Abstract
Horava-Witten spacetimes necessarily include two branes of opposite tension.
If these branes are BPS we are led to a puzzle: a negative tension brane should
be unstable as it can loose energy by expanding, whereas a BPS brane should be
stable as it resides at a minimum of the energy. I provide a detailed analysis
of the energy of such braneworld spacetimes in 5 dimensions. This allows one to
show that such spacetimes are stable by using a non-perturbative positive energy
theorem, or by a more intuitive ADM approach.