Complex sentence: The conclusion of Witten was that Minkowski's space was also stabl, because, perturbations in the form of gravitational waves should not decrease the total energy, due to the fact that it is known that gravitational waves have positive energy.
Split into simpler sentences: The conclusion of Witten was that Minkowski's space was also stable. Since perturbations in the form of gravitational waves should not decrease the total energy, because it is known that gravitational waves have positive energy.