In his popular-physics book "The Universe in a Nutshell", Hawking presents the creation/annihilation of virtual particle pairs in two ways: (1) one particle has positive energy, the other negative energy, and (2) a matter/antimatter pair. This leads to the obvious conclusion that he is identifying antimatter as particles with negative energy, out of a sort of Dirac sea. However, this identification makes me wonder: (a) on one side, negative energy results in repulsive gravity, but (b) on the other side, antimatter is supposed to have the same gravitation as matter. Something is not right. What? While I am at it, another question on such pairs, whatever they are. One explanation for Hawking radiation is that one of the particles, either the one with negative energy and/or the antimatter one, falls into the black hole, while the other particle, matter with positive energy, escapes as radiation. However, these explanations never mention why it is the antimatter and/or particle with negative energy that falls into the hole, and the other one escapes, instead of the other way around. Why? Thanks in advance.