It is known that in a medium of refractive index 'n' , the speed of light becomes c/n. So if we can make a medium of high refractive index, then the speed of light will be substantially less in that medium. Then it can be possible to accelerate particles inside that medium at speeds greater than the speed of light in that medium i.e. 'c/n'. Then what will be our observation from an stationary frame of reference ? What will be the observation from the particle's frame of reference ? Will the particle travel in time as it is crossing the speed of light in its surrounding medium ? Also one problem in reaching the speed of light 'c' in vacuum was that - as the speed of a particle approaches 'c' its relativistic mass increases and at v=c, its mass becomes infinite. But in the case as described above the particle is able to cross the speed of light in that medium (of refractive index 'n') without reaching 'c' , and thus its mass does not become infinite. So is time travel really possible for that particle in that medium of refractive index 'n' ?