On the origin of the outgoing black hole modes

The question of how to account for the outgoing black hole modes without drawing upon a trans-Planckian reservoir at the horizon is addressed. It is argued that the outgoing modes must arise via conversion from ingoing modes. It is further argued that the back reaction must be included to avoid the conclusion that particle creation cannot occur in a strictly stationary background. The process of ‘‘mode conversion’’ is known in plasma physics by this name and in condensed matter physics as ‘‘Andreev reflection’’ or ‘‘branch conversion.’’ It is illustrated here in a linear Lorentz noninvariant model introduced by Unruh. The role of interactions and a physical short distance cutoff is then examined in the sonic black hole formed with helium-II. © 1996 The American Physical Society.