  In this work, we study the core concepts of Hawking radiation in the
astrophysical and analogue systems. We focus on the definitions of negative
frequencies and negative norms: their relationship and their role in the
particle creation process of the Hawking effect. We characterize the dispersion
relation by the signs of the frequency and the norm. We conclude that the most
natural frame for studying the Hawking effect is in the frame in which the
horizon is static, where the sign of the norm can be made equal to the sign of
the Doppler-shifted frequency in that frame. We use as examples the four most
successful experimental analogue systems: water waves, Bose-Einstein
condensates, polaritons fluids, and optical fibers.
