Subject: Why is it impossible for a particule with a negative energy to escape a BH?

In an aswer to a previous question about hawking radiation, Ben Monreal writes "Empty 
space is full of virtual particle/antiparticle pairs; each pair has positive energy (...) Either 
the particle or the antiparticle gets sucked in, and the other one escapes with positive 
energy." But in "A Brief History of Time" Hawking clearly mentions the existence of 
negative energy particles (chapter 7).  I assume Hawking knows best. However, he does 
not explain why it is only the positive energy particle that can escape the black hole.

Re: Why is it impossible for a particule with a negative energy to escape a BH?