  Recently it was proposed that extraordinary light waves in hyperbolic
metamaterials exhibit two times physics behavior (Phys. Rev. Lett. 105, 067402,
2010). We report experimental observation of this effect via investigation of
gravity-like nonlinear optics of iron/cobalt-based ferrofluid hyperbolic
metamaterials. In addition to conventional temporal coordinate, the spatial
coordinate oriented along the optical axis of the metamaterial also exhibits
timelike character, which leads to very unusual two times physics behavior in
these systems.
