For what it's worth...

     To paraphrase...."Because energy cannot be created out of nothing, one of the partners in a... pair will have positive energy and the other partner negative energy.  The one with negative energy is condemned to be a short -lived virtual particle because real particles always have positive energy in normal situations. ...The gravitational field inside a black hole is so strong that even a real particle can have negative energy there. It is possible, if a black hole is present, for the particle with negative energy to fall into the black hole and become a real particle.  It then no longer has to annihilate its partner.  But its forsaken partner may fall into the black hole as well."