>v_phase = -k/ω = -1 + mω_+/ω >Note that if mω_+/ω > 1, then v_phase is positive
これは、位相速度が「外向き」になることがあると言っている。 ただし、その前で、
>v_group = -dk/dω = -1 >The group velocity agrees with condition (ii) above.
この(ii)というのは、
>Equivalently, way (ii) : demand that the radial group velocity of a wave packet, as measured by a physically well-behaved observer, be negative (i.e., signals can travel into the hole, but cannot come out).