2 Kaluza-Klein monopole and the charge 2 D6-brane. Such a ${\bf Z}_2$ quotient identifies the two sheets of the D2-brane, leading to a single D2-brane in the charge 2 D6-brane background. Thus, the confinement effect disappears in the charge 2 D8-brane background.

The future promise of the six-brane {#orient}
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In this final section, we give a preview of other results one may hope to obtain from a study of six-branes. This latter study is of quite a different nature, and involves the relationship between negative energy objects and the generalized second law of thermodynamics. Let us first state the issues involved, and we will then fore-shadow briefly why a study of six-branes, this time of the orientifold variety, should allow us to understand the situation.

The following discussion is motived by the recent appearances of negative energy objects in various large extra dimension scenarios (see, e.g. [@Rubakov:1983bb]-[@Corley:2001rt]). It is, of course, important that any extra-dimensional scenario be stable, and negative energies can be problematic. Placing a negative tension brane at a orbifold fixed plane has been shown 