Creating an analog gravitational potential in an optical system. (Left) A suitable change in the index of refraction in a moving medium creates an effective event horizon for photons that propagate with it. As indicated by the pink arrow, photons are forbidden from entering beyond the optical event horizon. The equation below relates the analog black body temperature of the optical white hole to how the change in the index varies in time (τ). (Right) Belgiorno et al. use a high-intensity light filament to perturb the index of refraction in glass and create an optical event horizon and measure the analog Hawking radiation emerging at right angles from the filament.