# Is it possible to create an optical medium where sound can travel faster than light?

The speed of light decreases with increase in the density of the optical medium whereas the speed of sound increases with increase in the density of the medium. So is there any possibility to construct a material where sound travels faster than Light.

• Well, if I just think of opaque iron block, that's the medium. But I guess you meant something transparent and really useful as an optical medium. – Victor Pira Dec 6 '15 at 22:35

It isn't possible to construct a material where the speed of sound is greater than the speed of light in a vacuum, i.e. $c$. That's because the interatomic forces that propagate the vibrations that make up the sound can themselves only propagate at the speed of light. Having said this you can get surprisingly close - in a neutron star the speed of sound can get pretty close to the speed of light.