Light experiences a moving medium as an effective gravitational field. In t
he limit of low medium velocities the medium flow plays the role of a magne
tic vector potential. We review the background of our theory [U. Leonhardt
and P. Piwnicki, Phys. Rev. A 60, 4301 (1999); Phys. Rev. Lett. 84, 822 (20
00)], including our proposal of making optical black holes.